Computer simulation of the behaviour of an LNG carrier moored to an open jetty

Abstract

In the design of oil and gas terminals Six Degree of Freedom (SDF) models are used to calculate the motions of the moored ship as well as forces in mooring lines. TERMSIM II (LNG) is one of those SDF-models, developed by MARIN under a Joint Industry Program with Shell as one of the main sponsors and users. Although the simulation programs have been used for more than twenty years, there is still need for more information about the accuracy of the models. The research objective is to verify the accuracy of the calculations of the SDF computer model TERMSIM II (LNG) simulating an LNG carrier moored to an open jetty. The accuracy is investigated by comparing calculated forces in mooring legs to prototype measurements. The prototype measurements have been carried out at the LNG jetty of the Withnell Bay loading terminal, located in the Mermaid Sound, West Australia. The moored ships were 125,000 m' Northwest LNG carriers. The measurements consist of average, minimum and maximum values per ten minute interval of the force in each mooring line. Simultaneously weather conditions were monitored near the terminal. These conditions serve as input for TERMSIM simulations. Simulation runs were carried out for different kinds of environmental loads. The dynamic loads from wind and waves (sea state) appeared to be small and difficult to be compared to the measured data. Dominant loads appeared to be swell waves with peak periods ranging from 14 to 18 s. Simulation under these weather conditions provided good results in case of loaded ships, i.e, the simulation results nearly match the measured data. In case of ships in ballast and partly loaded ships the simulations provide overestimating results. From a sensitivity analysis appeared that the results are rather sensitive to directional spreading of waves and the direction of the incident waves. From further investigation of the motions of moored ships exposed to swell waves followed that the yaw motion, due to first order wave forces, is dominant for forces in breast lines. A simple empirical expression has been derived to calculate the yaw motion of a moored 125,000 m3 LNG carrier exposed to swell waves. This expression can be used for preliminary harbour design to estimate maximum yaw motions and mooring forces. Computer simulations can be carried out during further stages of the design process when more detailed input parameters are available. The main conclusions of this research project are: 1) The difference between TERMSIM calculations and measured line forces, concerning a loaded 125,000 m3 LNG carrier moored to the LNG jetty in Withnell Bay exposed to moderate swell waves, is within reasonable limits. Underestimation of 10% to 20% for the calculated maximum forces in breast lines compared to prototype measurements has been found. 2) The prototype measurements show significant dependence between maximum line forces and the loading condition whereas TERMSIM simulations do not show this dependence. 3) The influence of directional spreading of waves on line forces is significant in case of wave directions nearly head- or stern-on. 4) Yaw motions appear to be dominant for the maximum line forces in breast lines in case of a 125,000 m3 LNG carrier moored to a jetty exposed to swell waves. 5) The yaw motion is dependent on the water depth, wave direction, wave period, wave height and stiffness of the mooring system. 6) Maximum allowable wave loads in Withnell Bay appear to be smaller than design wave loads.