Determination of a DP operability index for an offshore vessel in early design stage

For an offshore vessel, it is common practice to evaluate the ship's operability considering working criteria inherent to motions/accelerations in several operative conditions. However, to an offshore vessel, it is also required to keep an assigned position and heading during operations or in a rough sea. Station keeping and seakeeping problems are not usually considered as part of the same analysis, mainly because the station keeping issues are mostly related to surge, sway and yaw, while seakeeping deals more with heave, roll and pitch. It is also possible to perform combined analysis, determining seakeeping criteria for dynamic positioning (DP) due to ship motions, and use it as effective limiting environmental criteria for capability plots. In any case, even though seakeeping operational criteria are probably more restrictive than DP ones, it can be useful to investigate the effective operability of the DP system, considering all the possible wind wave combinations, the vessel can encounter during his operational life, together with the sea current. For this purpose, an enhanced and innovative procedure to establish the effective DP operability of an offshore vessel has been developed, by explicitly considering the vessel geographic operational area. The procedure has been tested on a pipe-lay vessel (PLV) and compared with the standard DP calculations, considering all the possible failures requested by regulations. Differences obtained from the comparison are significant and constitute a matter for further discussions.