Assessment and Evaluation of Deepwater, Continental Slope Geohazards

Abstract

Abstract As offshore pipelines begin to be routed between different continents, they often have to cross deepwater continental slopes. These regions pose numerous geological hazards for pipelines: submarine channels, unstable slopes, landslides, debris and turbidity flows, and faulting. Minimizing risk to the pipeline in these conditions is a basic criterion for a viable project. A properly designed route survey is a critical first step towards recognizing the hazards, and this must be followed up by proper analysis and modeling. This paper describes some of the geological processes found on an active continental margin and how they are identified from marine surveys. Recent advances have allowed the key filed data to be gathered in depths in excess of 2,000 m. The confidence in the model results is very dependent on the quality of the survey data. As projects go into more and more difficult terrain, the envelope of survey capability and personnel expertise has also expanded. Specialists model the processes in their offices far away from the site, but the survey personnel on site must be aware of the model's sensitivity and limitations. They must direct data acquisition location-bylocation in tune with the precise needs of the model. Because of the very high cost of ultra-deepwater pipelines, good confidence in predicting geohazard events is very critical. Uncertainty in loading can lead to either over-design or under-design. Reliable prediction of forces on the pipeline due to a high-velocity sediment flows is just as important as predicting the flow itself. Introduction When submarine pipelines are routed through difficult terrain, risks from localized geohazards can often be avoided by simply detouring around them. In some cases, however, the continental slopes may have uninviting features that stretch for tens of miles along their length, making re-routing a costly option. Sometimes it is not an option at all, for political or territorial reasons. Below the continental slope, a leveed channel system may be active for hundreds of miles, making it a question of where to cross rather than whether to cross. This has focussed attention on assessing the level of risk posed by geohazards, and providing results reliable enough for sound design of major deepwater gas pipelines. This paper concentrates on a specific class of geohazards - sudden, episodic sediment flows such as submarine landslides. When avalanches occur onshore, they may be only too well witnessed. Subsea, however, they nearly always occur unnoticed. A broken communications cable or a well-placed current meter may detect or even record the event, but this is rare. The tools we have for detecting flow events rely heavily on remote acoustic sensing, in-situ soils testing, and recovery of samples from the seabed. Since the sediment flow events are rarely actually witnessed, one has to infer what processes have caused the event, and from this predict when, where and how they might occur again over the lifetime of the proposed pipeline. The first priority is to identify the parameters controlling the flows.