generally called mass gravity flows. Slope failures ... mass gravity flow that might damage a pipeline is a key issue for .... using of an air-gun or sparker systems.
Proceedings of the Eleventh (2001) International Offshore and Polar Engineering Conference Stavanger, Norway, June 17-22, 2001 Copyright © 2001 by The International Society of Offshore and Polar Engineers ISBN 1.880653-51-6 (Set); ISBN 1-880653-53-2 (VoL !1); ISSN 1098-6189(Set)
Geo-Hazard Assessment for Pipelines Crossing the Continental Slope Marco Venturi and Sabrina Bughi Snamprogetti S.p.A. Fano, Italy
ABSTRACT
MASS GRAVITY FLOWS
The major geo-hazard for pipelines laid across the continental slope are the sudden and mostly unpredictable downslope soil movement generally called mass gravity flows. Slope failures developing into mass gravity flow that might damage a pipeline is a key issue for pipeline design. Geo-hazard assessment is a multi-disciplinary effort and it involves expertise in geology, geophysics, sedimentology, geotechnics, fluid dynamics and modelling. The present paper presents an integrated approach to assess geo-hazards across the continental slope. In addition to the individual aspects of geo-hazard assessment, such as potential flow pattern recognition, soil properties mapping, slope stability and mass flow modelling, the paper describes the importance of proper planning of the activities, starting at a very early stage to optimise the collection of survey data, and the interaction among the different disciplines, which are not sequentially linked but highly integrated.
The term Mass Gravity Flow (MGF) is a general term for a downslope movement of sediment. MGF's are classified according to their soil/water ratio, which also determines their physical behaviour. A Debris Flow (DF) is a Bingham flow composed of an upper viscoplastic body and a lower laminar shear layer in contact with the sea bed (Johnson, 1984). A DF is composed of clasts and sediments, it has a typical density of 1600-1800 kg/m 3, can reach very high velocities (Norem et al., 1990) on steep slopes (typically 20 m/s) and has a typical run-out distance of few kin. DF's recognition is easy from geophysical and morphological records, but there is no in situ measurement collected during their motion. The activation of a DF can be of seismic origin (Mulder and Cochonat, 1996; Hampton et al., 1996). Due to their inferred velocity and density, a DF lateral impact can damage a pipeline. A Turbidity Current (TC) is a turbulent flow in which sediment particles are maintained in suspension. It can reach densities of about 1100 kg/m 3, velocities over 10 m/s and has very long run-out distance, higher than 1000 km even on nearly flat seabed. They are documented as distinct layers in sediment cores, often with the presence of terrigenous material. Also few direct measurement are available, either current measurements or the breakage sequence of communication cables laid at different depths across continental slopes, (Krause et al., 1970). Many TC's result from the transformation of a DF, or can be the consequence of small soil instabilities initiating a downslope movement of sediment-rich water. The occurrence of a TC can affect the stability of a pipeline. A Mud Flow (MF) is a laminar flow of fluid mud. Poorly known in the marine environment, the evidence of MF is seen mainly in sediment cores by the presence of distinct layers of sediments of local origin, i.e. pelagic material. MF's occur when very soft sediments become unstable and move downslope. The typical parameters of a MF are believed to be: 1200 kg/m 3 density, 10 m/s maximum speed, run-out distance of few km, (Van Kessel and Kranenburg, 1996; Mei and Liu, 1987). Their impact on a pipeline has effects similar to a TC. In addition to MGF's, potentially unstable sediment in the continental slope are subject to movements due to creep, transport, deposition erosion, overpressure of internal fluid, and any other local mechanism able to cause shear stress increase or strength decrease of
KEYWORDS: Geo-hazard; debris flows; turbidity flows; pipelines.
INTRODUCTION Pipeline design across continental slopes has become a major area of interest of ocean engineering as a consequence of the rapid development of Oil & Gas Industry in deep and ultra-deep environments. In this context, the offshore pipeline industry is now offering new or upgraded lay vessels able to reach depths in excess of 1000 metres (lorio et al., 2000). The Continental slopes are geologically complex areas characterised by steeply sloping seabed, irregular bathymetry and locally abundant sediments. Offshore installations in the continental slopes of the Gulf of Mexico, West Africa, the Philippines, the northern margin of the North Sea but also of enclosed basins like the Black Sea and the Caspian Sea have focused the effort of engineers to study this geologically hazardous environments (Niedoroda et al., 2000a, 2000b, Reed et al., 2000). Their efforts are justified by the difficulty of deep sea intervention or protective works, which calls for more restrictive design requirements to ensure reliability over the operating life-span.
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planning of seabed sampling, have to be taken during the survey; an expert team able to analyse the preliminary data under the geo-hazard point of view is required during the survey.
involved soil. In particular, referring to seismic events, earthquake loading applied to potentially unstable slopes may not develop into a complete failure and in a flow but can induce significant permanent displacements. This downslope soil deformation can exert loads on the pipeline laid on it or embedded in it, particularly when the pipeline has some constraint.
IDENTIFICATION OF THE MGF REGIME Once the survey data are available, the pieces of the geo-hazard puzzle can be put together. The DTM is the first element to identify how relevant is the geohazard problem. The presence of a complex network of branching canyons (example in Figure I), can represent an evidence of the occurrence of MGF since the canyons can also be formed and maintained by downslope flow of sediments (Hampton and Locat, 1996). An apparently less active environment is shown in Figure 2, this slope does not show the marks of localised frequent MGFs. The geophysical records SBP and SSS supported by samples of sediments are used to identify past occurrences of MGF's. A DF deposit shows up as a rough surface indicating accumulation on the SSS records, while the SBP shows chaotic deposits without apparent layering. TC's are not as evident as DF's on geophysical records. In some cases their signature is a rippled seabed on canyon floors, due to strong currents; in other cases TC's can be identified, from the bathymetry and the SSS, by the presence of the lateral levees bordering a channel. TC can be identified in sediment cores, each event showing as a distinct layer. Hemipelagic sedimentation may occur between successive TC events. The layers can be dated with radiogenic isotope techniques (e.g. C 14 if some organic material, between 2000 and 35000 years old, is found in the layer) and by measuring the thickness of the pelagic ooze of which the average sedimentation rate should be known. Both dating techniques have a large degree of uncertainty. Firstly, because what is dated is the age of the sediments deposited by the TC and not the occurrence of the event. Secondly, because the pelagic sedimentation rate is not precisely known and because the layer is severely disturbed by the occurrence of a TC. The evidence of past MGF's in the geophysical records is very valuable for the calibration of MGF models: the observed DF and TC events are reproduced in terms of thickness of the deposit or run out distance by tuning the different parameters of the model, most of which relate to the rheologic characteristics of the soil (Reed et al., 2000).
BACKGROUNDINFORMATION The first step in geo-hazard assessment is the collection of background general and historical information to supplement the data collected during the surveys. The literature and government agencies normally have background information on geology, general seafloormorphology, tectonics, sedimentology. Seasonal and annual river discharge data (water and sediment) are also important to define possible sedimentation models at the shelf break. The reports of tsunamis and of submarine cable breaks (Krause et al., 1970) on the continental slope can identify the occurrence and possibly the frequency of MGF's in the area of interest. In addition to the pipeline landfall constraints, the background information can be a valid aid in the first selection of the crossing corridor of the continental shelf.
SURVEY REQUIREMENTS In a traditional pipeline project, surveying is limited to the definition of all the required parameters (geological, geophysical, geotechnical, bathy-morphological, meteocean) along a pipeline corridor that has a typical width of 1 km. In a project crossing the continental slope the view angle needs to be greatly enlarged. Once the pipeline corridor has been selected, the survey area for geo-hazard purposes must be extended, to include at least all the areas from which a MGF can start and all the MGF trajectories reaching the pipeline. The MGF trajectories are sometimes evident, as in the case of a slope with deeply incised canyons, in other cases they are not obvious and require the application of a simple gravity-inertia model to define it. Parametric modelling of DF and TC based on typical seabed profiles can be used to determine if a DF or TC has a maximum run-out distance that can reach the pipeline (Drago and Terenzi, 2001 ). The bathymetric information required for geo-hazard assessment is wide-range and low-resolution. A Digital Terrain Model (DTM) with a typical grid step of 25 m is sufficiently detailed. As an indication, the survey area of a continental slope is a square of 20 km side. Such dimensions are required also for selecting the best pipeline route. As such large survey area can be covered only with hull-mounted widerange multibeam bathymetric systems combined with side-scan sonar (SSS, measuring seabed morphology) and sub-bottom profiler (SBP, measuring high resolution stratigraphy) data are collected. The geophysical data are complemented with high resolution seismic data using of an air-gun or sparker systems. Using the wide-range bathymetric and geophysical data, a sediment sampling cruise and additional detailed geophysical surveys are planned. Samples (normally gravity cores), CPT's, and detailed geophysical data, chirp SBP with towfish flying at a short distance from seabed to increase the resolution and reduce possible side reflections m deeply incised bathymetry, will be collected in critical areas. The detailed surveys will focus on areas where a MGF is likely to start: soft sediment thickness, soil strength, seabed slope are the relevant parameters. In a pipeline project this new wide range surveying concept is expensive and requires great planning and decisional efforts. Most key decisions, e.g. extension of wide-range and detailed survey work,
SEDIMENT DISTRIBUTION AND PROPERTIES The scope is to define the soft soil thickness (SST) at any location in the geo-hazard area so that all potential MGF sources are identified. By definition SST is the distance from seabed to a level where there is a discontinuity of the soil strength, the lower material being either the bedrock or stiff material not prone to permanent deformation under seismic loading. This unconformity is identified in the SBP records by matching it with the soil mechanical data measured in situ (CPT) or in laboratory tests on soil samples (from piston corer). SST data are available along the survey tracks covered by high resolution SBP and from soil samples at particular locations. If the slope is nearly uniform (Figure 2), and the SBP records are undisturbed and vary slowly, SST mapping can be done by spatial interpolation; the same tools as these used to create a bathymetric DTM are used to create a Digital Sediments Thickness Map (DSTM). Unfortunately, in the continental slopes where geo-hazard is critical, this situation is not common. Going to the other extreme, Figure 1 shows a very complex configuration of successive canyons, ridges and canyon flanks. Spatial SST interpolation in such a complex system is meaningless and other tools need to be used to obtain a DSTM as 43
representative as possible. The situation shown in Figure 1 has been tackled by developing SST distribution criteria that can be applied to create the DSTM. The SST distribution criteria can be developed by: a) Statistical approach: SST data are used as dependent variable and they are tested against a set of independent variables, such as the seabed slope, the absolute depth, the geographical orientation of the slope, the seabed feature (canyon floor, ridge, canyon flank), relative to the same geographical location. The statistical tools used are simple correlation, regression analysis and cluster analysis. b) Integrated visual approach: the colour scale coded SST data are mapped on a bathymetric map along the survey tracks where they have been collected. This representation is used to interpret the sedimentation model and the SST distribution. In addition to the digital data (SST, depth, seabed slope, slope orientation, seabed morphology), any other available information is used in this integrated approach to understand the SST distribution pattern: the results of the statistical analysis, the SSS records to identify possible slump features, the soil sampling data, the river location and sediment discharge values. Sedimentation criteria, i.e. the criteria to assign a value of SST to any location, are developed and then used to draw isopach (equal SST) contours. The DSTM is then computed in a straightforward manner.
SEISMIC RISK ASSESSMENT The definition of the seismic risk, i.e. the seismic design parameters, follows a standard procedure called Probabilistic Seismic Hazard Analysis. The main steps in PSHA, not described in detail in the present paper, are the definition of the seismo-tectonic model, the propagation of the seismic motion from the earthquake epicentres to the study location, the probabilistic analysis to define the seismic parameters at prescribed annual probabilities or return periods. It is important to remark that for geo-hazard analysis the main seismic parameter is not solely the Peak Ground Acceleration (PGA), but also the acceleration time histories, which reflect frequency content and duration of the seismic event. They control the seismic response of soft soils and the permanent deformation of the sediment layer, in particular for thick sediment layers where the response is highly nonlinear.
LOCAL SLOPE STABILITY The scope is to investigate in detail the stability of soil resting on sloping seafioor referring to all representative locations of the study area. Limit equilibrium theory is applied and both static and dynamic conditions are studied. According to this theory, the slope equilibrium is controlled essentially by the slope angle, the sediment thickness and the soil strength parameters (i.e. friction angle and cohesion). On this basis, the whole considered area can be represented considering proper ranges for the relevant parameters. The slope profiles, characterised by the most critical conditions in terms of slope stability in the considered area (e.g. highest slopes, lowest cohesions, etc.), are selected as reference cases for more detailed analysis. The first step is a 2D analysis performed according to limit equilibrium slice method, like Morgestern and Price (1965) included in SLOPE/W model (Geo-Slope Int., 1998). Each reference case is analysed in order to assess the relative minimum static safety factor value. Then, the location, length and depth of the most critical slip surface due to seismic loading for the subject profile is identified. The effect of an earthquake is accounted for by a pseudo-static analysis where the seismic load is applied as a static horizontal force equal to the soil mass multiplied by the ground acceleration. The seismic acceleration, associated to a safety factor equal to unity, is the critical acceleration (or cut-off acceleration). It represents a measure of the • failure potential on a slope. Subsequently, 3D slope stability analysis is performed according to Hovland (1977) method referring to an ellipsoidal volume of sliding soil in which the middle longitudinal section of the critical slip surface has been identified using a preliminary 2D analysis and the width to length ratio is about 1.5. Static and pseudo-static conditions are considered and both the 3D safety factor and the 3D critical acceleration are determined. When the dynamic factor of safety is below 1, an instantaneous slope instability condition is occurring. Due to the transient nature of the peak ground motion, the soil structure may experience only a finite displacement rather than a complete structural failure, even when the dynamic factor of safety momentarily drops below I during an earthquake. Permanent slope displacements under earthquake condition are calculated according to Newmark integration method (Chen and Liu, 1990) and/or with simplified closed form solution, derived from Newmark (1965) integrated data fitting, referring to 3D cut-off acceleration, proper acceleration time histories and peak ground accelerations.
SEISMIC SITE EFFECTS Seismic site response analysis is performed to evaluate the modification of the seismic design parameters from bedrock to the seabed surface through sediment layers, taking into account local seismic transfer effects. Soil deformations after seismic loading and induced ground accelerations at different depths are considered. Actually, seismic site response and earthquake-induced slope instability are interconnected. The critical failure surface is located where the largest shear strains (cyclic and/or permanent) occur during an earthquake. The strain distribution along the soil profile is determined by the site response analysis and its value gives an indication of potential slope instability. The basic model is a layered soil profile behaving as one-dimensional shear beam, including nonlinear soil stress-strain behaviour, Nadim (I 991). The propagation of the seismic acceleration from bedrock to seabed is used to define expected peak ground acceleration for Newmark displacement calculation (Newmark 1965).
GLOBAL SOIL STABILITY The scope of the global soil stability analysis is to develop a Digital Soil Movement Map (DSMM), associated to each given earthquake with assigned return period. The first step is to identify the geotechnical properties of the sediments in the study area. When the configuration is complex, as in Figure t, several types of sediment can be identified, each with different shear strength related to depth below seafloor. The sediment types are further associated to one or more relevant parameters for which a digital map has been prepared (depth, seabed slope, soil thickness); these association criteria are used to build the Digital Geotechnical Soil Map (DGSM) in which each grid element is assigned to a soil type. The application of sophisticated 2D or 3D slope stability models to compute the soil movement (SM) at each grid-node of the digital model can not be done, because" of the large number of grid nodes (typically from l0 s to 106). In addition, it is no correct because of the simplifying assumption of uniform sediments and fixed volume in each grid cell. A simplified model based on Newmark's double 44
integration method (Cai and Bathurst, 1996) is used to compute the soil movement. The calibration parameters of the simplified method are tuned to match the results of 2D and 3D models for a wide range of situations (soil types, seabed slope, soil thickness, seismic forcing) encountered in the study area. The simplified soil movement model is a multi-dimensional matrix defining SM as a discrete function of the same parameters used for tuning. The DSMM is then computed, for each earthquake of relevant return period, using this simplified model and the associated grids: DGSM, DTM (for seabed slope) and DSTM. The DSMM gives the first element to compute possible loads on the pipeline as it shows the locations where downslope soil deformations is expected. The next step is to use the SM results to compute the volume of soil that will develop into a MGF. The basic concepts are: a) the soil will he subject to failure if its computed SM is greater than a critical value SMc defined from 2D and 3D slope stability modelling, b) a significant MGF will develop if the soil instability has sufficient spatial coherence and c)the soil volume contributing to a MGF must be directionally coherent, i.e. all grid-points feed the MGF. The above concepts are applied to the DSMM with a pseudo-cluster analysis: a control area, the extension of which has been defined from 3D slope stability modelling and is oriented along the slope gradient, is tested around each grid-point. Each grid-point within the control area will contribute to the MGF volume if SM>SMc and its slope direction does not differ from that of the central grid-point by more than 30° . The result of this pseudo-cluster analysis is the Digital Slide Volume Map (DSVM) that indicates the volume size ofa MGF from each gridpoint. Summarising for clarity, the digital maps used in the analysis are the following: DTM
Digital Terrain Model (depth)
From survey data
DSTM
Digital Sediment Thickness Map
From geophysical data, DTM
DSMM
Digital Sediment Movement Map
From seismic data, DTM, DSTM, DGSM
DGSM
Digital Geotechnical Sediment Map
From survey data, DTM, DSTM
DSVM
Digital Slide Volume Map
From DTM, DSTM, DSMM and simplified slope stability model
A detailed description of the models, including the calibration procedure with site specific data, is presented in a companion paper, Drago and Terenzi, 2001. MGF modelling is carried out for each location identified in the DSVM from which a significant MGF is expected. The input parameters are the slide initial geometry, sediment density and the seabed profile along the trajectory computed from the DTM with the trajectory model. Each model run consists of DF modelling first and then TC modelling via the DF/TC coupling module. The MGF modelling results are used to identify the locations along the pipeline route where a MGF lateral impact can be expected. The relevant impact parameters are the velocity near seabed (pipeline level), the angle of impact of MGF with the pipeline, the density at impact and the width of the flow. The model results are also used to define the axial loads that a MGF can exert on a pipeline. This situation occurs when the pipeline is laid in a canyon thalweg and the MGF flows along it either because it has been activated in the canyon itself or because the MGF continues its trajectory after merging in from a lateral canyon. For axial load assessment the relevant parameters are the MGF length (along the pipeline), its mean square velocity (averaged over the flow length) and the density. CONCLUSIONS Mass Gravity Flows are frequent events that have no direct observation in the marine environment. The problem to assess their quantitative effects on a pipeline project has been approached with both probabilistic and deterministic tools. Each aspect of the procedure described above has intrinsic uncertainties that can be partially estimated by calculation of errors or standard deviations, but they cannot be completely avoided. Nevertheless, a correct problem setting and a sound interpretation of the results can give the engineer valid indications for designing a pipeline in a geo-hazardous area. The availability of high quality survey data will greatly reduce the uncertainties. The survey activity should be flexible to adjust to specific requirements, such as extension of surveying and sample location that may arise during the survey execution. A pre-determined survey plan will rarely reflect the geo-hazard needs that arise in the course of the survey itself. The final results must be checked for consistency with the available data and with the overall sedimentary environment. The predicted MGF activity must be consistent with the MGF evidence in the geophysical records in terms of location, recurrence time (age dating of the sediment cores can be used for this purpose) and intensity.
All digital maps have the same representative grid size of about 25 meters.
AKNOWLEDGEMENTS The authors express their appreciation to Saipem for the support to this work during the Blue Stream Project. We wish to thank Alan Niedoroda and Chris Reed of URS for the valuable time spent together working on geo-hazard problems.
MASS GRAVITY FLOW MODELLING The final step in geo-hazard assessment is to model the MGF's activated by the soil volumes indicated in the DSVM. The scope of MGF modelling is to assess the trajectory and the physical parameters (velocity, density, width, run-out distance). MGF modelling is carried out with specifically developed numerical tools: MGF trajectory model
Simple gravity-inertia model
DF/MFmodel
Quasi 2D Visco-plastic Bingham fluid numerical model 2D (K-I) Turbulence closure numerical model Turbulent boundary layer development and sediment suspension model
TC model DF/TC coupling model
REFERENCES Cai, Z and Bathurst, RJ (1996). "Deterministic sliding block methods for estimating seismic displacement of earth structures", Soil
Dynamics and Earthquake Engineering, 15. Chen, WF and Liu, XL (1990). "Limit Analysis in Soil Mechanics"; Developments in Geotechnieal Engineering, Vol. 52, Elsevier. Drago, M and Terenzi, A (2001). "Mass Gravity Flow Modelling",
International Offshore and Polar Engineering Conference, ISOPE-2001, Stavanger. 45
FIGURES
Geo-Slope International Ltd (1998). "SLOPE/W v. 4.23 User Manual". Hampton, MA, Lee, HJ and Locat, J (1996). "Submarine Landslides". Review of Geophysics, 34, 1. Paper No. 95RG03287. Hovland, HJ (1977), "Three-Dimensional Slope Stability Analysis Method"; Journal of the Geotechnical Engineering Division, Volt 03, No GT9. Iorio, G, Bruschi, R, Donati, E (2000). "Challanges and Opportunities for Ultra Deep Water Pipelines in Difficult Sea Bottoms", 16th World Petroleum Conference, Calgary. Johnson, AM (1984). Debris Flow. Slope Instability (Eds. D. Brunden and D.B. Prior) 257-362, Wiley, Toronto. Krause, DC, White, WC, Piper, DJW and Heezen, BC (1970). "Turbidity Currents and Cable Breaks in the Western New Britain Trench". Geological Society of America Bulletin, v. 81, pp. 2153-2160. Mei, CC and Liu, K-F (1987). "A Bingham-Plastic model for a Muddy Seabed under Long Waves". Journal of Geophysical Research. Vo]. 92, No. C13. Morgerstern NR and Price, VE (1965). The Analysis of the Stability of General Slip Surfaces. Geotechnique, Vot. 15, pp.79-93. Mulder, T and Cochonat, P (1996). "Classification of Offshore Mass Movements". Journal of Sedimentary Research, Vo]. 66, No. 1. Nadim, F (1991), "AMPLE - A Computer Program for Amplification of Eearthquakes", Norwegian Geotechnical Institute, Report 525285-6. Newmark, NW (1965), "Effects of Earthquake on Dams and Embankments"; The Fifth Rankine Lecture of the British Geotechnical Society. Geotechnique 15 No 2. Niedoroda, AW, Reed, CW, Breza, J, Parson, BS, Badalini, M, Kruse, G, Mullee, JE, Parker, G, Forristat, GZ (2000a). "Developing Engineering Design Criteria for Deepwater Turbidity Currents
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", Offshore Mechanics and Artic Engineering Conference, New Orleans. Niedoroda, AW, Reed, CW, Parson, BS, Breza, J, Forristal, GZ, Mullee, JE (2000b). "Developing Engineering Design Criteria for Mass Gravity Flows in Deepsea Slope Environments", Offshore Technology Conference, Houston. Norem, H, Locat, J and Schieldrop, B (1990). "An Approach to the Physics and Modeling of Submarine Flowslides". Marine Geotechnology, Vol. 9, pp. 93-111. Reed, CW, Niedoroda, AW, Parson, BS, Breza, J, Mullee, JE, Forristal, GZ (2000). "Analysis of Deepwater Flow, Mud Flows and Turbidity Currents for Speed and Recurrence Rates", Deepwater Pipeline & Riser Technology Conference, Houston.
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