Geohazards and Ocean Hazards in Deepwater: Overview and Methods of Assessment

Abstract Drilling for hydrocarbons in the deep marine environment provides a unique setof challenges for industry. Amongst these are the distinct hazards caused bynatural geological and oceanic processes such as:semi-permanent bottomcurrents,episodic turbidity currents, slope instability and mass transportevents (slides, slumps, debris flows), andgas hydrate escape. We presentdata on the nature, effects and assessment of these deepwater hazards, including current velocities, transport/erosion capacities, recurrenceintervals, and hydrate distribution. It is of upmost importance that theoceanographic conditions are carefully considered prior to deepwater operationsto ensure the work can be carried out safely. Thorough risk assessment requiresknowledge of existing bottom currents, an assessment of potential masstransport events and turbidity currents, and understanding of conditions likelyto induce the formation and destabilisation of gas hydrates. Pipelines, cables, subsea installations, key connections such as the riser and any other seabedinfrastructure are all susceptible to damage. Correct assessment of hazard willallow for the right equipment to be used for the operations including blowoutpreventers, riser size, vibration suppressors, and for the safest siting ofcables and pipelines. It will also aid the subsea architecture design andplanning operations with minimal downtime. All these considerations lead to asafer exploration and production while eliminating unnecessary cost. Introduction Deepwater is defined by different communities in slightly different ways. Forthe marine geologist and oceanographer, it is often taken as that area of theocean beyond the shelf break, i.e. deeper than about 100–200 m water depth. Forthe sedimentologist, it is below storm wave base, which is also around 100–200m. For the industry, deepwater exploration is considered to be that in waterdepths in excess of 500 m. For the purposes of this paper, we consider theshelf-slope transition (i.e. shelf break) as the key boundary between shallowand deep water. This deepwater environment is far from tranquil. It is subject to a range ofprocesses that will significantly affect drilling operations for hydrocarbonexploration and recovery. Bottom currents are everywhere present and in someareas they are especially active and with considerably elevated velocities. Here we first focus on deepwater bottom currents and the hazards they present. Their occurrence and distribution, nature and variability, and damage potentialare discussed, followed by consideration of the methods of hazard assessment. We present a new method using the bedform-velocity matrix.