John Oliphant

Pipeline Unburial Behaviour in Loose Sand

Small diameter pipelines are routinely used to transport oil and gas between offshore production plants and the mainland, or between remote subsea well-heads and a centralised production facility. The pipelines may be placed on the soil surface but it is more usual that they are placed into trenches, which are subsequently backfilled. For the buried pipelines a well established problem has been that of upheaval buckling. This occurs because the fluid is usually pumped through the pipes at elevated temperatures causing the pipeline to experience thermal expansion which, if restrained, leads to an increase in the axial stress in the pipeline possibly resulting in a buckling failure. A secondary phenomenon that has also been identified, particularly in loose silty sands and silts, involves floatation of pipelines through the backfill material, usually shortly after burial. At the University of Oxford a project sponsored by EPSRC and Technip Offshore UK Ltd has commenced to investigate in detail the buckling and floatation problems. The main aim of the research programme is to investigate three-dimensional effects on the buckling behaviour. The initial experiments involve the more typical plane strain pipeline unburial tests to explore the relationship between depth of cover, uplift rate, pipeline diameter and pullout resistance under drained and undrained conditions. The second and main phase of experiments involves inducing a buckle in a model pipeline under laboratory conditions and making observations of the pipe/soil response. This paper will describe the initial findings from the research including a) plane strain pipe unburial tests in loose dry sand, and, b) initial small scale three-dimensional buckling tests. The paper will then describe the proposed large scale three-dimensional testing programme that will be taking place during 2006 and 2007.Copyright

CAISSON: A Suction Pile Design Tool

The two main suction pile design methods that are generally applied and accepted within the industry are 3D Finite Element analysis and limit equilibrium. The limit equilibrium method involves assuming a number of failure mechanisms with the mechanism offering the least resistance adopted for design. The limit equilibrium suction pile design software CAISSON has been developed and validated by Cathie Associates for Technip. It is currently in use for rapidly and reliably determining the critical failure mechanism and ultimate holding capacity of initiation, mooring and hold back suction piles in clay. CAISSON has been developed as a stand-alone program written in Visual Basic with a user-friendly program interface implemented to allow for efficient computations. The failure mechanisms employed in CAISSON were identified initially using 2D FE results from PLAXIS. The failure mechanisms identified were further calibrated using 3D FE modelling in ABAQUS and FLAC to account for the influence of side shear within the limit equilibrium equations adopted in CAISSON. The current version of CAISSON can analyse suction piles with L/D aspect ratios from 0.5 to 5 installed in clay of uniform or linearly increasing undrained shear strength. Additional program features include computation of inverse catenary shapes for anchor chains, anisotropic undrained shear strength profiles, pile tilt and pile misalignment. The development and validation of CAISSON is presented in this paper along with a case study and a short parametric study to identify the significance of the CAISSON input parameters that govern the ultimate holding capacity of suction piles. Planned upgrades to CAISSON will also be presented.Copyright

Pipeline Embedment Prediction Using As-Laid Data

Pipe embedment defines the initial condition of the pipeline following installation and is fundamental to the calculation of both axial and lateral resistance of pipelines. However, the initial embedment depth of a pipeline is notoriously difficult to predict. The influence of cyclic loading during pipe-lay is known to significantly increase embedment. However, there is no known empirical work or data from which to model or predict the additional embedment associated with the dynamics of the pipelay operation. As-laid data from pipelines must be viewed as the best guide. Field survey embedment data are presented for a rigid pipeline installed in deep and shallow water clay sediments. These data are used to re-evaluate the models used for predicting the initial pipe embedment. Recommendations are given on the dynamic embedment factor required to account for the dynamic effects at the touchdown region during installation. The results demonstrate that the initial pipeline embedment is much more dependent on the dynamic installation loads and less on the stress concentration at touchdown.Copyright

Reliability of Buckle Formation Accounting for Bathymetry

The mitigation of lateral buckling for subsea pipelines is achieved by making the pipe buckle at intended locations in preference to uncontrolled lateral buckles. At these engineered locations, the pipeline can be given enhanced characteristics (pipe tolerances, welds and Non-Destructive Examination (NDE) criteria). Alternatively, the buckle initiation equipment can be designed to distribute evenly the stress along the buckled pipeline and consequently reduce the maximum stress in the buckled pipeline section. As various parameters involved in the buckle formation process are uncertain (e.g. soil characteristics, residual horizontal imperfections after pipelay), a probabilistic approach is used to assess the likelihood that the pipeline will buckle at the intended locations.Up to now, the existing guidelines focus on in-plane residual Out Of Straightness (OOS) whereas the impact of bathymetry (i.e. uneven seabed) is only addressed in a high level fashion, with no clear methodology.The authors have therefore developed an original approach to account for the effects of bathymetry on the reliability of buckle formation. In a first step, the effects of bathymetry on buckle formation are discussed from a geotechnical and mechanical perspective. Then, the implementation of the buckling formation formulation in a probabilistic numerical subroutine is presented. Finally, a generic case study is considered to assess the impact of accounting for bathymetry on the buckle formation reliability analysis.Copyright

Influence of Iceberg Geometry and Scour Depth on Drag Force in Sand

The seabed disturbance in Arctic regions comes predominently through horizontal cutting mechanisms induced by natural processes such as iceberg scour. The zone of soil being disturbed by the iceberg and the soil resistance to the object movement is of interest to the geotechnical engineer. The influence of the iceberg on potential movement of the pipeline buried in the seabed has been of concern recently with recovery of the oil and gas assets situated in Arctic region.Despite the significant developments in recent years in numerical modelling the ice gouging process still remains a challenge and further work is required to obtain a fully calibrated and validated numerical model. This paper therefore aims at developing a validated numerical model of the interaction between the iceberg and seabed through a series of laboratory tests undertaken at the University of Aberdeen.This paper reports on a preliminary study focusing primarily on the comparison between laboratory and finite element (FE) modelling of the interaction between an iceberg and the seabed in terms of drag force and disturbance of the sediment. The FE model and experimental rig are described. The effect of the shape of the iceberg and the depth at which it penetrates has been investigated. Outputs of drag force from FE calculations showed a good agreement with the results obtained from the laboratory model giving the confidence to move to the next stage where both the inclusion of a pipeline segment and sand with different densities will be considered.Copyright

The Bearing Capacity of a Skirted Two-Foundation System

A foundation system comprising two closely spaced skirted rigidly connected foundations has been developed for in-line subsea structures on a number of offshore deepwater development projects off the west coast of Africa. Here the pipeline and its holding system are placed between the foundations and allowed to rest on the seabed during operation. This type of foundation system produces benefits during installation as the hydrodynamic forces on the structure are reduced and in operation a greater stability is achieved through a lower system centre of gravity. In addition, as the pipeline interacts with the seabed between the foundations, some of the pipeline loading is carried by the soil rather than the foundation system. There is no established design methodology for these foundation systems. Full interaction between the skirted two-foundation system occurs as the soil between them is constrained. Therefore, efficiencies in the vertical bearing capacity of a spaced skirted foundation system can be mobilised through foundation interaction provided by the structural connection. Previous work on uniform clay found that a maximum vertical bearing capacity efficiency of 1.05 was observed when the spacing between the two foundations or footings was approximately 0.25B. A series of plain strain FE analyses have been conducted to investigate the vertical bearing capacity of a skirted two-foundation system. Both uniform and non-uniform clay models have been studied. A parametric study has also been done to investigate the influence of the size of the spacing between foundations and skirt lengths on the overall foundation capacity.Copyright