Establishing Semi-Submersible Air Gaps

In response to changes in DNV GL technical guidance, Houlder has devised a more accurate methodology for establishing semi-submersible air gaps – the distance between the top of the waves and deck floor.

James Russell, Senior Naval Architect explains “DNV GL have asked for analysis to include roll and pitch slowly varying wave and wind motions and greater asymmetric factoring combined with a longer return probability. Typically this analysis is conducted in the frequency domain with linearized viscous damping and generic statistical assumptions. These assumptions inherently lead to uncertainty in any analysis.”

Without properly considering the effects of mooring lines and rig drag, roll and pitch slowly varying motions can be overestimated by up to 40%. This clearly impacts on the accuracy of air gap ultimately derived. The good news is, Houlder has devised a time domain and diffraction analysis method to negate this problem and produce a more accurate overall assessment.

James continues. “Using time domain simulations via our cutting edge software greatly improves the estimation of non-linear damping for which slowly varying roll and pitch motions are critical. We also capture interactions between system components and environmental actions. Another benefit of these simulations is the direct use of time histories allows for an improved statistical airgap prediction.

Frederic Perdrix, Houlder’s Chief Technical Officer adds “Houlder’s holistic analysis, comprising sound hydrodynamics and high level statistics, means operators can take a lot of the guess work out of establishing semi-submersible air gaps and move forward with more confidence.”

The analysis required to establish semi-submersible air gaps is undertaken by Houlder’s Advanced Analysis team of specialist naval architects based in Aberdeen and London.

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