SS: MODU Risk- Evaluation of Low Frequency damping and Analysis Methods for MODU Moorings
Authors: Kwan Chi-Tat Thomas , Chen Donghui , Cheng June , Guan Julie 
Published: July 2009

As higher environmental criteria are now required for GOM MODU mooring operations, the accuracy of the mooring analysis for site assessment becomes more critical. Inaccurate mooring analysis may lead to wrong conclusion concerning the acceptability of a MODU for a specific operation. This paper summarizes two studies conducted for the MODU Mooring Strength and Reliability JIP for improving the accuracy of mooring analysis.

The first study used frequency domain coupled analysis software, which was first benchmarked against damping research results, to generate low frequency (LF) damping estimates for a number of representative MODUs and moorings under various environmental conditions. The purpose of the study is to develop understanding and practical guidance for LF damping. The second study compares mooring analysis results from six analysis methods allowed by API RP 2SK for MODU moorings and investigates the relative conservatism among the six methods. The study provides guidance to designers on the selection of analysis methods and also provides data for API to develop guidance on analysis methods in future revision of API RP 2SK. Major conclusions include:

Low frequency damping study: LF damping has many components, and each component is a complicated issue. Although a significant investigation has been carried out in this study for current damping and mooring line damping, uncertainties still exist because of the complexity of the issues and lack of physical data for verification. To compensate for the uncertainties, a number of simplified assumptions have been used to present the data in a user friendly and conservative manner for current and mooring line damping. This approach, combined with neglecting the damping from other sources, should give conservative damping estimates despite the uncertainties. The damping estimates were generated for the specific vessels, mooring systems, water depths, and environments included in the study matrix, which may not be the same as those for the mooring operation under consideration. This study also raises a question on the validity of Cross Flow Principle for typical inclined offshore slender bodies and points out a need for future investigation.

Analysis methods study: The simplified methods (steady wind and quasi-static analysis) are in general more conservative with very few exceptions, although there is a large variation in the conservatism from case to case. For the exceptions, the non-conservatism is small. The Designers should consider all factors such as efficiency, accuracy, software cost and availability, when selecting an analysis method. The most efficient method can be very conservative and may lead to incorrect conclusion on the acceptability of the mooring system for a MODU operation. The study has been conducted for semisubmersible MODUs operating in tropical revolving storm conditions. The results could be significantly different when the mix of vessel type, mooring type, and metocean parameters is changed.