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

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UPWIND WP4: Offshore Support Structures and Foundations As far as the wave-induced forces on the floating bodies are concerned, OrcaFlex relies on an external code (AQWA and WAMIT are the ones it explicitly supports) to provide the hydrodynamic coefficients. The following data can be imported from an external hydrodynamic solver: • Displacement RAOs (amplitude and phase). • Load RAOs. • Quadratic transfer functions (QTFs) for wave drift calculations. • Added-mass and damping coefficients. • Hydrostatic stiffness. Figure 8.39: OrcaFlex screenshot showing both wire-frame and shaded 3-D views. Key findings - Orcaflex OrcaFlex is suitable for comparison with the look-up table approach because of the contrasting modelling method (mooring lines are modelled by means of Morison elements). It has all the necessary functionality to produce a sufficiently detailed simulation of the floating platforms and their moorings. 8.3.2 Simulation results: MBS approach An approach for modelling mooring lines originally described by Kreuzer and Wilke for oil platforms [116], is to divide the mooring line into rigid (or flexible, modal reduced) multi-body elements connected by spring-damper elements. The line seabed interaction is modelled with a coulombic friction element including spring and hysteresis characteristics as a function of the translational forces. This MBS approach is currently investigated by Matha et al and Azcona. A multi-purpose commercial Multi Body code (Simpack) has been extended to model offshore floating wind turbines. An originally implemented quasi-static mooring line model (NREL’s HydroDyn) has been replaced by a MBS based model. With this approach, no interface between separate programs is necessary since the turbine’s structure and mooring lines are modelled within one code using the same mathematical MBS formulation. This MBS formulation is numerically stable and also allows for a simple implementation of line-seabed interaction, required for catenary systems. First results for the OC3-Hywind spar buoy in 320m water depth show significant differences in the floating WT system’s response between both modelling approaches. The MBS-model is built up of three mooring lines which are discretized into separate rigid bodies. Every single body is modelled as a cylindrical structure and has the gross properties of the particular part of the mooring line it represents. They are connected by spring-damper-elements to simulate the extensional stiffness and the ac- 128
UPWIND WP4: Offshore Support Structures and Foundations cordant damping by using the linear beam theory. Although this is a significant simplification of the structural arrangement of the line’s fibers, it is necessary to reduce the model’s complexity and thereby simulation time. Using chain mooring lines, this stiffness and damping could be neglected. Spar Buoy Fairlead Joint 129 ϕ x ϕ z u y rigid body 3 DOF c r, d r Seabed c t, d t c s d s Figure 8.40: MBS mooring line configuration Anchor The degrees of freedom (DOF) are reduced to a minimum of one translational and two rotational directions. To enable the simulation of the line’s elongation the DOF in cylinder’s longitudinal direction has to be maintained. Also the transverse rotational movements have to be enabled. The line’s twist DOF could be eliminated because there is no significant effect on the hydrodynamic behaviour due to the use of symmetric cylinders for discretization. The very complex behaviour of the seabed is reduced to a unilateral spring-damper-model with high stiffness and damping to represent a rigid floor. The lateral friction is modelled by a simple coulombic element with an empiric friction coefficient. The hydrodynamic effects to the mooring line are represented by a variation of the Morison-Equation. Östergaard and Schellin describe this variation for slender hydrodynamic transparent cylindrical structures with arbitrary orientation to the current of the surrounding fluid. Where is the fluid’s velocity, the relative velocity between structure and fluid and index the normal direction to the segment. The drag and inertia coefficients and are chosen empirically which could lead to further uncertainties. By using the potential theory the Morison-Equation considers hydrodynamic drag and inertia but neglects effects based on dissipative flow, like vortex induced vibrations. Investigations of the discretization show only small effects on the results when increasing the number of elements beyond a certain number of discretized elements. This number has to be identified by a sensitivity analysis. Following this procedure enables to come up with a moderate discretization and limit simulation time while keeping accuracy of the results. Comparisons in Figure 8.41 of the platform’s surge motion to quasi-static models show significant differences caused by the non-linear additional hydrodynamic damping of the MBS model. Accordingly, similar differences in the fore-aft bending moment at the tower base can be identified in Figure 8.42.
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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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It is directly visible that there i
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ated eigenfrequency is shifted into
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Water levels For the calculation of
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guidelines for the inclusion of cli
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damaging if the frequency of the ex
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uncertain parameters carefully. Thr
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Table 5.3 shows annual reliability
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Steel substructure for offshore win
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Table 5.16 shows the required FDF v
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A Fracture Mechanical (FM) modellin
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Figure 5.4: Annual reliability indi
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Further, the effect of possible ins
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Rainflow evaluation The load cases
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For DLC 7.2 it has to be considered
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Guideline, DLC 1.5). Furthermore, t
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Table 5.28: Output locations for da
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From these results it can be clearl
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Figure 5.11: Normalised DELs with v
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Figure 5.14 presents normalized DEL
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Figure 5.16: Output locations for e
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UPWIND WP4: Offshore Support Struct
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Final report for WP4.3: Enhancement of design methods ... - Upwind

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