Auxiliary Moorings for Dynamically Positioned Drilling Vessels

Dynamically positioned (DP) drilling vessels were introduced as a solution for perceived shortcomings of mooring in deep and ultra deep water. However, since the introduction of the DP vessel, mooring technology has moved on.

Major advances in synthetic rope and high uplift anchors have enabled preset moorings to develop a strong track record worldwide for safety and cost-effectiveness in deep and ultra deep water, even in crowded seabeds.

An argument for the ship-shaped DP vessel, apart from in-transit advantages and the ability to keep station headed into the weather, is that it can safely shut down drilling operations, uplift its riser, and move off station if threatened by severe storm or hurricane conditions, while multiple equipment redundancies take care of system failure so that loss of station-keeping capacity becomes a low probability risk. Nevertheless, this is a risk that merits serious attention as the economic and environmental consequences could be as catastrophic as those of the Deepwater Horizon blow-out that eight purpose-designed technical systems failed to prevent.

If a DP drilling vessel is caught by circumstances which prevent shut down and disable propulsion and riser uplift (for example, an explosion onboard), unless the vessel sinks, it will drift off station at a rate determined by weather and current conditions. Its riser may be torn free and precipitate an oil-spill, the effects of which would be compounded should the riser be dragged into and damage a pipeline, an occurrence made more likely where the vessel is being used specifically because of crowded seabed conditions.

This particular risk of station-keeping failure for a DP drilling vessel can be avoided by preset auxiliary moorings that would buy time for fire-fighting, emergency repairs, and safe shut down, and would avoid the risk of its riser being torn free as a result of the vessel drifting off station.

A strong case can therefore be made for DP drilling vessels to have auxiliary moorings as routine station-keeping backup. For DP semi-submersibles, these could be conventional presets. For ship-shaped vessels, the presets could be based, for example, on the Petrobras Differential Compliance Anchoring System (DICAS). DICAS has a track record for reliability that has been proven for permanent mooring of ship-shaped storage and processing vessels since 2004.

Adopting auxiliary presets for DP drilling vessels has a cost implication. However, this is trivial when compared to the potential economic and environmental consequences of ignoring the risk of station-keeping failure until it occurs.