Reliability of Very Large Crude Carrier and Single Anchor Leg Mooring Buoy Mooring System During Squall Event

Abstract

Mooring systems are integral and sophisticated components of offshore terminals, which are vital elements of the hydrocarbon supply chain. The significant risks to asset integrity and personnel safety associated with compromised mooring systems makes it of paramount importance to evaluate their reliability in multiple scenarios. Because much larger hawser forces occur during sudden changes of wind direction than during steady-state storm conditions, fully-coupled hydrodynamic analyses and simulations were conducted for a 420,000 DWT tanker connected to a single anchor leg mooring (SALM) buoy when subjected to a sudden squall event. The global performance of the SALM and tanker coupled model was evaluated based on consistent analysis of extremes while solving the equations of motion for a system of rigid bodies, each oscillating in six-degrees-of-freedom (DOF) in time domain. The dynamic model conveniently and consistently calculated and documented vessel responses for motion and mooring tension under the environmental conditions specified in each loadcase. A range of wind, wave, and current headings were analyzed to capture the most onerous design conditions for the tanker; and the maximum vessel motions, and mooring line tensions were determined. The analyses were carried out for the vessel in its loaded and ballasted conditions, as well as for two water depths. Results showed that in winds over 50 knots, weathervaning of the tanker to the position of least resistance produced mooring and motion responses exceeding the safe working envelope, while break-out due to tautening of the bow's mooring hawsers occurred in winds over 77 knots. Significant buoy overtopping was also observed during the 3-hour time domain simulations. Mitigations to avoid break-outs and maintain a safe-working environment included a constant stern thrust of 50 tonnes throughout the loading/offloading operations, resulting in a 30% reduction of the total bow mooring force.