Towards Reliable Prediction of Wave-in-Deck Loads and Response of Offshore Structures
Creators: name: Abdussamie, Nagi
Publisher: name: University Of Tasmania
Year: 2023
Subjects: Ocean engineering

description: Offshore structures need to survive whilst being exposed to extreme wave events, which can potentially threaten workers, environment and the structure itself. Despite the increase in regulating air gap requirements, numerous offshore installations around the world continue to suffer damage due to wave-in-deck loads, and yet the prediction methods for these loads are still not mature. This thesis reports on the development of reliable experimental and numerical techniques for the analysis and prediction of wave-in-deck loads and the resulting response of different types of offshore structures. The investigated structures included a fixed platform deck, a fixed multicolumn platform (rigidly mounted Tension Leg Platform) and a compliant TLP. Experimental investigations were conducted at the Australian Maritime College (AMC) towing tank at a model scale of 1:125 to examine extreme wave events associated with a 10,000-year tropical cyclonic condition offshore Western Australia. All the investigated models were subjected to long-crested irregular waves. The compliant TLP model was also subjected to several deterministic unidirectional regular waves aimed at validating two-phase flow numerical models. The scope of the experimental part was to obtain the magnitudes and trends in the wave forces, discrete local pressures and the platform dynamics and to obtain high-quality data for the purpose of validation of numerical predictions. The effect of the deck clearance reduction on the magnitudes of forces and pressure acting on the fixed structures was also examined. Model accelerations were monitored for each wave impact event so that the inertial force effects due to the structural dynamic response could be identified. Uncertainty analyses conducted in this work demonstrated that variability in the measurements of wave elevations, global loads and motion responses were minimised using highly-controlled model tests of 4 – 5 repeated runs for each test condition. The experimental results for a fixed platform deck showed that a reduction of deck clearance (up to 2.5 m in full scale, ≈17% of the original deck clearance) significantly increased global loads due to wave impacts (by a factor of 2). However, reducing deck clearance did not result in increased impact pressure magnitudes for all locations. In contrast, for a fixed multicolumn platform, a reduction in deck clearance was found to have no clear effect on either global or local vertical wave-in-deck loads. For a compliant floating TLP, wave-in-deck impact events were found to have a significant effect on the tendon tensions. The experiments showed that the maximum tension in the up-wave tendons usually occurred when the wave crest reached the deck leading edge. The down-wave tendons experienced lower tensions and frequently became slack when the wave crest excited the platform deck, and ringing responses were produced in both the up-wave and the down-wave tendons. The slam pressure was found to correlate with wave steepness; the steeper waves tended to cause higher pressures. The numerical part of the investigation used the commercial Computational Fluid Dynamics (CFD) code STAR-CCM+ to simulate the characteristics of a unidirectional regular wave impact on the floating TLP model. The numerical results of surge motions, tendon tensions and deck slamming pressures were compared against the measurements acquired in model tests. The CFD simulations showed that the model’s motions and tendon tensions predicted by CFD were in good agreement with the measurements, except for the initial transient periods caused by the start-up condition of the wavemaker. Using CFD results, it was revealed that the downward component of the vertical wave-in-deck force caused tendon slack situations in the down-wave tendons. The consequences of wave-in-deck impact events were identified and a better understanding of the problem for different types of offshore structures was gained. CFD simulations in regular waves developed a starting point towards reliable prediction of such loads. The results of the present investigations provide statistically reliable force (global) and pressure (local) values which can be used for the validation of advanced CFD models of wave-in-deck impact problems in irregular waves. Hence, the wave-in-deck loads associated with extreme wave conditions can be assessed to evaluate the risk for local damage to structural members as well as platform structural integrity. Overall, the knowledge gained in this project contributes towards broadening the understanding of the wave-in-deck impact of offshore structures.