Potential Damage to Offshore Oil Structures as a Result of Underwater Slumps and Slides

Introduction

In respect of the submarine slumps two particular hazards are relevant to the safety of offshore oil structures. First it is possible that the foundations of individual oil rigs may be emplaced on fossil landslide deposits on the sea bed and that these sediments may move in the future. Secondly, there is the possible danger that other areas of the continental slope may be susceptible to slumping and sliding in the future with the obvious dangers that oil rigs would encounter. Any future submarine slide or slump in this area is most likely to take place along the areas of the continental slope. In the following section, the most likely triggering mechanisms for such slope failure are considered - these being the release of methane gas compounds (clathrates) and/or the occurrence of offshore earthquakes of sufficient magnitude to trigger submarine slope failure.

One of the characteristic features of the Norwegian Sea is the widespread occurrence of underwater slump and slide sediments. These were first described by Kenyon (1987) and more recently newly-identified underwater slides have been identified along the continental margin NE of the Faeroe Islands (van Weering et al., 1998). In addition, evidence for long-term instability in the Storegga Slide region on the continental slope of W Norway has recently been summarised by Evans et al. (1996). The continental slope region of the Norwegian Sea and Greenland Sea is therefore well-known as an area characterised by underwater slide deposits and points to the occurrence in this area in the recent geological past of a series of submarine slope failures. The area is therefore potentially susceptible to underwater slope failures and therefore there is a danger that an oil rig whose foundation piles are constructed in landslip sediments are susceptible to movement in the future. Assessment of the geotechnical properties of these sediments is beyond the scope of this report, but is presently an area of experimentation undertaken by engineering geologists at various academic institutes in Norway. Suffice it is to state here that there is a real risk that future sediment failures could take place on the continental slope west of Norway and lead to the collapse of an oil rig. By contrast, those oil rigs located on the continental shelf west of Norway would appear to be less susceptible to submarine sediment slumping and sliding.

Submarine Sediment Instability and Gas Escape

It has been known for a number of years that thicknesses of submarine sediment may become destabilised by the sudden release of gas mixtures trapped within the sediment. In general the gas mixtures principally consist of compounds of methane gas that have accumulated within isolated pockets within the sediments during the recent geological past. However, the areas of bedrock from which oil and gas is extracted are unrelated to the movement of methane gas compounds in that are present within overlying sediments. Thus the physical extraction of oil and gas does not affect the stability of oil rigs the piles of which are generally lodged within the younger accumulations of sediment that mantle the bedrock. There is always the danger that a sudden localised release of clathrate compounds may trigger submarine slope instability and cause the sudden movement of an oil rig. However, throughout the history of oil and gas exploration in the North Sea and North Atlantic regions there has not yet been an account of this process having taken place.

Yet in certain areas the localised release of methane gas compounds may account for the occurrence of pockmarks, semi-circular depressions on the seafloor surface. Pockmarks have been mapped for many continental shelf areas of the world and are indicative of former episodes of gas escape. They highlight the potential dangers that this process may present to the offshore oil industry.