Chew, Kok Hon (Nanyang Technological University) | Ng, E.Y.K. (Nanyang Technological University) | Tai, Kang (Nanyang Technological University) | Muskulus, Michael (Norwegian University of Science and Technology) | Zwick, Daniel (Norwegian University of Science and Technology)
Studies were conducted on a bottom fixed offshore wind turbine with jacket type substructure, including structural optimization and parametric studies for a newly developed three-legged jacket. The four-legged jacket adapted for the NREL 5MW reference turbine within the IEA Task 30 OC4 project was selected as the reference support structure design. Coupled aero-hydro-servo-elastic simulation was carried out in the time-domain to model the dynamic response of the turbine. Fatigue (FLS) and ultimate limit state (ULS) analyses were performed for the substructure members. By comparing against the reference model, the three-legged designs were iteratively optimized to obtain Pareto optimal designs (with lowest material consumption). Under the load cases studied, the three-legged jacket can save up to 55 per cent (LC 5.6) and 13 per cent (LC 5.7) of structural mass and is feasible as an interesting alternative to the four-legged jacket. Further analyses were carried out to evaluate both design options in terms of structural stability and vibrational frequencies. Finally, parametric studies were carried out to investigate the sensitivity of the performance of the jacket substructures with respect to different load cases, loading directionality, and wind-wave misalignment. It is concluded that wind-wave misalignment effects can be neglected, but directionality effects can lead to differences in joint fatigue lifetimes of up to 60 per cent. It is therefore important to account for these effects in the design phase.
A motion and riser design assessment study of a specialized dual-use transport and offshore support vessel (with drilling, mining, and construction capabilities) is presented. To support the mining equipment, the beam of the vessel is proposed to increase by adding sponsons on port and starboard sides. For vessel motions, an approach combining state-of-the-art frequency- and time-domain tools is utilized. Riser strength and fatigue analyses are performed in time-domain. It is found that vessel response is non-linear due to the presence of sponsons that participate in hydrodynamic mass and inertia, thereby increasing the vessel excitation, particularly, for the roll degree of freedom. Overall, it is concluded that the riser system design for this specialized ship conversion is considered to be feasible, regardless of the variations in the input data.
Due to the recent invigoration in the development of coastal and marine areas, many sea-crossing bridges are being designed and constructed all over the world. A large-diameter drilled shaft is commonly used in the construction of a sea-crossing bridge; a permanent steel casing is required when the drilled shaft must be installed in water, and the protruding portion of the casing is used as a form. The selection of an appropriate thickness for the permanent steel casing is an important factor for cost savings. However, no specific approach exists to determine the thickness of the permanent steel casing for large-diameter drilled shafts. Hence, this study proposes a method for rationally determining the thickness. This method was verified in field tests and successfully applied in practice.
The three dimensional simulations of the flow around cylinder at Re = 3900 by using the large eddy simulation solver in OpenFOAM are presented in this paper. Extensive analysis of characteristics and properties of the turbulent wake flow behind the cylinder is also given. The presented numerical results show that the large eddy simulation employed could capture the subtle structure in the flow field. All of the presented numerical results are in good agreement with the experimental ones, which could help us to understand the complicated 3D turbulent wake flow around a 3D cylinder.
The paper investigates the dynamic response of single and multiple rotor Darrieus-type vertical axis water turbines in order to determine the best configuration. Three configurations are considered: a single two-bladed rotor, a twin two-bladed rotor with blades aligned, and a twin two-bladed rotor with rotor blades offset by 90 degrees in azimuth. The Double-Multiple Streamtube model is employed to compute hydrodynamic loads and turbine performance. The structural dynamic response of the turbines is determined through the spinning finite element approach in which the turbines are modeled using 3D beam elements. The study reveals that the twin two-bladed rotor with blades aligned is structurally more resilient than the other two configurations.
The Kuakata beach, located at northern part of Bay of Bengal and at southwest of Bangladesh, lies between latitude N21°48'05" to N21°51'36" and longitude E90°05'06" to E90°15'07". It is one of the most attractive tourist destinations and perfect place for holidaymakers and sun-seekers, because both the sunrise and sunset in the sea are visible from this place. It is the second most famous beach in the country located some 320 kilometers from the city of Dhaka, the capital city. From the present study it has been found that the western part of Kuakata beach is exposed to continuous erosion due to wave actions and storm surges. The study finds the shore line shifting of Kuakata beach during the period of 1973 to 2010 by satellite image analysis. To protect the central 5 km reach of eroded beach this study also investigates the design aspects of artificial beach nourishment. The half-life of the designed nourished beach has been found as 3.52 years assuming a rectangle nourished beach profile and the nourishment factor is estimated as 0.87.
Short-crested wave in real sea condition is defined as the linear summation of a series of long-crested waves propagated to different directions, where the magnitude and directions are randomly generated. Also, it is complex and three dimensional. In this paper, an experimental and numerical study on the truss spar responses subjected to long and short-crested waves are presented. The experimental study on a typical truss spar model subjected to long and short-crested waves were performed in the wave tank of Offshore Laboratory in Universiti Teknologi PETRONAS. In the study, physical motions in surge, heave and pitch of the model with four linear spring mooring lines restrained were measured. Also, a numerical MATLAB code incorporating the short-crestedness of the waves to predict the dynamic analysis of the truss spar was developed. The results were obtained in terms of Response Amplitude Operator (RAO) for surge, heave and pitch motions using both methods. The predicted RAO were compared with the measured RAO and they agreed fairly well. Also, it was observed that the responses for short-crested waves were much lower compared to the responses for long-crested waves.
In this paper, a generic logical model of the liquefaction cycle in an LNG FPSO is proposed to automatically generate various alternatives to such cycle. The generic logical model, which has rules for combining equipment for the liquefaction cycle, is represented by the system entity structure (SES), an ontological framework that hierarchically represents the elements of a system and their relationships. Using this generic logical model, process simulation was performed to determine the optimal—i.e., the most effective—operating conditions for the liquefaction cycle. The method based on sequential modular simulation was used for the process simulation. All the equipment for this method were modeled on the DEVS (Discrete Event System Specification) formalism and connected with the logical models of the liquefaction cycles. Various types of liquefaction cycles for the LNG FPSO, including a dual mixed-refrigerant (DMR) cycle, were synthesized, and the optimal operating conditions for these feasible liquefaction cycles were determined through sequential modular simulation in a DEVS environment.
Perforated free surface semicircular breakwater is a good energy dissipater, but its wave suppression ability is relatively low, particularly when subjected to lower breakwater immersion depths and waves of longer period. To overcome the limitations of the breakwater, addition of wave screens of different configurations and porosities below the breakwater caisson is proposed in this study. Experiments have been conducted to investigate the hydraulic performance of the test models and to identify the optimum design for the breakwater. The experimental results show that the breakwater with double screen of 25% porosity offers a reasonably good hydraulic efficiency when immersed in limited water depths.