Mooring Line Reliability Analysis of Single Point Mooring (SPM) System under Extreme Wave and Current Conditions

Single Point Mooring (SPM) is a type of offshore facility that is used for the loading and unloading of oil and gas tankers in the sea. In Iran, it is briefly called “floating buoy”. The present study discussed floating type, fixed to the seabed with mooring chain. The mooring chain of SPM, which is considered one of the important parameters of its design, will be evaluated with regard to reliability in different environmental conditions. Reliability is the likelihood of a healthy and flawless functionality for a specific time according to existing and predetermined conditions. OrcaFlex Ver. 9.4e (UK) software, by Orcina Company was used for moorings analysis. For this purpose and to calculate the environmental forces in the mooring chains (in our case 6 mooring chains with a 60 degree angle), the researchers used the diffraction analysis and time history. The results revealed that; by decreasing and increasing the diameter of the mooring chain, the force applied to the mooring also decreased and increased, respectively. Also, the effects of chain diameter, direction, and amount of wave and current were studied. The behavior of wave and current showed that when the direction of the wave and the current were closer to the direction of each mooring chain, the force applied to the mooring increased. To calculate the reliability, the FORM method was employed, and to find the target point on the function, the MPP method was used. The reliability of mooring was intended to control, the value of reliability index (β), using a software program written in MATLAB, for the environment loads with a return period of 100 years. The reliability index for mooring lines of SPM was greater than 3.51, representing the safe performance of the mooring lines under environmental load.