Hideyuki Suzuki

An Approach for the Optimum Design of TLP Type Offshore Wind Turbines

In this paper, we will show a new approach to design a Tension Leg Platform (TLP) type offshore wind turbine. Generally, TLPs are used in deepwater oil and gas development fields due to their favorable motion characteristics. In this field, they have high set up costs. An upper structure of 5MW wind turbine, however, is only 450tons at its total weight, which is much lighter than that of oil and gas platforms. Therefore the displacement and water plane area of the platform might be smaller. As a result, wave forces could decrease and it could lead initial tensions to be lower. This idea that leads to low set up costs will be discussed and also principal particulars of two types of TLP prototypes will be proposed in the present work. A tank test using 1/100 scale models was conducted under combined wind and wave conditions in this work. We measured not only motion characteristics, tensions on tendons, but also rotation speed of the turbine blades. Important phenomena in terms of a gyro effect, snapping and so on could be observed, and will be introduced in this paper.Copyright

Development of an Analysis Code of Rotor-Floater Coupled Response of a Floating Offshore Wind Turbine

Coupled rotor-floater response analysis is essentially important for the design of Rotor Nacelle Assembly (RNA) and floating support structure of Floating Offshore Wind Turbine (FOWT). The authors have developed an analysis code UTWind for analysis of the coupled structural response. Blades and floater are modeled as frame structure with beam elements. Lumped mass model is use for mooring. Aerodynamic load on blade is calculated by Blade Element Momentum Theory (BEM), and hydrodynamic load is calculated by Hooft’s method, and Morison equation was modified to be applicable to cylindrical element with cross section with two axes of lines symmetry. The equations of motion of rotor, floater and mooring are solved in time domain by weak coupling algorithm. The numerical results by the code were compared with responses measured by experiment in wave and wind-and-wave coexistence field with/without blade pitch control and showed good agreement. Response by negative damping was reproduced by the code and showed good agreement with experiments.Copyright

Collision of a Drifting Ship With Wind Turbines in a Wind Farm

A multiple collisions caused by a drifting ship which lost control and entered into a wind farm (WF) may cause relatively large risk for a WF comprised of bottom mounted type offshore wind turbines. A bottom mounted type wind turbine will be installed relatively close to shore in Japan and sometimes close to dense marine traffic area. Consideration of the risk will be necessary in planning a WF.This paper presented an estimation of a damage caused by collision with a drifting ship accidentally entered a wind farm. The WF is assumed comprised of bottom mounted type offshore wind turbines. The size of the drift ship considered in the analysis is 6788 GT. For smaller ships, damage to wind turbine considered to be small. Entry of ships from sides other than the one facing sea route was ignored because the number of ship entries from the sides was considered small. Under a number of limitations, risk of multiple collisions in WF by a drifting ship was formulated and quantitatively estimated.Copyright

Very large floating structures

Very Large Floating Structure (VLFS) is a unique concept of ocean structures primary because of their unprecedented length, displacement cost and associated hydroelastic response. International Ship and Offshore Structures Congress (ISSC) had paid attention to the emerging novel technology and launched Special Task Committee to investigate the state of the art in the technology. This paper summarizes the activities of the committee. A brief overview of VLFS is given first for readers new to the subject. History, application and uniqueness with regard to engineering implication are presented. The Mobile Offshore Base (MOB) and Mega-Float, which are typical VLFS projects that have been investigated in detail and are aimed to be realized in the near future, are introduced. Uniqueness of VLFS, such as differences in behavior of VLFS from conventional ships and offshore structures, are described. The engineering challenges associated with behavior, design procedure, environment, and the structural analysis of VLFS are introduced. A comparative study of hydroelastic analysis tools that were independently developed for MOB and Mega-Float is made in terms of accuracy of global behavior. The effect of structural modeling on the accuracy of stress analysis is also discussed. VLFS entails innovative design methods and procedure. Development of design criteria and design procedures are described and application of reliability-based approaches are documented and discussed.

Optimization of the dynamic response of a semi-submersible type megafloat

In recent years, many researches have been made on the middle-scale floating airport using semi-submersible type megafloat. However, few researches have been done from the viewpoint of the design of elastic response. An elastic response characteristic must be designed under the constraints that the structural response is small enough and the structure is safe. This is a subject to be investigated further. In this paper, in order to find the optimized form of semisubmersible type megafloat, an optimization procedure more efficient than before is proposed. An objective function based on the concept of risk which rationally unifies the structural weight, strength and functionality aspect of the structure such as motion and elastic response is proposed. To reduce heavy burden of calculation, a simplified analysis model with sufficient precision and short calculation time was developed and used for the optimization. The simplified analysis model was developed based on the equation of motion of rectangular plate. Using this model, semisubmersible type megafloat was optimized and new form of semisubmersible type megafloat was obtained.

Numerical Simulation of Vortex-Induced Motion With Free Surface by Lattice Boltzmann Method

In this study, numerical simulation of the fluid flow by using lattice Boltzmann method is carried out and the vortex-induced motion (VIM) of a cylindrical floating structure is calculated. The way of calculate the fluid flow, fluid force and floating body’s movement is introduced.The fluid flow with free surface is also calculated. The height change of water surface exerts the effect to the evaluation of hydrostatic pressure and wave resistance. In this study, the method to express the movement of free surface is introduced.Copyright

Numerical Study on Vortex Induced Motion of Floating Body by Lattice Boltzmann Method

Flow around a solid cylinder is numerically simulated with allowance of the movement of the cylinder by the lattice Boltzmann method. The drag force of the fixed cylinder is in good comparison with the literature showing that the fluid force acting on the cylinder is accurately estimated in the computation. It is confirmed that as the body oscillates, the drag and lift forces increases and decreases, respectively. By changing the mooring tension, the locked-in phenomenon and the nonlinear interaction between the vortex-shedding and the mooring system are found.Copyright