Nam Abdussamie

Bio: Nam Abdussamie is an academic researcher. The author has contributed to research in topics: Slamming & Deck. The author has an hindex of 1, co-authored 1 publications receiving 2 citations.

Towards reliable prediction of wave-in-deck loads and response of offshore structures

Abstract: 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.

Loads and Response of a Tension Leg Platform Wind Turbine with Non-Rotating Blades: An Experimental Study

Abstract: This paper describes model testing of a Tension Leg Platform Wind Turbine (TLPWT) with non-rotating blades to better understand its motion and tendon responses when subjected to combined wind and unidirectional regular wave conditions. The TLPWT structure is closely based on the National Renewable Energy Laboratory (NREL) 5 MW concept. Multiple free decay tests were performed to evaluate the natural periods of the model in the key degrees of freedom, whilst Response Amplitude Operators (RAOs) were derived to show the motion and tendon characteristics. The natural periods in surge and pitch motions evaluated from the decay tests had a relatively close agreement to the theoretical values. Overall, the tested TLPWT model exhibited typical motion responses to that of a generalised TLP with significant surge offsets along with stiff heave and pitch motions. The maximum magnitudes for the RAOs of surge motion and all tendons occurred at the longest wave period of 1.23 s (~13.0 s at full-scale) tested in this study. From the attained results, there was evidence that static wind loading on the turbine structure had some impact on the motions and tendon response, particularly in the heave direction, with an average increase of 13.1% in motion amplitude for the tested wind conditions. The wind had a negligible effect on the surge motion and slightly decreased the tendon tensions in all tendons. The results also showed the set-down magnitudes amounting to approximately 2–5% of the offset. Furthermore, the waves are the dominant factor contributing to the set-down of the TLPWT, with a minimal contribution from the static wind loading. The results of this study could be used for calibrating numerical tools such as CFD codes.

The effect of air content and compressibility on wave-in-deck impact pressures

Abstract: Numerical simulations of an extreme wave impact on a topside deck structure were conducted to ascertain the effect of air content and its compressibility on the magnitude of the wave-indeck impact (slam) pressure. The topside deck was investigated as both a fixed structure and as a topside structure of a typical Tension Leg Platform (TLP) exposed to unidirectional regular waves. The volume of fluid model implemented in STAR-CCM+ was used to capture the free surface interface. CFD results were validated using different levels of mesh resolution against 1:125 model-scale experiments. In all simulated cases, the deck area exposed to a wave slam event was found to be in contact with a water-air mixture with a significant proportion of air, which revealed that two-phase models are necessary to accurately simulate wave-in-deck problems.

Research on the Water Ridge and Slamming Characteristics of a Semisubmersible Platform under Towing Conditions

Abstract: : During the towing of semisubmersible platforms, waves impact and superpose in front of the platform to form a ridge shaped “water ridge”, which protrudes near the platform and produces a large slamming pressure. The water ridges occur frequently in the towing conditions of semisubmersible platforms. The wave–slamming on the braces and columns of platform is aggravated due to the water ridges, particularly in rough sea conditions. The effect of water ridges is usually ignored in slamming pressure analysis, which is used to check the structural strengths of the braces and columns. In this paper, the characteristics of the water ridge at the braces of a semisubmersible platform are studied by experimental tests and numerical simulations. In addition, the sensitivity of the water ridge to the wave height and period is studied. The numerical simulations are conducted by a Computational Fluid Dynamics (CFD) method, and their accuracy is validated based on experimental tests. The characteristics of the water ridge and slamming pressure on the braces and columns are studied in different wave conditions based on the validated numerical model. It is found that the wave extrusion is the main reason of water ridge. The wave–slamming pressure caused by the water ridge has an approximately linear increase with the wave height and is sensitive to the wave period. With the increase of the wave period, the wave–slamming pressure on the brace and column of the platform increases first and then decreases. The maximum wave–slamming pressure is found when the wave period is 10 s and the slamming pressure reduces rapidly with an increase of wave period.