Description

A Study on the Sensitivity of Dynamic Behavior of
Jacket Type Offshore Structure
Choong-Yul Son, Kang-Su Lee, Jung-Tak Lee, Keon-Hoon Kim
(INHA UNIVERSITY Department of Naval Architecture & Ocean Engineering Inchon 402-751, Korea)
Abstract : Unlike strucutres in the air, the vibration analysis of a submerged or floating
structure such as offshore structures is possibly only when the fluid-structures is understood,
as the whole or part of the structure is in contact

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A Study on the Sensitivity of Dynamic Behavior of Jacket Type Offshore Structure
Choong-Yul Son, Kang-Su Lee, Jung-Tak Lee, Keon-Hoon Kim
(INHA UNIVERSITY Department of Naval Architecture & Ocean Engineering Inchon 402-751, Korea)
Abstract : Unlike strucutres in the air, the vibration analysis of a submerged or floating structure such as offshore structures is possibly only when the fluid-structures is understood, as the whole or part of the structure is in contact with water. Through the comparision between the experimental result and the finite element analysis result for a simple cylindrical model, it was verified that an added mass effects on the structure. Using the commercial FEA program ANSYS(v.11.0), the stress matrix considering an load and underwater added mass was superposed on the stiffness matrix of the structure. A frequency response analysis of forced vibration in the frequency considered the dynamic load was also performed. It was proposed to find the several important modes of resonance peak for these fixed type structures. Furthermore, it is expected that the analysis method and the data in this study can be applied to a dynamic design and dynamic performance evaluation for the ground and marine purpose of power generator by wind.
Key words: Natural Frequency, Wind Turbine Jacket, Finite Element Method, Beam Theory, The static analysis (email : soncy@inha.ac.kr
)
1. INTRODUCTION
Because of unlimited resources, cleanness of energy and advantage of technical commonness, Wind Turbine System is one of the future oriented techniques as spotlighted alternative energy technique converting wind energy into electrical energy. Modal test which is one of the examination assessments is the method to analyze the dynamic characteristics. . Its purpose is to avoid the resonance which, finding the natural frequency of the wind tower and forecasting the vibration phenomenon for mode shape. In case of domestic, study for Wind Turbine System has been preceded actively in some big corporation, small-medium enterprises and national researcher. But it was impossible to obtain systematic data. Today, the research field is very numerous unlike an advanced country oversea. Therefore, it is necessary to study the Wind Turbine System as stated above. Based on this design we calculated the complex load on the tower off- and onshore. The onshore load is calculated using aerodynamic load(caused by wind) and gravity load(caused by the upper structure). Calculations in the offshore case have to take into account aerodynamic load, wave load(caused by waves) and current load(caused by the current). However, since current load is insignificant compared to wave load, it can be ignored
2. ENVIRONMENTAL LOADS
The external loads include hydrostatic pressure, wind, wave, current, tide, ice, earthquake, temperature, fouling, marine growth and scouring.
2-1. The load calculation in on shore
We calculated the gravity load of the upper structure, which consists of the wind turbine system (i.e. blade, nacelle and generator). In order to carry out the structural analysis of the tower we first divided it into sections of height 3m each. Then the feasibility of the
load was determined and the resulting stress and deflection analysed. For the purpose of calculating the section loads in the tower the tower can be viewed as a cantilever beam as shown in figure 1. Fig. 1 Cantilever beam model of Tubular tower
When the tower is analysed structurally, the following three main loads have to be considered:
2-1-1. Impellent force
The impellent force caused by a rotating blade can be calculated using the dynamic pressure of a rotating blade or the drag force affecting the tower.
2-1-2. Distribution force
The tower is of a cylindrical shell type. Assuming a maximum wind speed of 23m/s and the tower being divided into 3m spacing sections, the load that affects the tower can be obtained by evaluating each section area.
2-1-3. Gravity force
The gravity force can be calculated as follows. (The weight of nacelle + blade + generator)
!
9.8m/s
2
2-2 The load calculation in off shore.
To calculate wave load we assume the water to be on average 5m deep, maximum wave height of 10m and maximum wind speed 23m/s. Because the ratio of horizontal dimension (D) to wave length (L) is smaller than 0.05, we can calculate the wave load with Morrison’s Formula. Wave load depends on the form of the structure (here : the tower), the form of the current, Inertia force due to wave particle velocity, the roughness of the surface and Drag force depending on Reynold’s number. Wave load per unit length is as follows: F = 0.5
C
D
Au
2
+
C
m
Vdu/dt
C
m
and C
d
are a coefficients determined by shape, condition of the surface and Reynold’s number. They are calculated using the ABS rule; C
d
is 0.5 and C
m
is 1.5
2-3. Wind Load
Since the wind acts as an external force to the upper structure, above sea level, the wind velocity is determined to estimate the wind generated force (Lee, 1989). The sustained wind speed is the average velocity during 1 min and that is used to determine the wind force acting on the whole structure. The gust wind speed is the average velocity during 3 sec and is applied on planning deck facilities. The wind force, acting on the structure, is largely divided into drag force and lift force. The drag force is a force that is created in the flow direction by pressure difference and lift force created in the vertical flow direction by shape or orientation of object. Total drag force from seabed to height z above the surface is
(1) Total life force from sea bottom to height z above the surface of ocean is
(2) The wind force can be applied to upper structure above M.W.L. The length of a pile for wind force calculation can be determined by considering the maximum wave elevation and the clearance under the super structure. Therefore, the buoyancy uplift and
direct wave force that could occur on the deck structure can be avoided. The air gap is also considered in determining fixed platform height. Commonly 1.5m of air gap and 1/10 wave height is applied.
2-4. Wave Load
A number of wave theories such as Airy, Stoke, Stream Function, Cnoidal and Solitary Wave Theory, enable a suitable wave theory to be applied for the estimation of wave load. The appropriate wave theory can be determined by water depth, wave length and wave period. Stoke wave theories are valid for d/L>0.039, and Cnoidal or Solitary wave theories for shallow sea of d/L>0.04. After selecting the approximate wave theory, the wave force can be calculated by the Morrison equation (Sarpkaya and Issacson, 1981). Considering the energy conservation law, boundary conditions, initial conditions and Bernoulli equation, the following expressing for wave elevation, can be obtained. (3) From the above equation, the following relationship can be obtained + (4) Representing wave and potential as power series: (5) (6) Each potential has to satisfy Laplace’s equation and the boundary conditions. If the potential is represented as a Taylor-series of still water surface in the free surface then, (7) The wave force is approximated by using stokes wave theory which can resolve the non-linear wave motion(Dawson, 1983). To simulated the actual ocean wave, this theory is applied in the study. Wave celerity, can be calculated as (8) Surface elevation, is (9) Horizontal particle velocity, is (10) Vertical particle velocity, is (11) Horizontal particle acceleration, is (12) Vertical particle acceleration, is
(13) Wave force in the horizontal direction on the vertical pile can be classified as an inertia force by acceleration and drag force caused by the boundary layer effect (Clasuss,et. Al., 1988). The inertia force can be expressed as (14) Where, = Mass Coefficient, determinated by experiment. The maximum inertia force is (15) The inertia force is generated between the boundary layer and the fluid layer with the assumption that an infinitely thin fluid layer is stuck on the cylinder side and the velocity is exponential is increased by the distance from the cylinder. Fig. 2 shows the schematic diagram of the wave force on a pile. Fig. 2 Wave load for pile The drag force can be expressed as (16) Where, = Drag Coefficient, determined by experiment. The unit area of a member is
d
A which is projected on the vertical plane of force direction. Therefore the maximum drag force is (17) The total wave force on a pile can be represented as + (18) The calculation of the wave force on a cylindrical object can differ by the ratio of member diameter/wave length, D/L. When this value is lower than 0.05, the pile does not effect the wave property, thus Morrison equation can be applied. However, The Morrison equation can be utilized up to ratio of 0.2. For a large body in the calculation of the wave force. The and vary as the pile roughness, degree of fouling, aspect ration(the ratio of width length), cross-sectional shape, body orientation, relative flow velocity, and Reynolds number etc.. In offshore steel structures = 0.1 and = 2.0 are recommended. These values consider marine roughness.
2-5. Current load
Because the actual current is composed of the various sums of currents coming from multi-directions, it is common to measure the current speeds at several depths of the region (Lee, 1989). Should this data be unavailable, the following equations are used to estimate the current speed; (19)

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