Stability of subsea pipelines during large storms.

On-bottom stability design of subsea pipelines transporting hydrocarbons is important to ensure safety and reliability but is challenging to achieve in the onerous metocean (meteorological and oceanographic) conditions typical of large storms (such as tropical cyclones, hurricanes or typhoons). This challenge is increased by the fact that industry design guidelines presently give no guidance on how to incorporate the potential benefits of seabed mobility, which can lead to lowering and self-burial of the pipeline on a sandy seabed. In this paper, we demonstrate recent advances in experimental modelling of pipeline scour and present results investigating how pipeline stability can change in a large storm. An emphasis is placed on the initial development of the storm, where scour is inevitable on an erodible bed as the storm velocities build up to peak conditions. During this initial development, we compare the rate at which peak near-bed velocities increase in a large storm (typically less than 10(-3) m s(-2)) to the rate at which a pipeline scours and subsequently lowers (which is dependent not only on the storm velocities, but also on the mechanism of lowering and the pipeline properties). We show that the relative magnitude of these rates influences pipeline embedment during a storm and the stability of the pipeline.