Final report for WP4.3: Enhancement of design methods ... - Upwind

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UPWIND WP4: Offshore Support Structures and Foundations tion. In this approach the wave energy spectrum is altered to give the same resulting hydrodynamic load on the structure as the standard MacCamy-Fuchs method, in which the Cd and Cm coefficients are modified in the frequency domain. For non-linear waves the wave particle kinematics are calculated using stream function theory. The order of the solution is chosen based on the input values of wave height, wave period and water depth. Mooring lines The Bladed code uses a user-defined force-displacement relationship to model non-linear restoring forces from mooring lines. The relationship between the displacement of the platform and the applied force from the mooring line is calculated separately by the user and implemented via a stiffness matrix at the fairlead position. SIMO/RIFLEX by MARINTEK SIMO (Simulation of Marine Operations) is a general-purpose time-domain program developed by MARINTEK for the modelling and simulation of offshore structures. It is used extensively to model motions and station keeping of floating structures in the offshore industry. The code has been extended to enable modelling of floating wind turbines by the addition of an external module for the simulation of rotor aerodynamic forces [86]. SIMO has also been coupled with non-linear finite element code RIFLEX [87], also developed by MARINTEK, a tailor-made code for the static and dynamic analysis of slender marine bodies such as risers and mooring lines. Structural dynamics The SIMO code uses interconnected multibody systems to model structural dynamics. In order to model a floating offshore wind turbine multiple bodies may be defined and coupled together. In [86] the turbine and support structure are defined using a small number of rigid bodies (2-body and 4-body configurations are investigated). In this case the rotor loads are transferred to the support structure using three flexible coupling elements consisting of two radial bearings and one axial bearing. In [87] the coupling with RIFLEX enables a finite element formulation of the structure, allowing for unlimited displacements and rotations in 3D space. The rotor is still modelled as a rigid body but the tower is made up of flexible beam elements, each with 12 degrees of freedom, which means that the elastic behaviour of the tower can be investigated. Aerodynamics The aerodynamic forces are calculated in a separate module and implemented in SIMO as a user-specified external force. Blade Element Momentum theory is used to calculate the forces on the rotor blades, with dynamic inflow effects included. Individual blade element forces are then summed together and applied in SIMO as a six-component external load on a rotating body. The drag force on the tower and nacelle is also accounted for in the aerodynamic loading. Hydrodynamics The hydrodynamic forces are modelled within the standard SIMO code. Linear Airy wave theory is assumed for calculating wave kinematics. The calculation of hydrodynamic loads takes into account linear and quadratic potential forces including frequency-dependent excitation, added mass and damping contributions (calculated in the frequency domain using WAMIT) and slow drift. Viscous drag forces from Morison’s equation, mooring line forces and body-to-body hydrodynamic coupling force models are also included. Mooring lines The mooring lines are modelled using the RIFLEX code. This enables the representation of mooring lines as flexible finite elements, incorporating non-linear material properties and dynamic properties. A separate mooring system module is not required as it is an integrated part of the RIFLEX code. SIMO/RIFLEX with HAWC2 coupling The SIMO/RIFLEX code has also been coupled with the HAWC2 code in [88] and [89]. HAWC2 is an aeroelastic simulation tool developed by Risø National Laboratory for the dynamic analysis of fixed-bottom wind turbines [90]. The coupling of these two codes enables detailed modelling of both the aerodynamic and hydrodynamic forces acting on a floating offshore wind turbine. The HAWC2 code has also been used to directly model a floating wind turbine in [91], with the mooring line analysis performed separately in SIMO/RIFLEX. Structural dynamics The HAWC2 code uses a combined linear finite element and nonlinear multibody representation to calculate the structural dynamics of a wind turbine. A number of separate bodies can be defined, consisting of an assembly of linear Timoshenko beam finite elements. The bodies are connected by algebraic constraint equa- 80
UPWIND WP4: Offshore Support Structures and Foundations tions, which can take the form of flexible joints, bearings or rigid connections. Internal forces are calculated from these algebraic constraints. In order to couple the two codes together the position, velocity and acceleration vectors and rotation matrix at the interface point are passed to HAWC2 by SIMO/RIFLEX and the reaction force at the interface point is returned to SIMO/RIFLEX by HAWC2 at each time step. Aerodynamics The aerodynamic forces on the rotor are calculated in HAWC2 using Blade Element Momentum theory. The classic approach has been modified to include the effects of dynamic inflow, dynamic stall, skewed inflow, shear effects on induction and effects from large deflections. The aerodynamic calculation points are positioned independently of the structural nodes to provide and optimal distribution of these points. Hydrodynamics and mooring lines In [88] and [89] the modelling of hydrodynamics and mooring lines is performed in SIMO/RIFLEX, as described in Section 3.4 above. In [91] the hydrodynamics are calculated using Morison’s equation based on the instantaneous position of the platform. The mooring lines are modelled in SIMO/RIFLEX using a finite element model and the resulting force-displacement relationship applied as an external force at the fairlead position. 3Dfloat by UMB 3Dfloat is a code developed by the Norwegian University of Life Sciences (UMB) for the modelling of floating offshore wind turbines with full coupling between structural dynamics, aerodynamics, hydrodynamics and control system actions. The code has been used to analyse floating offshore wind turbine models and compare conceptual designs [92]. Structural dynamics 3Dfloat uses a finite element method for modelling the structural dynamics of a floating wind turbine. Euler- Bernoulli beams with 12 degrees of freedom are used, and geometric non-linearities in the elements are taken into account by casting the model in a co-rotational framework. The rotor and drive train are modelled as rigid, with no interaction between the rotor and the tower. Flexibility is included in the tower. The global motion of the structure is taken into account using structural modes. Aerodynamics The rotor aerodynamics are calculated in 3Dfloat using Blade Element Momentum theory. Extensions for dynamic inflow and large yaw errors are also included. Hydrodynamics The hydrodynamic forces are calculated in 3Dfloat using Morison’s equation, with wave particle kinematics derived using linear Airy wave theory. The hydrodynamic loads include terms for added mass of water from the acceleration of the structure, linear hydrostatic restoring and non-linear viscous drag. Mooring lines The mooring lines are modelled using finite elements in 3Dfloat with bending stiffness neglected. The mooring lines can also be replaced by linear stiffnesses at the fairlead positions for the purposes of eigen-frequency analysis. SIMPACK by SIMPACK AG SIMPACK is a commercially available general purpose multibody dynamics code developed by SIMPACK AG. The code is used by the automotive, railway, aerospace and robotics industries. A version of SIMPACK, SIM- PACK Wind, offers extensions to the original code which allow integrated wind turbine simulation. Several wind turbine manufacturers and suppliers are using the code, primarily for drive train analysis, but also for integrated wind turbine load simulations. Structural dynamics The SIMPACK code uses a multibody representation to allow a large number of structural configurations and degrees of freedom. In SIMPACK the parts or bodies of the wind turbine structure are connected using complex joints with different types of force elements acting from the inertial system on the bodies (for example aerodynamics on the rotor, hydrodynamics on the support structure) and between bodies (for example springdamper elements). Parts of the WT, where the relative deflection of the bodies is small in comparison to the 81
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Project UpWind Contract No.: 019945
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Acknowledgement The presented work
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Summary The overall aim of the UpWi
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Table of Contents Acknowledgement..
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Task 4.1 Integration of support str
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Sequential coupling The sequential
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analysis. In the new multibody code
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Figure 2.4: 2nd global bending mode
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3.2 Comparison of deterministic loa
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Figure 3.5: Time series of axial fo
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• Time domain simulation in ADCoS
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• A supplementary load vector for
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The following results are found: Fi
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Figure 4.5: Output positions in the
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Page 32 and 33: ated eigenfrequency is shifted into
Page 34 and 35: Water levels For the calculation of
Page 36 and 37: guidelines for the inclusion of cli
Page 38 and 39: damaging if the frequency of the ex
Page 40 and 41: uncertain parameters carefully. Thr
Page 42 and 43: Table 5.3 shows annual reliability
Page 44 and 45: Steel substructure for offshore win
Page 46 and 47: Table 5.16 shows the required FDF v
Page 48 and 49: A Fracture Mechanical (FM) modellin
Page 50 and 51: Figure 5.4: Annual reliability indi
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Page 54 and 55: Rainflow evaluation The load cases
Page 56 and 57: For DLC 7.2 it has to be considered
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Page 60 and 61: Table 5.28: Output locations for da
Page 62 and 63: From these results it can be clearl
Page 64 and 65: Figure 5.11: Normalised DELs with v
Page 66 and 67: Figure 5.14 presents normalized DEL
Page 68 and 69: Figure 5.16: Output locations for e
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Final report for WP4.3: Enhancement of design methods ... - Upwind

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