Within the mapping area, there are seven wells are proposed. Single and ... Deva, Taufan, Rezky Zaky, Becky, Kemal, Bengki, Lia, Elyas, Robby, Fahmi,. Tommy ...
SEABED MORPHOLOGY MAPPING FOR JACK-UP DRILLING RIG EMPLACEMENT
UNDERGRADUATE THESIS
Academic Writing as one of the requirements to obtain the degree of SARJANA TEKNIK at Geodesy and Geomatics Engineering Study Program
By Faber William Marsahala 151 10 047
GEODESY AND GEOMATICS ENGINEERING STUDY PROGRAM FACULTY OF EARTH SCIENCE AND TECHNOLOGY BANDUNG INSTITUTE OF TECHNOLOGY 2015
AUTHORIZATION
UNDERGRADUATE THESIS
Undergraduate Thesis entitled “Seabed Morphology Mapping for Jack-up Drilling Rig Emplacement” is actually made by myself and has not been formerly written or submitted, both in ITB and other educational institutions.
Bandung, March 2015 Author,
Faber William Marsahala 15110047
Approved by, Supervisor I
Supervisor II
Dr. rer. nat. Wiwin Windupranata, M. Si 19740504 199903 1 001
Dr. Ir. Dwi Wisayantono, MT 19641008 199403 1 002
Authorized by, Head of Undergraduate Study Program of Geodesy and Geomatics Engineering Faculty of Earth Science and Technology Bandung Institute of Technology
Dr. Ir. Kosasih Prijatna, M. Sc 19600702 198810 1 001
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Abstract
Most of the potential oil and gas reserves are located mostly in offshore area. Jack-up drilling rig is one type of many other oil rig type. To locate a jack-up rig, a sea mapping process must be conducted to get information about the seabed surface and what kind of morphology there are. The seabed condition must be clearly identified in order to know if there are any endangering objects or obstructions within the emplacement area
The instruments used are single beam echo sounder, multi beam echo sounder, side scan sonar. Single beam echo sounder have a role to validate the result of the multi beam echo sounder. To know about the existing features, side scan sonar is used to make a high resolution seabed image. Also if the result must be interpreted by a geophysicist. To know about the soil structures, then a soil boring was conducted to know the site soil structure.
Within the mapping area, there are seven wells are proposed. Single and multi beam echo sounder provide bathymetric data and side scan sonar provides seabed features information. Side scan sonar also provides information about the existence of some rock/coral outcrops and high reflectivity seabed that may be an obstruction. Based on the bathymetric, seabed features, and sub-seabed features, the seven proposed wells comply the requirement of jack-up drilling rig emplacement, but based on soil boring data only well four, five, and six comply.
Keywords: singlebeam echosounder, multibeam echosounder, side scan sonar, Qinsy, bathymetry, seabed features, soil boring.
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Abstrak
Cadangan minyak dan gas bumi terletak di daratan maupun lepas pantai. Sebuah rig diperlukan untuk mengekploitasinya. Jack-up drilling rig merupakan salah salah satu jenisnya. Kondisi dasar laut harus teridentifikasi dengan baik untuk mengetahui apakah pada lokasi peletakan terdapat objek atau halangan yang dapat membahayakan, mulai dari lokasi masuknya jack-up rig hingga kaki rig melakukan penetrasi ke dasar laut.
Peralatan yang digunakan untuk mendapatkan informasi tersebut adalah single beam echo sounder, multi beam echo sounder, dan side scan sonar. Single beam echo sounder berfungsi untuk memvalidasi nilai kedalaman yang diperoleh oleh multi beam echo sounder. Untuk mengetahui fitur-fitur yang terdapat di dasar laut, side scan sonar digunakan untuk menciptakan gambar beresolusi tinggi. Hasilnya nantinya diinterpretasi oleh ahli geofisika. Proses soil boring dilaksanakan untuk dapat mengetahui struktur tanah dasar lautnya.
Pada area pemetaan, telah terdapat tujuh lokasi yang sudah diproposed. Single beam dan multi beam echo sounder menghasilkan data batimetri dan side scan sonar menghasilkan informasi fitur dasar laut. Hasil side scan sonar juga mendeteksi keberadaan akan fitur-fitur dasar laut berupa rock/coral outcrops dan high reflectivity seabed. Berdasarkan data batimetri dan citra dasar laut, keseluruhan proposed well memenuhi kriteria lokasi penempatan jack-up drilling rig, akan tetapi berdasarkan data soil boring, hanya proposed well empat, lima, dan enam yang memenuhi. Kata kunci: single beam echo sounder, multi beam echo sounder, side scan sonar, qinsy, batimetri, fitur dasar laut, soil boring.
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Acknowledgement
The biggest gratitude I would like to express to my Lord, Jesus Christ, for all his blessings.
First and foremost, I would say my gratitude to my mom and dad, Mr. Djunggu Harungguan Sitorus and Ms. Ria Minar Sitohang for any kind of their supports, morraly and finacialy, while I was working on this undergraduate thesis. To my elder sister, Polina Ursula Melissa Sitorus for her encouragement every day. To my brother, Timothy Sintong Marullam Sitorus for his support, also to my beautiful younger sister, Kezia Alusianna Michella Sitorus for her prayers and laughs.
Secondly I would like to express my gratitude to Dr. rer. nat. Wiwin Windupranata, M. Si as my primary supervisor. Thank you for all the advice and suggestion for this thesis. Also to Dr. Ir. Dwi Wisayantono, MT as my secondary supervisor for all the advice and input to my undergraduate thesis.
I would like to deliver my gratitude to PT. Pageo Utama for facilitating this undergraduate thesis. Special thanks to Mr. Sanny Samudra and Mr. Agung Prasetyo for all the input and evaluation during this research work, and to Mr. Marino Abubakar, Mr. Deni Satria, Mr. Imra, Mr. Angga, Kang Dindin, and Kang Sukma for the ideas and knowledge you have shared.
To Dr. Ir. Kosasih Prijatna, M. Sc as the Head of Undergraduate Study Program of Geodesy and Geomatics Engineering for the knowledge that has been given to me during the lectures. For Dr. Ir. Agustinus Bambang Setiadji, M. Si as my faculty trustee, and all lecturers in Geodesy and Geomatics Engineering Department. Special thanks to Mr. Dudung Suhendar as administration officer.
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For my dearest friends, “kamerad IMG-ITB” and especially all members of Ikatan Mahasiswa Geodesi Institut Teknologi Bandung 2010, Gilang, Said, Adi, Zandy, Yafet, Husein, Aan, Arifin S., Joshua, Tari, Windie, Caroline, Fabian, for all the supports and laughs during the lectures. For all my fellow graduates Miko, Najib, Deva, Taufan, Rezky Zaky, Becky, Kemal, Bengki, Lia, Elyas, Robby, Fahmi, Tommy, Aat, Dirga, Faisal, Marlina, Benyamin, Mei, and, Mila thanks for the companion during the research work.
Thank you for all Badan Pengurus Harian IMG-ITB 2013/2014 and Extracampus Division members that have become my learning companion inside and outside the classroom.
To all friends that has involved in this undergraduate thesis that the author cannot explain one by one. Thank you for all the contributions and suggestions for the author.
Bandung, February 2015
Faber William Marsahala Sitorus
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Table of Content
Abstract ..................................................................................................................... i Abstrak..................................................................................................................... ii Acknowledgement ................................................................................................... iii Table of Content ....................................................................................................... v List of Figures ........................................................................................................ vii List of Tables .......................................................................................................... ix Chapter 1 Introduction ............................................................................................. 1 1.1 Background ............................................................................................ 1 1.2 Research Questions ................................................................................ 2 1.3 Research Objectives ............................................................................... 2 1.4 Scope of Research .................................................................................. 3 1.5 Research Methodology ........................................................................... 4 1.6 Structure and Contents ........................................................................... 6 Chapter 2 Methods and Data .................................................................................... 7 2.1 Site Survey............................................................................................. 7 2.2 Jack-up Drilling Rig ............................................................................. 11 2.2.1 Components of Jack-up Drilling Rig ........................................... 12 2.2.2 Jack-up Drilling Rig Emplacement Phases .................................. 13 2.2.3 Jack-up Drilling Rig Emplacement Criteria ................................. 14 2.2.4 Jack-up Drilling Rig Emplacement Proposed Locations .............. 16 2.3 Acquisition Instruments and Data Processing ....................................... 16 2.3.1 Instrument’s Sensor Static Offset ................................................ 17 2.3.2 Echo Sounder .............................................................................. 17 2.3.3 Side Scan Sonar .......................................................................... 28 2.3.4 Sub-Bottom Profiler .................................................................... 30 2.3.5 Soil Boring .................................................................................. 30 2.3.6 Magnetometer ............................................................................. 31 v
Chapter 3 Result and Discussion ............................................................................. 33 3.1 Bathymetry ........................................................................................... 33 3.1.1 Single Beam Echo Sounder ..........................................................34 3.1.2 Multibeam Echo Sounder.............................................................35 3.2 Seabed Features .................................................................................... 40 3.3 Sub-seabed Features ............................................................................. 44 3.3.1 Sub-bottom Profiler .....................................................................44 3.3.2 Magnetometer ..............................................................................45 3.3.3 Soil Boring ..................................................................................45 Chapter 4 Conclusions and Recommendations ........................................................ 47 4.1 Conclusions .......................................................................................... 47 4.2 Recommendations................................................................................. 48 References .............................................................................................................. 49 Appendix A: Multi Beam Echo Sounder Bathymetric Map ....................................... I Appendix B: Multi Beam Echo Sounder Seabed Image ............................................ II
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List of Figures
Figure 1.1 Study Case Location ................................................................................ 3 Figure 1.2 Research Methodology Flowchart ............................................................ 5 Figure 2.1 Site Survey Process ............................................................................... 10 Figure 2.2 Typical Jack-up Drilling Unit ................................................................ 11 Figure 2.3 Jack-up Drilling Rig with Spudcans ....................................................... 12 Figure 2.4 General Jack-up Rig Emplacement Phases ............................................. 14 Figure 2.5 Static Offset for the Instrument’s Sensor ................................................ 17 Figure 2.6 Multibeam Echo Sounder Working Principal ......................................... 23 Figure 2.7 Vessel Local Coordinate System (X, Y, and Z) ...................................... 24 Figure 2.8 Roll Motion Effect to the Depth Value................................................... 24 Figure 2.9 Sail Pattern of Roll Calibration .............................................................. 25 Figure 2.10 Pitch Motion Effect to the Depth Value ............................................... 25 Figure 2.11 Sail Pattern of Pitch Calibration ........................................................... 26 Figure 2.12 Yaw Motion Effect to the Position and Depth Value ............................ 26 Figure 2.13 Sail Pattern of Yaw Calibration............................................................ 27 Figure 2.14 Side Scan Sonar Working Principal ..................................................... 29 Figure 2.15 Piston Corer ......................................................................................... 31 Figure 3.1 Single Beam Echo Sounder Plotting Result ............................................ 34 Figure 3.2 Roll, Pitch, and Yaw Calibration Result (a) Roll, (b) Pitch, (c) Yaw ...... 36 Figure 3.3 Multibeam Echo Sounder Depth Spots................................................... 37 Figure 3.4 Multibeam Echo Sounder Image ............................................................ 37
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Figure 3.5 Multibeam Echo Sounder Bad Data (a) DTM, (b) Profile View, (c) Plane View ........................................................................................... 39 Figure 3.6 3-dimensional Visualization of The Bathymetric Data (a) 3-D View, (b) Location of Proposed Wells ............................................................. 39 Figure 3.7 Side Scan Sonar Images Mosaic Result .................................................. 41 Figure 3.8 Side Scan Sonar (SSS) Processing Result (a) Side Scan Sonar Mosaic, (b) Seabed Features Interpretation, (c) Side Scan Sonar (SSS) Mosaic and Interpreted Seabed Features Overlay .............................................. 41 Figure 3.9 Side Sonar Images Around Proposed Well Depict (a) High Reflectivity Seabed, (b) Coral Outcrop..................................................................... 42 Figure 3.10 Conditions around Proposed Wells (a) Seabed Features (b) Overlaid to MBES Image .................................................................................... 44 Figure 3.11 Magnetometer Data on Proposed Well 1............................................... 45 Figure 3.12 Soil Boring Result (a) Borehole Log, (b) Tabulation of Design Parameters ............................................................................................ 46
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List of Tables
Table 2.1 Conditions to Be Addressed By a Marine Site Survey (OGP, 2013) .......... 8 Table 2.2 Position of the Proposed Wells................................................................ 16 Table 2.3 Single Beam Echo Sounder Data............................................................. 20 Table 2.4 AutoCAD Scrip file Content ................................................................... 22 Table 3.1 Single Beam Echo Sounder High and Low Frequency Depth .................. 35 Table 3.2 Roll, Pitch, and Yaw Calibration Angle .................................................. 36 Table 3.3 Water Depth in Proposed Wells .............................................................. 40
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Chapter 1
Introduction
1.1 Background In recent years there has been a large increase in offshore oil and gas exploration activities. About 60% of the work is being performed with jack-up drilling rigs (Dennes, 1984). Some variabels must be considered i.e. rig dimension, jack-up rig type, and the seabed morphology. This research is focused on to determine the seabed morphology, so seabed mapping must be conducted to make sure the rig emplacement process is safe.
Indonesia is an archipelagic country, which consists of more than 13.466 islands and has approximately 3.7 millions barrel oil reserves. Indonesia needs some researchers and experts in sea mapping field to support the national infrastructure development, especially development in the coastal and ocean areas, scoping either the sea surface, seabed surface or sub-seabed surface. This research will focus on the discussion about the procedures in moving and placing on Mobile Offshore Drilling Unit (MODU) which named Jack-up Drilling Rig.
This research will focus on the description of the seabed morphology which was obtained by single and multi beam echo sounder and how the equipments works. The result will be combined with another data such as side scan sonar, sub-bottom profiler, magnetometer, and soil sampling with soil boring method. This whole process is considered to be done so that the position of the jack-up drilling rig’s legs could be placed and fitted around the proposed area before.
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Also to decrease the risk that may occur due to the seabed and sub-seabed condition, such as (Basuki, 1986): 1. A disadvantage seabed soil conditions. 2. Some gas pockets that contain a high pressured gas. 3. Incompatible seabed topography condition. 4. Some things that has been existing on the seabed and may cause a danger.
Information of bathymetry and seabed morphology information is important in order to make a rig’s maneuver design and which area is feasible enough for the jack-up type drilling rig’s emplacement. This research may provide some informations about seabed topography and its features, and also the perspective from the geoscientist to understand what kind of seabed condition that appropriate for this kind of drilling rigs. This research may result a proper procedure to be used in seabed clearance survey in seabed morphology mapping for jack-up drilling rig emplacement.
1.2 Research Questions The research questions of this Undergraduate Thesis are: 1. What is the seabed topography and morphology within the case study area and what is the comparison between the safe area and area that has obstructions? 2. Does the study area, including the proposed wells comply the jack-up drilling rig emplacement criteria?
1.3 Research Objectives The objectives of this research are to determine the seabed morphology at and around the proposed well location with accurate water depth, to assess shallow geotechnical condition for the jack-up drilling rig’s emplacement combined with the soil boring results, to assess the shallow sub-bottom geological condition including the presence of any shallow sub-bottom anomalous such as shallow gas and/or shallow faults.
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1.4 Scope of Research The scope of this undergraduate thesis research are: 1. The survey area is located in the eastern part of Central Sulawesi offshore (Figure 1.1).
Figure 1.1 Study Case Location 2. The data was processed are single beam echo sounder and multibeam echo sounder. Side scan sonar, sub-bottom, magnetometer, and soil boring are supporting. 3. Synchronize the processed data with sub-bottom and magnetic anomalous informations within the survey area. 4. Describe the role of each technology in obtaining the informations. 5. Definition of oil and gas drilling rig jack-up type, its parts, and emplacement stages. 6. Seabed and sub-seabed hazard identification. 7. Water depth analysis combined with other supporting informations. 8. Describing the acquisition procedures and the result of sub-bottom profiler and magnetometer as interpreted by the geophysicist and soil sampling by soil boring method as interpreted by the geotechnical engineers.
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1.5 Research Methodology Methodology of the research is literature study and data processing. The methodology started from studying about site survey activity in offshore area. The literature and references was sourced from related research, international and domestic text books and research papers, also some research in the internet. The literature study interspersed with discussions with the supervisors and practioners in the industry. The research continued to data procurement stages which was provided by PT. Pageo Utama. The data is raw data from acquisition activity and some geophysicist’s interpretation results. Started from multi-beam echo sounder data calibration and continued to filtering. In this process, the sounding tracks must be suitable to particular calibration factor. After multi-beam echo sounder data processing, the research continued to single beam echo sounder data processing and side scan sonar images mosaicking and make recommendation for rig’s manuever and emplacement based on bathymetric information from echo sounder, seabed features information from side scan sonar image, sub-bottom features from subbottom profiler and soil boring, and magnetic anomalous from magnetometer. The methodology implemented in this research can be seen in Figure 1.2.
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Start
Literature Study
Data Procurement Raw Data Singlebeam Echosounder Multibeam Echosounder Data Processing
Interpreted Data Side Scan Sonar Sub-bottom Profiler Magnetometer Soil Boring
Bathymetry
Seabed Features
Sub-seabed Features
no
Quality Control
yes Results and Proposed Well Analysis
Conclusion and Recommendation
Finish
Figure 1.2 Research Methodology Flowchart
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1.6 Structure and Contents This Undergraduate Thesis is divided into four chapters:
Chapter 1 Introduction This chapter would contain and discuss the background, research purpose, and the scope of research, research methods, and structure and the contents of the research.
Chapter 2 Methods and Data In this chapter the basic theory and concept that are used in this undergraduate thesis will be described, particularly the theory and methods which are used as the scientific reference. The basic theory about jack-up drilling rig, site survey, the basic theory about acoustic wave propagation with its parameters that has been the basic principles of the data acquisition instruments. Also how to classify the formation of the seabed morphology.
Chapter 3 Results and Discussions This chapter presents the result of the data processing, specifically the echo sounders processing result will be figured out and retold as an intact and coherent story. Also included the data from side scan sonar, sub-bottom profiler, magnetometer, and soil sampling by soil boring methods.
Chapter 4 Conclusion and Recommendation This chapter answers the purpose of the research based on the findings, also provides recommendation for future research sustainability.
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Chapter 2
Methods and Data
2.1 Site Survey This following subchapter, the explanation about site survey was summarized from International Association of Oil and Gas Producers (OGP) (2013). In seabed morphology mapping, site survey is one of the most important survey phase that must be done. Site survey are performed to minimise the risk of harm to personnel and equipment, and to protect the natural environment (OGP, 2013). Every site surveys have their own objectives that is to identify all possible constraints and hazards from man-made, natural and geological features which may affect all of the operational or environmental integrity of a proposed drilling well operation, and also to allow appropriate operational practices to be put in place to mitigate any risks identified. In addition, the proposed site survey area that contains the proposed wells should be of adequate coverage to plan any potential relief well locations and provide sufficient data to assess the fully potential top-hole drilling hazards at these locations.
Another objective of site survey activity is to assess the physical environment, depend on the physical environment and the intended operation, the scope of a site survey may need to review any, or and all of the following as listed in Table 2.1.
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Table 2.1 Conditions to Be Addressed By a Marine Site Survey (OGP, 2013) Man-made Features Platforms: active, abandoned, or toppled Pipelines: on or buried below the seabed Power and umbilical lines
Natural Seabed Features
Subsurface Geological Features
Seabed topography and relief
Sedimentary sequence
Seafloor sediments
Stratigraphy
Sand: banks, waves, and
Shallow gas charged
mega-ripples
intervals
Communications cables and
Mud: flows, gullies
Archaeological remains
volcanoes, lumps, lobes
Gas chimneys
Fault escarpments
Shallow water flow zones
Diapiric structures
Over-pressure zones
Gas vents and pockmarks
Buried infilled channels
Unstable slopes
Boulder beds
Wellheads and abandoned well location Manifolds and templates Pipeline terminations, valves and protection frames Subsea isolation valves Rock dumps, Ordnance and chemical dumping grounds Scour protection material
Jack-up rig footprints
Slumps
Collapse features Chemosynthetic communities
Buried slumps and mass transport complexes Gas Hydrate sones and hydrate soils Faults
Non-oil and gas infrastructure such as
Gas hydrate mounds and
Erosion and truncation
navigation buoys, wind
reefs, hard grounds
surfaces
Rock outcrops, pinnacles and
Sal or mud diapirs and
boulders
diatremes
turbines etc. Shipwrecks
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In this research, no manmade and sub-surface geological faults are identified, only coral outcrop and high reflectivity seabed, which are one of the natural seabed features. Coral outcrop is a large mass of rock that stands above the seabed surface and high reflectivity seabed may be interpreted as sediments that may be dangerous for the rig. Site survey are performed before the installation of a platform jacket, template, jack-up drilling rig or manifold to establish the precise and detailed bathymetric, topographic and geotechnical nature of the location. As figured in Figure 2.1 and 2.2, the Site Survey is performed for the following reasons (Lekkerkerk, 2006a): 1. To identify any sub surface objects which would damage the installation of an underwater structure, 2. To identify shallow gas areas prior to a drill rig entry, 3. To identify, with the means of an Remotely Operated Vehicle (ROV), hazardous objects on the seafloor which may endanger the installation of underwater structures, 4. To identify with seismic equipment the sub strata of the seabed to identify faults for the determination of mineral resources 5. Investigate slope stability.
A site-specific survey for a production platform, including jack-up drilling rig, must cover at least the area within a 1,000 meter radius of the proposed drilling rig or the platform location because in jack-up drilling case, technically the spudcan (As shown in Figure 2.4) will make a contact with the seabed within 400 meter radius of the proposed well, also 100 meter beyond the furthest anticipated anchor location for a drilling rig, derrick barge and pipe lay barge, whichever is greater. It might be required to do the survey in a larger area if sensitive habitat or other conditions need to be evaluated. Primary and tie line spacing for all acoustic data acquisition systems for a platform site survey may not exceed 150 meters intervals, depending on the water depth.
Based on Oil and Gas Producers Standard Guidelines for the conduct of offshore drilling hazard site survey report no. 373-18-1 April 2013 version 1.2, the scope of 9
site survey must include a review of all seafloor conditions and geology to a depth at least 200 meters below the preferred setting depth of the first pressure containment string or to a depth of 1000 meters below the seabed, whichever is greater. Identification and assessment process of all relevant geological features is suggested be performed within the context of a geological model that takes into account depositional and post-depositional processes. The output of the site survey activity is a report that must include a discussion of all relevant geological and/or man-made features that have a direct bearing on operational risk.
Generally a site survey activity process may be considered to consist of four phases as shown in Figure 2.1.
Figure 2.1 Site Survey Process (OGP, 2013)
From Figure 2.1 above, the desk study should be considered as an integral part of the project planning process. In this phase, some decisions will be made as to whether new data – and which type of data – must be acquired. In deep water areas, according to OGP (2013) the water area that has >150 meters depth, the desk study and any ensuing acquisition may need to address a semi-regional scope to consider topographic or geological matters that may be a threat to operations from outside of the direct area of proposed operations. The next phase is the acquisition of new data coverage. All the existing and the new data that has been processed or reprocessed to improve their value, and interpreted to produce an integrated geological model of the 10
seabed and subsurface conditions are done in the third phase. The last phase holds the integrated report that describes the condition and operational risk identified across the site and – specifically – at the proposed drilling/well location.
2.2 Jack-up Drilling Rig Oil rig is a structure and associated machinery used in drilling for oil and/or gas. A drilling rig can be also described as a machine which creates a hole in the earth’s sub-surface. Offshore drilling rig commonly are divided to fixed platforms, compliant towers, semi-submersible platform, jack-up drilling rig, drill ships, floating production systems, and spar platforms. They are divided based on the capability to accommodate the water depth condition. The type of drilling rig discussed in this research is the jack-up type one or also called jack-up drilling rig. Jack-up rig is a mobile drilling rig and has a long structures which is lowered to and into the seabed and raising the rig’s body out of water. This rig is a type of the many other rig’s types and it operates only in offshore area and need bottom supporting system to support the body.
Figure 2.2 Typical Jack-up Drilling Unit (Casidy, 2011)
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The picture of a jack-up drilling rig may be presented in Figure 2.2. The main characteristic of this type of rig is this rig has the capability to adjust its elevation until it fits the water depth in the drilling activities location. Jack-up rig will only operate in shallow water area with average depth within 90-110 meters and maximum depth of about 150 meters (Chakrabakti, 2005); where the depth is classified as a shallow water; and therefore the water depth in which a rig can operate will depend on the extent of the penetration of its leg into the seabed and the air gap required at the give location (Dennes, 1984).
Jack-up drilling has some advantage in the shallow water area because it is a stable work platform, relatively inexpensive mobilization costs, and its availability. Jack-up drilling rig is the stable one because its legs come in contact to the seabed and free of the heaving motion that is commonly experienced by any other drilling rig type.. In general type of jack-up unit, the legs will penetrate down to the sub-seabed layer about 12 meters under the seabed and a condition where quite seabed is hard enough to support the legs is necessary to be assessed.
2.2.1 Components of Jack-up Drilling Rig
Figure 2.3 Jack-up Drilling Rig with Spudcans (Chakrabakti, 2005)
Figure 2.3 describes the components of jack-up drilling rig that generally can be found in general. It consists of seven components as follows:
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1. Derrick, load-bearing tower like framework over an oil/gas well which holds the hoisting and lowering equipment. 2. Cantilever Beam, the platform carrying the drill floor and derrick. Skids in and out of the rig. 3. Leg, the three legs of a jack-up rig are lattice structures made from vertical, horizontal and diagonal tubes. They can move up and down using jacking motors/gears. 4. Quarters, a room where the rig’s crew lives, usually it can be up to 120 men onboard. 5. Helideck, for reception of helicopters delivering supplies and change of crew. 6. Hull, the main structure of the rig. Triangular rigid and water tight. 7. Spud can, circular shoes of the rig’s legs. It is designed to penetrate up to the determined depth under the seabed for good foothold. (Source: http://www.ppl.com.sg/technology_jack-up.htm?reloaded=true) 2.2.2 Jack-up Drilling Rig Emplacement Phases The jack-up drilling rig emplacement consist of several phases. Firstly, the jack-up unit is towed from the jetty and assisted by around 2 up to 4 tug boats. In this phase, the jacks down until the hull is in contact with the water and then free to float under tow to the next installation location. Some jack-ups usually have an independent propulsion to move itself in the water, but on moving location require the support of the tug boats. Before moving the unit, it must be clear what Geodetic Datum is used in determining the Drilling target position. The Datum might be different with the one the positioning system is using considering the original Seismic data was done a long time ago. It is important to know the transformation parameters from the vessel’s positioning system is used to the Geodetic Datum used while planning the drilling programme.
The next phase is when the jack-up unit is approaching the proposed well. When a rig arrive at a drilling location, the jack-up legs will be mechanically lowered (Figure 2.4 (a)) until a contact with the seabed is made and nominal penetration of the legs has been achieved. Contact is made within radius 400 meters from the
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proposed drilling well and the legs are dragged until they reach the fixed location (Figure 2.4 (b)). After the position of the unit is cross-checked, the leg are jacked down and penetrate going through the seabed into the sub seabed. The legs will keep penetrating until sufficient bearing resistance is encountered by the footings, the hull will start to rise clear of the water (Figure 2.4 (c) and Figure 2.4 (d)).
(A) Jack-up Rig Arrival
(b) Legs Contact the Seabed
(c) Jack-up Rig arrives at the Proposed Well (d) Jack-up’s Legs Penetrate to Seabed
Figure 2.4 General Jack-up Rig Emplacement Phases (Ti, 2009) 2.2.3 Jack-up Drilling Rig Emplacement Criteria The selected route for jack-up rig moving must be the safest and shortest possible. The jack-up drilling rig’s stability will be varying depend on the cantilever type in correlation to the behavior and mechanical characteristics of the seabed soil. The soil investigation must be held before the jack-up unit is installed.
In the jack-up drilling rig emplacement process, some things need to be considered related to the seabed conditions itself. Information about characteristic of the seabed 14
is substantial to know about seabed and sub-seabed geological condition, such as stratigraphy, shallow faults, sediment strength, old river and glaciers, and water bottom anomalies and non-geological condition such as man-made objects. The location must be spared from those conditions so they do not endanger the rig during the emplacement and production process.
There are four conditions to determine the location of the jack-up drilling rig in order to be feasible for the emplacement phases (Mandasari, 2012): 1. If the location is a new drilling location, then the conductor (well) location and the position of any subsea structures (debris, existing pipeline and/or subsea cables) around the platform must be considered and a soil investigation survey may be held within the location. 2. If the location has already been used by the same structure before, then it is recommended to use the same position as the existing jack-up footprint. 3. If the location is an open sea drilling, the existing subsea structure, also the bathymetry, any slope or channel around the drilling location must be considered to know whether the seabed is relatively flat or not, also the soil investigation as a supporting data to know the sub-seabed geological condition. 4. Within proposed wells must have maximum 150 meters water depth and no existing features that may endanger the rig’s structure. While choosing the rig’s emplacement location, a seabed clearance survey must be done to make sure the site is clear from these following things (Mandasari, 2012): 1. Existing subsea cables and pipelines. 2. Airplane and/or ship’s wrecks. 3. Debris, rubbish/garbage or pieces of material that are left or unwanted from the previous installation. 4. Pockmark and pockmark cluster, craters in the seabed caused by fluids (gas and liquids) erupting and streaming through the sediments. 5. Mound, a large pile of soils or stones.
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6. Shallow gas and water flow, an outburst of the natural gas or water that may harm the jack-up rig’s foundation. 7. Booby trap. 8. Rock outcrop, a large mass of rock that stands above the seabed surface. 9. Mudflows, seabed mud outburst. 10. High reflectivity seabed, may be interpreted as sediments that may be dangerous for the rig. Besides some few things explained above, then some further investigations is necessary upon the seabed surface to ensure the emplacement area is genuinely clear from any obstructions.
2.2.4 Jack-up Drilling Rig Emplacement Proposed Locations The jack-up rig will be located in some certain locations that has been proposed by the geologist and geophysicist. The location is named “proposed well”. The rig is planned to be located in seven locations and each location has grid coordinates in Universal Transverse Mercator (UTM) projection system using WGS-72 datum as listed in Table 2.2.
Table 2.2 Position of the Proposed Wells Well
Easting
Northing
1
398137.93m
9804317.33m
2
397915.72m
9804307.40m
3
397917.52m
9804307.34 m
4
397620.52m
9804527.03m
5
397484.12m
9804630.83m
6
397318.42m
9804643.65m
7
397545.72m
9804358.46m
2.3 Acquisition Instruments and Data Processing The data has been derived cannot be replaced from the role of some certain technologies to obtain the information. The technologies or instruments that get
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involved in this research are single and multibeam echo sounders, side scan sonar, sub-bottom profiler, magnetometer and soil sampling with soil boring methods. 2.3.1 Instrument’s Sensor Static Offset The single beam and multibeam echo sounder’s transducers, side scan sonar tow fish, sub-bottom profiler, and magnetometer are commonly installed at different locations on the survey vessel. All the instruments must refer to a certain referenced point by vectors and expressed in the vessel’s body-frame system as shown in Figure 2.5.
Figure 2.5 Static Offset for the Instrument’s Sensor (Godin, 1998) It can be said the offset static is to combine all the acquisition’s sensors into an integrated system. The determination of offset static may use classic measuring tape, total station, and differential GPS. To determine the vessel’s heading, the gyrocompass must be calibrated while vessel is still leaning on the harbour by applying terrestrial measurement method.
2.3.2 Echo Sounder One of the methods to get an idea about the topography of the seafloor that describes the closest condition to the reality may be reached by measuring the depth of the sounding points and the horizontal position on the sea surface in some certain ways. This kind of work is commonly known as pemeruman or soundings which is a part of sea surveying and mapping of the sea.
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One type of the equipment that is often used to measure the depth is called an Echo Sounder. This equipment works by utilizing the wave propagation characteristics in the vertical direction from the sea surface down to the seafloor. The sea depth may be determined through the following relation:
(1)
Where: d = a measured water depth V = sound velocity in sea water
t = time interval between the emitted and received sound wave 2.3.2.1 Single Beam Echo Sounder Single beam echo sounders use one emitting and receiving “transducer”, which releases a series of energy pulses in the form of sound waves, “ensonifying” (filling with sound) a small area underneath the boat. If multiplied by the sound velocity, the time lag between the sound that being emitted and its returning echo is used to calculate water depth beneath the transducer. Single beam echo sounder, will be summoned SBES, usually has beamwidth of the order of 30 degree. Narrow beam echo sounder operation requires the SBES transducer to be mechanically or electronically stabilized for vessel’s roll and pitch motion.
Single beam echo sounder needs to be calibrated before it begins to be used. The calibration is called “bar check”, the bar check will measure the actual depth relative to a recorded depth on the echo sounder with an assumed average sound velocity in a particular water. Bar check calibration is done by lowering a bar or plate underneath the transducer at several depth (for instance, every two meters) either recording the depth error to apply afterwards during the data processing or forcing the echo sounder to record the correct depth from the bar or plate through the adjustment of the sound velocity parameter. In such cases the value adopted for calibration is the mean value of the observations. (Jong, 2010). The sound velocity determination is much more critical on multibeam echo sounder survey.
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In general, a single beam echo sounder must also corrected from the heave motion and the magnitude of the heave is calculated by a motion sensor or heave compensator. The correction of the heave compensates for the vertical displacement of the sounding vessel from the mean water surface. Heave is the oscillatory rise and fall of a vessel due to the vessel’s body is being lifted by the force of the sea and it can be compensated during the acquisition or filtered manually thereafter.
Like the basic operation of echo sounding instruments, SBES is usually operated in mode of dual frequency. According to (Lekkerkerk, 2006b); a practical book for offshore surveying; if both the high frequency and low frequency transducers have to be installed, both of them must be mounted as close as possible. The installation is a necessity to enable comparison between both channels. If the SBES applied in shallow waters, the transducer should be either mounted or retraceable above the deepest point of the vessel. A list of mounting considerations is provided below (Lekkerkerk, 2006b): 1. The transducer should ideally be located at a third or half the length of the vessel measured from the bow. 2. It should also be installed as close to the centreline as possible. This way the effect of roll and pitch will be minimized. 3. The transducer should have an unobstructed field of vision over the entire bundle angle and not be obstructed by the hull of the vessel. 4. The position of the transducer in relation to other survey system should be determined as accurately as possible. 5. The transducer should be located as far away as possible from source of noise (propeller, engine, aeration) that generate noise at or around the transducer’s frequency of operation. 6. In order to optimize the performance of the transducer, it should protrude somewhat from the hull of the vessel (1 mm) and be angled slightly forward (0.50). This ensures positive pressure on the active transducer face, which is vital for the effective transmission of the acoustic signal into the water. If aeration under and along the ship’s hull is a problem, the transducer should
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be installed clear of the hull; possibly in a housing that can be refracted when the transducer is not in use. The data used in this undergraduate thesis research was derived from PT. Pageo Utama as a secondary data. The SBES data is used to validate the depth results from Multibeam Echo Sounder acquisition and data processing. The survey was conducted in an offshore area in Central Sulawesi and data was acquired using a dual frequency Single beam echo sounder Odom Echotrac MK III, also this system was equipped by a heave compensator to compensate the heave motion. The transducer used was a dual frequency transducer, the 12 kHz low frequency and 200 kHz high frequency. The surveyed area is 2 km x 2 km wide and consist of 27 main lines and 17 cross lines. That number of lines was applied not only for Single Beam Echo Sounder but also Multibeam Echo Sounder, Side Scan Sonar, Sub-bottom Profiler, and Magnetometer. The data is a database from the acquisition and contain not only Single beam echo sounder, but also another instruments and filtered to generate only the Single beam echo sounder data. The file is data acquisition per survey lane, as shown in Table 2.3.
Table 2.3 Single Beam Echo Sounder Data Time
6:12:37
Date
6/20/2014
Easting
398904.3 meters
Northing
9803493.99 meters
High Frequency Depth
165.1 meters
Low Frequency Depth
170.3 meters
Tide Prediction Value
2.764 meters
Water Depth After Correction
162.3 meters
Table 2.3 shows the sample point information of the single beam echo sounder data from one of the survey lanes. It is a cell formatted data and was processed using Microsoft Excel Software to generate the depth value which referenced to the chart datum. The chart datum used is Lowest Astronomical Tide (LAT). LAT is the lowest tide level which can be predicted to occur under average meteorological conditions 20
and under any combination of astronomical conditions (IHO Dictionary, S-32, 5th Edition, 2936). In this area, the chart datum is 2.32 meters below the mean sea level (MSL). In addition to the depth data, each point also have horizontal position which was obtained from the differential GPS (DGPS) and has been projected to Universal Transverse Mercator (UTM) map projection system. Before the processing, the data must free of undefined depth points.
For engineering purpose, related to this research is to emplace a jack-up drilling rig construction, the water depth value used is the high frequency depth because it has a better resolution than the low frequency. In accordance with acoustic wave propagation theory, the higher the frequency the shorter the length of wavelength. It also applies vice versa, the lower the frequency, the longer the length of the wave length. So it can be said frequency is inversely proportional to the ability of the sound waves to penetrate through the water and seabed. But, in some points, there are some low frequency depths that a shallower value than the high frequency value. Besides it affects the depth resolution, it also affects the sensitivity of the acoustic wave to detect objects.
Table 2.3 shows the observed data and depth value after the tide correction. The depth value after tide correction derived from calculation of high frequency depth value minus the predicted tide value. So then, point coordinates 397448,6mE and 9803427mN has a depth value of 162.3 meter. The time is displayed in Table 2.3 is the local time whereas the predicted tide refers to UTC or Greenwich time, so the processing process must be done careful enough to obtain the right tide.
After the processing is done and the tide reduced depth has been derived, then the data must be plotted. The cell formatted SBES data then must be converted to script file (.scr) AutoCAD format, because the plotting process is using AutoCAD Map 3D software belongs to PT. Pageo Utama. The script file making first is made using Microsoft Office Excel also belongs to PT. Pageo Utama, the script making based on the type of the vector, alignment, point positions (coordinates), size, rotation, and the
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vector that want to be displayed, in this case it is a depth value. The content of the script file’s shown in this following Table 2.4.
Table 2.4 AutoCAD Scrip file Content Type of the Vector
Text
Centre Point
Justify
Text Alignment
Centre
Point Position (Coordinates)
398904.3,9803493.99
Text Height
2
Rotation Angle
0
Depth Value
162.3 m
The data in Table 2.4 above is the same point as in Table 2.3. Because AutoCAD software produce a graphic, either vector or raster, so the all the depth value was presented in computer-aided design (CAD) format.
2.3.2.2 Multibeam Echo Sounder Multibeam echo sounders are used to measure multiple depths from one transducer array. The depths are measured along a swath fanning out from the transducer array. Multibeam echo sounders are characterized by the following parameters: 1. Frequency, typically ranging from 12 to 500 kHz. 2. Swath sector/swath width, typically ranging from 90 to 180 0. (2 to 12 x water depth) 3. Beam width, typically ranging from 0.5 to 30 4. Range resolution, depending on depth, best resolution 1—15 cm.
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Figure 2.6 Multibeam Echo Sounder Working Principal An acoustic signal is sent from the transducer to the bottom. Depending on the type of multibeam, this signal will cover the entire swath, or just part of it, can be seen in Figure 2.6. The signal reflects on the bottom and returns back to the transducer. Every beam which is transmitted by the multibeam echo sounder transducer, has one transmitter and receiver. The multibeam echo sounder transducer consist of a set of element that transmit sound pulse within different angles.
Multibeam echo sounder calibration intend to correct the depth data so it has high level of confidence, usually the calibration implemented before the survey was conducted. The calibration is aimed to determine the error value that may occur during the transducers installation, so that create a deviation angle against the supposed axis, as figured in Figure 2.7.
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Figure 2.7 Vessel Local Coordinate System (X, Y, and Z) (Courtesy of Godin, 1998) In multibeam echo sounder, the series of calibration is patch test consist of roll, pitch, and yaw calibration as shown in Figure 2.7. Roll of the vessel’s motion rotating towards the X axis of the vessel due to the water surface motion. Roll motion may cause the depth value measured, especially in the outer beams as figured in Figure 2.8
Figure 2.8 Roll Motion Effect to the Depth Value
In Figure 2.8 can be displayed that there is a difference between the green and red lines. The green line show the actual depth value, while the red line show the measured depth value instead. The roll motion can be detected by conducting a survey along a flat sea bottom area. The illustration to detect the roll motion may be seen in Figure 2.10.
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Figure 2.9 Sail Pattern of Roll Calibration To detect the roll motion, the vessel must sail in along the same survey lines and the survey must be conducted minimum twice with opposing direction as illustrated in Figure 2.9. Pitch is the vessel’s motion rotating to vessel’s Y axis (Figure 2.7). This motion may cause the depth value is shallower or even deeper than the real depth value if there is a slope on the seabed measured during the survey, like in Figure 2.10.
Figure 2.10 Pitch Motion Effect to the Depth Value In Figure 2.10 can be seen that there is a difference between the measured and actual depth value if there is a pitch motion effect, the multibeam echo sounder may detect a shallower or deeper in the slope area. The pitch motion effect can be obtained by applying this following sail pattern in Figure 2.11.
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Figure 2.11 Sail Pattern of Pitch Calibration In pitch calibration survey, the vessel must sail along the same line minimum twice in opposing direction also the area chosen must have an object or slope around the flat area. Yaw is the vessel’s motion that rotating toward the Z axis of the vessel (Figure 2.8). Yaw motion will make a position error at the measured point. Generally, there is only a position error within the flat survey area, also position and depth value error within area that has slope or object as displayed in Figure 2.12.
Figure 2.12 Yaw Motion Effect to the Position and Depth Value
The yaw calibration can be erased by conducting a survey within an area that has a slope or object around the flat area as shown in Figure 2.13.
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Figure 2.13 Sail Pattern of Yaw Calibration The vessel must sail in different two survey lines and unlike the roll and pitch calibration, it must direct to the same direction and the object must be covered by the two survey lines.
The multibeam echo sounder data used in this research is the result of the survey which consist of calibration survey line and main survey line. First data obtained is a database format data which is a raw data and a combination of all acquisition data like GPS, vessel instrument offset, vessel’s name, and another data measured in the survey. In this processing step, the data will be processed is only multibeam echo sounder data. First the raw data is replayed in the validator using QINSy software. After the calibration value obtained, the data is calibrated with roll, pitch, and yaw deviation angle. The processing continued to data filtering process. In the main lines, filtering was done by erasing the bad data and noise. Filtering stage is important because later either the multibeam echo sounder data processing is successful or not is depend on this process. After filtering process, data processing continued to sound velocity configuration and applying the tide prediction data by applying the harmonic constants, so the depth value is reduced to it. The harmonic constants was obtained from tide observation in Tiaka tide station in a certain observation time. Because of the difference of temperature, salinity, and pressure in each water column, the sound velocity also could be different. Then the sound velocity become one of the correction of multibeam echo sounder data. After passing a quality control, the result is presented in depth values, text and image formatted.
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2.3.3 Side Scan Sonar Side scan sonar is used to produce images of the sea-bottom, which in turn are used for geological investigation and the search for objects like wrecks, mines and pipelines. Side scan sonars are designed to provide “acoustic image” of the seabed, with high definition. Side scan sonars are used to give a near-visual representation of the geological faces and also give general indications about the nature of the waterbottom interface, directly linked to the “reflectivity” of the signal. For example, a soft sedimentary bottom (silt, mud) will send back little energy because of its low impedance contrast with water and its interface smoothness, on the other hand a rocky or gravelly bottom will have the opposite effect, with strong impedance contrast and high roughness (Lurton, 2002). Side scan sonar usually is used for object detection like minerals, shipwreck, pipelines, cable lines and bottom rock classification like the sediment type, rock outcrop, sand suns, also for underwater constructions inspection activities. In general, side scan sonar system consist of three main components: 1. Tow fish 2. Transmission cable 3. Topside Processing Unit
The tow fish has the function to transmit acoustic waves from the tow fish to the seabed surface. It transmits hundreds of acoustic wave in every minute so it is a multi-ping system. The acoustic wave that reach the seabed or any object in the seabed will be reflected and received by a receiver which then will be visualized by the recorder unit in an image that describe the seabed surface condition. The side scan sonar transmits the acoustic wave like a “fan” that sweep the seabed in its side beside the tow fish, commonly until 100 meters to each side. In data acquisition, the tow fish keep transmitting the acoustic wave so it create a seabed picture. The tow fish is usually towed in the 10% depth of the total water depth. While interpreting, the object in form of a mound will have a darker colour and the object in form of cavity will have a brighter colour. The brighter colour shows the reflection is stronger and faster, while the darker shows less strong and slower reflection or even
28
no bounce back. The side scan sonar working principal is describe in this following Figure 2.14.
Figure 2.14 Side Scan Sonar Working Principal Side scan sonar is using dual frequency, the high frequency and low frequency. This system must be applied to obtain a better acquisition result. In high frequency, side scan sonar may produce a good resolution image, but the wave penetration will not be too deep and cover a short distance. The low frequency the acoustic wave may penetrate deeper the high frequency, but as the consequences the image resolution is not as good as the high frequency.
Some terms are typical for side scan sonar operations, and originate from the geometry typical for side scan sonar (Lekkerkerk, 2006b): 1. Slant Range: Distance from the sonar fish to any given point at the bottom. The slant range is equal to the path travelled by the sound wave from the sonar to that given point at the bottom and back. 2. Horizontal range: The horizontal distance between the position exactly beneath the sonar fish and that same given point at the bottom. The value of the horizontal range can be calculated from the sonar height and the slant range by using the Pythagorean Theorem 3. Maximum Range: The maximum value of the slant range, telling how far the side scan sonar should scan. 4. Insonified area: The total area that is insonified by the sonar system.
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After acquisition, side scan sonar will produce a high resolution image that later will be interpreted if there is any object identified. 2.3.4 Sub-Bottom Profiler Sub bottom profiler is one of the shallow seismic systems. Long before the introduction of high frequency echo sounders, the echo sounders used strong, low to high frequency sound burst to measure the depth by recording the travel time. These primitive echo sounders were the first sub-bottom profilers. The characteristic of the sound pulse was such that did not only reflect but also penetrate into the bottom and reflect on geological boundaries below the bottom. It did not take long to realize that sound was also a good instrument to map structures well below the bottom. (Lekkerkerk, 2006b).
Sub-bottom profiler aims at exploring the first layer of sediment below the seafloor, over a thickness commonly reaching several tens of meters. Echo signal in subbottom profiler is acquired from the reflection on the interfaces between layers, which correspond to acoustic discontinuities. Those echoes would be gathered when the ship is moving are set side by side graphically, reconstituting a vertical crosssection of the sediment layer discontinuities. 2.3.5 Soil Boring Soil boring provides geological samples required for geotechnical testing for engineering design as well as to verify the geophysical data, in the research context, based on multibeam echo sounder, side scan sonar and sub-bottom profiler. For jackup drilling rig installation, geotechnical data will facilitate successful foundation design, site or route selection, choice of foundation type, dimensioning, installation and operational integrity of the proposed structure.
The geotechnical survey method for jack-up rig is soil boring. Soil boring is a hole in the ground drilled, bored, cored, washed, driven, dug or jetted, the intended use of which includes obtaining data for engineering, geophysical or geological exploration, or prospecting for minerals or products of mining or quarrying. To get the data the soil boring implements standard penetration testing, a method is used to obtain soil 30
density data by pounding specified tube into a bore hole. The soil boring mechanism may be displayed in this following Figure 2.15.
Piston corer may penetrate up to 30 meters below the seabed with unlimited water depth capability. The soil boring methods may be applied by varying equipment like grab sampler, vibrocorer, box corer, etc. Each equipment is distinguished based on the penetration capability.
Figure 2.15 Piston Corer (Lekkerkerk, 2006b)
2.3.6 Magnetometer Magnetometer is an instrument used to detect the existence of any metal objects by measuring its magnetic anomaly. Magnetic survey data was processed by calculating the magnetic anomaly measurement (Umam, 2011). Survey using magnetometer must be perform in this research to measure magnetic value and define magnetic anomaly within the survey area. The increased magnetization usually is caused by the presence of ferrous iron located on the seafloor. The iron objects could be a shipwreck that made from metal, or a volcanic rock containing grains of magnetite (a highly magnetic material) or any existing subsea cables and pipelines. The magnetometer fish is surface-towed. Even it has lower sensitivities than the near bottom, but it is less expensive and cover larger areas than the near-bottom towed.
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The amount of magnetic field intensity anomaly produced by correcting the local magnetic intensity from acquisition compared to daily correction and International Geomagnetic Reference Field (IGRF). The magnetic field data from magnetometer is processed by geophysical engineers and will bring out an intensity field magnetic anomalous.
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Chapter 3
Result and Discussion This chapter will describe the processing results that required for jack-up drilling emplacement. The information must be related each other. The method used for the discussion is comparative and verify each other and discuss about five information, i.e. site bathymetry, seabed features, sub-seabed features, magnetic anomalous, and geotechnical information within the research are. The single beam and multibeam echo sounder produce a bathymetry information, side scan sonar results a seabed feature information, sub-bottom profiler results sub seabed features information, magnetometer results a magnetic anomalous information, whereas the soil boring data will result the physical morphology of the research. Single beam echo sounder is used to validate the multibeam echo sounder processing result, and side scan sonar used to verify if there is any seabed objects. Magnetometer produce information about magnetic value and geotechnical is used to help the geophysicist in interpreting the sub-bottom profiler result.
3.1 Bathymetry Bathymetry information in jack-up drilling rig emplacement is needed to obtain the description or topography of the seabed and also to know the seabed condition around the working area. The depiction of the seabed topography for jack-up drilling rig emplacement will be visualized as depth contour and/or also digital terrain model. Every depth information must have a horizontal position. In this research, the visualization will be displayed is a depth spot map with contour and digital terrain model for the multibeam echo sounder result and a survey lines for single beam echo sounder result. Positioning and bathymetric data acquisition must be done in a same time within the survey area, so in the same time the horizontal position of the depth spots can be obtainable directly. After both the single beam and multibeam echo
33
sounder data has been processed, reduced to the chart datum, corrected to the sound velocity profile, and plotted in the previous chapter will be displayed in this chapter.
3.1.1 Single Beam Echo Sounder As stated in the previous chapter, the result of the single beam echo sounder is the result of the high frequency transducer and in format of survey lines. After it has been processed and corrected to the chart datum, which is Lowest Astronomical Tide, then the Figure 3.1 below will show the plotted depth points using AutoCAD Map 3D software.
Figure 3.1 Single Beam Echo Sounder Plotting Result The survey main lines consist of 27 lines and the cross lines consist of 18 lines and the depth result is presented in meter unit. The depth points must free from any error and has been corrected.
After the processing result, also founded some anomaly from the data like jumping data as stated in Table 3.1.
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Table 3.1 Single Beam Echo Sounder High and Low Frequency Depth High Frequency Depth
152m
Low Frequency Depth
67.3m
The high frequency must result a shallower depth value than the low frequency because it penetrate less deep than the low frequency. Table 3.1 shows the result of one of the data within the survey lines. Some depth value has the high frequency value deeper than the low ones. This anomaly may be happened due to the transducer’s error while conducting the survey or the single beam transducer passed through an object. Ordinarily, the used data is from the high frequency transducer, but if data like Table 3.1 acquired, the echo roll must be rechecked to find out the real depth. From the echo paper can be interpreted any object that might disturb the acquired data. 3.1.2 Multibeam Echo Sounder The multibeam echo sounder data was processed using QINSy 8.0 software belongs to PT. Pageo Utama. The data was in acquisition database format and was a raw data and equipped with sound velocity profile which was observed by sound velocity profiler (SVP) instrument. SVP is also applying acoustic wave to observe the sound velocity in the water. The following depth spots has been reduced to chart datum, Lowest Astronomical Tide.
Before processing the main line multibeam echo sounder data. The data must through a calibration process. The calibration results a calibration graphics so it can be analyzed how well the line chosen. The calibration results a deviation angle i.e. roll, pitch, and yaw angle. The deviation angle of the roll motion is -1.280. The angle shows that the multibeam echo sounder transducer rotated to X axis of the vessel is -1.280. The following graphics describe the calibration accuracy and its suitability toward the calibration line. In Figure 3.2 (a), the green line is parabolic enough, means that the calibration process is accurate enough and the red line is coincide to the green line means that the survey line is suitable to the roll calibration mechanism. 35
(a) Roll Calibration
(b) Pitch Calibration
(c) Yaw Calibration
Figure 3.2 Roll, Pitch, and Yaw Calibration Result (a) Roll, (b) Pitch, (c) Yaw In Figure 3.2 (b), can be known the pitch deviation angle is 1.60. Pitch angle deviation means that the multibeam echo sounder transducer rotated towards the Y axis of the vessel in the amount of 1.60. From the graphics can be analyzed that the green line is in shape of parabolic, this means the calculation is accurate enough and the red line is not coincide with the green line. It indicates the pitch calibration line that was chosen is not suitable enough to pitch calibration mechanism. Yaw calibration process results a deviation angle in the amount of -0.890. The angle deviation explains the difference between the transducer’s angle and Z axis of the vessel is -0.890. From Figure 3.3 (c), the green line is parabolic enough and means it is accurate enough while the red lines is not coincide the green lines, means the calibration line was chosen is not suitable to yaw calibration mechanism. The deviation angle is presented in Table 3.2. The deviation angle also can be said as roll, pitch, and yaw correction and presented in degree unit. The following Table will conclude that the roll correction is -1.280, pitch correction is 1.60, and yaw correction is 0.890.
Table 3.2 Roll, Pitch, and Yaw Calibration Angle Roll
-1.280
Pitch
1.60
Yaw
0.890
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Multibeam echo sounder result from the processing is a multibeam echo sounder depth spots and Digital Terrain Model (DTM). Figure 3.3 is the picture of multibeam echo sounder depth spots which consist of thousands depth spots, and also the result of the DTM was exported in an image format as shown in Figure 3.3 below.
Figure 3.3 Multibeam Echo Sounder Depth Spots
Figure 3.4 Multibeam Echo Sounder Image
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Figure 3.4 above is the picture of the interpolated depth spots and exported to .tiff extension image. The red area is the shallower depth area whereas the darker blue color shows a deeper water depth. From this bathymetric map, the morphology can be interpreted that the red coloured area has more hazard potential for jack-up drilling emplacement because it has an extreme slope changes and may endanger the rig construction.
In the eastern and southern part of the survey area is a blank area. It is because the R2 Sonic multibeam echo sounder cannot detect the depth and caused a bad data. Within the red circled area, the acoustic wave cannot reach any object or seabed within the area so the acoustic wave did not reflect back to the transducer. It indicates the depth is out of range, so in processing will shows this following Figure 3.5. If the bad data is made to a Digital Terrain Model then it will appear like the Figure 3.5 (a) and from the QINSy 8.0 profile view will appear like Figure 3.5 (b) and the plane view is appeared like Figure 3.5 (c).
(a) Multibeam Echo Sounder Digital Terrain Model (DTM)
(b) Profile View of Multibeam Echo Sounder Data 38
(c) Plane View of Multibeam Echo Sounder Data
Figure 3.5 Multibeam Echo Sounder Bad Data (a) DTM, (b) Profile View, (c) Plane View The data in eastern and southern part of the surveyed area indicates the heterogeneity from the area around. The reason why the multibeam echo sounder data is out of range because it has been validated by the single beam echo sounder data, and it show the depth in the eastern and southern area is greater than 200 meters. It is clearly is not a proper area for jack-up drilling rig emplacement because jack-up rig can only accommodate maximum water depth of 150 meters. Beside the digital terrain model (DTM), the bathymetric data also can be made to three-dimensional model to make the visualization of the digital terrain model. The three-dimensional view of the seabed topography may be seen in Figure 3.7.
(a) 3-D View
(b) Location of Proposed Wells
Figure 3.6 3-dimensional Visualization of The Bathymetric Data (a) 3-D View, (b) Location of Proposed Wells The bathymetry data processing has obtained information about water depth within the proposed wells as listed in Table 3.3 below.
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Table 3.3 Water Depth in Proposed Wells Proposed Well Location
Water Depth (below Chart Datum)
1
94.4 meters
2
97.0 meters
3
97.0 meters
4
97.7 meters
5
97.9 meters
6
97.5 meters
7
98.1 meters
In Table 3.3 above, the average water depth is below 100 meters and relatively flat areas, means that it is a safe location for jack-up drilling rig emplacement location, because average jack-up drilling unit can accommodate water depth up to 150 meters.
3.2 Seabed Features The result of side scan sonar in this research is a seabed images which depict the seabed features such as, the result of this research, coral outcrop and high reflectivity seabed. The result of side scan sonar is a result from interpretation and mosaicking process per survey line. The images was combined become a mosaic so the survey area can be depicted as a whole.
The mosaic result in Figure 3.8 has already had coordinates because it is a georeferenced image and geotif formatted. Mosaic in Figure 3.7 consist of 50 side scan sonar images which overlaid each other. In Figure 3.8 (a), the mosaicked image was cut as wide as the research area which only 2 km x 2 km wide. Figure 3.9 also provides information about the interpreted seabed features which already interpreted by the geophysicist. The red area indicates a coral outcrop and the grey area indicates high reflectivity seabed.
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Figure 3.7 Side Scan Sonar Images Mosaic Result
(a) SSS Mosaic
(b) Seabed Features Interpretation
(c) SSS Mosaic and Seabed Features Interpretation Overlay
Figure 3.8 Side Scan Sonar (SSS) Processing Result (a) Side Scan Sonar Mosaic, (b) Seabed Features Interpretation, (c) Side Scan Sonar (SSS) Mosaic and Interpreted Seabed Features Overlay Side scan sonar (SSS) image in Figure 3.8 (a) is in geotif image format and interpreted based on the color and the shape of the objects, whether it is interpreted as seabed features or not depend on the interpreter’s ability and experience. One interpreter to another may obtain a different interpretation. Figure 3.8 (b) is the result of the side scan sonar image’s interpretation. The red and grey area is interpreted a potential hazardous area. If it is the area that planned to be jack-up rig emplaced, 41
then it might endanger the jack-up rig construction. In Figure 3.8 (c) the side scan sonar mosaic and the interpreted seabed features is overlaid so the position of the seabed features may be considered for jack-up drilling rig emplacement.
(a) High Reflectivity Seabed on Side Scan Sonar Image
(b) Coral Outcrop on Side Scan Sonar Image Figure 3.9 Side Sonar Images Around Proposed Well Depict (a) High Reflectivity Seabed, (b) Coral Outcrop
Also in southern and eastern part show brighter image that indicates the side scan sonar acoustic wave did not reflect back onto the sonar fish. It can be verified by the bathymetry data that that area has water depth more than 150 meters.
Based on the processing and interpretation of side sonar image in Figure 3.10, the result was analyzed by the geophysicist is resulting a quite good image because most 42
of the recorded seabed features are clearly identified. The seabed side scan sonar images are characterized as low to moderate and moderate to high sonar reflectivity. It indicates the non-uniformity of seabed lithology. Low to moderate is interpreted as clay to sand sediment, meanwhile the moderate to high reflectivity sonar is interpreted as coral and/or rock outcrops (Figure 3.9 (a) and Figure 3.9 (b)). The side scan sonar image provides the seabed features information included carbonate, rock outcrop, and coral reef, etc. which normally considered as an obstruction to jack-up drilling rig emplacement. A good sediment for jack-up rig’s leg penetration is expected to be clay sediment. Sand may be unable to support the rig’s construction and may cause rig collapse. Also the coarser sediment’s high reflectivity seabed is considered to be a hard surface and may break the jack-up rig’s spudcan. The coral/rock outcrop may be a coral reef or any hard rock, so the area must be considered in jack-up rig’s emplacement. High reflectivity seabed interpreted as a hazard because it reflected more acoustic wave towards the fish.
From the side scan sonar image in Figure 3.10 (a) and 3.10 (b) above may be seen there has never been drilling activity exist before, because usually if the location has ever been a drilling activity before, the side scan sonar image will show some seabed features like seabed scar, jack-up footprint, and either existing platform or pipelines. From the side scan sonar image, all the proposed wells is free of any hazard also pockmark and pockmark cluster that may indicates shallow gas which may cause a rig’s blowout. Seabed features from the side scan sonar like interpreted in Figure 3.10 (a) can be used to verify the seabed features information from multibeam echo sounder (MBES) as shown in Figure 3.11 (b). From Figure 3.10, side scan sonar image and multibeam echo sounder may verify each other in detecting seabed features.
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(a) Seabed Features
(b) MBES Image and Seabed Features
Figure 3.10 Conditions around Proposed Wells (a) Seabed Features (b) Overlaid to MBES Image
3.3 Sub-seabed Features The information of sub-seabed features is one of the supporting information in this undergraduate thesis. The information of sub-seabed features is provided by subbottom profiler and magnetometer, if there is any metal object on the seabed and presented in magnetic anomalous number. The sub-bottom profiler result is interpreted by the geophysicist to know whether there is any object or obstruction located below the seabed. Beside the sub-seabed features, jack-up drilling rig emplacement also need information about the soil structure provided by geotechnical survey, in this case, was held by soil boring activity. The soil boring must be interpreted by the geotechnical engineers to know the bottom soil condition.
3.3.1 Sub-bottom Profiler Sub-bottom profiler results a sub-bottom profile. The result was also interpreted by the geophysicist. Based on the Geophysicist’s interpretation the sub-bottom profiler penetrate maximum to 14.6 meters below the seabed. In all over sub-bottom interpretation, there is no fault or any shallow gas are identified within the site which may endanger the jack-up drilling emplacement.
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3.3.2 Magnetometer Magnetometer result is also interpreted by the geophysicist and within this research location, there is no magnetic anomaly found which could be associated to any metal objects and may endanger the emplacement process.
Figure 3.11 Magnetometer Data on Proposed Well 1 Figure 3.11 shows the variation of the magnetic field around the proposed well 1. The fix axis describes the number of the survey line and magnetic field axis describes the number of magnetic field. Based on geophysicist interpretation, the variation of the magnetic field looks natural which coming from sand, silt, and clay distributions on seabed and sub-seabed geology. Stronger magnetic field value around proposed well 1 may associated with coral and or rock located on the seabed.
3.3.3 Soil Boring The soil boring result as shown in Figure 3.12 was interpreted by the geotechnical engineer. In this research, the soil boring result is presented in borehole log and the tabulation of design parameters as presented in Figure 3.12 (a) and Figure 3.12 (b). Soil boring is one of the supporting data from geotechnical aspect and used to validate the interpretation from side scan sonar and sub-bottom profiler. The soil boring result was analyzed by geotechnical engineering and may provide physical information up to tens of meters below the seabed. For jack-up drilling rig leg’s penetration, the information must be analyzed up to 12—15 meters. The soil condition within the survey area is dominated by clay and
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sand. The proper soil condition for jack-up rig is dominated by clay to ensure the jack-up leg’s stability. Sand-dominated may be endangering the rig because sand is easy to get carried away by the sea currents.
(a) Borehole Log
(b) Tabulation of Design Parameters
Figure 3.12 Soil Boring Result (a) Borehole Log, (b) Tabulation of Design Parameters
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Chapter 4
Conclusions and Recommendations
4.1 Conclusions 1. The bathymetry within the mapping area is varying from 36.81 meters minimum and 197.91 meters. The proper area for jack-up drilling emplacement is relatively a flat area and has water depth less than 100—150 meters. The water depth in Proposed Well 1 is 94.4 meters, Proposed Well 2 is 97.0 meters, Proposed Well 3 is 97.0 meters, Proposed Well 4 is 97.7 meters, Proposed Well 5 is 97.9 meters, Proposed Well 6 is 97.5 meters, Proposed Well 7 is 98.1 meters, so that can be concluded that all the Proposed Well comply the jack-up drilling rig emplacement criteria because the water depth on average is below 100 meters also the mapping were planned for an area of 4,000,000 m2, but 3,492,881.2616 m2 comply the emplacement criteria.
2. The seabed conditions is dominated by clay and sand. In the eastern and southern part of the mapping area the seabed cannot be identified clearly due to the bad data, 17.89 percent area is interpreted as coral outcrop and 0.51 percent area as high reflectivity seabed which may become an obstruction to the jack-up drilling rig emplacement, mostly located in eastern and southern part. Seabed condition in Proposed Well 1—7 has no obstruction and the seabed conditions are clearly identified also comply the jack-up drilling rig criteria, also the sub-seabed condition does not indicate any obstruction or hazard like buried metal object or faults. From soil boring, only proposed well four, five, and six comply the emplacement criteria.
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4.2 Recommendations 1. The single beam and multibeam echo sounder data processing must be done by more expert person, because experience is necessary so data is not over smoothed or no unfiltered bad data left, so there will not be misinterpretation. 2. More advance acquisition instrument is required, so the deeper water depth can be measured. 3. After the jack-up drilling rig is moved from the current location, similar seabed morphology mapping should be done because the jack-up rig emplacement may change the seabed morphology. 4. Further research about pipelines installation is needed to support the oil and gas distribution from the offshore platform. That research will consist of pipelines pre-lay survey as can be seen in Utomo (2014) and post-laid survey to verify the pipeline’s free span.
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References
Basuki, S. 1986. Kebutuhan Survei Geodesi Dalam Industri Minyak dan Gas Bumi di Lepas Pantai. Media Teknik. Casidy, J. M., and C. Gaudin. 2010. "Recent Contributions of Geotechnical Centrifuge Modelling to the Understanding of Jack-up Spudcan Behaviour." Ocean Engineering 2. Chakrabarti, S. K. 2005. Handbook of Offshore Eingineering Volume I. Amsterdam: Elsevier. Godin, A. 1998. The Calibration of Shallow Water Multibeam Echo-Sounding Process. Fredericton, New Brunswick: University of New Brunswick. Jong, C. D. de, Lachapelle, S. SKone, and I. A. Elema. 2010. Hydrography. Delft: Delft University Press. Dennes, B., Kee, R., and B. W. Ims. 1984. "Geotechnical Hazard Associated with Leg Penetration of Jack-up Rigs." In Seabed Mechanics, by Bruce Dennes, Len Maunder, Sir Alan Muir Wood, Akio Nakase and Adrian Richards, 169. Boston: Graham and Trotman. Lekkerkerk, Huibert-Jan. 2006a. Handbook of Offshore Surveying Vol I. London: Skilltrade. Lekkerkerk, Huibert-Jan. 2006b. Handbook of Offshore Surveying Vol II. London: Skilltrade. Ltd, PPL Shipyard Pte. 2006. What is a Jack-up Rig. Accessed February 13, 2015. http://ppl.com.sg/technology_jack-up.htm?reloaded=true. Lurton, X. 2002. An Introduction to Underwater Acoustic: Principle and Application. Chichester: Praxis Publishing. Mandasari, S. 2013. Studi Kelayakan Lokasi Rencana Peletakan Jack-up Drilling Rig Menggunakan Hasil Pencitraan Side Scan Sonar. Surabaya: Teknik Geomatika Institut Teknologi Sepuluh Nopember.
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OGP. 2013. "Guidelines for the Conduct of Offshore Drilling Hazard Site Surveys." Report No. 373-18-1, April. Organization, International Hydrographic. 1994. "Hydrography Dictionary Part I Volume 1 5th Edition." Special Publication No. 32. Accessed February 13, 2015. http://www.iho.int/iho_pubs/standard/S-32/S-32-eng.pdf. Ti, G. C. 2009. Centrifuge Model Study on Spudcan-Footprint Interaction. PhD Thesis, Singapore: National University of Singapore. Umam, S. K., Yuwono, and Subarsyah. 2011. Studi Penggunaan Magnetometer Dalam Pembuatan Peta Sebaran Logam Untuk Mendukung Pemasangan Pipa Bawah Laut. Undergraduate Thesis Paper, Surabaya: ITS Library Utomo, C. P. 2014. Pemetaan Morfologi Dasar Laut Untuk Perencanaan Jalur Pipa Bawah Laut. Bandung: Teknik Geodesi dan Geomatika Institut Teknologi Bandung.
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Appendix A: Multi Beam Echo Sounder Bathymetric Map
I
Appendix B: Multi Beam Echo Sounder Seabed Image
II
