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Lookup NU author(s): Dr Arun DevORCiD
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Semisubmersibles are now often considered as strong contenders for not only drilling but also for Floating Production Systems (FPS). However, motions and station-keeping capability remain as the biggest challenges for such column stabilised platforms. For a Dry Tree Semisubmersible, heave motions still need to be considerably improved. Similarly, for station-keeping like mooring or Dynamic Positioning (DP) capability, more accurate computations of environmental forces are necessary as columns of a semisubmersible have the potential for attracting large viscous origin drift forces in storm conditions (low frequencies).Deep Draft Semisubmersibles (DD-Semi) and Extendable Draft Semisubmersibles (EDSemis) are known for their usefulness in deep waters. Their deep drafts lead to a lesser heave response. This article presents the ED-Semi and DD-Semi with a comparison with the heave motion of the Conventional Semi (CV-Semi). It was found that an increase in the mass and added mass of the submerged portion of the semisubmersible, which resulted in a lower natural frequency, aided in the lowering of the heave response. The mathematical analysis and the numerical analysis (computational methods) proved that the ED-Semi and DD-Semi have lesser heave response as compared to that of the CV-Semi. The DD-Semi was found to be better than the ED-Semi in terms of heave response.One of the main design parameters of mooring analysis or DP capability for semisubmersibles is assessing environmental forces quite accurately. Of the three main environmental forces (wave, wind and current), the horizontal wave drift force is quite important. 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 quite accurate in predicting wave drift force for a ship-shaped body, the same may not be true for a semisubmersible 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 that plays an important role. Surprisingly, the viscous effects also influence this low-frequency part. A serious design deficiency will result in wave drift force prediction, if these viscous effects are not considered for column stabilised semisubmersibles.Part 1 of this article, which highlighted the above motion aspects, was published in the October 2015 issue of ‘The Singapore Engineer’. empirically using wave drift force coefficients. While this method is quite accurate in predictingwave drift force for a ship-shaped body, the same may not be true for a semisubmersible wherethe slender column structure especially in low frequency (long waves) will cause the drift forceto be influenced by viscous effects. In irregular waves, it is not only the wave drift force butalso the low-frequency part that plays an important role. Surprisingly, the viscous effects alsoinfluence this low-frequency part. A serious design deficiency will result in wave drift forceprediction, if these viscous effects are not considered for column stabilised semisubmersibles.Part 1 of this article, which highlighted the above motion aspects, was published in the October2015 issue of ‘The Singapore Engineer’.
Author(s): Dev AK
Publication type: Article
Publication status: Published
Journal: The Singapore Engineer
Year: 2015
Volume: 2015
Issue: December
Pages: 32-39
Print publication date: 01/12/2015
Online publication date: 01/12/2015
Acceptance date: 01/08/2015
Publisher: The Institution of Engineers Singapore
URL: https://www.ies.org.sg/publication/se/dec15.pdf
Notes: This article has been published in The Magazine (THE SINGAPORE ENGINEER) of The Institution of Engineers, Singapore in December 2015
