Yasunori Nihei

Motion Characteristics of TLP Type Offshore Wind Turbine in Waves and Wind

In this research, we propose a new offshore wind energy generation system that uses a Tension Leg Platform (TLP) and performed the experimental test of the TLP type wind turbine both in waves and in wind. The authors used a 1/100 scale wind turbine. Not only the motion characteristics, but also the loads of the tension legs and the bending moments of the tower were revealed in this paper. From the research, the following conclusions were mainly obtained. 1) In the case of waves-wind coexisting condition, the wind effect stabilizes the pitch motion compared with in only waves. 2) The wind effect is decreasing the vibration of the mooring lines in waves and wind coexisting field. Especially, the springing (2nd order or 3rd order force) is also decreasing in this field. 3) It can be estimated that the amount of reduction rate of electricity generation power is up to about 6% from the results of the heel angle.Copyright

A Comparative Study of Motion Performance of Four Different FOWT Designs in Combined Wind and Wave Loads

This paper summarizes our recent collaborative/competitive works on floating offshore wind turbines (FOWTs) among four universities including Osaka Prefecture Univ., Osaka Univ., Yokohama National Univ., and Nihon Univ. The tasks assigned to each member were to develop the respective FOWT designs which could support 5MW class wind turbine, then to fabricate a scale model based on their own concept, and finally to evaluate the performance by tank tests under prescribed environmental (wind and wave) conditions. Osaka Prefecture Univ. adopted TLP concept, Yokohama National Univ. semi-submersible concept, Nihon University SPAR concept while Osaka Univ. also adopted semi-submersible, however, with single-point mooring. All the measured data were collected and compared among the four designs. It turned out that: (1) All the proposed deigns suffice criteria in terms of motion performance which were assumed at the beginning of the study. (2) The TLP type shows the most favorable performance among the four while the SPAR type shows largest acceleration in almost all the range of environmental conditions. The large acceleration may pose a problem of maintainability. (3) The SPAR type suffers the gyration effects more than the other types. (4) The RAOs of motions under combined wind and wave loads are almost the same as those under only wave loads for all the concepts but the single-point moored semisubmersible. (5) The difference of the RAOs for the single-point moored semisubmersible may be ascribed to the larger coupling effects between the main floater and the mooring system under the combined loads.Copyright

Conceptual Design of a Single-Point-Moored FOWT and Tank Test for Its Motion Characteristics

A new design concept of a semi-submersible type floating offshore wind turbine (FOWT) moored by a single-point mooring is proposed. The FOWT model adopting 5MW class wind turbine is designed. The motion characteristics of the FOWT are evaluated by a series of tank tests. To this end, a scaled model with a scale ratio 1/100 is fabricated. The scaled mode tests are performed under winds, waves, and combined winds and waves to check its fundamental feasibility. It is observed that the motion characteristics under wind and waves are acceptable in general, and the combination of the single point mooring and the down-wind type rotor is effective in terms of weathervane. It is also shown that the difference between the two transfer functions to wave loads, one with and the other without wind loads, is small except pitch response at low frequencies.Copyright

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

Strongly Coupled Method for Predicting the Response of Flexible FOWT With Mooring and its Experimental Validation

In this research, a numerical simulation method for a coupled system of a Floating Offshore Wind Turbine (FOWT) and its mooring system is developed. Flexibility of the platform and the nonlinear properties of mooring can be accounted for by the proposed method. A series of scaled model experiments which include the TLP and SPAR types of FOWT are also performed to evaluate the response of the FOWTs under combined wind and wave loads. Steady wind and regular waves are applied to the models. Measurements are made on strains in the structure, tension variation in the mooring as well as the rigid body motions of the platform. For validating the numerical model, comparison between the experimental and simulation results is made. An acceptable correlation between the experimental and the simulation results is obtained. It is shown that the flexibility of the platform may affect the tension variation in the mooring.Copyright

Designing process and motion characteristics of spar type offshore wind turbines

In this paper, we will discuss about the designing process and the motion characteristics of the spar type offshore wind turbines. When considering a spar type structure for offshore wind turbines, it is important to take a lot of elements into consideration which have not yet been considered in the case of oil and gas platforms. In this research, we used the following standards to conduct our tests. The limit of the heel angle was 5 degrees when the wind turbines are generating in the rated state. When designing the substructure for this research we have decided to go with a substructure that operates in depth of 100m or more. Following the conditions above we have designed the spar type offshore wind turbine used for this research. In order to compare the simulated result we have created a scale model and performed tank tests under various conditions. Also we observed unexpected motion characteristics in certain mooring arrangement. So we will touch these subjects in this paper.Copyright

Non-Linear Wave Forces Acting on a Body of Arbitrary Shape Slowly Oscillating in Waves

In the present study, non-linear wave loads such as the wave drift force, wave drift damping and wave drift added mass, acting on a moored body is evaluated based on the potential theory. The body is oscillating at a low frequency under the non-linear excitation of waves. The problem of interaction between the low-frequency oscillation of the body and ambient wave fields is considered. A moving coordinate frame following the low frequency motion is adopted. Two small parameters, which measure the wave slope and the frequency of slow oscillations (compared with the wave frequency) respectively, are used in the perturbation analysis. So obtained boundary value problems for each order of potentials are solved by means of the hybrid method. The fluid domain is divided into two regions by an virtual circular cylinder surrounding the body. Different approaches, i.e. boundary element method and eigen-function expansion, are applied to these two regions. Calculated nonlinear wave loads are compared to the semi-analytical results to validate the present method.Copyright

Comparison of Weathervane Performance Between Two Types of FOWT Systems Moored to SPM

This paper addresses the weathervane performance of Floating Offshore Wind Turbines (FOWTs) moored to single-point mooring (SPM) systems. A system of equations of motion to describe the motions in horizontal plane around the mooring point in the combined environmental conditions is derived. Wind, wave and current loads are considered. Two types of SPM-FOWT systems proposed by the present authors are considered for comparison. It is found out that the weathervane performance of the SPM-FOWT systems is acceptable in a point that the power generation efficiency does not reduce significantly. The stability is also checked based on the eigenvalue analysis for the linearized equations of motion around the equilibrium point. The stability and the responsivity are discussed in comparison between the two systems. Sensitivity of the stability and responsivity to design parameters such as length between the buoy and the main floater to the turning motion performance is discussed, too.Copyright

Investigations Into Motion Characteristics of a Multi-Column Type FOWT in Waves and Winds

Many researches about various type of FOWT have been already carried out by many researchers including the authors of this paper. As a result, we know that each type has own advantages and own disadvantages. On the other hand, in general, because the floating type is different for each project, it has not compared its performance among those projects in the same condition. Therefore, the authors have examined the performance of the various types of FOWT under the same experimental conditions.In this paper, the performance of the FOWT focusing on Semi-submersible type of multi-column type in particular is reported. First, we examined as design requirements the shape of a column with footing, the distance between the columns and the number and the arrangement of the multi-column. Within it, we investigated the combination of the variables for good performance in waves and winds. We manufactured a 1/100 experiment model which is based on the design requirements and carried out the experiment in waves and winds. Then we discuss the effectiveness of the model using the experimental results and the numerical results.Copyright

Discussion on Hybrid Utilization for Offshore Wind and Wave Energy

In this paper, we will discuss the concept of combined utilization of offshore wind and wave energy in terms of both tank test and numerical simulation. There are some possibility that combined utilization of ocean renewable energies can avoid some disadvantages that single utilization of them may have, such as cost problem associated with the stability of power and power supply. In this study, we focus on the motion of an offshore wind turbine in waves associated to wave energy converters, are installed in the same area in order to improve its motion by reducing the effect of incident waves. We carried out wave tank test and numerical simulation, and investigated the motion of wind turbine in waves. In addition, numerical simulation allows to visualize the wave fields around those devices. Appropriate arrangement of them is discussed for the most efficient utilization of offshore wind and wave energy.Copyright