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Prediction of Environmental Forces for Station Keeping: Ships versus Semi-Submersibles

Lookup NU author(s): Dr Arun DevORCiD

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Abstract

The dynamic positioning (DP) is a fast-growing technology, which started exactly 50 years back in the USA. In the last two decades, tremendous growth has been noted in DP because of rapid development in both hardware and software associated with dynamic positioning. Dynamic positioning is primarily used in ships; mainly offshore support vessels like AHTSVs, PSVs, AHTs, pipe laying vessels, diving support vessels, etc. due to their operational requirements. However, synonymous with drillships, DP is now being attempted in many deepwater semi-submersibles deploying 6 – 8 thrusters. One of the main design parameters of DP capability for either ships or semi-submersibles is assessing environmental forces quite accurately before selecting the thrusters’ power for the ship or the semi-submersible. Of the three main environmental forces (wave, wind and current), the horizontal wave drift force is quite essential. Calculation of this force is mainly done using 3-D radiation-diffraction theory under potential theory or sometimes empirically using wave drift force coefficients. While this method is entirely accurate in predicting wave drift force for a ship shaped body, the same may not be right for a semi-submersible where the slender column structure, especially in low frequency (long waves), will cause the drift force to be influenced by viscous effects. In irregular waves, it is not only the wave drift force but also the low frequency part also plays an important role. Surprisingly, the viscous effects also influence this low frequency part. A severe design deficiency will result in wave drift force prediction if these viscous effects are not considered for column stabilised semi-submersible type floating structures.

Publication metadata

Author(s): Dev AK

Publication type: Article

Publication status: Published

Journal: Annual Journal of Society of Naval Architects and Marine Engineers

Year: 2019

Volume: 38

Pages: 91-107

Online publication date: 01/06/2019

Acceptance date: 11/01/2019

Date deposited: 18/06/2019

ISSN (print): 0217-4294

Publisher: Society of Naval Architects and Marine Engineers Singapore

URL: http://snames.org.sg/publications/annual-journal/

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