A global operability index for an offshore vessel

For the operability evaluation of an offshore unit, it is usual to adopt specific working limiting criteria dealing with ship motions and accelerations. These criteria given by operators reflect the ability of the vessel (or crew) to accomplish a precise target in an operative scenario with different meteorological conditions. However, an offshore vessel needs also to keep a determined position and heading during working operations. That means designers or operators have to consider criteria associated both with seakeeping and station keeping to evaluate the effective operability of the vessel. Traditionally, early design stage operability analyses do not combine seakeeping and station keeping predictions, which are referring to different working environments. Here, an innovative procedure allows to evaluate the joint operability of an offshore unit, adopting scatter diagrams for local environmental conditions. The method has been applied on a Pipe-Laying vessel, assessing the global operability for different areas of commercial interest.