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Performance prediction for subsea structures during lowering operations

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Zhang, Wei ORCID: 0000-0001-8015-8585 2018 , 'Performance prediction for subsea structures during lowering operations', Research Master thesis, University of Tasmania.

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Abstract

Accurate predictions of the hydrodynamic loads experienced by offshore structures with large horizontal surface are crucial for offshore installation operations, especially when the structures are crossing the splash zone. The structures may encounter various wave conditions and experience significant impact forces which lead to unexpected impulsive loads on the hoisting system, and subsequently, the operation window may be limited by the impulsive loads.
The numerical simulations of the water entry process for different objects, such as a cylinder and a wedge, have undergone substantial development in recent years. However, there have not been studies conducted on perforated plates in terms of quantifying the slamming coefficients for various layout configurations. The main objective of this work is to find a viable solution to predict the impact force acting on subsea structures, to improve the safety of offshore lowering operations, especially during the water entry process. To achieve the research objective, the Unsteady Reynolds-averaged Navier-Stokes equations (URANS) solver STAR-CCM+ has been used to predict the slamming coefficients for perforated plates of various perforation ratios and layout configurations (i.e. phase one). OrcaFlex was then used to model the splash zone crossing under different sea states, using the slamming coefficients obtained via STAR-CCM+ and DNV-RP-H103 (i.e. phase two).
In phase one, the URANS solver was verified and validated by predicting the slamming coefficient of a circular cylinder during the water entry process. A good agreement has been achieved between the numerical and experimental results. Upon validating the numerical model, the water entry of perforated plates has been simulated to predict the slamming coefficient and free surface profile at full-scale, where the influence of different layout configurations was investigated. In addition, the effect of air compressibility was found to be important when studying the water entry of the flat plate.
In the second phase, the impact of the slamming coefficient on splash zone crossing has been studied, through a series of time domain simulations for the perforated plate under different sea states using OrcaFlex. The results obtained using the slamming coefficient predicted via STAR-CCM+ and the slamming coefficient recommended in DNV-RP-H103 were compared. The results suggest that the probability of slack occurring is lower when using the recommended slamming coefficient from DNV-RP-H103 for offshore structures with a large horizontal surface, such as the perforated plate in this study. In addition, the influence of a passive heave compensator (PHC) has been investigated through a series of deployment simulations for the perforated plate with and without a PHC. Based on the results, the presence of the PHC is crucial for installation operations where the wave period ranges from 3 to 10 seconds in order to achieve a safe operation.
In summary, the presented work has provided some insights into the slamming coefficients predicted for perforated plates with various layout configurations. The probability distribution of slack occurring on the hoisting system can assist the field engineer to perform the risk assessment for lowering operations.

Item Type: Thesis - Research Master
Authors/Creators:Zhang, Wei
Keywords: perforated plates, slamming, splash zone crossing
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Copyright 2018 the author

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