Abstract First-order, second-moment (FOSM) approximations provide an efficient way to assess submarine slope stability across large areas for which digital bathymetric data are available. This is demonstrated using 20m bin 3D seismic seafloor data for a deepwater area with typical geotechnical soil properties. Results are obtained in terms of a factor of safety mean and standard deviation for an infinite slope with pseudo-static seismic loading. From this the probability of sliding is calculated for each bin without the computational burden of Monte Carlo or other iterative methods. Because these types of probabilistic model incorporate parameter uncertainty into their input and output, they can be used to support decisions about the value of additional data collection, or justify more sophisticated analyses that may help to reduce output uncertainties. In addition to providing detailed maps of the probability of sliding, the analysis produces global statistics that allow insight into the broader response of the system to seismic shaking. 1. Introduction Evaluation of deepwater geohazards commonly entails assessment of slope stability either to understand the geologic history of a project area, or to anticipate the risk associated with future events, such as major earthquakes. This can be done qualitatively based on the presence or absence of past landslide deposits; semi-quantitatively using simple measures such as slope angle or gradient; or quantitatively using limit equilibrium slope stability analysis (e.g. Mackenzie et al., 2010). Limit equilibrium methods are widely known and attractive because they integrate the essential physics of sliding and allow evaluation of rare or unprecedented conditions (for example the effects of a large future earthquake). However, they also require specification of geotechnical variables, such as sediment shear strength, thickness and unit weight, in addition to some description of slope geometry (minimally the slope angle).

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 Omen Lange gas field is located within the scar created by the Storegga Slide. This gigantic submarine slide occurred about 8.200 years ago, and caused a tsunami that also reached the surrounding coasts. Extensive work has been performed to explain the prehstoric sliding and to evaluate the present stability conditions m the vicinity of the Ormen Lange gas field. Based on regional seismic mapping and "geoborings" (combined geotechnical and geological borings to depths of several hundred meters below the seafloor) a geological explanation model for the sliding has been established This model explains the location of the two major slides in the area, the Storegga Slide and the Trienadjupet Slide The two major slides are located m two large depressions in the More-Voring area. These depressions form sediment traps for soft clays continuously transported along the slopes by the North Atlantic current During the short periods of peak glaciations, the Scandinavian ice sheet reaches the shelf edge and rapid deposition of glacial sediments takes place at the outer shelf and upper continental slope. The sudden load from these sediments on the soft clays m the upper margin, increases the pore pressure in the soft clays, which becomes unstable over time. The most likely trigger for the sliding is one or a series of strong earthquakes caused by glacio-isostatic rebound following glacial retreat. This explains the tendency of sliding to occur after glacial periods. The Storegga area has experienced sliding through the last 1.3 mill years, a period dominated by the effects of glacial linter-glacial climatic cyclicity. Slide of similar type and sue as the Holocene Storegga Slide occurred repeatedly in the same area during the last 0.6–0.5 million years Since the soft unstable clays were essentially removed from the Storegga margin during the last slide, it is concluded that a new cycle with sedimentation of fme-gramed marine clays followed by rapid deposition of glacial sediments m the upper slope is needed to create a new large scale unstable situation in the Storegga area. Geotechnical parameters from a number of geoborings together with seismic reflection data and slope geometry are used to calculate the present stability of the steepest slopes in the vicinity of the field development area Potential slide triggers like earthquakes and high pore pressure have been included m this calculation. Based on the stability calculation and the geological model the risk for new sliding in the area is considered very low. However, work is still ongoing to verify the final conclusion of the risk analysis. INTRODUCTION The Ormen Lange gas field was discovered in 1997 with the scar of the Storegga Slide. The recoverable gas reserves are 400 billion scm and production is planned to start in 2007. Norsk Hydro is the operator for development and construction and Norske Shell will be the operator for the production. The other partners m the Ormen Lange license are: Statoil, BP, ExxonMobil and Petoro.

ABSTRACT The Ormen Lange gas field is located in about 900 to 1100 m water depth in the slide scar of the enormous Storegga Slide that occurred about 8000 years ago. The slide left steep and high headwalls above and below the planned field development area. Today's stability of the headwalls is a major concern for the field development work The area under consideration is large and has been mapped extensively with 2D and 3D seismic profiling The number of geotechnical borings is limited and integration of geological, geophysical and geotechnical information was required to develop a geotechnical model of the area. Stability analyses have been carried out for critical sections of the headwalls. These involved long-term drained analyses under gravity loading and undrained analyses considering the effects of earthquake-loading and possible Influence from field installations like rockfill supports for pipelines and anchors. Focus has been set on explanation of slide mechanisms involved in the Storegga slide and comparison of the stress-strain-strength conditions in the headwall at the tune of the slide and today. Work is still ongoing and under review and the conclusions presented here are thus to be considered as preliminary. INTRODUCTION The Ormen Lange gas field was discovered m 1997 within the slide scar of the Storegga Slide The recoverable gas reserves are 400 billion sm3 and production is planned to start in 2007. Norsk Hydro is the operator for development and construction, and Norske Shell wll be the operator for the production. The other partners in the Ormen Lange license are Statoil, BP, ExxonMobil and Petoro. Three papers are presented on the Ormen Lange and Storegga Slide topic at the 2002 International Conference on Offshore Site Investigation and Geotechnics. These are Establishing a geological model for the regional understanding of the Storegga Slide, by P Bryn et al. Ormen Lange geobonngs by T.I Tjelta et al. Ormen Lange slope stability (this paper) by T J. Kvalstad et al. The Ormen Lange gas field is located in the Norwegian Sea about 130 km WNW of Kristiansund. Figure 1 shows the location and the approximate extension of the Storegga Slide The Storegga Slide is one of the largest submarine slides m the world (Bugge, 1983 and Bryn et. al., 1998) and occurred about 8200 calendar years ago. The upper headwall scar has a total length of about 290km. The estimated soil masses involved were (according to Bugge, 1983) in the order of 5600 km3 More recent updates indicate somewhat lower volumes Evidence of a tsunami generated by this slide event has been found along the coast of Norway, Scotland and the Faeroe Islands. Figure 2 shows an exaggerated bathymetric picture of the central, upper part of the slide showing the upper and lower headwalls of the slide.

Abstract The Terang-Sirasun development is reservoired in the Pacuan limestones of the Plio-Pleistocene. Development of the 1 TCF GIIP is complicated by a range of seismically driven overburden and surface processes that constitute a substantial geohazard risk to planned well bore and gas gathering installations. Terang is characterised by significant shallow gas in the overburden and faults, some of which have seabed expression. Sirasun is characterised by fewer faults and little shallow gas but by the location of the present day shelf-slope break This exhibits extreme erosion and potential mass flow features. The challenge for the business unit is to create a development scenario that, where possible, avoids all geohazards for the lowest CAPEX. This involves identification of the risks from conventional geohazard survey datasets totalling some 2 terabytes of data. Manipulation and integration of the data and the associated interpretahon, has involved a multi-discipline team ranging across 3 different companies using a number of software packages such as Fledermaus, Landmark's Earthcube and 3D Drillview KM to define this development scenario. This paper attempts to detail the methods used to deliver the data sets into visualisation packages, identify and analyse the geohazards, the geohazards themselves and the development scenario through visualisation in bp's visualisation centre based on a novel risk assessment approach of the geohazards themselves. INTRODUCTION The Terang - Sirasun-Batur (T-S-B) fields exist within an area of extremely active geological hazards (geohazards). Indeed the area was ranked second m the world in terms of geohazard seventy as effecting bp developments, second only to Azerbaijan. Eight primary contractors were assembled within a BP management environment and tasked with the key objectives of qualifying the HSE and financial performance risks and, where necessary, revising the development layout based on a programme of geohazard identification and risk quantification. The primary contractors comprised: JP Kenny - Pipeline & Subsea Engineering K & M Technology - Drilling Engineering Hydrosearch - Data Quality & Control Thales (Racal) - Offshore Data Acquisition and Mapping GMI - Subsurface Geotechnical Engineering Landmark Graphics - 30 Visualization Software IVS (Interactive Visualization Systems) - 30 Visualization Software URS - Earthquake Engineering and Geohazards Interpretation BP Indonesia - Project Management