SS: MODU Risk- MODU Mooring Reliability Analysis for Hurricane Condition in the Gulf of Mexico
Authors: Ku Albert Pao-Ding , Stiff John James , Huang Beiqing , Puskar Frank 
Published: July 2009

This paper presents a probabilistic study of various Gulf of Mexico (GoM) MODU mooring systems that was performed as part of the MODU Mooring JIP (Joint Industry Project). The intent was to determine the relative changes in reliability between different types of MODUs, with different mooring systems, in different water depths, designed for different hurricane conditions and located at different regions in the Gulf. In addition, the effects of assuming reduced mooring line strength were investigated.

Twenty cases were selected which cover four different classes of MODUs, four different types of mooring systems, three different numbers of mooring lines, as well as three different water depths. These base cases were chosen so that trends or differences between different hull shapes and mooring systems could be identified. These twenty base cases are further expanded into sixty cases according to location in the Gulf as well as the return period used to design the mooring system.

The results indicate that on average the " notional?? (i.e., relative) annual failure probability of mooring line failure decreases by one order of magnitude when increasing the MODU mooring system design return period from 10-yr to 50-yr. Two orders of magnitude decrease are observed if the design return period is increased from 10-yr to 100-yr. In addition to the above findings, reliability sensitivity to four different GoM Metocean regions is also provided. Redundancy of the mooring system for 8, 12 and 16-lines is also examined in terms of the additional reliability gained by the extra lines.

Introduction

The probabilistic reliability approach was performed using an updated procedure that was originally developed in prior work by DeepStar [1]. The results can be used by operators, drilling contractors, API work groups and regulators alike to understand, on a comparative basis, the performance of various MODUs and mooring systems.

A base set of 20 separate MODU and associated mooring cases were evaluated as shown in the first twenty cases of the matrix in Table 1. These selected twenty base cases cover four MODUs (or MODU classes), four types of mooring systems, three numbers of mooring lines and three water depths. The footnotes of Table 1 provide definitions of these cases. These base cases were chosen so that trends or differences between different hull shapes and mooring systems can be identified.

This base set of cases was then further defined into a larger set of 60 according to location in the Gulf as well as the return period used to design the mooring system. The Gulf locations were defined as the Central, West Central and West according to MODU JIP metocean study [5] and the latest API guideline [6], and the design return periods were defined as 10, 25, 50 and 100 year. The remainder of Table 1 shows the resulting matrix of cases studied.

The full set of 60 cases was initially evaluated for reliability using 100% CBS (Catalogue Break Strength) for the mooring line. The entire set of 60 was then re-evaluated using 80% CBS to determine the sensitivity of the results to lower than expected mooring line strength.

For the 8-10 mooring line cases, the first and second line failures were determined. The second line failure was used to represent the mooring system failure. For the 12 and 16 line cases, the third line failure was also determined, with the third line failure representing the mooring system failure. This allowed a comparison of the effect of mooring line redundancy in terms of the first line failure, versus the mooring system failure.