Numerical simulations of a conventional tension leg platform (TLP) were generated to investigate its response to wave-in-deck-loads under extreme wave conditions. The model was setup as a fixed body and as a floating body connected to a fixed boundary by flexible mooring cables. The overset mesh technique was utilised to account for large platform motions in the floating body CFD simulations. The validity of the CFD simulations was extensively studied using 1:125 model-scale experiments. Special attention was given to ensure the accuracy of the simulated wave profile, global and local loads on the fixed structure as well as motions and global loads on the moored floating structure. The global horizontal wave impact loads and motions were found to agree well between simulations and experiments.
Discussion Discussers’ reports and floor discussions, including the replies by the committees, are included in this Volume 3 discussion paper. Committee Mandate Concern for advances in structural model testing and full-scale experimentation and in-service monitoring and their role in the design, construction, inspection and maintenance of ship and offshore structures. This shall include new developments in: best practice and uncertainty analysis; experimental techniques; full field imaging and sensor systems; big data applications for ship and offshore structures; and correlation between model, full-scale and numerical datasets.
Prediction of the hydrodynamic efficiency of a Wave Energy Converter (WEC) device is crucial to evaluate the design and the concept of the device. Experimental and numerical techniques are the main tools currently available for WEC designers; however, these techniques are still costly and too time expensive to be used for optimisation and commercial purposes. It is, therefore, important to develop an efficient and cost/time-effective technique in order to investigate the hydrodynamic characteristics of WEC devices. In this work, an Adaptive Neuro-Fuzzy Inference System (ANFIS) technique was developed to predict the hydrodynamic efficiency of WEC devices. ANFIS models were designed, trained and tested using published experimental datasets for the hydrodynamic efficiency of fixed Oscillating Water Column (OWC) devices, and different types of membership functions were examined to develop the best accurate model. ANFIS technique was found to provide good estimates in comparison with experimental results and can be used to predict the hydrodynamic efficiency of WEC devices during the early stages of design.
The reduction in design air gap has been considered as a threat in the amplification of wave-in-deck loads on both fixed and floating offshore platforms. In this paper, the wave-in-deck forces on a fixed plate of a rigidly mounted box-shaped structure due to monochromatic regular waves are computed by means of a computational fluid dynamics (CFD) approach based on the volume of fluid (VOF) method implemented in the commercial CFD codes. Different parameters including wave steepness and air gap are tested. The obtained results are validated against tank experiments. The measured peaks of force components were analysed using one-way ANOVA (analysis of variance) in order to test the force variation over time. CFD force predictions were found to be in good agreement with the measured forces.
In recent years, increasing attention has been paid to the effects of pitting corrosion in ships’ hull structures. Unlike general corrosion, where the thickness loss is normally assumed to be uniform, pitting corrosion is extremely localised which poses difficulties in the ultimate strength assessment of structures suffering from it. The use of an Adaptive Neuro-Fuzzy Inference System (ANFIS) method is proposed in this paper to predict the ultimate strength reduction of locally corroded plates. Published ultimate strength data sets for unstiffened plates with pitting corrosion subjected to uniaxial inplane compressive loads were used to create and test ANFIS models. A number of input variables were used including the ratios of the plate slenderness, pit breadth to plate width, pit length to plate length and pit depth to plate thickness. Rule-based fuzzy sets were used for mapping the inputs to the output which is the reduction in the ultimate strength. Different types of membership functions were tested to find the best accurate model. The two-sided Gaussian-type function was found to be more effective and less sensitive to the sample size than other functions tested in this study. It was concluded that the developed model can predict the ultimate strength of structures locally corroded.
Abstract Any offshore structure requires engineering feasibility and fatigue study during the design and/or maintenance process. The studies mentioned above require an understanding of wave loading on the structure. Most wave loading studies consider wave scatter diagrams. However, the probability of wave occurrence depicted on wave scatter diagram change over region, season and period. Hence, the study based on any static wave diagram doesn’t reflect the probability of wave occurrence for a required region. Further, these waves scatter diagrams are not readily available for the specific local region of interest. Hence, in this study, an algorithm is developed to make a wave scatter diagram using hindcast Australian Meteorological data for a local region for a particular season or period of time. The data has 0.4 × 0.4 degrees spatial and one-hour temporal resolution and contain mean, wind and swell sea state; hence enables to make wave scatter diagram for mean, wind and swell sea for any offshore application. The advantage of the proposed hindcast wave scatter diagram includes flexibility and ease of customising wave scatter for any region, season or period of time compared to available sea area-specific wave scatter diagram. Also, the hindcast model is very well tested and updated regularly and hence wave scatter diagram is developed based on recently validated data. Since these data are free, making a more reliable and accurate wave scatter diagram is free. Additionally, in this study, the graphical comparison of the hindcast wave diagram with two additional sources for the North Atlantic wave and Australian water is provided for validation. The graphical comparison shows the differences and similarities of the wave scatter diagrams over different regions, seasons and periods.
This paper presents an experimental and numerical investigation into the magnitude and distribution of the hydrodynamic loads affecting a fixed multicolumn offshore platform (rigidly mounted tension leg platform) when subjected to extreme wave events. All wave load components, including wave-in-deck slamming pressures, were predicted using a commercial computational fluid dynamics (CFD) code STAR-CCM+ and compared against experimental measurements. Slamming pressures were calculated using both data obtained locally at discrete points and globally averaged over the whole exposed area of the deck. In all simulated cases, the deck area exposed to a wave-slamming event was found to be in contact with a water–air mixture with a significant proportion of air phase. It was concluded that the slamming pressure data for the exposed area provided better insights into the pressure changes due to air compressibility and its content.
The study of rogue waves is becoming increasingly important, as the offshore oil and gas, as well as renewable energy industries, expand. The unpredictability of such disastrous waves poses a significant risk to floating and fixed structures, making it necessary to develop methods capable of recreating rogue waves for model testing purposes. In this paper, an investigation into the useability of the NewWave theory, a theoretical formula for producing focused waves, was conducted in model test facilities with a wavemaker. The numerical modelling of rogue waves was performed using MATLAB codes developed to create several types of wave packets. The success of the numerical generation of design rogue waves was dependent on the number of wave components used during construction such that a suitable rogue wave (Hmax/Hs > 2.0) could be created using 400 or more components. It was found that the NewWave technique could construct and physically generate design rogue waves within a close range of the predicted height provided the main wavemaker stroke was smooth enough (at around 0.8 s trough-crest for the tested model scale). The measured rogue waves were found to be complex; highly non-linear in amplitude with the behaviour of up to the 3rd order. Furthermore, it was observed that rogue waves, created based on a 100-year sea state, were very similar to the New Year Wave confirming that such extreme waves, approximately 25–27 m high at full scale, can indeed occur in severe sea states.
In bad weather conditions, manoeuvring of a liquefied natural gas (LNG) carrier in open sea at floating LNG (FLNG) terminals can be associated with operational risks of potential consequences to human safety, environment and economy. Due to limited knowledge and experience with the FLNG technology, it is difficult to perform risk evaluations. Expert judgments are often employed to analyse and assess LNG risks using the risk matrix technique. However, human behaviour may influence the outcome of risk analyses resulting in an uncertain risk level. To deal with the uncertainty of expert opinions, two fuzzy set approaches are introduced in this paper. The first is a rule-based fuzzy model using Sugeno method for fuzzy inference. The second is a trapezoidal fuzzy set approach simplified approach. Risk parameters were translated into a number of fuzzy sets, and a fuzzy risk value was calculated for several hazardous scenarios at different phases during ship berthing operations. The calculated fuzzy risk values were found to be consistent with the results of risk matrix technique.
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