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

Dynamic Effect in Heading Analysis of Turret Moored FPSOs
Youngkook Kim and Kwang-cheol Ha
North Asia Technology Centre, Lloyd’s Register Asia
Busan, Korea

environmental parameters and this approach has been accepted byindustry due to its fast calculation as a large number of sea states are tobe processed for offshore projects. Milne et al. (2016) compared thequasi-static heading analysis method with full scale measurements of aFPSO in relatively benign sea conditions and showed good accuracy atstable time histories. This approach also provides good estimates ofpossible heading distributions of metocean data, however, it can neglect
the events of lower probability by taking only a single mean heading fora sea state. Since, the design loads for local scantling, hull girder ultimate
strength assessment and strength analysis should be based on the mostprobable extreme response, all the probable events need to be taken into
account if they are still likely to occur during the operation.

ABSTRACTAs turret moored units can weathervane against environmental
parameters, heading angles play an important role for the design ofFPSOs. The offshore structures need to withstand the most probable
extreme responses of a target return period that the asset may encounterat an operational site. It is common that their structures are designed formean heading angles obtained from heading analysis. However, it is aquestion whether the mean heading angles used for the long-term
response calculations can represent the extreme event that the asset canencounter during the operation. In general, heading angle finds a new
equilibrium angle due to weathervaning from environmental parameters.
However, when environmental parameters have relatively large angles
between them, equilibrium heading may not be stable enough but canshow continuous slow yaw motions. Therefore, the variance of theheading angles was studied by time-domain dynamic analysis to be taken
into account for long-term response calculations in this study. Due to thelarge amount of metocean data and required time for the dynamic
analysis, ANN (Artificial Neural Network) regression technique wasimplemented to supplement the analysis. Several parameters were used
to train the datasets and predict which sea state would lead to wider
heading angles by ANN. Such results were compared to the long-term
responses calculated with the mean heading angles in this study.

Although turret-moored FPSO units can weathervane so that the heading
angle can be directed to minimise vessel motions, unexpected heading
angles and associated responses may appear for the weathervaning units.
The UK Health & Safety Executive (2005) highlighted happenings ofabrupt change of heading or rapid changes in direction of environment
during the offshore operations. The study also reported that someresponses are not well recognised under the prescribed return period.
Thiagarajan and Tian (2013) studied about weathervaning instability that
occurred beyond a certain wave length to vessel length ratio. The study
also found the notable deviation between quasi-static/quasi-dynamic
analysis and dynamic analysis when the ratio between wave length andvessel length is large.

KEY WORDS:floating offshore structure, heading analysis,weathervaning, artificial neural network

Furthermore, real life environments are more complex than thesimplified conditions used for a quasi-static analysis. Apart from thepossible weathervaning instability, various uncertainties lie in thedetermination of equilibrium heading angles. Therefore, how heading
angles can affect long-term calculations of turret-moored offshore units
should be evaluated. In this study, the heading variances around theequilibrium of weathervaning are investigated with various
combinations of environmental parameters. Long-term response
calculations are performed with wider heading angles to be comparedwith the results with mean heading angles in this study.

INTRODUCTION
Floating offshore structures operating at fixed locations are designed towithstand site-specific loads. Classification societies have tailored theirRules and procedures for floating offshore units so that the site-specificenvironmental loads can be used for the structural design. Long-term
responses of hull structures are derived statistically in the target return
period, typically 100 year, to be used for either local scantlingassessment or structural analysis. As turret-moored Floating Production,Storage, and Offloading (FPSO) units can weathervane against
environmental parameters, a mean heading angle estimated for each sea
state plays an important role in the determination of the design loads.
The mean heading angles used for the short-term response calculations
are normally determined by quasi-static equilibrium from various

LONG-TERM RESPONSE CALCULATIONS
Hull structure is assessed by Response Based Analysis (RBA) that aims
to evaluate the hull structure for long-term extreme responses and toderive associated loads that are occurring simultaneously using design

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