The effects of hurricanes on mooring systems

When hurricanes occur in the Gulf of Mexico, permanent production units and moored drilling vessels are mandatorily prepared and evacuated to fare the storms without risk to personnel and the environment. Kent Longridge, Principal Engineer at InterMoor, a brand in Acteon’s Engineering, Moorings and Foundations division, analyses the impact hurricanes can have on mooring systems.

What happens during a hurricane?

Mooring systems commonly consist of on-vessel winches, chain-jacks, fairleads and bending shoes. Off-vessel line components include stud-link and studless chain, six-strand and spiral strand wire (i.e., jacketed and unjacketed), polyester rope, and appropriate connecting links (e.g., H-links, spelter sockets, Kenter Links, shackles, etc.).

Suction embedded and driven piles that can resist vertical and horizontal loads and drag embedment anchors are used to hold systems in place. Suction Embedded Plate Anchors (SEPLAs) are also used for temporary and permanent moorings. Combining these components in different ways enables the development of safe, robust and reliable catenary, taut leg and semi-taut mooring system designs that can resist extreme environment loads, satisfy station-keeping requirements and realise cost efficiencies when properly inspected and maintained.

As moored vessels encounter hurricane conditions, they are exposed to extreme wind and wave loads (e.g., strong wind gusts, wave run-up and green water overtopping, etc.) and they experience large vessel motions with maximum line tensions. Large yaw motions can exceed a fairlead’s azimuth angle limits and cause localised bending in the mooring line and large stresses in the fairlead itself.

There is also the possibility of damage from dropped objects falling on mooring lines and floating debris. Conversely, dynamically positioned drilling units that are augmented with disconnectable mooring systems can disconnect from their moorings to move out of a hurricane’s path. However, the disconnected portions of their mooring systems that are left behind and exposed to extreme waves must be designed to remain intact without causing damage to any adjacent or underlying infrastructure (e.g., pipelines, subsea equipment, etc.).