Reliability of Very Large Crude Carrier and Single Anchor Leg Mooring BuoyMooring System during Squall Event

Mooring systems are integral and sophisticated components of offshore terminals, which are vital elementsof the hydrocarbon supply chain. The significant risks to asset integrity and personnel safety associatedwith compromised mooring systems makes it of paramount importance to evaluate their reliability inmultiple scenarios. Because much larger hawser forces occur during sudden changes of wind direction thanduring steady-state storm conditions, fully-coupled hydrodynamic analyses and simulations were conductedfor a 420,000 DWT tanker connected to a single anchor leg mooring (SALM) buoy when subjected toa sudden squall event. The global performance of the SALM and tanker coupled model was evaluatedbased on consistent analysis of extremes while solving the equations of motion for a system of rigidbodies, each oscillating in six-degrees-of-freedom (DOF) in time domain. The dynamic model convenientlyand consistently calculated and documented vessel responses for motion and mooring tension under theenvironmental conditions specified in each loadcase. A range of wind, wave, and current headings wereanalyzed 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 andballasted 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 resistanceproduced mooring and motion responses exceeding the safe working envelope, while break-out due totautening of the bow's mooring hawsers occurred in winds over 77 knots. Significant buoy overtopping wasalso observed during the 3-hour time domain simulations. Mitigations to avoid break-outs and maintain asafe-working environment included a constant stern thrust of 50 tonnes throughout the loading/offloadingoperations, resulting in a 30% reduction of the total bow mooring force.