Mooring system fatigue analysis for CALM and SALM oil terminals

Offshore oil terminals are a cheaper and safer solution than conventional shore terminals for unloading and loading tankers. There are several types of offshore terminals, including Catenary Anchor Leg Mooring (CALM) and Single Anchor Leg Mooring (SALM). Safety is crucially important for offshore terminals. However, over the past few decades, mooring accidents of permanent floating structures have occurred quite frequently in the last few decades. Most of these failures have been caused by fatigue load. T-N curves-based mooring system fatigue analyses for a CALM and SALM oil terminal are presented. Stress amplitudes are calculated based on the tension amplitudes of the mooring lines under the combined loading process due to wave frequency (WF) and low frequency (LF) motion. A comparison is made between CALM and SALM mooring fatigue designs based on the conditions of the Persian Gulf region. For simulation, the hydrodynamic response characteristics of terminals and tankers are first calculated using ANSYS AQWA software, and then the outputs are imported into ORCAFLEX software for fatigue analysis. The results show that under the same environmental conditions with the same tanker tonnage, the SALM terminal mooring system shows a greater fatigue life. The minimum fatigue life of the mooring system for CALM and SALM terminals occurs at near the touch-down position (TDP) and the near of connection to the seabed, respectively. It is revealed that by changing the value of minimum breaking strength (MBS) the fatigue life of the CALM and SALM terminals changes by 119% and 100%, respectively. It is also observed that by changing the amount of K value (the value for platted T-N curve), the fatigue life of the CALM and SALM terminals changes by the same amount. In all cases, the value of R (the ratio of tension range to reference breaking strength), in the mooring line of SALM terminal, although more tension is generated, the ratio of R is less and will improve the life of fatigue.