In the offshore industry, the thruster layout for dynamic positioning (DP) is not optimal to reach the maximum station-keeping capability. It is possible to optimise the position of the thrusters since early design stage applying a genetic algorithm-based optimisation procedure. The enhancements due to an optimal thruster configuration can be reported in the standard capability plot format as commonly done in the offshore industry. However, besides standard capability plots, additional calculations can be performed by explicitly considering the vessel’s geographic operational areas. By adopting this novel approach, it is possible to associate to vessel’s DP capability an operability index which is specific for the considered sea area and is in analogy of that adopted for ship motion criteria. In this way, the improvements of the optimised thruster configuration can be represented in terms of the increment of DP operability index in a selected sea area, instead of the capability plot area increase for standard conditions. The procedure is here applied to a reference Pipe-lay vessel, where the thruster layout has been optimised. The improvements are reported through both conventional capability plot and newly proposed DP operability index for the North Sea area.@en

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.

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The analysis of time records, coming from seakeeping experiments in irregular waves, is used to determine the occurrence of extreme events. The common procedure used for data analysis is to assume that the statistics of record’s peaks is following two or three parameters Weibull distribution. For particularly severe sea states, it can happen that the peaks assume a multi-modal distribution and the use of a Weibull distribution may lead to errors in the extreme value estimate. It is than possible to use multi-modal distributions, or to change the peaks extraction technique, adopting a certain threshold. Here, the determination of extreme values probability distribution parameters by genetic algorithm is applied to improve the methodology of extreme sea state prediction. A data analysis procedure is here proposed and tested on a time record coming from seakeeping model-scale experiments and on a set of wave heights record, in comparison with standard Weibull approach.

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