ABSTRACT This paper reviews some key issues regarding the cyclic loading response of offshore piled foundations. Starting with axial loading it considers: the cyclic loading that can be expected; the fundamental responses of piles driven in clays and sands; frameworks for understanding axial cyclic response and specifying cyclic soil testing; and approaches for practical application in design. The review then moves to consider pile responses to moment and lateral loading, distinguishing between flexible and relatively rigid piles and anchors. A range of possible design approaches is considered and it is argued that current routine practice needs to be reconsidered. Practical methods now exist to address the potentially highly significant effects on axial capacity of piles that experience high ratios of cyclic to average loads. New research and calculation procedures are emerging that offer significant improvements in a broad spread of topics. 1. Introduction Interest in the behaviour of piles under cyclic loading grew in the 1980s to meet challenges posed by inherently fail-unsafe Tension Leg Platforms (with the first TLP being installed at Hutton in 1984) and heavily loaded deeper water fixed platforms, such as the Cognac jacket set in 320m water. Briaud and Felio (1986) assembled for API a database intended to resemble fine marine sediments covering the cyclic behaviour of clays in: laboratory tests, cyclic model experiments and axially cyclic field pile tests. They considered 16 studies on piles with diameters greater than 150mm, most of which were strain-gauged to measure axial load distributions. Local shaft friction, pore pressure and radial stress measurements were attempted in some cases, although these parameters are notoriously hard to sense reliably. The response of piles driven in sands was not addressed. The piles were submitted to significant numbers of load cycles (typically 100 to 1,000) with frequencies generally around 0.1 Hz.

ABSTRACT This paper describes field tests in which a 250mm steel sphere was allowed to free fall through water from drop heights of up to 2m and dynamically embed the soft clay underlying the water. Instrumentation housed within the sphere measured accelerations in three orthogonal axes, as well as rates of rotation about those three axes. The data were used to calculate velocities and displacements of the sphere during free fall in water and embedment in soil. Reasonable agreement was obtained between the measured velocity profiles and velocity profiles predicted using a simple approach based on strain rate dependent shearing resistance and fluid mechanics drag resistance. 1. Introduction Understanding the processes associated with dynamic penetration of rigid bodies from water into soft soil is challenging. There are a number of applications for dynamic penetration of rigid bodies, including installation of dynamically installed anchors, free-fall gravity core samplers and in situ characterisation tools. Previous work in this arena include centrifuge studies reported by Poorooshasb and James (1989), Richardson et al. (2006), O'Loughlin et al. (2004, 2009); field tests reported by Freeman et al. (1984), Lieng et al. (2010); and numerical studies reported by Einav et al. (2004), Nazem and Carter (2010), Raie and Tassoulas (2006). In these studies, the geometry of the rigid body tends to be rather complex, to the extent that a number of simplifying assumptions are required in order to address the problem. In this paper, the geometry is simplified to a sphere for which the soil mechanics is quite well behaved (Randolph et al., 2000). This permits a more rigor rigorous assessment of dynamic penetration effects. The data are then used to validate an embedment model based on strain rate dependent shearing resistance and fluid mechanics drag resistance.

ABSTRACT Centrifuge tests were performed to investigate the cyclic response of piles in soft clay under lateral loads. Typical data are presented and discussed. The accumulation of pile-head displacements and maximum bending moments with number of cycles in particular are examined. Power law models fitted on experimental data are proposed to describe the effect of large numbers of cycles. 1. Introduction Pile foundations of offshore structures used for the oil and gas or wind farm industries are subjected to large cyclic horizontal loads resulting mainly from the action of waves and wind. Pile design procedures are essentially based on the application of recognised standards or professional recommendations, among which the American Petroleum Institute (API) RP 2A (2000) is the most commonly used. It proposes a design methodology for horizontally loaded piles based on the use of local pile-soil transfer ( p-y ) curves. Monotonic, as well as so-called cyclic, p-y curves are provided for both sands and clays. The cyclic p-y curve concept requires some attention. It was derived in the 1970s on the basis of pile tests performed on relatively small-diameter (123/4–24-inch) piles for soft clays (Matlock, 1970), stiff clays (Reese et al., 1975; Reese and Welch, 1975) and sands (Cox et al., 1974; Reese et al, 1974). The piles were subjected to a series of cyclic loads representative of load histories imposed by Gulf of Mexico storms to jacket piles. The final result was an ‘envelope curve’, which is aimed at reproducing the response of a pile monotonically loaded at the end of the extreme event (e.g. the centennial storm). More recent laboratory tests (Craig and Kan, 1986; Kitazume and Miyajima, 1994; Jeanjean, 2009; Zhang et al., 2011) emphasise the significance of progressive accumulation of pile head displacements with applied number of load cycles.

ABSTRACT This paper presents the results of a series of laboratory tests performed on model piles in dense sand to investigate the axial response and tensile capacity. The test schedule was designed to impose varying cyclic loads followed by static tensile tests to measure the improvement or degradation in pile capacity. The degree of degradation was calculated by using a simplified axial degradation model, a formulation that encompasses average and cyclic loads along with a prediction of the number of cycle to failure of the pile. The results indicated that few intensity cyclic loading rapidly leads to shakedown behaviour. The model predicted no capacity degradation during low-intensity cycling at low load levels with minimal degradation in the high-intensity cyclic loading tests. Measurements of tensile capacity after high intensity compressive cycles, which showed varying amounts of degradation, indicate that very low cyclic load levels need careful consideration in any degradation formulation. Load levels that did not cross a threshold, dependent on the load application history, showed little or no degradation. This threshold will be higher if low load level cycling has improved capacity and lower if degradation has occurred in previous loading cycles. 1. Introduction A large body of literature is available on the subject of loading of piles used for offshore structures of different types, such as oil and gas platforms. In the case of sand, cyclic loading is expected ultimately to reduce the axial capacity of the pile itself. As noted by Abdel-Rahman and Achmus (2011), there is a need for a method of calculating axial pile capacity degradation with regard to load magnitude and number of load cycles. This is important in relation to the improvement of economic design methods and the number of wind farms proposed to be constructed in the coming years.

ABSTRACT As part of the French national research project SOLCYP, an extensive series of static and cyclic axial pile load tests has been carried out in the overconsolidated Flandrian clays from northern France (Merville experimental site). Tests were performed on 4 driven closed-ended pipe piles (with a depth of 406mm, length of 13m and wall thickness of 15mm); 4 bored piles; and 2 screwed piles (with a depth of 420mm and length of 13m). All piles were instrumented with retrievable extensometers for measuring the load distribution along the pile wall. The paper describes the context of the pile tests and presents preliminary static and cyclic test results obtained on the driven piles. The example of pile B4 under one-way tension loading is taken to illustrate the type and quality of data acquired. 1. Introduction 1.1 Background Cyclic axial and lateral loading on offshore, near shore and onshore structures may be essentially of environmental (e.g. wave, wind) or industrial origin. Loading histories include a large variety of loading modes (e.g. tension, compression, one-way, two way), cycle amplitude and frequencies. A keynote paper in these conference proceedings by Jardine et al. (2012) presents the potential effects of cyclic loading on the response of offshore piles. Practical engineering methods in use in the offshore industry (oil and gas, wind farms) are reviewed. Key aspects that are of interest to the pile designer are as follows: (i) the potential reduction on the ultimate axial capacity; (ii) the number of load cycles of a given load that the pile can sustain before cyclic failure; and (iii) the evolution of displacements of the pile head during cyclic loading (pile stiffness). In the building and civil engineering domain, the effects of cyclic loading on foundation piles are poorly understood and largely ignored in practical design.

ABSTRACT The behaviour of ‘non-displacement’ piles installed in dense Fontainebleau sand and subject to both static and cyclic vertical loads is investigated in a geotechnical centrifuge at a scale of 1:23. Different loading sequences with several combinations (one- and two-way) have been applied. The influences of the cyclic load amplitude and mean amplitude are analysed. 1. Introduction Numerous types of structure experience significant cyclic loading. Cyclic loads may be essentially environmental (e.g. wave, wind) or operational in origin. The oil and gas industry has developed procedures for considering the effects of large wave cyclic loads on foundations for offshore structures (e.g. American Petroleum Institute (API), 2011). Comparatively, the effects of cyclic loading on foundations are largely ignored in most civil engineering and building activities. The French national programme, SOLCYP (Puech et al., 2012), aims to investigate the effects of cyclic loadings on piles. The experimental part of the project includes both large-scale and model pile testing, which will be performed in calibration chambers and in the centrifuge. The main objective of the centrifuge tests is to study the stability of piles in sands and clays subject to a large variety of cyclic loading histories. Centrifuge testing has proved its efficiency in conducting extensive series of tests, as it allows a large range of parameters to be varied while keeping associated costs relatively low, when compared to in situ testing (e.g. Craig, 1988). This paper focuses on the results of the first series of tests conducted at the IFSTTAR (formerly LCPC) centrifuge facility in Nantes, France. The mode of installation of the model piles is intended to simulate soil-pile interactions relevant for cast-in-place piles. The tests, conducted in dense Fontainebleau sand, covered the full range of loading modes (monotonic versus cyclic; compressive versus tensile; and oneway versus two-way).

ABSTRACT Various foundation types for offshore wind energy converters (OWEC) are currently discussed and used. Shallow offshore foundations can rarely be placed on the seafloor, as weak soils usually need to be excavated to place the foundation structure on more stable ground, thus resulting in anthropogenic submarine pits. Steep but stable slopes of the pit meet both economic and ecologic aims as they minimise material movement and sediment disturbance. Natural submarine slopes of sandy soils are usually less steep in reality than in theory. According to Terzaghi (1957) the angle (β) between slope and the horizontal ground surface of cohesionless soil is at most equal to the critical state friction angle (φ crit ). However, hydrodynamic forces act on the slopes and natural submarine slopes are already shaped by perpetual loads of waves, tide and mass movements. Artificial slopes of foundation pits do not appear and behave as natural submarine slopes, or as theoretically predicted. Physical simulations of different scales were used to analyse the stability of artificial submarine slopes with sandy soil of the North Sea. The laboratory tests focused on gravitational forces and impacts from the excavation processes. In addition, numerical simulations of wave-induced bottom pressure supported considerations of suggested submarine pit slope angles. Based on these slope angles, in situ tests will be performed and both dredging process and resulting foundation pits will be surveyed. 1. Introduction Submarine foundation pits are usually necessary for gravity based foundations to place the structure on stable ground. The required depth of projected pits in the German North Sea varies from 2m to 7m, and the excavation of temporary submarine foundation pits results in huge amounts of dredged material. From economic and ecologic points of view, this amount should be reduced to the only what is most necessary.

Abstract The interaction between the seabed and unburied pipelines is one of the key uncertainties associated with deepwater and/or high pressure, high temperature (HPHT) pipeline design. Emerging research is shedding light on this important topic that is not currently well understood. BP has amassed a considerable quantity of data from different basins ranging from conventional in situ measurements of soil properties (supplemented by two campaigns using Fugro's SMARTPIPE®) to model testing at the Norwegian Geotechnical Institute (NGI) and the University of Western Australia (UWA). Added to this, standard and advanced low stress interface shear testing has been carried out in laboratories at the University of Texas, Austin, UWA and Fugro. These tests were designed to address the key mechanisms governing axial pipe-soil interaction behaviour, spanned a large range of pipe properties and sliding velocities, and included episodes of post-sliding reconsolidation. New data from all of these sources are given in this paper, along with a theoretical framework developed to synthesise these results and contribute to improved practice for the design of pipelines that are susceptible to walking, buckling and other forms of movement. 1. Introduction It is well known that the interaction between surfacelaid pipelines and the seabed is extremely complex. The challenge for geotechnical and pipeline engineers is to model this interaction in a manner that can be efficiently applied in design calculations and analyses. In addition, this modelling will need to produce safe and reliable results, while faithfully reproducing the relevant physical processes in play. Design calculations for buckling and walking can be classified as either analytical or numerical. In analy-tical calculations, the pipe-soil response is modelled as rigid-plastic, characterised by a single parameter - the ultimate resistance. In numerical calculations, the response may be elastic-plastic (i.e. bi-linear), or may be a more complex piecewise linear variation.

Abstract To satisfy flow assurance criteria, it is sometimes required to restrain the pipeline against any movement that could prevent free drainage of the pipeline during a shutdown. Rock dumping of a surface pipeline is a viable restraining solution, provided it is properly designed to withstand lateral (and upheaval) buckling load coming from the pipeline. The available solutions for lateral resistance have been developed for buried pipelines under a flat seabed, and consequently do not take account of the geometry effect relevant to rock berms. They have also been mostly developed for in situ sand conditions, which do not necessarily reflect the properties of the rock or gravel forming the rock berm. In this paper, the lateral resistance of a pipeline buried under a rock berm is investigated using the finite element (FE) method. The developed FE model is validated against the buried pipeline problem with available solution. The effects of geotechnical properties such as friction and dilation angles, and also geometry of the rock berm, are then investigated using the developed model. The findings of this paper are finally discussed towards appropriate engineering design of rock-dumped pipelines. 1. Introduction Rock-dumping a pipeline on the seabed is often undertaken to provide resistance against lateral buckling of the pipeline. As such, it is essential to have an understanding of the lateral resistance provided by the rock-dump. Experimental and numerical research on the lateral resistance of buried pipelines (Trautmann and O'Rourke, 1983, 1985; Yimsiri et al., 2004; Karimian et al., 2006; Di Prisco and Galli, 2006; Badv and Daryani, 2010) has thus far focused on a flat seabed, and consequently does not take into account the rock-berm geometry effect. In addition, the research has mainly looked at in situ sand, not rock, conditions.

Abstract Offshore shallow foundations may be subjected to uplift due to overturning or buoyancy loading. Peripheral (and often internal) skirts can enable transient tension loads to be resisted because of negative excess pore pressures developed between the underside of the foundation top cap and the soil plug confined by the skirts. Uncertainty exists with regard to the duration over which these negative excess pore pressures can be maintained and the effect of a gap forming along the skirt-soil interface on the transient and sustained holding capacity. This paper presents results from drum centrifuge tests carried out on a shallow skirted foundation subject to transient and sustained uplift in a lightly overconsolidated clay. Results from baseline tests with an intact skirt-soil interface are compared with tests in which a gap was created along the skirt-soil interface prior to transient and sustained uplift. The results are promising, showing for example that uplift loads of 40% of the peak undrained capacity were maintained for up to two years without significant foundation displacement when an intact foundation-soil interface was maintained. However, they also reveal that the presence of a gap may halve the time to reach similar displacements. Introduction Shallow skirted foundations are widely used offshore to support small platforms, seabed protection structures, storage tanks, and subsea frames for oil wells and pipelines, as well as for larger fixedbottom and floating structures (e.g. Støve et al., 1992; Tjelta, 1994; Bye et al., 1995; Watson and Humpheson, 2007). Skirted foundations are also an attractive option for mooring or supporting current meters and wind turbines offshore. Skirted foundations may comprise a top plate a peripheral skirt, and sometimes internal skirts or a cluster of individual skirted units connected together. Regardless of configuration, the foundation penetrates the seabed, confining a soil plug inside the structural members.

Abstract The performance of a hybrid foundation for subsea systems, defined as a shallow skirted mat foundation featuring short piles at the mat corners, has been investigated through a series of centrifuge tests. The foundation, with and without corner piles, was subjected to eccentric monotonic loading along the x , y and z axes, resulting in combined loading over 6 degrees of freedom. Modes of yielding were identified and the contribution of the corner piles to the bearing, sliding, overturning and torsional capacities of the hybrid subsea foundation was quantified. Results revealed that (a) centrifuge modelling could capture the strain hardening arising from the plunging nature of the mat foundation yield; and (b) the addition of the corner piles to the shallow mat resulted in a change of yielding mode from shearing at the mat invert to overturning, with a significant increase in the foundation capacities. Consequently, corner piles appear to be an efficient option to reduce the size of the subsea system foundation required to withstand a given set of combined loading. 1. Introduction Subsea mats are used in deep waters as foundations in soft normally consolidated (or lightly overconsolidated) clay to support facilities such as pipeline terminals, jumpers, riser bases and manifolds. They are typically subjected to various combinations of loading in all six degrees of freedom. In some cases, a mat may not provide sufficient resistance against bearing, sliding or overturning failure. The use of pinned piles in each corner of the mat may then be considered to increase the sliding and overturning capacity of the foundation. Such a foundation is defined here as a hybrid subsea foundation. Hybrid subsea foundations have already been deployed in situ , but neither formal guidance nor experimental data existed at the time to assist in their design.

Abstract Flowlines and pipelines installed in deep-sea waters are submitted to axial and lateral loads due to the effects of flow stoppages and starts, thermal influences and internal pressure. To study the phenomenon of soilpipeline interaction, a physical model was used and special emphasis was given to the application of large horizontal loads coupled with vertical loads. This paper focuses on the experimental results of two sideswipetype tests with a pipeline model in a very soft soil with undrained shear strength of ~3kPa. Experimental yield envelopes are also included. The experimental tests revealed that for very shallow pipe embedments the maximum horizontal load is obtained for a value of V/Vmax = 0.5, and that for larger embedments this value is in the order of 0.2. 1. Introduction The design of pipelines installed in deep-sea waters is still a challenge for offshore geotechnical engineering. Flowlines and pipelines can be submitted to combined vertical and horizontal loads, thermal expansion and internal pressure, among other loadings. The pipeline laying installation is not a guarantee of their penetration embedment and, consequently, their stability. The problem of untrenched pipelines has been studied and reported in literature (Murf et al., 1989; Brennodden and Stokkeland, 1992; Cassidy, 2004; Fontaine et al., 2004; Cathie et al., 2005; Zhang and Erbrich, 2005; Cheuk and Bolton, 2006; Dendani and Jaeck, 2007; Bruton et al., 2008; Tian and Cassidy, 2008; among others). A physical model was used to understand the mechanisms of interaction between pipe and soil through a lateral visualisation, and then to simulate the pipe response under combined vertical and horizontal loads. This paper concentrates on the results obtained using sideswipe tests with large and short horizontal displacements, in order to obtain an experimental yielding envelope of the pipe.

Abstract The vast majority of offshore wind farms constructed to date are supported on monopile foundations. These monopiles consist of a large diameter (>4m) open-ended steel piles driven into the seabed to a specified penetration. While laterally loaded piles have been used for many years in the offshore oil and gas sector, they typically have diameters below 2m and a slenderness ratio (ratio of pile length to diameter) in excess of 20. In contrast, monopiles used in the offshore wind sector typically have slenderness ratios of 5 to 8. Design methods developed for relatively slender flexible piles are unlikely to provide accurate predictions of the response of more rigid monopiles to loading. This paper presents the results of a field test performed on an instrumented monopile installed at a dense sand research site in Blessington, Ireland. The pile, which had an external diameter of 340mm, was driven into the dense sand to a slenderness ratio of 6. It was also instrumented with 11 levels of strain gauges to capture the load transfer and bending moments along the shaft. The load test results show that conventional design procedures, such as the Det Norske Veritas (DNV) or the American Petroleum Institute (API) approaches, grossly underestimated the lateral capacity of the monopile. At the end of the paper, a 3D finite element analysis of the pile load test is presented. 1. Introduction There has been a significant drive to develop offshore wind energy resources in the last 20 years. To date, most offshore wind turbines (>75%) have been constructed on monopile foundations in relatively shallow water (<30m). Monopiles are large openended steel tubes, with typical diameters ( D ) that are between 4m and 7m, and embedded lengths ( L ) typically less than 30m. This gives a resulting slenderness ratio ( L/D ) of between 5 and 7.

Abstract Significant research effort has been put into the formulation of improved design procedures for the calculation of the axial capacity of open-ended piles in sand. Methods, such as the Imperial College Pile (ICP) and the University of Western Australia (UWA-05), estimate the medium-term capacity of piles at relatively large displacements (typically 10% of the pile diameter). To use these approaches in pile driveability analyses, some additional factors need to be taken into account. In addition to the significant influence of dynamic effects, such factors include the absence of pile ageing and the relatively low displacement mobilised during individual hammer blows. In this paper, pile driving records from the installation of two open-ended steel piles installed in medium dense to dense North Sea sand are considered. The piles are a 0.762m-diameter skirt pile supporting a jacket structure and a 4.2m-diameter monopile, respectively. The piling records are used to test the accuracy of existing pile driveability models by comparing the measured and predicted blow counts. The latter are determined by incorporating the static resistance to driving (SRD) and dynamic damping parameters in a 1D wave equation analysis program. The possibilities of using recent static capacity formulae (e.g. ICP and UWA-05) to predict driveability were also explored. 1. Introduction Pile driveability is an integral component of the process of offshore pile design and is often the determining factor in the selection of an appropriate driving system. Accurately predicting the pile response to driving is becoming increasingly important, with the use of large 3–6m diameter steel monopiles becoming commonplace. Pile driveability assesses the ability for a pile to be economically driven with an acceptably low risk of refusal and, ultimately, to reach a desired penetration or capacity within a reasonable number of blows without overstressing the steel.

Abstract A new Interstate 10 (I-10) Twin Span Bridge over Lake Pontchartrain was recently constructed to replace the old bridge that was heavily damaged by Hurricane Katrina in 2005. A large portion of the bridge is supported by batter pile group foundations. To evaluate the performance of batter pile foundations under lateral loading, a selected pier (M19 eastbound) of the new bridge was instrumented and used to monitor the pier during a unique full-scale lateral load testing. The M19 pier foundation consists of 24 precast prestressed concrete (PPC) 33.53m (110ft) long batter piles, among which 8 piles were instrumented with microelectromechanical sensor (MEMS) in-place inclinometers (IPI), and 12 piles were instrumented with strain gauges. The test was conducted by pulling the M19 eastbound and westbound piers toward each other by using high-strength steel tendons. A maximum of 8320kN (1870 kips) lateral load was applied in increments. A high-order polynomial curve fitting method was applied to fit the measured rotation profiles from the IPIs. The fitted rotation curves were then used to deduce the bending moment, shear force and soil reaction profiles. The calculated moments from curve fitting were compared with the moments calculated from strain gauges, and the results showed good agreements. The p-y curves of the soils at different depths were back-calculated, and the results showed little evidence of group effect. 1. Introduction Pile foundations are usually designed to support highway bridges and other structures, primarily to safely carry superstructure axial loads deep into the ground. However, in many cases structures (such as bridges, quays and harbours) are subjected to lateral loads caused by high winds, wave action, water pressure, earthquakes and ship impacts. Therefore, it becomes essential to understand the resistance behaviour of piles and pile groups to lateral loads.

Abstract Probabilistic slope stability analysis has been performed for the West Nile Delta (WND) deepwater area, offshore Egypt. It has been used to minimise geohazard risk for the WND subsea development. The analysis was premised on earthquakes being the predominant trigger of slope instability, exacerbated in the past by excess pore pressures generated by relatively rapid sediment accumulation. Earthquake-induced slope displacements calculated using the Newmark method were mapped to a pseudostatic safety factor to facilitate area-wide characterisation of slope failure probability. The annual probability of slope failure was assessed across the WND development area within ArcGIS, incorporating mapped variable geological controls (e.g. soil strength, slope angle) as inputs. Failure probabilities were spatially generated using the concept of slope facets, defined as areas of seabed with similar expected susceptibility to slope failure, based on geomorphological examination. 1. Introduction BP Egypt and its equity partners, RWE Dea Group and Egyptian Natural Gas Holding Company (EGAS), have embarked on a programme of subsea natural gas developments in the West Nile Delta (WND) deepwater area. A systematic method has been adopted to mitigate geohazard risk in the area, using a multi-disciplinary team approach to risk identification and quantification (described in Evans et al., 2007; Evans, 2011). The various geohazards identified have been highlighted by Moore et al. (2007), foremost of which is the potential for slope instability. This failure type has been the subject of both an expert judgment assessment and probabilistic stability analysis (PSA) to best constrain the associated risk. The former was conducted in multi-disciplinary workshops, based primarily on historical event frequency and informed by variations in environmental controls (including sedimentation rate and related pore pressures). The latter, and the subject of this paper, is considered a geotechnical assessment of slope failure probability.

Abstract Submarine landslides are one of the major hazards for offshore pipelines. Progressive differential ground movements caused by earthquakes can initiate run-out/debris flows that can impact and damage pipelines. Hence, both the stability of submarine slopes caused by earthquakes and the potential run-out distances must be assessed. This is particularly important for deepwater pipelines, whose routes often cross areas that are prone to landslide and debris flow. This paper presents an overview of slope stability and run-out assessment for offshore pipelines using both analytical and numerical methods. Analytical slope stability assessment is based on guidelines for seismic design and assessment of natural gas and liquid hydrocarbon pipelines, and SLOPE/W and QUAKE/W are used for the numerical assessments. The paper provides a review of run-out assessments in literature and summarises the best methodology through a case study. 1. Introduction Submarine landslides, which can be triggered by many factors such as shallow gas release and seismic events, pose a threat to offshore pipelines, and the outcome of such event could be catastrophic. Progressive differential ground movements, such as those caused by landslides, earthquakes and runout/debris flows, can cause pipeline deformations that may impact serviceability of pipelines. Hence, it is essential to assess the stability of submarine slopes and study the risk associated for pipelines stability. There are mainly two types of slope failures usually associated with submarine slopes undergoing earthquake shaking: coherent failure and disruptive failure. Coherent failure is where the soil moves as a single solid mass (Figure 1a), and disruptive failure is where the soil loses the majority of its strength and flows as individual particles (Figure 1b). Hence the assessments need to be conducted firstly to identify the stability of slopes during shaking and then to assess the expected run-out.

ABSTRACT Various observations pointed out that cores performed with gravity piston corer show significant distortions mainly located at the top of the core. A series of 15 cores were performed at the same location on a submarine sand wave (Var canyon, France). Six different settings of the corer "three freefall heights and three slacks of the piston cable" were tested, including duplicates. Two accelerometers recorded simultaneously the movements of the core tube and the movements of the triggering arm. Then the z displacements were obtained by a double integration versus time of the measured acceleration. The analyses of results allowed the authors to estimate the amplitude and the duration of the elastic recoil of the aramid cable, and to distinguish four steps during the 4 seconds of penetration, including a distortion phase followed by a normal sampling phase linked to the status of the piston. The analyses of the quality and benchmark layers from recovered cores highlight the major role of the piston driven by the lengths of the counterweight and piston cables. The recovered thickness of a given layer can vary from 0.8 to 1.3 depending to the settings. A cone penetrometer test (CPT) trial at the same location gives a good estimation of the absolute geometry of the layers. The settings for cores with geotechnical purpose (better quality) will be different from settings for cores with sedimentological or palaeoclimatological purposes (better geometry). A compromise is proposed. Introduction Gravity coring with stationary piston is an efficient way to recover long sedimentary cores 1 . However, various observations pointed out consequent distortions 2 based on different techniques: magnetic orientation 3 , and comparison of different corers or with sub-bottom profiler 4 . Authors described the ?over-sampling? and the ?under-sampling 5 , and others proposed recommendations for improvements of corers 6 . The Stacor corer gets round these disturbances with a truly stationary piston 7, 8 . The device provides high quality Sampling 9 especially for soil investigation 10 , But the duration of the deployment (8–10hr) and the size of vessels and cranes are constraining factors for the use of such a device by scientific community. The trial of a new aramid cable onboard the R/V Le Suroit was an opportunity to examine the effect of the elastic recoil of such a cable on the recovered sediment. The use of accelerometers allowed for the recording the behaviour of a corer and understanding the effect of settings on the recovered sediment. Method Devices and sensors The stationary piston corer of the R/V Le Suroit consists of a 994kg weight and a 10m steel tube with a plastic liner (maximum recoverable length of sediment of 9.55m), as well as a platform for the release or triggering arm linked to the main cable and the counterweight linked to the plat-form with a cable (Figure 1). The piston is linked to the platform via the piston cable and slides freely inside the liner. The main cable is a 17mm aramid cable with a weight in water of 0.056kg/m.

ABSTRACT The mechanical properties of hydrate bearing sediments may deteriorate as a result of hydrate dissociation due to heating of the wellbore casing, with possible adverse effects on the stability of the casing. The authors have created a numerical model in plane strain in order to simulate the stability of a well-bore supported with a casing. The model captures the interaction between the formation, casing, cement and cement-casing bond. The simulation results demonstrate that the safety factor of the casing is very dependent on the quality of the cement, for with strong cement, it is multiples of the safety factor of the casing with weak cement. The safety factor for the casing with weak cement, in the presence of hydrates in the formation, reduces due to hydrate dissociation and the subsequent deterioration of the mechanical strength of the formation. This paper shows that in this case an effective cement placement around the casing becomes crucial in order to keep the casing factor within acceptable limits. INTRODUTION Casing deformation resulting from reservoir compaction and surface subsidence during the primary production stage has been analysed and reported by many investigators 1,2,3 . Generally the following effects can be considered: Pressure depletion in the formation, which can lead to subsidence and possible shearing of the casing Heating the formation, which can generate high stresses in the formation Deterioration of the sediment mechanical properties due to heating as for hydrate bearing sediments, which can induce large stress in the casing. Casing integrity for well drilled in hydrate bearing sediments presents a challenge that is explored in this paper. Gas hydrate, or clathrates, are ice-like deposits containing a mixture of water and gas, the most common gas being methane. Other gases of small molecular size such as CO 2 , H 2 S, ethane and higher hydrocarbons are also found in natural gas hydrates. Gas hydrates are stable under high pressure and low temperature conditions, and hence they can be found in deepwater settings at relatively shallow depths below the seafloor and in permafrost regions 4 . The dissociation of gas hydrates can take place due to a reduction in pressure and/or an increase in temperature, or due to the injection of certain chemicals that may affect hydrate stability. Once melted due to the circulation of hot fluid in the casing. Hydrate bearing sediments will suffer degradation in their mechanical properties (e.g, strength and stiffness), accompanied by overpressure development 5 . Moreover the gas released by hydrate dissociation may undermine the quality of the cement supporting the casing (if dissociation takes place before the cement is fully cured) and hence reduce the collapse resistance of the casing 6 . This the well by the surrounding sediments in response to heating. Some of the sediment considered are hydrate bearing sediments (HBS). The cements examined could be strong cement (foam cement) or weaker cement (hard cement). The cement-casing bond strength is investigated in the modelling. The modelling is performed using FLAC3D, which is a continuum code developed by Itasca Consulting Group 7,8 .

ABSTRACT The calculation of axial resistance of a pipeline is a key component in lateral buckling design, upheaval buckling (UHB) analysis, axial walking assessment, pipeline anchoring and pull-in and retrieval analyses. The level of axial resistance will depend on the rate and duration of pipeline loading, pipeline displacement and on the relative roughness of the pipe-soil interface which is normally addressed by an interface friction factor. The former relates to the understanding that the soil response can be bounded by drained and undrained conditions. A method is described which allows the application of drained or undrained conditions to be estimated for pipelines in clay/slit soils. The main purpose of the paper is to present a method for estimating the axial soil resistance using a relative roughness parameter, as well as making recommendations on the displacements necessary to mobilize peak and residual drained and undrained resistances in sand and clays. Both untrenched and trenched pipelines with various types of backfill will be considered. INTRODUCTION Understanding pipe-soil interaction behavior is important in the design of subsea pipelines for : Lateral buckling design Steel catenary riser (SCR) design On-bottom stability analysis Installation analysis The main design aspects involving pipe-soil interaction for each of these design are summariesed in Figure 1. The lateral buckling solution is very sensitive to soil-pipe interaction. The SAFEBUCK JIP 1 has been the focus of research and testing of deepwater very soft clay to improve the understanding of pipe-soil response. Sands are generally considered to be better understood than cohesive soils. Guideline for pipe-soil interaction associated with SCR design have been developed from the findings of STRIDE JIP and CARISIMA JIP. The CARISMA soil-pipe interaction testing has allowed the development of mathematical expressions for vertical (compression and suction) and lateral resistances of risers. These expressions have been encoded in the finite element program RIFLES as part of a SCR design approach. The design approach is relevant to very soft clays with intermediate to high plasticity. However, in general, deepwater very soft clays are characterised with an extremely high plasticity. Oliphant et al 2 has extended the original work by SCR design approach within Abaqus. Two pipe-soil interaction models are generally used on-bottom stability analysis programs. The first and simplest is an equivalent 2D Coulomb friction model, while the second 3D analysis model adopts a more realistic non-linear lateral force displacement relationship. In upheaval buckling (UHB) analysis the interaction between the pipeline and surrounding soil will mobilize uplift resistance generally in the vertical plane and axial resistance in the longitudinal plane. Technip has developed state-of-the art uplift resistance equations for a range of different backfill types, such as blocky clay and post-jetted very loose sand. The development of accurate and realistic pipe-soil interaction models is critical to the safe and cost effective design of subsea pipeline. The scope of this paper is to make recommendations on calculating the axial resistance Figure 1: Pipe-Soil interaction aspects of subsea pipeline design (available in full paper)