Drilling Technologies for Offshore Foundation

Proceedings of the ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering OMAE2013 June 9-14, 2013, Nantes, France

OMAE2013-10305

DRILLING TECHNOLOGIES FOR OFFSHORE FOUNDATION ENGINEERING Giovanni Spagnoli Dept. Maritime Technologies BAUER Maschinen GmbH Schrobenhausen, Germany

Leonhard Weixler Dept. Maritime Technologies BAUER Maschinen GmbH Schrobenhausen, Germany

ABSTRACT Offshore piles are normally installed by driving using overwater or underwater hammers. However, there are many situations where pile reaches refusal before the installation depth. This paper briefly describes the current offshore foundation practice, the BAUER technology for onshore pile installation by drilling, the BAUER experiences in the offshore foundation and geotechnical fields and a new technology for supporting offshore pile installation when refusal is prematurely reached by means of the Dive Drill.

range from a standard conductor, 0.76 m diameter, up to over 2.5 m. The wall thickness of the pile will generally vary along the length, with thicker walls used near the pile head where bending moments are maximum. Through the body of the pile, typical diameter to wall thickness ratios (d/t) are ~40 (range 30 to 50), giving a net steel area, termed the area ratio, ρ, of 10 % of the overall pile cross-section (Randolph et al., 2005).

INTRODUCTION Jacket structures are still the most common form of fixed offshore platform, with the design evolving gradually from their first use for the shallow offshore fields in the Gulf of Mexico, and are now used in water depths of up to 400 m. The platforms are fixed to the seabed by piles inserted through sleeves attached to the jacket, with the piles eventually grouted to the sleeves after installation (Randolph et al., 2005). Offshore oil and gas structures typically involve smaller diameter (about 2 m) and longer piles (about 100 m). About 95% of the offshore platforms in the world are jacket designed. The jacket consists of an open-framed steel structure made of tubular leg chords, horizontal bracing and diagonal bracing. It supports a deck and topside modules, usually including a helideck for access and drilling rig (Dean, 2010). The majority of piles used offshore are steel pipes, driven open-ended into the seafloor. Pile sizes

Figure 1. Distribution of interface friction on shaft of pile driven into sand (after Tomlinson and Woodward, 2007)

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Copyright © 2013 by ASME

pile toe, after which the hammer is again used to drive the pile to its final foundation level, or to the next level of pile refusal, where after the drilling process is repeated. Besides, pile driving installation in carbonate sands leads also to problems. Their unconventional behavior, characterized by an important compressibility as opposed to the dilating behavior of the dense siliceous sands remains a challenge for geotechnical engineering. The calcareous sands tend to collapse, resulting in a reduction in confining stress in the field and hence in a lower strength that would be expected from the relatively high peak friction angle indicated by triaxial tests (Poulos, 1989). Colliat et al. (1999) reported extremely low skin friction obtained from model or field pile tests in highly crushable and non-cemented carbonate sand (less than 15 kPa or even less of 5 kPa). In addition to contraction of the soil-pile interface, one should consider also that the effect of driving destroys the carbonate sand fabric with large production of fines and decreases the sand relative density immediately around the pile shaft. Besides, Colliat et al. (1999) states that the properties of carbonate sands with low Poisson ratio and very high particle friction combines with possible local cementation may result in very low or zero lateral stresses at the pile-soil interface. When the piles were driven at the North Rankin A platform on the North-West Shelf of Australia, some piles penetrated over 100 m penetration in just a few blows, with deduced shaft friction of just a few kPa (Dolwin et al., 1988). The low shaft friction is associated with very low radial effective stresses around the pile, a situation remedied by drilled and grouted pile construction, where the original horizontal effective stresses in the ground can be restored by appropriate grouting design. Dutt et al. (1985) described experiences when driving 1.55 m diameter steel piles with open ends into carbonate soils derived from coral detritus. The piles fell freely to a depth of 21 m below sea bed when tapped by a hammer with an 18 ton ram. At 73 m the driving resistance was only 15 blows/0.3 m. Bowles (2001, with a number of references) reported skin resistance values fs to in order of 15 to 30 kPa and point bearing values q0 in the range of 4 to 6 MPa. As the mean effective stress p’, increases, the carbonate soil behavior was found to change from one which dilated at failure to a more plastic material exhibiting volume reduction during shear. This transition was found to occur at low confining pressure, of the order of 0.2 MPa, which is in contrast to the behavior of terrestrial silica sands, which typically exhibit such a transition at confining pressure of the order of 2 MPa (Vesic and Clough, 1968). This characteristic of volume reduction at low confining stresses is of great significant in foundation design (Poulos, 1988). Drilled piles are, therefore, preferred in calcareous sediments, and potentially other crushable material, where the shaft friction obtained with driven piles can be extremely low. Drilled shafts can be installed by a variety of onshore drilling rigs. Offshore equipment is generally the same with the exception of being mounted on a barge or other floating platform. Casing is driven or drilled past the mudline. If the casing can be sealed into an impervious stratum, it can then be dewatered and drilled.

In marine structures piles are subjected to uplift and lateral forces caused by wave action. It is therefore necessary to drive pile to much greater depths than those necessary to obtain the required resistance to axial compression only. Closed-end piles are however not suitable for offshore structures because of the high base resistances which can be reached in dense sands making it impossible to drive piles to a sufficient depth to obtain the required resistances to uplift and lateral loading (Tomlinson and Woodward, 2007). Driving a closed-end pile into sand displaces the soil surrounding the base radially. The expansion of the soil mass reduces its in-situ pore-pressure, even to a negative state, again increasing the shaft friction and greatly increasing the resistance to penetration of the pile. Tests on instrumented driven piles have shown that the interface friction increases exponentially with increasing depth as shown in Figure 1 (Tomlinson and Woodward, 2007). According to Dean (2010) in a pile drivability, upper and lower bound predictions are made of the number of hammer blows per foot of penetration needed to drive the pile into the seabed and maximum compressive and tensile stresses induced in the pile during driving. Predictions depend on the characteristics of the hammer, the pile dimensions, the soil properties and how far the pile has penetrated into the seabed. With onshore piling, final pile penetrations are usually determined empirically according to the driving performance, with the capacity verified by dynamic testing. By contrast, offshore driven piles are driven to a specified penetration, calculated from pile design algorithms (Randolph et al., 2005). Therefore, accurate prediction of pile drivability remains an uncertain art. There are many situations where piles cannot be driven to their full penetration without the need for ‘drillingand-driving’ techniques. Tomlinson and Woodward (2007) reported an example of times required for welding add-on lengths of 1.37 m tubular piles; they varied from 3 ¼ hours for 25 mm wall thickness to 10 ½ hours for 64 mm thickness. Such delays, which are sometimes inevitable, cause increased driving resistance due to ‘set-up’ (i.e. the increase of shaft friction). Setup is the increase in soil resistance that can be developed after a pause in driving through strong clayey soils (e.g. Aurora, 1980). The high cyclic stresses in the soil close to the pile can cause excess pore pressure to build up in clayey soils, usually resulting in easier driving. Therefore, soil resistance during driving can be less than the ultimate axial capacity of the pile (Dean, 2010). Pore pressure dissipate quickly during a pause in driving, resulting in increased frictional resistance when installation restarts. Dean (2010) reports set-up factors from PDI of 1 for sand, 1.5 for silt and 2 for clay, indicating that the driving resistance can double for clay as a result of set-up. For instance, the stiff clays of the Midwestern states of America and the Great Lakes area of Canada favored large-diameter bored piles, and mobile rotary drilling machines were developed for their installation. Drilling-and-driving method is used with steel tube piles in hard driving conditions. Then a drill is used to remove the plug of soil, which has formed inside the hollow pile section. Drilling is continued to a limited depth below the

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Otherwise, drilling is conducted through casing using the muddrilling technique. Cuttings are removed using air or water ejectors or by mud circulation techniques. After cleanout and inspection, steel reinforcing and concrete are then placed into the casing to form the shaft (Greer and Gardner, 1986). BAUER Maschinen GmbH is therefore currently developing alternative offshore piles installation based on its experience on onshore geotechnical engineering and previous offshore geotechnical works.

depth of – 110 m from G.L. This corresponds to a drilling depth of 80 m. The subsoil overlying the limestone bedrock is formed of highly weathered and decomposed sedimentary rock (shale, sandstone, quartzite).

BAUER TECHNOLOGIES FOR PILES INSTALLATION Bored pile dimensions with 25 m length and a diameter of 1.2 m were outstanding figures when introducing the first BAUER hydraulic rotary drill rigs BG 7 and BG 11 around 1980. All rotary drilling techniques share a common characteristic, which requires torque and a vertical force to be applied to the soil by way of a tool. During the process of borehole construction, the following steps have to be executed either concurrently or successively: • Loosening of the soil • Conveying the spoil material out of the borehole • Stabilizing the borehole wall The two most significant systems for the construction of bored piles are the kelly system and Top drill systems which work on the principle of reverse circulation. As far as the spoil removal is concerned, the Kelly system is classified as “intermittent system”. After filling the drilling tool with soil, the tool is extracted out of the borehole with the Kelly bar and the spoil is dumped at the surface. Reverse circulation systems are considered as “continuous systems”, as the spoil is re-moved continuously from the bottom of the borehole to the surface with a fluid or air flushing system (Schoepf, 2010). The drilling process most frequently employed with rotary drilling rigs is still the kelly drilling technique. The main components of a Kelly drilling rig and its functions are: 1 Hydraulic base carrier; 2 Mast; 3 Rotary drive; 4 kelly bar; 5 Drilling tool; 6 Temporary casing (Fig. 2). The kelly drilling is a universal drilling technique for virtually all soil conditions due to the big variety of drilling tools. Drilling diameters from 0.5 m up to 3.5 m can be achieved by simply changing of the tool. The drilling tools can be changed quickly due to a fast and simple bolt connection between Kelly bar and tool. This system allows an exact adaption of appropriate tools at any depth. Telescopic kelly bars can cover drilling depths up to 100 m. The system requires much less site installation area in comparison to reverse circulation methods. Therefore it is the prime choice for piling works in confined inner city situations

Figure 2. Main unit components (left: kelly bar retracted, right: kelly bar extended) (after Schoepf, 2010) The rock is transformed into hard clay and sand and gravel with irregular distribution of density and layering. These formations are known as “Kenny Hill” formation (Fig. 3). The top of the bedrock was extremely varying in height – a wellknown fact in Kuala Lumpur. It was very difficult to predict the final pile length. The design requested an embedment depth of 2 – 4 m into bedrock. In case that the bedrock could not be encountered after 80 m of drilling, the piles could be terminated at 80 m.

CASE HISTORIES IN ONSHORE APPLICATIONS Two towers with 47 floors and a 6-storey basement were built in Kuala Lumpur in 2000. The loads of the two towers are transferred by a bored pile foundation consisting of 418 piles with a diameter of 1.5 m. For a safe transfer of working loads of 11500 kN per pile, the base level of the piles was designed at a

Figure 3. Cross-section of foundation (Berjaya Star Center, Kuala Lumpur) lying on the Kenny Hill bedrock (after Schoepf, 2010)

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The rotary drilling rig BG 40 was selected as the most appropriate rig for executing this difficult work in terms of depth and soil strata. Stability, a winch with a pulling capacity of 300 kN and efficient engine and hydraulic performance were the key factors. But it was the progress in designing and manufacturing of long Kelly bars which made it possible to execute the project. It was for the first time that a fourfold Kelly bar could reach a drilling depth of 80 m and transfer a torque of 300 kN·m in fully extended position. A comparison of the “simple” drilling process in Kuala Lumpur (one rig and one kelly bar) with the rather complex work procedure with kelly bar and auxiliary kelly extension for reaching the same pile depth in Taiwan ten years earlier are a clear demonstration of the rapid development in drilling rig and kelly bar design within a period of ten years.

reduce the frictional resistance on the drill casing, three hydraulically operated roller bits were extended laterally during drilling. They underreamed the casing shoe (Fig. 5). Embedment depth of drill casing in conglomerate was up to 20 m. It was drilled with fully extended roller bits to final depth. Finally, the drill string and drill head were removed. A reinforcement cage, which extends from the base of the pile to 5 m above the top of the enlarged pile head, was installed and it was finally grouted.

CASE HISTORIES IN OFFSHORE APPLICATIONS From a jack-up barge the BAUER drill rigs were used several times, for instance, for foundations of a bridge in Gottfrieding (Germany) in 2006 for installing piles with diameter of 1.5 mm with a length of 25 m or for sheet piles predrilling in Mainz (Germany) in 2009. Pile length was of 20 m, diameter 813 mm. A very important project was the construction of the Chiapas Bridge in Mexico (Fig. 4). The foundation piles for seven jackets piers were drilled with the top drill unit BA2500 and a casing oscillator of BAUER Maschinen. The maximum water depth was 93 m and the maximum drilling depth into the claystone was 62 m. The full face roller bit was equipped with underreaming rollers to allow casing penetration into the rock.

Figure 5. Hydraulically underreaming the casing shoe.

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The BAUER drill rigs from a jack-up barge might be a valid alternative because torque is directly diverted into the ground, and it is possible to have a quick access to the borehole and quick exchange of the drill bit. However, the positioning from the barge is time consuming and the drill rigs are limited to a certain water level. Another technology is the Fly Drill which was used for a construction of an offshore wind farm in UK coastal waters at the Barrow Offshore Wind Farm site in the East Irish Sea (Fig. 6).

Figure 4. The Chiapas Bridge in Mexico Once the floating jacket was positioned four tubular support leg piles with structural connections to jacket of 1 m diameter was driven. The working platform on top of jackets was constructed; hydraulic casing oscillator was lifted and fixed onto platform. 2.5 m diameter drill casing was installed onto the bottom of the main casing section. The top drill unit BA2500 was mounted and connected on drill casing. Then, the drill head, ballast rods and air-lift pipe assembly was installed with casing guides. It was drilled down with air-lift flushing as the casing oscillator rotated the 2.5 m diameter drill casing and jacketed it down with its powerful hydraulic rams. In order to

Figure 6. Fly Drill during the excavation of the monopile at the Barrow Offshore Wind Farm Offshore wind farm consisted of 30 wind turbines (3.0 MW each), with 30 monopile foundations. Tubular steel monopiles of 4.75 m diameter and varying wall thickness ranging from 45 mm to 80 mm provided the wind turbine foundations. The monopiles varied in length between 49.5 and 61.2, weighing

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452 tons and penetrated up to 40.7 into the seabed. The geology consisted of sedimentary sequences of cross-bedded and interbedded sands, silts and periglacial clays varying in thickness between 20 and 40 m, followed by 30-40 m glacial tills comprising boulder clays with varying amounts of sands, gravels, cobbles and boulders. The northern site comprised medium to very dense gravely sands inter-bedded with silts followed by very firm to hard clays. Whereas the southern site was characterized by firm to hard clays with layers of very dense sands overlying very stiff of highly weathered mudstone/siltstone and weak siltstone/sandstone. For this project driven piles were chosen, however the presence of the sedimentary rocks above pile toe levels, had the potential of resulting in pile refusal before ending the pile installation. When refusal occurred, the Fly Drill 5500 was used. The rotary drive produced a torque of 462 kN·m at 320 bar; two hydraulic crowd cylinders each producing a crowd pressure of 40 tons; a clamping device delivering clamping force at 32 tons. The progress rates were 1 m/hr for the stiff/hard clays and dense sands, whereas for mudstone/siltstone the rates were of 0.35 to 0.65 m/hr. BAUER developed for rock layers the BSD3000. Spagnoli et al. (2013) described the successfully installation of a 1 MW tidal energy turbine in the test area of EMEC (European Marine Energy Center) off the island of Eday in the Orkneys (Scotland) in 2011. Due to the large loads, a monopile was chosen as the foundation structure. The water depth at low water was about 33 m, the sea floor consisted of sedimentary rocks from the Silurian and Carboniferous, in general sandstone and siltstone of medium strength (up to 150 MPa) in alternating strata. monopile was installed by means of a drilling tool (BSD 3000) developed by BAUER Maschinen GmbH (Fig. 7). Spagnoli et al. (2013) reported the operational work.

A template was deployed on the seabed. A conductor casing was inserted within the template and with a generated torque of 50-60 ton·m penetrated about 0.5 m in the surface weathered rock in order to ensure a stabilization of the tool. A drilling bit was deployed within the conductor casing in order to drill the sandstone. With a drill rate of max 1.90 m3/hr, the design depth (11 m) was reached. The drill head was removed and the pile was inserted in the hole and grouted. The approximate dimension of the monopile was 2 m outer diameter with a wall thickness between 60 and 90 mm. The drilling technology in offshore foundation industry may be also advantageous from an environmental point of view. During the pile driving operations huge sound levels are reached. According to the German government the threshold level must be 160 dB measured at 750 m from the source. Not always are such limits respected. For instance, Norro et al. (2010) reported maximum peak noises exceeding 192 dB were recorded already at 520 m from source at the offshore platform Blighbank (Belgian part of the North Sea, BPNS) construction site for the installation of 56 monopile foundations. Normally, the sound exposure level (SEL) is a measure of energy (see equation 1).

Figure 8. Drilling data of BSD3000 in Scotland Specifically, it is the dB level of the time integral of the squared-instantaneous sound pressure normalized to a 1-s period. The SEL metric also enables integrating sound energy across multiple exposures from sources such as pile driving.

(1) where instantaneous sound-pressure (p) is measured in µPa for n exposures and the reference pressure (pref) is 1 µPa under water and 20 µPa in air. This summation procedure essentially generates a single exposure “equivalent” value that assumes no recovery of hearing between repeated exposures (Southall et al., 2007). During the installation of the monopile in Scotland,

Figure 7. BSD3000

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drilling noise measurements were performed. Fig. 9 shows a spectrogram of an extract from drift Track 6 during the drilling operation and at 100 m from the drilling centre.

The MeBo 200 is the next evolution of the successful MeBo, developed and manufactured in close cooperation with MARUM, with enhanced application parameter, such as deployment and drilling depth. The previous generation of the MeBo 200 was tested successfully in July/August 2005 in deep water at the continental slope off Morocco with the German research vessel Meteor (Wefer et al., 2006) after an engineering and construction phase of about one year. Within the following year, two further expeditions were conducted with the Irish research vessel Celtic Explorer: one for shallow water tests of the MeBo system in the Baltic Sea, and a first scientific cruise on the Porcupine Bank west of Ireland for the Irish Shelf Petrol Studies Group (ISPSG) of the Irish Petroleum Infrastructure Programme Group 4. During these two cruises, the MeBo was deployed twenty times between 20 m and 1700 m. Push coring for soft sediments and rotary drilling for hard rocks, as well as stabilization of the drilled hole by setting casings, were successfully accomplished. The recovery rate was especially good for hard rocks and consolidated cohesive sediments (Freudenthal and Wefer, 2007). From 2008 to 2012 the first generation of MeBo took part at 9 expeditions on five different vessels, 66 deployments between 10 and 2050 m water depth, 1649 m were drilled in cristalline and sedimentary hard rocks, gravel, sand, till and hemipelagic mud and 1135 m core were recovered, with a mean recovery rate of 79%. Spectrum gamma ray bore hole logging is used since 2010 and in-situ temperature measurement since 2012. Remote controlled from a vessel, the MeBo 200 is deployed to the sea bed, using a steel armored umbilical to depths up to 4000 m. Four legs are extended before landing to increase the stability of the rig and leveling due to possible unevenness of the sea bed. The mast with the feeding system and the power swivel forms the central part of the drill rig. It is mounted on a guide carriage that moves up and down the mast with a maximum push force of 5 tons. A water pump provides sea water for flushing the drill string, for cooling of the drill bit, and for drill cuttings removal. The system utilizes rotary core barrels with diamond or tungsten carbide bits. The MeBo 200 stores drilling rods and core barrels on two rotating magazines that may be loaded with a mixture of tools as required for a specific task. With a storing capacity for core barrels with a length of 4.7 m, the MeBo 200 is capable of drilling down to 200 m into the sea floor, recovering cores with 54 – 63 mm diameter and stabilizing the drilled hole down to a depth of 200 m. MeBo 200 can be used for offshore geotechnical campaigns for offshore structures or also for methane hydrate explorations. Four components are required to form gas hydrates: water, light hydrocarbon gases, low temperature and high pressure. The conditions that are more favorable to the formation of methane hydrates are to be found on a large scale beneath the seabed at depths ranging from 300 to 3000 – 4000 m. However, already at 190 m water depth it is possible to encounter such deposits with overall temperature of 2 – 4° C.

Figure 9. Spectrogram of drilling noise at 100m off the drilling template The measurements concluded that there was little risk of any auditory impairment of harbour seals even in close proximity to the drilling operations. The zone of mild disturbance was limited to at most a few metres of the drill bit and thus the physical presence of the jack-up drilling platform may have a more significant influence on seal behaviour than the noise generated during drilling. Besides, the attenuation of noise from drilling is such that levels of background noise are likely to be reached within 100 m of the drilling platform (preferred model case) or within 1 km of the platform (worst case model). In the field of offshore geotechnical exploration BAUER Maschinen GmbH is currently developing the MeBo 200, a drill rig for geotechnical explorations (Spagnoli and Weixler, 2012) (Fig. 10).

Figure 10. Sketch of the MeBo 200 (after Spagnoli and Weixler, 2012)

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DIVE DRILL The Dive Drill has been developed mainly for supporting the pile installation when base resistance increases and refusal prematurely occurs. For supporting offshore petroleum production platforms, the use of piled foundations for offshore has necessitated driving hollow tubular piles with open ends to very great depths below the sea bed to obtain resistance in shaft friction to uplift loading. The assumption of a constant unit base resistance below a penetration depth of 10 to 20 diameters has been shown to be over conservative (Tomlinson and Woodward, 2007). At some stage during driving a plug of soil tends to form at the pile toe after which the plug is carried down with the pile. At this stage the base resistance increases sharply from that provided by the net cross-sectional area of the pile shoe to some proportion (not 100%) of the gross cross-sectional area. The stage when a soil plug forms is uncertain; it may form and then yield as denser soil layers are penetrated. It was reported by Tomlinson and Woodward (2007) that 1067 mm steel tube piles showed little indication of a plug moving down with the pile when they were driven to a depth of 22.6 m through loose becoming medium dense to dense silty sands and gravels in Cromarty Firth. Cleaning out the soil plug is an effective way of reducing the driving resistance, thus obtaining deep penetration, because of the elimination of base resistance. The Dive Drill has been developed for this specific aim (Fig. 11). The Dive Drill weighs approximately 35 tons. The rotary drive produces a torque of 275 kN·m. If needed, a second torque unit can produce the same torque with a feed force of 1000 kN. Pump, hydraulic and electrical installations are located in the central installation unit. The equipment is operated by a BAUER Maschinen MC 128 crane with a HDS 150 on a jack-up barge. Pump power is 300 m3/ hour. The drilling tool of the Dive Drill has an approximate length of 1.5 meters, and it can be adapted for a 2-to-3-meter diameter with underreaming function. The Dive Drill is also able to drill the bore hole continuously. The torque is transmitted to the drill by means of telescoping drill string (Spagnoli et al., 2013).

The dissolved cuttings are supplied to a cone crusher by means of a screw flight and hydraulic support, where larger cuttings are broken down (Fig. 12). The Dive Drill has two clamping systems for the ablation of the torque and thrust forces in the casing: one located at the torque unit and the second at the drill head. After drilling out of the complete feed, the drill unit is clamped with the second clamping system. The first clamping system, located at the torque unit, is released, and the drive unit can be topped by feed cylinder. Each clamp cylinder reaches 300 bar pressure. The drill process continues by clamping the torque unit and releasing the drill head unit. The Dive Drill can currently work for depths up to 250 m because of the hose handling system. The Dive Drill, in the current state, is thought to work from a jack-up, whose currently working water depths is up to 150 m, the Dive Drill can operate without problems up to 100 m from the subsea soils. Connected to a crane, the Dive Drill can be deployed inside the tubular pile. Once the plug is reached, the clamping system will open, and by means of telescopic rods, the drive process begins. When the feed system reaches its limit, the second clamping system opens, allowing the first clamping system to be retrieved and the feed system can reach the original dimension.

Figure 12. Drilling tool of the Dive Drill In the case higher depths are requested, i.e. more than 200 m for deep and ultra-deep water operations, templates must be installed (Fig. 13). Template openings can be deployed on the seabed with conductor casing inserted inside and with torque oscillation can penetrate in the seabed. The conductor casing is needed for the Dive Drill in order to provide the transmission of the torque to the drill bit. The Dive Drill might be used in order to remove soils ahead of the pile tip, by drilling an undersized hole ahead of the pile to improve drivability by reducing external and internal soil resistance. Piles for marine structures are sometimes installed by driving a steel tube to a limited penetration below seabed, followed by drilling-out soil plug then continuing the drilled hole with a support of a casing. On reaching the design penetration depth a smaller steel tube insert pile is lowered to the bottom of the borehole and a cement-sand grout is pumped-in to fill the annulus around the insert pile.

Figure 11. Sketch of the Dive Drill

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Insert piles are used where piles driven to their full design penetration fail to attain a satisfactory resistance, or where drilling and driving techniques are unable to achieve the required penetration (Tomlinson and Woodward, 2007). Since the drilling tool of the Dive Drill can expand, belled piles can be installed. API standard RP2A describes belled piles as they are used to give increased bearing and uplift capacity through direct bearing on the soil (Gerwick, 2007). The primary piles are driven down in the bearing stratum and it serves as a casing through which a rig drills a slightly oversized hole ahead. A pilot hole is drilled below the base of the driven pile with the Dive Drill. Then the bell is drilled, expanding the drill bit. Normally RC is employed in order to gain discharge velocity. With the Dive Drill the cuttings are removed by using the pump. Then an insert pile is run down. The bell and pile are filled with concrete. Underreamed bell piles were used for the Ekofisk Platform in the North Sea (Gerwick, 2007).

such limitation, so that in the case of jackets, where piles are always under water, the machine has practically no limitation. According to experiences gathered by BAUER, during trench cutter job sites, the amount of soils in the soil/water mixture is up to 10%. Taking into account the pump power alone, in one hour between 15 and 30 m3 materials can be excavated. According to the GOPAL Project (2000) unit production rates for pile installation for 39 platforms installed in the Adriatic Sea, Mediterranean Sea and off the West Africa coasts ranged between 100 m/24 hr for easy driving to less than 20 m/24 hr in difficult conditions. Taking into account a pile of a platform located in the central Adriatic Sea with a pile diameter of 2 m with pile penetration to 100 m, the pure drilling operation of a Dive Drill for a volume of soil corresponding to this pile geometry, would be in less than 11 hr. However, these are only indicative values based on previous experiences and in situ tests are therefore needed, in order to estimate the exact excavation capacity of the Dive Drill during offshore piles installation. Current research is in progress for the application in very soft sediments and for ultra-deep water applications thanks to the know-how gathered with the MeBo application mainly in soft sediments and in ultradeep water with electrical umbilicals. CONCLUSIONS BAUER Maschinen GmbH is currently developing new technologies for offshore pile installation or for supporting the pile installation. Based on its successfully experiences both onshore and offshore a new powerful tool, the Dive Drill has been designed. Dive Drill can be currently used up to depth about 250 m. Many soil mechanical issues have still to be solved such as the effects of loosening of the soil by drilling technique on the interface shaft friction and base resistances in dense materials. Therefore, further research will be carried out in order to improve this interesting technique for both offshore oil&gas and wind industry. ACKNOWLEDGEMENTS Thanks to Dr. Freudenthal and Prof. Wefer of Marum, University of Bremen for the cooperation regarding the development of the MeBo.

Figure 13. Sketch of the templates in deep-water for the Dive Drill A great advantage of the Dive Drill with respect to the top drill unit is that it works faster. One of the main disadvantages of the top drill unit technology is that extra time is required to add and remove rods during tripping in and tripping out. Besides, top drill units use the air-lift system, which can be done only in semi-consolidated or consolidated materials (i.e. rocks). As in this case the excavated material is normally hard, the drill bit is full-face. The main advantage of the Dive Drill is that the drill head can be easily exchanged according to the geology encountered. Current top drill units do not work in water. The pile top is normally outside water, so that the unit can be clamped above on top of the pipe. The Dive Drill has not

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