Mooring Safety Assessment Using Reliability Techniques
Authors: Luo Yang , Ahilan R.V. 
Published: October 2008

The paper deals with the reliability of offshore mooring systems. Reliability techniques have been applied to assess mooring systems designed by the quasi-static method. The study identifies that the variability of mooring line strength due to uncertainties in link strength and dynamic tension response are the two crucial parameters which affect the system safety, and further study is carried out to investigate their effects. It has been concluded that due to the simplistic and deterministic approach adopted, the quasi-static method is unable to address the variability of design parameters, and therefore mooring systems so designed often have inconsistent safety levels.

1. INTRODUCTION

Mooring system failure frequently occurred in the past (about average once every 2 or 3 years per rig) and questions have been raised as to whether the presently used quasi-static method is adequate to cope with mooring system designs in extreme conditions. In this paper, mooring systems designed using this method have been assessed using reliability techniques.

The main objectives of the study are:To perform a reliability analysis of a typical semisubmersible mooring system in order to identify the general acceptance level of mooring system safetyTo assess the reliability of mooring systems designed according to the present quasi-static methodTo identify the critical design parameters and assess their effects on mooring system safety.

To perform the reliability analysis, typical North Sea environmental conditions are modelled and the mooring system static and dynamic responses are predicted. Probabilistic descriptions of tensions and line strength are then defined to compute the probability of failure.

The technique is first applied to the mooring system of a semisubmersible unit, Sedco 700 series in 150m water depth designed according to the quasi-static method. The reliability analysis has shown that the variability of chain link strength and the line dynamic response are the two most important parameters which affect the probability of failure.

The study has concluded that mooring systems considered to be at the limit using the quasi-static method have inconsistent safety levels. The study has also identified a number of areas which warrant future studies.

2. PROBLEM DESCRIPTION

As most engineering design problems, the design of offshore mooring systems can be broadly categorised into two main approaches:Allowable working stress approachProbabilistic or reliability approach

In design approach (a), the system resistance capacity required can be determined by:(mathematical equation) (available in full paper) where Q is the resistance, L is the load and SF is the safety factor.

This approach implies that both the load and resistance can be perfectly predicted as shown in Fig.1, and when successfully designed, the probability of system failure is zero. However, in reality, due to the randomness of environment and our ability to predict the system response precisely, uncertainties are inevitable. Therefore every system has a finite probability of failure. This is why the design method (b), reliability based approach can be utilised for a better description of system safety.