Design optimization of offshore construction vessels
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- Institutt for marin teknikk 
The topic of this thesis is optimization of ship design with regards to operational performance. 4 VARD Offshore Subsea Construction Vessels are analysed to find the optimumcompromise of operability versus required power, building costs and machinery costs. Operability is assessed with regards to subsea lifting operations using the ShipX pluginVERES. Criteria is established for lifting phases in air, splash-zone and loweringthrough the water column. Parametric variations of beam, draught, GM and radius ofgyration for roll motion is done to find their effect on operability. Required power attransit speed for beam and draught variations is found using experimental results fromMARINTEK and the empirical method Holtrop 84. Building and machinery costs areassessed using empirical methods. Statistics from the North Sea and Norwegian Sea is applied to determine operability.Results from these areas is found to differ significantly. The only parameter resultingin consistent change in operability is the vessel length. The remaining parameters varyin terms of their effect on operability depending on vessel and operational area. Thelength is found to have the largest impact on costs, and the beam is found to be a morecost effective parameter for increasing operability compared to draught. It is shown that variation of beam is more difficult compared to draught if a constant GMis required without large modifications of general arrangement. The applied methodologyof assessing operability differs from the procedure of planning and executing liftingoperations since weather windows and -factors are neglected. This lead to an overestimateof operability represented as the expected percentage of time the vessel is capableof performing operations.