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Superior Motion and Station Keeping Aspects of Semisubmersibles (Part 1)

Lookup NU author(s): Dr Arun Dev


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With the discovery of oil fields in deep waters offshore, semisubmersibles are now often considered as strong contenders for not only drilling but also for Floating Production Systems (FPS). However, motion and station-keeping capability remain 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 environmental forces computations are necessary as columns of a semisubmersible offer potential for attracting large viscous origin drift forces in storm conditions where the wave energy is dominated by low frequencies.Deep Draft Semisubmersibles (DD-Semi) and Extendable Draft Semisubmersibles (ED-Semi) are known for their usefulness in deep waters. Their deep drafts lead to less heave response. This article presents the DD-Semi and ED-Semi, with a comparison done of their heave motion with that of a Conventional Semisubmersible (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 less heave response as compared to 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 for mooring analysis or DP capability for semisubmersibles is assessing environmental forces quite accurately before selecting the mooring line characteristics or the thrusters’ power for the semisubmersible. 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 the low-frequency part also 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 semisubmersible type floating structures including tension leg platforms.

Publication metadata

Author(s): Dev AK

Publication type: Article

Publication status: Published

Journal: The Singapore Engineer

Year: 2015

Volume: 2015

Issue: October

Pages: 34-40

Print publication date: 01/10/2015

Online publication date: 01/10/2015

Acceptance date: 01/08/2015

Publisher: The Institution of Engineers Singapore


Notes: This article has been published in The Magazine (THE SINGAPORE ENGINEER) of The Institution of Engineers, Singapore in October 2015