KOMtech’s new semi- submersible design under test in the Offshore Basin
Report describes the unique features of KOMtech’s new semi-submersible
and how they contribute to improving operability. Jorrit-Jan Serraris,
j.w.serraris@
marin.nl
To make offshore operations economically viable in the present challenging oil market, high operability is required. In order to achieve this Keppel Offshore & Marine Technology Centre (KOMtech) has developed an innovative semi-submersible design (see interview page 6). MARIN performed model tests on this new design to verify the improved performance.
Design distinguishing features The operations of a semi-submersible are typically limited by the vertical motion response and airgap of the platform. Four innovative technologies are incorporated in the present semi-submersible design to limit the vertical motions. These are: deeper pontoon submergence to reduce wave excitation forces; multi-column spacing and column sizing to achieve heave
cancellation effects at critical operating frequencies; a multi-sided polygon column helps to reduce and equalise environmental loads from different directions and finally, pencil columns with a passive air chamber to artificially increase roll and pitch stability and thus improve airgap.
Numerical analysis The hydrodynamics of this innovative semi-submersible concept, including all its design distinguishing features, have undergone numerical analysis by KOMtech engineers by means of state-of-the-art frequency domain, time domain and CFD simulations.
Concept verification tests As a verification of the design KOMtech asked MARIN to perform wave basin model tests in the Offshore Basin to verify if all the
hydrodynamic effects were in line with expectations. The four design distinguishing features were all represented on the model. To scale the effect of compressed air in the air chambers the volumes of air were increased at model scale. The results of the model tests showed low vertical motion behaviour due to the deep draught of the ring-shaped pontoon and an increase of the roll and pitch stability due to the passive air chambers.
The concept verification model tests demonstrated the favourable performance of the design. Furthermore, the tests provided KOMtech and MARIN with a better understanding of the complex hydrodynamics, allowing their engineers to further optimise the design.
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