Motohiko Murai

Experimental and theoretical analysis of the wave decay along a long array of vertical cylinders

A row of fifty identical, truncated vertical cylinders is submitted to regular head waves, with wave periods in a narrow range around the period of the so-called Neumann trapped mode. The free-surface elevation is measured at 14 locations along the array. Response amplitude operators of the free-surface motion are compared with numerical predictions from a potential flow model. Resonance effects, at wave periods equal to or larger than the critical one, are found to be much less than given by the numerical model. It is advocated that these discrepancies are due to dissipative effects taking place in the boundary layers at the cylinder walls. An artificial means is devised to incorporate dissipation in the potential flow model, whereby the cylinder walls are made slightly porous; the inward normal velocity of the flow is related to the dynamic pressure. The coefficient of proportionality is based on existing knowledge for circular cylinders in oscillatory flows. With this modification in the numerical code, excellent agreement is obtained with the experiments. The numerical model is further used for the case of a very long array composed of 1000 cylinders; it is found that with dissipation at the cylinder walls, the wave action steadily decreases along the array, even for wave periods substantially larger than the critical one. On the other hand, at wave periods less than the critical one, dissipation plays a negligible role; the observed decay is solely due to diffraction effects. Implications of these results for very large structures such as column-supported floating airports are discussed. In particular, it is concluded that scale effects may be an important issue in the experimental analysis of such multi-column structures.

Theoretical and experimental predictions of the hydroelastic response of a very large floating structure in waves

Abstract A prediction method for the hydroelastic behavior of a very large box-shaped flexible structure in regular waves is proposed. The structure considered is representative of such structures as a floating international airport and thus the horizontal dimensions are expected to be as large as several kilometers in both length and width. In the analysis, the structure is divided into a number of substructures and the continuous deformation is approximated by the succession of a discrete displacement of each substructure. The displacement of each component is determined from the equation of motion of a uniform free-free plate representing the substructure, while taking structural constraints into account as an additional restoring force. The hydrodynamic forces on the substructure are determined by enforcing the normal velocity of the flow to be equal to that of the corresponding body surface. Thus, the fluid motion and the body motion interact with each other, which is termed ‘hydroelastic interaction’, requiring the simultaneous solution of the structure and fluid problems.

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

A study on an experiment of behavior of a SPAR type offshore wind turbine considering rotation of wind turbine blades

Many From environmental problem such as the global warming, the time to utilize renewable energy is ripe in substitution for fossil fuel now. The marine space is a treasure house of renewable energy of various forms. In Japan holding EEZ more than 10 times of the national land, the inflection of the renewable energy is extremely important from the viewpoint of self-supply of the energy. Above all, wind-generated electricity is success of the large-scale industrialization in Europe, and the introduction is examined positively in Japan. However, in the sea near Japan and Japanese EEZ, there is little shallow for a good distance from the shore sea area where large-scale industrialization was made in Europe. Therefore, in the expansion of the offshore wind-generated electricity in Japan, expansion of the offshore wind-generated electricity facility which is available for the very deep waters is necessary

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

An experimental study of the interaction between pipe structure and internal flow

Nowadays pipes are widely deployed in the offshore environment especially in the petroleum industry where rigid and flexible pipes are used for well drilling and hydrocarbon production. Whereas during drilling, a mixture of drilling mud, rock cuttings and sometimes gas flows through the drilling riser, during production mono or multiphase (comprising oil, water and gas) flow takes place within the system. However up till now, most of the studies on offshore pipelines and risers have been focused on the pipe structure and its interaction with hydrodynamic forces and offshore platforms. In particular for numerical computation studies and reduced scale model experiments, the pipe is usually modeled as a tensioned beam and sometimes only the internal pressure is taken into account with other effects due to its internal flow being neglected. This paper deals with the interaction between the pipe structure and its internal flow. In order to verify the internal flow effects, an experimental analysis was carried out not using a reduced scale model. In particular, mono-phase fluid flows into the pipe and a parametric analysis using the flow rate was carried out. Discussion about the experimental results and numerical applications is also included.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

A Study on the Optimization for the Arrangement of Two Types of Supporting Columns for VLFS Using GA

In the recent years, there are many researches about hydroelastic responses of Very Large Floating Structure (VLFS). As a result, we are now in a position to say that VLFSs can be used as airport, city base, energy facilities and so on. There are several supporting structures of VLFS. In this paper, we shall focus only on the column type and the pontoon type. These two types of supporting structures have different hydroelastic behavior. For example, wave-induced elastic responses of column type are usually smaller than the response of the pontoon type. On the other hand, large amplitude responses at both ends of the column type VLFS are expected in certain frequencies. Then we thought that combining the two types of supporting structures could increase or decrease the VLFS hydroelastic response. Because there is uncountable number of combinations, we could not evaluate the hydroelastic responses of all combinations. In order to achieve the optimum combination in realistic calculation load, the Genetic Algorithm (GA) is applied. In this research, we discuss the combination of the two types of supporting structures in order to achieve smaller hydroelastic response of the VLFS in regular and irregular waves. GA is implemented as a computer simulation in which a population of abstract representations for the supporting structures are generated (in this case, the representations are the combination of pontoons and columns), then these abstract representations are recombined creating a new generation. At each generation, the best individual (the VLFS with the minimum hydroelastic response) is chosen. After run several generations, the optimum solution is reached.Copyright