A mooring system is designed to control horizontal motion of a floating structure connected to it. It will be considered to be disabled when the horizontal displacement of the floating structure exceeds the limited value. Besides, the breaking and fatigue damage of mooring lines also lead to failure of the mooring system. This paper presents the multi-mode reliability assessment method. Firstly, coupled dynamic responses of the floater/mooring system is obtained from numerical simulation based on the coupled program. Then, distribution functions, which are utilized to describe long-term statistical distribution of the floater motion response and mooring line tension, are discussed. Finally, the mooring system reliability during the whole design life is estimated combining the three failure modes mentioned above based on the weakest link theory.
The mooring system, which is a critical component of floating structures, is a major target of research in the industry, particularly with its increasingly recognized importance and the development of deeper water exploration. To ensure operating safety, reliability against mooring line failure is a major concern in design. It is to say that the mooring system responses should be checked against the mooring strength (extreme strength and fatigue strength) and hull offset limit to ensure safety probability.
Recently, a lot of attention has been given to the reliability analysis of mooring lines (Vazquez-Hernandez et al., 2006). Some of these studies focus on the application of reliability technologies to the offshore engineering in order to improve the prediction accuracy of the mooring system (Luo & Ahilan, 1991; Larsen & Mathisen, 1996). Others used the reliability techniques to assess the reliability of the mooring system based on the criteria about floater offset limit (Fang, et al., 2013), or to analyze the risk probability of the mooring lines against its breakage (Montes-Iturrizaga, et al., 2007), or to estimate the mooring line fatigue damage probability (Jameel & Ahmad, 2011), following the mooring system design standard such as API (API, 2005) and ABS (ABS, 2003).
