Design optimisation of a fabricated waterjet transition duct.
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Context ‒ One of the key elements of the waterjet system is the transition duct, which provides the key structural interface between the boat hull and the waterjet inlet and pump. There is a potential issue with inadequate stiffness in the duct region, which may have reliability implications.
Need ‒ There is a need to determine the optimum arrangement for the stiffeners that are required to reinforce the transition duct, considering all loads, ship classification society requirements for stress, installation constraints and manufacturing considerations such as weld types, weld access and stiffener profiles.
Approach ‒ A finite element analysis was used to address the needs of stiffening the roof-plate and optimise the arrangement and geometry of stiffeners. The risk of metallurgical damage due to weld-on-weld was investigated by examination of the grain morphology using optical microstructure test in conjunction with hardness test. A risk management method was used to estimate the financial cost for solution implementation.
Findings ‒ The study found significant roof-plate deflection caused by negative pressure generated by water flow. Therefore, the application of three stiffening plates is necessary to reduce this deflection. An arrangement of the stiffeners is unique to transition duct for the HT810 waterjet and the spacing between stiffeners is not equal. A fatigue endurance of the welded details is significantly influenced by the geometry of the stiffeners. The optimum geometry of the stiffeners was determined and adequate fatigue life was illustrated.
Implications – There are several important implications revealed for HamiltonJet production practices as well as for installation techniques for the shipyards. Firstly, there is a need to regularly monitor the weld quality at the HamiltonJet production line. This might be done by performing microstructural testing of the weld regions using techniques developed in this study. Quality of the welds might be improved by applying the weld toe grinding method as advised by DNV-GL guidelines. Secondly, there is the importance of analysing and including the detailed pressure distribution for each jet model during the optimisation of stiffening the transition ducts. Finally, the additional stiffening of the roof-plate by shipyards at the jet installation stage is no longer required and any field modifications to the transition duct must be consulted with HamiltonJet.