Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Airgap Analysis of Semisubmersible in Time Domain Considering Multiple Nonlinear Properties
Jinguang Wang

Shan Ma

Quanming Miao

College of Shipbuilding Engineering,Harbin Engineering University,
Harbin, Heilongjiang, China

College of Shipbuilding Engineering,Harbin Engineering University,
Harbin, Heilongjiang, China

College of Shipbuilding Engineering,Harbin Engineering University,
Harbin, Heilongjiang, China
Jiangsu Ocean University,Lianyungang, Jiangsu, China

the low frequency rotational motion, and the total low frequency
contribution could reach up to one fifths of the upwell extreme. Pan et al.
(2019) further demonstrated fair agreement between the recommended
procedure and a proposed approach, which takes into account thecorrelation of the wave frequency and low frequency upwells but withhigher non-exceedance probability. Machado-Damhaug et al. (2020)
pointed out that six random seeds were sufficient to derive stable airgap
extremes by using the peaks over threshold method rather than thesuggested Gumbel approach.

ABSTRACTThe airgap extremes are keys design parameters for the semisubmersible,
while uncertainty remains in the airgap prediction due to the highcomplexity and nonlinearity of this physical mechanism, especially
under severe weather conditions. The main objective of the present paper
is to explore the feasibility to improve the numerical predication
accuracy by the means of time domain analysis. Making use of thehydrodynamic transfer functions up to second order, analyses in bothfrequency domain and time domain were performed to derive the airgapresponses, followed by an extensive comparison with experimental
measurements. In contrast to the analysis procedure in frequency domain
as suggested by OTG-13, multiple nonlinear properties closely related tothe airgap responses were evaluated in time domain, including theviscous drag on slender elements and the abrupt variation of fluid loadon the braces during their emergence through the water surface. Besides,
the correlation between wave frequency and low frequency could be
inherently taken into account in time domain analysis.

For the airgap analysis of a semisubmersible drilling unit, much of thefocus has been on the survival condition with the corresponding draft
adjusted to strike the balance between air gap requirement and adequate
submergence of pontoons and braces. The survival condition,
characterized by suspension of drilling operation, is used to evaluate theunit’s performance in the expectation of extreme weather conditions
normally with 100 years return period. As the wave steepness plays animportant role in air gap analysis, several sea states along the 100-year
environmental contour line are selected. The survival weather condition
may lead to large movements and even frequent emergence andsubmergence of braces, pontoons and deck box. This strong nonlinear
phenomenon makes the prediction by linear analysis questionable.
Kvaleid et al. (2014) revealed that the occasionally emergence of thebraces and the pontoons would significantly increase the pitch restoring
force and hence reduce the pitch natural period. Wei et al. (2020)
captured the characteristic resonance features of pitch motion through amore sophisticated numerical model.

KEY WORDS: Semisubmersible; Airgap; Upwell; Nonlinear;
Morison Model; Brace; Correlation.

INTRODUCTION
The airgap responses of a semisubmersible drilling unit are highly
dependent on motions in the vertical plane, namely heave, roll and pitch.
The roll and pitch motion are characterized by low-frequency narrowband resonances. The linear potential flow analysis has been adopted as
a common practice for airgap prediction, but the full calculation of thequadratic transfer functions (QTFs) is becoming more widely used byvirtue of efficiency improvement.

It is meaningful to evolve a more robust and reliable air gap prediction
procedure in time domain, with higher accuracy but more computational
efforts. At this stage of study, several nonlinear characteristics areemphasized due to their significant contributions to the air gap responses.
The viscous drag on the slender elements is modelled by Morison
elements with reasonably tuned drag coefficients, while theinstantaneous fluid load on the braces is quantified by 6D buoy model.

After the fatal incident of COSL Innovator in 2015, OTG-13 (DNV,2017) were released to guide the airgap prediction for semisubmersible,
incorporating the achievements from the Wave Impact Loads JIP,EXWAVE JIP and WADECK JIP, etc. In wake of the issuance of thistechnical guideline, lots of attempts have been made to improve therecommended design procedure from different aspects. Pan et al. (2018)
revealed that the low frequency heave motion was equally important asEXPERIMENT SETUP

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