SWATH SOV Hull Concept and Optimisation for Seakeeping

Examensarbete för masterexamen
Macedo, Paulo
There is a fierce competition for the European market of offshore wind OMS (Operation, Maintenance and Service), expected to a significant growth in the coming years. Shipowners are constantly pursuing lower costs, increasing efficiency and profitability of its vessels in order to endure through time in the industry. In this thesis project, an innovative SWATH (Small Waterplane Area Twin Hull) hull form is investigated as an SOV (Service Operation Vessel) and compared to operating monohull SOVs. The hull form was idealised by members of Rolls Royce Marine Norway and inspired in semi-submersible platforms and SWATH vessels. The alternative hull claims to allow the reduction of the vessel size, currently a request from shipowners, whilst maintaining the ability to operate in rough seas. In order to investigate this claim, guidelines given by the interested partner were followed to develop a parametric model of a SWATH hull in CAESES® and connected to NEWDRIFT+, a seakeeping calculation software. With the goal of reducing heave motions at the connection point between the vessel and the offshore wind turbine, the parametric model was optimised and compared to two different sized monohulls, designed for the same purpose. From the optimisation process, optimisation patterns towards lower heave motions were identified and an additional SWATH variant was developed. This variant was also optimised and compared, totalling four vessels evaluated in this project. The comparison evaluated seakeeping performance in different sea states, heading angles and sailing speeds by means of heave RMS (Root Mean Square) motions, RAOs (Response Amplitude Operators) of motions and forces and moments caused by added wave resistance. Results obtained indicates that optimised SWATH hull forms have high potential to have a larger operational window than longer monohulls by heaving, rolling and pitching less when excited by waves in several different sea conditions. Not with standing superior seakeeping performance, SWATHs are likely to be more susceptible to wave drift forces, therefore possibly requiring higher propulsive capacity. Additionally, indepth studies of weight placement for hydrostatic stability requirements, as well as, estimations of towing resistance and propulsion system efficiency, are essential to be evaluated as they will definitely impact the hull form, construction and operational costs. These factors were not entirely considered in this project. The conclusions reached are definitely not enough to neither praise nor condemn the utilisation of a SWATH hull form as an SOV. On the other hand, they are the starting point for a series of future studies before it can be officially declared impracticable or the future of SOV designs.
parametric modelling, SWATH, SOV, walk-to-work, offshore wind energy, CAESES®, NEWDRIFT+, seakeeping, optimisation, simulation-driven design
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