ABSTRACT A relatively new technology outside of military applications is low logistics autonomous underwater vehicles (AUV). These vehicles can be programmed with a mission, deployed from a vessel or directly from shore, execute their mission and return with a variety of different data. Using a suite of sensors these vehicles provide a quiet and stable platform for the acquisition of high-resolution data. This paper covers the capabilities of the systems and presents some applications and advantages to the user, as well as some data from projects performed to date. This includes rig site surveys, some of which are in restrictive locations, route surveys and general offshore subsea surveys. 1. Introduction Autonomous underwater vehicles (AUV) have been operating in the upstream oil and gas sector for well over 10 years. In 1995 Kongsberg started the HUGIN project with the first commercial survey being performed in 1997 for Statoil's Aasgaard pipeline route. Since then, the HUGINs have built an impressive track record of performing large area survey very effectively, often in deep water and generally operated from dedicated, specialised vessels. There is no doubting the benefits of these vehicles for large surveys, especially in the deep waters of oil and gas exploration, however, there is a developing need for smaller surveys in relatively shallow water. The manufacturers of some smaller vehicles have realised the benefits of their vehicles, which have primarily serviced military applications (e.g. mine counter measures (MCM)). Similarly, the end clients, the oil and gas operators, have seen the benefits of using these smaller vehicles for some of their survey requirements. These smaller vehicles - often referred to now as low logistics, lightweight, compact or mini AUVs - are now being used more often especially by a few key end clients who have realised their benefits.

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 The axial sliding resistance between a surface-laid pipeline and the seabed is an important design parameter that influences the pipeline end expansions, as well as lateral buckling and cyclic walking responses. This paper describes a new framework for quantifying axial pipe-soil interaction and reports data from interface shear box testing of three carbonate soils in support of this approach. The framework uses concepts from critical state soil mechanics, and encompasses the influences of stress level, pipeline roughness and shearing rate - including drainage and viscous effects. Each of these effects is independently quantifiable and relevant to design. Typical pipeline expansion rates span drained and undrained behaviour on deepwater mud, and typical pipeline coatings are often in the transitional range between fully rough and fully smooth. In addition, at the stress levels relevant to pipelines, the effective stress failure envelope is non-linear. The laboratory data comprises interface shear box tests on a range of carbonate sediments spanning from sand to fine silt. Novel procedures are used to replicate the history of loading and consolidation experienced by soil beneath a pipeline, revealing the consequent changes in the interface strength. The new framework provides a basis for integrating characterisation data from soil element tests and model tests of pipe-soil interaction. This has led to an improvement in design practice, through the use of recommendations that are more robust. 1. Introduction The axial resistance between a surface-laid pipeline and the seabed affects the in-service end expansions and cyclic axial walking, as well as the initiation and growth of lateral buckles. This paper describes a new theoretical framework that captures the various mechanisms that control axial resistance. Results from interface shear box tests on three different carbonate soils are used to illustrate some of the mechanisms that control axial pipe-soil resistance.

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 Pipelines are often buried in the seabed using ploughs to protect against man-made and natural hazards. The depth of burial is influenced by the soil conditions and plough progress rates required to meet contract deadlines. Plough progress rates may be a function of the towing vessel capabilities. Being able to reduce plough tow forces, increase the rate of progress or increase ploughing depths without significant increases in tow forces may offer considerable commercial advantages to the ploughing industry. One possible means of improving pipeline plough performance is through inducing vibration in the plough and surrounding ground. This paper reports the findings of a pilot study looking at the effects of plough vibration on tow forces and plough depth. The results show that vibration at 77–102Hz resulted in increased plough depth by up to 38% in dense sand, with limited average increases in tow forces of 4–6%. 1. Introduction Offshore pipelines may be installed below the seabed using ploughing techniques to ensure protection against human activity, such as fishing and anchoring. Burial also reduces thermal losses when pumping hot product and the associated problems of upheaval buckling (UHB) as a result of thermal expansion during pipeline start-up and operation. Ploughing in difficult and unforeseen ground conditions may require higher tow forces than anticipated, which may lead to slower ploughing rates than is commercially desirable. It may also result in single plough passes that are not at the required burial depth, and thus the need for multi-passing. It may also become difficult to control the ploughs depth which may result in trench and pipeline out-ofstraightness (OOS) that can increase the risk of UHB. This could require further additional mitigation measures, such as rock dumping, which may not have been anticipated when costing the project.

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 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 The disturbance of the seabed that comes from natural processes, like iceberg keels, presents a potential issue for the installation of pipelines. The zone of soil disturbed by the iceberg and the soil resistance force to the object movement is of interest to the offshore geotechnical engineer. This paper reports experimental investigation of an iceberg keel shape penetrating sediment and discusses the influence of the depth of penetration on the drag force with its corresponding components. The study was aimed at estimating the drag force and its components in relation to scour developed and the shape of the iceberg. It involved laboratory experiments in which the effects of scaled iceberg models of different shapes were examined. These models were tested in sand channel. The results indicate an increase in penetration and drag with increasing front face angle for the free penetration case, but an opposite trend for the fixed penetration case. 1. Introduction 1.1 Ice gauging Determining the burial depth of a pipeline at which it remains safe from any contact with an iceberg has been of concern in recent years. A number of experimental and numerical studies have been undertaken to address this issue. During the movement of an iceberg, the soil below the scouring ice keel will displace laterally, in the direction of ice keel movement, transverse to the ice movement and vertically (as reported by Palmer et al., 1990; Woodworth-Lynas et al., 1996). The seabed affected by an ice scour process is described generally in terms of three zones (Palmer et al., 1990). Zone 2 is situated below the level of the base of the ice keel and is subjected to large plastic soil deformations. Zone 3 is where the soil deformation is limited to elastic behaviour and is usually deemed as a safe zone.

ABSTRACT Flowlines and pipelines in offshore areas are generally laid on the surface of the seabed under their own weight without being buried in a trench, these pipelines may experience lateral buckling and axial walking due to expansion caused by their operating temperatures and pressures. Analysis of pipe-soil interaction is required to assess, control and limit buckling. This analysis should include geotechnical data collection and interpretation, and evaluation of the lateral and axial resistance that a pipeline develops to resist the expansion forces. This paper consists of three parts. The first part discusses the importance of site-specific geotechnical characterisation in order to identify strength and deformation parameters of the very shallow layers. Reference is made to recent project in deepwater offshore West Africa. The second part is a summary of results of pipe-soil interaction experiments conducted on reconsolidated soft clay recovered from deepwater offshore West Africa. These laboratory experiments included measurements of axial and lateral resistances of pipe sections partially embedded in clay. Extensive instrumentation was used to measure forces and displacements of the tested pipe sections. In the third part, simplified methods and practical calculations are proposed for the evaluation of the axial and lateral pipeline resistance mobilization curves. These calculations are based on general soil mechanics rules and calibrated using the results of the pipe-soil interaction experiments presented in the second part. Parameters such as pipe over-penetration and the development of soil berms during lateral movement are briefly presented, as they may greatly influence the lateral soil resistance. INTRODUCTION Flowlines and pipelines are commonly used in deepwater oil and gas developments. They may operate at high temperatures and may be subjected to lateral buckling and excessive walking. Pipeline buckling occurs when the compressive effective axial force becomes large enough due to thermal expansion, for example. For deepwater flowlines and pipelines laid on the seabed, buckling is generally produced in the horizontal plane (horizontal buckling). Pipeline ‘walking’ us a ratcheting response consisting of axial displacements of the pipe-line and is caused by start-up/shut down cycles. General pipeline models (finite element or other models) are used to check pipe integrity. Realistic rules for interaction laws between the soil and the pipeline should be integrated into these models. The objective of this paper is to propose simplified methods and practical calculations to evaluate the axial and lateral pipeline resistance curves to be integrated into the general pipeline models. Soil Investigation Reliable information concerning soil layering and properties should be collected. The soil investigation programme should include high quality geophysical and geotechnical surveys. A geophysical survey allows detection of ?geohazards? and provides regional geological overview. The geotechnical survey should allow collection of high quality soil samples. In situ geotechnical tests should be also performed, such as cone penetrometer tests (CPT), T-bar test, etc. Particular attention should be paid to investigate the very shallow strata)the first metre). Characterizing the top metre of deepwater sediments for geotechnical engineering purposes can be achieved by two complementary methods. Performing in situ testing using lightweight seaved modules

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)

ABSTRACT This paper address the influence of pipe-soil interaction on the design of pipelines susceptible to lateral buckling and pipeline walking. The pipe-soil response is the largest uncertainty in the design of such systems, and so generic guidance has been developed to guide the design process. Force-displacement-response models were developed during Phase 1 of the SAFEBUCK joint-industry project (JIP) based on large- and small scale tests carried out by the SAFEBUCK JIP and project-specific test data donated by JIP participants. These models are currently being applies by JIP participants on a number of projects, to quantify the susceptibility to lateral buckling and pipeline walking, and design safe and effective means to control these phenomena. However, of all the design parameters, the soil responses cause the greatest uncertainty in design because of the extreme sensitivity of design solution to the axial and lateral resistance imposed by the soil. Improving the understanding of pipe-soil response provides the greatest scope for refining the design of such systems. The purpose of this paper is to outline the significant influence that pipe-soil interaction has on the pipeline design process and to highlight the way in which the inherent uncertainty in pipe-soil resistance severely complicates pipeline design. The paper than describes the research, development and model refinement that is ongoing to reduce the uncertainties. INTRODUCTION Subsea pipeline are increasingly being required to operate at higher temperatures and pressures. The natural tendency of a hot pipeline is to relieve the resulting high axial stress in the pipe wall by buckling. Such uncontrolled buckling can have serious consequences for the integrity of pipeline. Consequently, the industry has generally sought to restrain pipelines by trenching, burying and rock dumping, or to relieve the stress with inline expansion spools. A far more elegant and cost effective solution is to work with rather than against the pipeline by controlling the formation of lateral buckles along the route. Controlled lateral buckling is an efficient solution for the relief of axial compression. Indeed, as operating temperatures and pressures increase further, lateral buckling may be the only economic solution. This challenges has led to a radical advance in pipeline engineering with a greater need for robust lateral-buckling design solutions. The SAFEBUCK JIP 1 was initiated to address this challenge and aims to raise confidence in the lateral-buckling design approach and to improve under-standing of the related phenomenon of pipeline walking. The pipe-soil force displacement response is the largest uncertainty in the design of such systems. With lateral buckling it is necessary to understand the soil behaviour at large displacement and through many cycles of loading well beyond the point of failure. Such behaviour is outside the bounds of conventional geotechnics or extensive earlier research on pipeline stability. Most previous research into pipe-soil interaction has been related to stability under hydrodynamic loading, with the aim being to ensure the pipe remains in place. A lateral buckling design required the pipe to break out from the as-laid position and move across the seabed, typically by several diameters.

ABSTRACT In most offshore design projects, in situ penetrometer tests are first correlated with element test data in order to assess a suitable resistance factor, so that effectively the absolute value of penetration resistance is not used directly. However, given the difficulty in obtaining high quality samples from deepwater sites, the direct field measurement of penetration resistance may provide a better guide for estimating the response of pipelines and shallow foundation or anchoring systems than laboratory shear strength data obtained from specific strain paths. The key to making better use of in situ penetrometer data is to improve methods of interpretation to allow evaluation of the separate effects of the high strain rates imposed during the test, as well as the gradual softening of the soil as it is sheared. The paper explores how the penetration resistance might be ‘scaled’ for direct application in design, with appropriate adjustments to allow for the timescale of loading (or rate of straining); whether the design event involves steady-state shearing or initial shearing from a previously installed position; and the geometric scale of the design object relative to the size of the penetrometer. New approaches to interpretation are presented to facilitate this process. The question of accuracy of the in situ test data is also addressed, with particular reference to strength measurement at very shallow depths. Introduction Much of offshore design in soft sediments hinges on a design shear strength profile, which is generally based on results from laboratory or vane shear strength data, supplemented by profiles of penetration resistance. The latter provide a means of interpolating strengths, in terms of relative values, but the resistance factor relating penetration resistance to shear strength measured in an element test is generally derived for the given site by correlation, so that the absolute value of the penetration resistance is not used directly. Design calculations, however, are usually based on theoretical relationships, such as the standard Prandtl bearing capacity formula. There is, therefore, an inconsistency between the interpretation of data from in situ penetration tests, where the correlated resistance factor may not accord with theory or a design process based on theoretical solutions. The question may be posed as to which provides a more reliable basis for estimating the capacity of an offshore system, such as a pipeline, shallow foundation or anchoring system ? the penetration resistance measured during an in situ test, or the shear strength measured in a laboratory test or vane shear test. Penetrometer tests may be viewed as a form of model test, but one that may not reflect precisely the timescale of loading (or rate of straining) of the design application. In addition, the soil is disturbed and partially remoulded as the penetrometer passes, so that the average shear strength reflected in the penetration resistance is less than the intact value. However, the simple geometry of full-flow devices, such as T bar and ball penetrometers, renders them amenable to analysis, incorporating constitutive models that allow for strain rate dependency of shear strength and gradual softening as the soil is sheared.

ABSTRACT Pipeline and cable plough stability is an important issue that must be considered when planning trenching operations in soft clays. Of a plough has insufficient bearing capacity, the result can be over deep trenching, high out-of-straightness, a poor quality trench and higher than expected tow forces. Excessive sinkage can also render pipeline or cable loading impossible. Plough stability is mainly affected by the plough characteristics, the applied forces and the undrained shear strength of the soil. When operating correctly, the skids and/or share are in state of continuous sliding failure and do not cause a bearing failure of the soil. This paper discusses the mechanics of trenching in soft soil, and presents examples of how the applied loads and load distribution on the skids and share can be calculated and compared to the failure enveloper to determine satisfactory ploughing conditions. A basic framework for rational selection of factors of safety is discussed. INTRODUCTION Background Pipeline and cable plough stability is an important issue that must be considered when planning subsea trenching operations in soft clays. If a plough has insufficient bearing capacity the result can be over deep trenching, high out-of-straightness, a poor quality trench and higher than expected tow forces. Furthermore, upon plough deployment, excessive sinkage may endanger a pipeline or render the product loading impossible. Although this paper concentrates principally upon pipeline ploughs, the concepts can also be applied to cable plough (Section 6). Plough stability A pipeline or cable plough typically operates using the ‘long beam’ plough principle. This means that the cutting depth of the plough share, which excavates and displaces the soil to form a trench, is controlled by the height of skids which run ahead of the share. The plough share and body can freely rotate relative to the skids, which are designed to slide along the natural seabed without sinkage. The principal components and configuration of a typical pipeline plough are illustrated in Figure 1. A typical cable plough operates in a similar manner. In order to retain proper control of the plough it is important that the skids do not experience significant sinkage. Excessive sinkage of the share must be avoided to obtain a high quality trench. The plough is considered ‘stable’ if both the skids and the share operate as intended, either at seabed level for the skids or at a controlled dept below seabed for the share. Plough stability is mainly affected by the plough characteristics (submerged weight, skid or share dimensions), the applied forces on the skids and share, and the bearing capacity of the soil under undrained conditions. In order to ensure successful operation of plough (i.e. stability) it is necessary to consider the two main operating scenarios: plough deployment on the seabed and pipeline loading; and normal trenching. These two operating scenarios are discussed further. Illustrating the difference between them. Ploughing Operations Deployment In order to deploy the plough over a pipeline, the share is opened and the plough is lowered over the pipeline.

ABSTRACT Offshore construction design codes, such as API and DNV, provide guidance for gravity based design They are formulated for single foundation sand contain only general guidance for many smaller subsea construction application in use today. It is commercially attractive to Contractors and Operators alike to install structures with gravity base rather than piled foundations w here horizontal applied loads are low The normal layout of these structures is to have two or four mudmats placed on the edges or comers of the structure Analysing these structural layouts for foundation stability is actually more complex than single foundations. Drawing on experience with the design of a variety of subsea foundations, the authors address some of the important issues that have arisen recently and suggest methods and approaches for dealing with them. These include Design basis and geotechnical design methodology, Selection of soil parameters if limited data are available, Load distribution and limit state for structures with multiple mudmats Bearing capacity and skirt design issues, Cyclic loading and influence on shear strength, Foundation stability under impact loads The Intent of the paper is to prevent delays in the design and review process, thus reducing costs, by encouraging the use of geotechnical design methodology as a basis for technical communication, and outlining some of the practical design methods that can be adopted for small subsea gravity structures INTRODUCTION The various codes of practice (for example DNV, 1995, API, 2000) provide design guidance for gravity base structures. While the principles are also applicable to smaller subsea applications such as protection structures, riser bases, pipeline end manifolds etc, there are a number of geotechnical issues that arise which are particular to these smaller, less sensitive structures. Because they are smaller does not mean that the geotechnical issues are necessarily simpler. This paper sets out to highlight and propose solutions to some of the design issues related to the foundations of such structures Foundation design should never be separated from the overall design process except that it may require a geotechnical engineer to perform the work. The structure and foundation act together, not apart. To pass loads from one to the other without consideration of the overall behaviour of the system may not provide the best solution. This is particularly true for some subsea structures In which two or more separate bearing are as are involved This paper seeks to face some practical issues related to certain areas of gravity base designs that are particularly important for small subsea structures The main steps of the foundation design process for small structures are considered to be Agree design basis, Define soil parameters, Agree design loads and serviceability limitations, Perform foundation design analysis for limit states, Confirm stability and service ability, Achieve independent verification Aspects of these steps will be addressed in this paper. No attempt has been made to guide the reader through all aspects of design-this would be far too extensive Rather, certain topics have been chosen that are unusual or contentious and not dealt within the codes.

ABSTRACT The paper explains issues associated with pipeline uplift during backfilling and floatation, specifies the high risk conditions and presents analytical work and full scale laboratory testing carred out to enhance the current state-of-the-art. Pipelines are buried to provide on-bottom stability, physical protection and thermal insulation. If pipeline uplift during backfilling or floatation occurs, the beneficial effects of soil cover can be partially or completely negated. Quantifying and designing to mitigate against these risks is therefore essential In most cases this can be achieved by ensuring the pipeline has sufficient weight to resist any uplift or buoyancy forces. However, this may not always be possible If a pipeline is deemed too light to be backfilled, additional remedial measures are required. Since the installation of remedial measures is a high cost operation, optimisation is essential. Research has been pioneered into the field of pipeline movements both during and after backfilling A finite element modelling study has been carrd out to assess the forces experienced by pipelines during backfilling using computational fluid dynamics. Full scale testing has also been carried out to assess pipeline floatation potential. The paper concludes with a discussion of new analysis tools which can be used to predict potential floatation or uplift during backfilling in the future. Upward movement of pipelines can represent a high risk to the assumptions made for pipeline design Therefore, it is imperative that phenomenon such as floatation and uplift during backfilling are understood to avoid significant increases m project costs. INTRODUCTION As smaller hydrocarbon reserves are progressively developed, particularly in the North Sea, small diameter pipelines with thick coating systems are required to provide flow assurance. This not only makes for lighter pipelines but invariably introduces the need to trench and backfill them (OTH 1999), increasing the importance of intelligent geotechnical design. Pipelines used m sub sea environments are trenched and backfilled to provide stability, physical protection, thermal insulation and download to prevent upheaval buckling. One or any combination of these may be required for a given pipeline, but all are interrelated and have a significant effect on the pipeline design process (Finch et al., 2000) In any pipeline design the method of trenching must therefore be given due consideration. There are three mam types of trenching (Fisher 1998): Pipeline ploughing Jet trenching Cutter trenchers / Cable ploughs Each method carries a degree of risk which if not addressed could cause upward movement of the pipeline. If the pipeline does move up from the base of the trench the beneficial effects of the soil cover could be partially or completely negated. It is therefore imperative that phenomenon such as uplift during backfilling and floatation are understood and accounted for in pipeline design to avoid significant increases in project costs from remedial works (Finch et. a1 2000).