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

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UPWIND WP4: Offshore Support Structures and Foundations subsequently installed. However, for wind turbines to be installed on floating platforms the marine conditions have a non-negligible influence on the structural integrity of the RNA. This means that the marine conditions need to be accounted for in the initial design of the wind turbine. Preliminary results show that for spars or trifloat structures e.g. heel angles of up to 10° can occur. Due to this, relative motion has to be analyzed and mechanical parts have to be reviewed. The analysis requirements do not differ to those of floating structures of the oil & gas industry. 2. Inclusion of low-frequency components in wind and wave conditions shall be considered. These can be important in evaluating the sway motion of the mooring system. 3. The influence of the water depth on the heave eigenperiod is a crucial issue for platforms with compliant heave, roll and pitch modes. Increasing the water depth, the heave eigenperiod increases. The choice of the design water depth will be directly related to the environmental conditions, with the aim to avoid heave, roll or pitch resonance. 4. The relation between the heave and the roll/pitch eigenperiods shall be checked in order to avoid or at least to estimate the consequences of parametric excitations. For new concepts of floating wind turbines it must be ensured that the susceptibility of the structure to those parametric excitations and Vortex Induced Motions are unlikely to occur or can be avoided due to proper choice of operational conditions. Platforms with abrupt changes in the waterplane area and in metacentric height should be especially investigated. 5. Influence of wave drift forces on mean response e.g. for catenary moored systems shall be considered. 6. Influence of low frequency motions on the control system shall be analysed. The slow drift motions of the floating structure may interfere with the control system since their periods are in the typical range of coherent gust rising time. 7. Extension of the frequency range for wind/wave conditions to higher levels shall be taken into account. High frequency excitation forces, generally of small amplitude, may be of importance for floating platforms kept in place by tethers. The restrained modes (heave, roll and pitch) have eigenperiods below the typical wave periods between 5s and 15s. Those platforms can experience high order effects like springing (fatigue) and ringing. Ringing is understood as a natural frequency response of the structure due to the weak impact, caused by steep asymmetric waves. This shall be considered for the design assessment of TLPs. 8. Gust periods larger than 12 seconds shall be included in the analysis to cover the frequencies found from the dynamic analysis of floating offshore wind turbines. 9. Inclusion of two-peaked spectrum formulations that properly reflect the influence of swells is essential for the assessment of the mooring forces and the large period motion behaviour. 10. As mentioned in previous chapters the hydrodynamic and aerodynamic aspects of a FOWT are much more complex than fixed offshore wind turbines. Minimum requirements on simulation tools for the simulation of the coupled aero- and hydrodynamics of moored floating structures shall cover properly the different issues related to this. The main issues for the aerodynamics are the large amplitudes of low frequency motions combined with high heel angles. The wake formulation may have to be adapted for these high angles. 11. The complexity of the system may even require model tests in the basin to analyse the relative motion and impact load on deck structures. Until now no standard method to scale model tests for offshore wind turbines has been developed. 12. Simultaneous time simulations of wind, wave and current shall be necessary for proper fatigue analysis. 134
UPWIND WP4: Offshore Support Structures and Foundations 13. Longer simulation requirements and how this relates to wind/wave stationarity has to be discussed. Wind and sea spectra shall be extended to cover the requirements of simulations with proper time length. It is proposed to extend the minimum simulation time of every single time series from 10 to 20 minutes to capture slow drift motions. The total required simulation time at every wind speed bin, for e.g. DLC 1.1 to reach statistical convergence should not be less than 3 hours. 14. Importance of radiation and diffraction effects as well as wind/wave misalignment is to be considered on the hydrodynamic model. For some types of structures the radiation and diffraction forces have the same order of magnitude. As a result the Morison’s equation neglecting the diffraction components is not applicable anymore. Enhanced panel methods may be applied. Hybrid formulations applying the drag forces according to the Morison’s equation but including diffraction and radiation terms as analysed by classic methods may be a convenient solution for part of the hydrodynamic analysis. 15. Mooring and riser systems shall be considered as a new component with associated failure modes in the wave turbine concept design. Failure modes shall be considered in the turbine safety system with a following safety chain release (safety system activation) if threshold values are reached: a. e.g. extreme heel angle during operation b. vertical motion c. extreme yaw of the floater d. station keeping (deviation from reference point) 16. Condition monitoring systems may have to be extended to cover mooring specific issues (e.g. pretension control for taut moorings). 17. Ice loads can be important for the mooring systems and shall be further analyzed. The presence of ice will change the dynamics of a floating wind turbine considerably, as the significant horizontal displacement experienced by the structure under normal conditions will be impeded. This effect shall be considered for the design of floating wind turbines to be installed at sites where sea ice is expected to occur. This shall include guidance on modelling ice loads for floating turbines and possible extension of the ice design load cases. 18. Inclining, stability and watertight integrity requirements shall be considered according to the IMO intact stability code IS-Code 2008, unless other national regulations are mandatory on where the device will be installed. The unmanned platform should be treated as a pontoon. Damage instability is to be considered according to Offshore Rules and approved by National Authorities. 19. For transport ships that brings personnel to the site, the SPS-Code is to be applied, if no national regulations are mandatory. The IS-and the SPS-codes refer to the SOLAS document (definitions, etc...). a. IS-Code 2008 International Code on Intact Stability, Res. MSC.267(85) adopted 2008 b. SPS-Code Code - Code of Safety for Special Purpose Ships, Res. MSC.266(84) adopted 2008 c. SOLAS Consolidated Edition, 2010 Consolidated text of the International Convention for the Safety of Life at Sea, 1974, and its Protocol of 1988: articles, annexes and certificates 20. Safety system, sensors for water in tanks, draft control, deck clearance shall be included. Alarm operator or activation of the safety system if threshold values are reached should be specified. 21. Safety factors (including non-redundant mooring systems) shall be considered for moorings. Guidance is given in ISO 19901-7. 22. For station keeping systems where no redundancy is available higher safety factors should be applied and need further consideration. 23. Assessment of seismicity may be of significant importance for design of tension leg platforms. 24. Ship impact loads due to normal operations should be revised, since more severe consequences are expected as for fixed offshore structures and due to the fact that two bodies are moving. Another issue 135
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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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