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The push of the petroleum industry into greater water depths has lead to a tangible increase in project geohazards. Some of the most significant hazards on the continental shelf and slope are submarine landslides. Deepwater pipelines are at greater risk from landslide impact than other subsea structures. This is due to two factors: the length of installed pipeline increases exposure to landslide hazard, and the structural resistance of pipelines is small compared to landslide forces. This paper examines the behavior of surface laid submarine pipelines under shallow landslide impact based on the results of parametric studies performed for BP. A simple closed form solution is developed, modeling the pipe as an elastic cable, the landslide as a distributed load and the soil as a rigid-plastic resistance. The model is calibrated against finite element analysis. Design charts showing the maximum landslide pressure which the pipe can resist are presented. This approach can be used as a simple screening tool to assess pipeline vulnerability during routing in typical deep-water soil conditions and to develop hazard mitigation strategies in such settings. A case study is presented for a small (2-3 m thick) landslide impacting flowlines and an export line. The conclusions of the study are:when a landslide displaces a pipeline it causes the pipe to stretch with resulting tension forces. Ultimately failure is due to tension effects, either as a full bore rupture of the pipe or by disconnecting at the pipeline end termination (PLET) or riser;the landslide induced tension is resisted by axial soil forces along the pipe. As the pipeline is strong in tension and soil resistance is low, these forces are transferred long distances. If the landslide occurs near the PLET, the soil resistance will not be sufficient to fully resist the in-line tension and the end of the anchor will drag unless restrained. In this case the landslide forces will act against the PLET or riser. This is important as the PLET is not generally designed to resist significant tension or accommodate large displacements;geometry of the pipeline is important. If there are horizontal curves they will be " pulled in??, making the pipeline more compliant and slide resistant. For a specific example of a 2-m thick, 400-m wide landslide impacting a 24 inch diameter pipe, it is shown that slide displacements of up to about 500 m can be resisted with limited pull in. A considerable amount of additional slack is required, however, to mitigate greater landslide runouts. The results of these analytical preliminary studies suggest that landslides risks may be reduced by laying pipes with curves in landslide-prone areas. Introduction The push of the petroleum industry into ever greater water depths has lead to a tangible increase in project geohazards. Some of the most significant hazards on the continental shelf and slope are submarine landslides. Deepwater pipelines are at greater risk from landslide impact than other subsea structures. This is due to two factors:the length of installed pipeline increases exposure to landslide hazard, andthe structural resistance of pipelines is small compared to landslide forces. There is limited reporting of geohazard damage for deepwater pipelines in the technical literature. The majority of published data pertains to hurricane damage in shallow water in the Gulf of Mexico. In reviews of damage for hurricanes Andrew, Lili, Ivan, Katrina and Rita there were more than 1300 reported cases of damage (DNV, 2007). The main types of damage are rupture of risers and piping, although cases of full-bore rupture have also been reported (DNV, 2006). A number of researchers have examined pipelines subject to landslide impact. Swanson and Jones (1982) developed an elastic cable model for submarine pipelines under landslide impact, and implemented the solution in a dedicated computer program. As part of a general review of geotechnical issues for offshore pipelines Bea and Aurora (1982) examined the stability of pipelines to mudflows. These authors considered the loading of a flexibly supported cable. Summers and Nyman (1985) applied the theorem of stationary total potential to assess the equilibrium condition of a pipeline with mudslide forces.

Suction piles are widely used in deepwater engineering both for anchoring and as foundation systems. In the first case the piles serve as anchor points for mooring systems in alternative to more standard drag anchors or piles. More recently, however, they have been used as structure foundations. In this role suction piles are a competitive alternative to the more traditional solutions of driven piles or mudmats, for platform jackets, subsea systems and subsea equipment protection structures. This solution provides cost savings in fabrication and required installation equipment. Furthermore, the foundations are relatively easy and rapid to install and can be positioned with high precision by controlled and simple marine operations, and they can be removed for reuse. This paper describes the use of steel suction piles for deepwater subsea Manifolds, Tie-in Spool Bases and Subsea Control Distribution Assemblies, in the West Delta Deep Marine (WDDM) and Rosetta concessions offshore Egypt. Most of the structures were supported by a single suction pile foundation; pile diameters ranged from 4 m to 8 m and penetrations from 8 m to 12 m. One of the larger units was supported by a “quad” foundation frame with four suction piles. Soils in the area are very soft, normally consolidated clays typical of deepwater conditions. Design is complicated by seismicity of the area, which required the foundations to resist significant horizontal dynamic loads in addition to the normal vertical operating loads. The solution adopted utilized an internal top plate in contact with the soil allowing full development of base bearing capacity. As the pile skin friction in these soils is very low, the increased end bearing leads to significant savings on foundation weight and cost. The paper discusses the main aspects of foundation design, covering the installation process with expected self weight penetration and the required suction to achieve the target design penetration, the retrieval operation for repositioning in case the final inclination is out of tolerance, the assessment of the bearing capacity and the stability under the combined vertical, horizontal and overturning loads during operation and earthquake conditions. Seismic design was based on a nonlinear dynamic analysis. In some cases the seismic loads were comparable to the ultimate foundation capacity and the final acceptance criteria utilized a Performance Based Design philosophy. In this approach the foundation is considered acceptable if the deformation experienced by the structure, during and after the seismic event, does not jeopardize structural integrity.

The past ten years have seen a considerable advance in the industry with respect to the identification and quantification of geohazards. One issue which is lagging and neglected in the technical literature is the ability of subsea infrastructure to survive geohazard processes and events. This paper summarizes our recent work on the subject that is used in a risk-based approach to foundation design and, where practicable, to improve the geohazard resistance of structures.The paper discusses the vulnerability of subsea infrastructure to impact of geohazards. A case study is presented for a deepwater manifold subject to landslides and turbidity currents. The vulnerability of the structure is evaluated comparing available resistance of foundation and jumpers to the anticipated loadings. The results give guidance that informs early field planning and risk mitigation strategies. Possibilities for geohazard resistant design are explored.Results and conclusions: 1) Critical issues for structural performance are the capacity of the foundation to resist geohazard loads without failure or excessive deflection, and of the connections (jumpers and flying leads) to withstand deflections or distributed loads caused by the geohazard event; 2) Geohazard resistant design for ground movements should increase foundation capacity and increase flexibility of connections. In this case, driven piles are likely to be more effective than suction piles given the smaller area exposed to soil forces. However , for loads impacting above the mudline foundation capacity may be increased using either suction piles or driven piles. The strength of connections should also be increased. The conflict between connection strength and flexibility is a design challenge; 3) The best mitigation strategy for geohazards is avoidance. This is generally possible for manifolds and other fixed structures. Location of wells and flowlines is dictated by reservoir constraints and relocation is generally not an option; so geohazard design design should be concentrated on accepting and controlling repairable damage. Export pipelines and umbilicals are critical, as loss or damage results in an interruption of production. The main geohazard mitigation strategy in this case is careful routing.

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