Operability robustness index as seakeeping performance criterion for offshore vessels

Abstract

The offshore industry operates increasingly large installations in exposed areas requiring high reliability and availability. Downtime of complex offshore systems leads to significant financial losses. Towards year-round offshore installation and maintenance service, this research focuses on the identification of weather-robust vessel designs. Even though it might seem that the motions of a larger vessel will be more favorable than those of a smaller vessel, this research shows that this hypothesis is not necessarily true. It will be shown that for certain vessel parameters the performance of a larger vessel is not better than that of a smaller vessel. This investigation aims to provide knowledge for a more holistic vessel design optimization approach to enable ship designers and operators to design and select an offshore vessel with main dimensions and hydrostatic parameters providing optimal seakeeping performance for a given operation and environment. The key aspect is a mission-dependent optimization of hull dimensions, including loading condition parameters, aiming for a hull design where natural periods of important responses such as pitch and roll are significantly distinct from the dominating wave periods. For this purpose, a novel parameter for seakeeping performance evaluation, the Operability Robustness Index (ORI), will be used.