STANDARD DEVELOPMENT FOR FLOATING WIND TURBINE STRUCTURES
Anne Lene H. Hopstad Det Norske Veritas N-1322 Høvik, NORWAY E-mail:
[email protected] Phone: +47-98626178
Knut O. Ronold Det Norske Veritas N-1322 Høvik, NORWAY E-mail:
[email protected] Phone: +47-67577311
Carl Sixtensson Det Norske Veritas N-1322 Høvik, NORWAY E-mail:
[email protected] Phone: +47-90476755
Johan Sandberg Det Norske Veritas N-1322 Høvik, NORWAY E-mail:
[email protected] Phone: +47-91877047
Summary The floating wind energy market is attracting increasing interest throughout the world and floating wind turbines is a field undergoing major development. Several companies and research institutes worldwide are engaged in research programs, pilot projects and even planning of commercial floating wind farms. Developing standards for design of floating wind turbine structures is crucial and necessary for the industry to continue to grow. In this paper, the status of the floating wind turbine industry is presented with emphasis on current efforts to establish a standard for safe design of floating wind turbine structures. DNV’s efforts to develop a new standard for floating wind turbine structures are particularly addressed and it is outlined how this new standard complements existing technical standards such as the IEC standards for wind turbines.
INTRODUCTION Until recently available standards for design of offshore wind turbine structures were limited to the following four standards for design of bottom-fixed structures, [1], [2], [3] and [12]: • IEC61400-3 • DNV-OS-J101 • GL (IV Part 2) • ABS #176 The IEC standard, IEC61400-3, was issued in 2009 close to ten years after the decision was made to develop this standard. The DNV and GL standards, DNV-OS-J101 and GL (IV Part 2), were first issued in 2004 and 2005, respectively. The ABS standard was issued in 2010. A DNV Guideline for Offshore Floating Wind Turbine Structures was developed in 2009 and issued as a technical report, [4,6]. This guideline, which is a less formal document than an official full-fledged standard, addresses some of the key issues of importance for design of floating wind turbine structures. Used in combination with DNV-OS-J101 [2] for bottom-fixed structures, the guideline represents a first step towards a new standard to fully cover floating wind turbine structures. Over the past few years, however, work has been undertaken to develop standards for floating wind turbine structures, [14]. NKK and ABS released new guidelines dealing with design of this type of structures in July 2012 and January 2013 [15,13], respectively. DNV is currently close to finalizing the work on a new standard for floating wind turbine structures for publication later in 2013. Also, in December 2012, GL released its updated Guideline for the Certification of Offshore Wind Turbines (IV Part 2) to also include design of floating structures, [16]. The new
standards for floating structures capitalize on the existing standards for bottom-fixed structures and in addition address floater specific issues such as stability, station keeping and systems for control of floater motions. A Joint Industry Project (JIP) was initiated by DNV in September 2011 with the aim to develop a full-fledged standard for structural design of floating wind turbine structures. Ten important players in the industry are taking part in this JIP. These are Statoil, Navantia, Iberdrola, Alstom, Gamesa, Sasebo Heavy Industries, Nippon Steel Corporation, STX, Principle Power and Glosten Associates, hence including developers, utility companies, wind turbine manufacturers, steel manufacturers and yards. Valuable data from demonstration projects have been placed at the JIP’s disposal by developers participating in the project. In the spring of 2013, the project report resulting from this JIP and containing the draft standard will be subject to external and internal hearing. Important industry players will receive the draft standard for their review. The standard will be updated in light of hearing comments received and, subject to DNV management approval, the standard will be issued as a public available DNV Offshore Standard. The DNV standard is expected to be ready before summer 2013. The following sections present the key issues that have been dealt with during the development of the new DNV standard for design of floating wind turbine structures.
DESIGN PRINCIPLES AND SAFETY LEVEL The safety philosophy is intended to follow the same principles as those used for fixed wind turbine structures in DNV-OS-J101, i.e. a safety class methodology is applied, in which three safety classes are considered, viz. low, normal and high safety class, depending on the severity of the failure consequences. The target safety level expressed in terms of a target failure probability depends on the required safety class. Design rules are given on a partial safety factor format and requirements for the partial safety factors are set depending on the required safety class. DNV-OS-J101 [2] is developed and calibrated for unmanned wind turbine structures to be designed to normal safety class, i.e. failure consequences are of a purely economic nature. The standard gives some recommendations for when it is advisable to design to high safety class and provides a requirement for an increased load factor when design to high safety class is aimed for. The target safety level of the existing standards, including DNV-OS-J101, is originally taken as equal to the safety level for design of wind turbines on land as given in IEC61400-1 [5], viz. normal safety class. The scope for this target safety level has been expanded several times over the years: (1) Extrapolation from smaller turbines to larger turbines (2) Extrapolation from onshore turbines to offshore turbines (3) Extrapolation from only turbine and tower to cover also offshore support structures and their foundations (4) Extrapolation from individual structures to multiple structures in large wind farms. As part of the JIP, a study has been undertaken to assess what the target reliability should be for the structures in a large floating wind farm. Under assumptions of unmanned structures and purely economic losses associated with structural failures, the study was based on cost-benefit analyses. These analyses were carried out, based on available cost data, to determine the expected net present value of a wind farm project as function of the structural reliability required for design. The target reliability was subsequently interpreted as the particular reliability that would optimize the expected net present value. The results depend on the size of the wind farm in terms of the number of floating wind turbine structures and on assumptions made regarding the redundancy of these structures. The results have been used to formulate requirements for safety class depending on the size of the wind farm and depending on the redundancy of the floating structure. The associated target reliabilities have been used as a basis for calibrating
requirements for partial safety factors as function of safety class. An anonymized example of the results from the cost-benefit analyses is shown in Figure 1.
Figure 1 Example of results of cost-benefit analysis: Expected net present value of wind farm project vs. required failure probability in structural design
SITE CONDITIONS For floating wind turbine structures, in contrast to bottom-fixed structures, low frequency response is an issue, and this implies a need for models to adequately capture environmental conditions in the low frequency range, beyond what is included in DNV-OS-J101 [2]. This need includes, but is not limited to: • adequate representation of wind in the low frequency range, where some of the commonly applied power spectral density models for wind are known not to provide a particularly good representation • gust events based on gust periods in excess of 12 seconds need to be defined; they must cover expected events and reflect frequencies encountered for dynamics of floaters.With its duration of 10.5 sec, the Extreme Operating Gust event defined in DNV-OS-J101 and based on IEC 61400-1 is inadequate for design of most floating support structures. • for floaters which can be excited by swell, the uni-modal JONSWAP wave spectrum which is commonly used for representation of wave energy will be insufficient and an alternative twopeaked spectral density model would need to be applied • water level and seismicity may be of significant importance for tension leg platforms. For water level, set-down effects are the major issue.
LOADS AND LOAD MODELLING In order to adequately capture effects associated with the natural frequencies of floating support structures when loads and responses are to be determined, simulation periods need to be increased from the standard default of 10 minutes. A minimum of 3 to 6 hours is recommended to adequately capture effects such as nonlinearities, second-order effects, and slowly varying re-
sponses. This poses some challenges, since wind cannot be considered stationary over time scales as long as 3 to 6 hours. DNV-OS-J101 [2] provides a number of proposed environmental load cases by combining various environmental conditions. For floating wind turbine structures, this set of design load cases will be supplemented with load cases accounting for: • Changes necessitated by new or revised representations of gust events • Changes necessitated by floating wind turbines, referring to the fact that the control system may be used to keep the turbine in place by preventing excitations • Pressure loads on the hull of the floating structure. Accidental loads are loads related to accidental events, abnormal operations or technical failure, i.e. events that occur more rarely than the 50- or 100-year loads usually used as characteristic loads for design in the ULS. Examples of accidental loads are loads caused by: • impacts from unintended collisions by drifting service vessels • unintended change in ballast distribution (e.g. failure of active ballast system) • change of intended pressure difference • loss of mooring line or tendon • dropped objects • fire and explosions • accidental flooding.
LOAD FACTOR REQUIREMENTS Load factor requirements for the floating structure will be given as function of safety class – low, normal and high – with requirements for normal safety class being compatible with the load factor requirements given in DNV-OS-J101. Load factor requirements for mooring lines and their anchors deviate from those applicable for design of the floating structure itself, see the below section on station keeping.
STABILITY Floating stability implies a stable equilibrium and reflects a total integrity against downflooding and capsizing. Satisfactory floating stability of floating wind turbine units is necessary in order to support the safety level required for the involved structures. Static floating stability needs to be demonstrated in the early stages of design. This is merely a matter of determining where the COG of the floating unit should be located in order to ensure that the unit is stable. Floating stability of the floating unit will be required in the following service modes for the unit: • • • •
operation, i.e. a normal working condition with the wind turbine operating temporary conditions, i.e. transient conditions such as installation and changing of draught survival condition, i.e. conditions during extreme storms transit, in particular tow-out
For permanently manned floating wind turbine units, sufficient floating stability is an absolute requirement. For unmanned floating wind turbine units, i.e. for units which are unmanned during extreme environmental conditions and during normal operation of the wind turbine, sufficient floating stability is an absolute requirement in the intact condition. This applies to the operational phase as well as any temporary phases. For unmanned units in the damaged condition, sufficient floating stability is not a requirement, but an option which may be considered.
STATION KEEPING Station keeping in the context of floating wind turbines implies catenary or taut systems of chain, wire or fibre ropes, or tendon systems for restrained systems like TLPs. The station keeping system is vital for keeping the wind turbine in position such that it can generate electricity and such that the transfer of electricity to a receiver can be maintained. Redundancy is a key issue for station keeping systems and governs the safety requirements for such systems. For station keeping systems without redundancy, it will be required that all structural components in the station keeping system are to be designed to a higher safety class than the rest of the floating structure. This requirement reflects the risk for collision with adjacent wind turbine structures, should the floater happen to disengage from its station keeping system and float about within the wind farm that it constitutes a part of, for example in the event of a mooring line failure. This requirement can be relaxed when there is no risk for collision with adjacent wind turbine structures. In this case, it will be required that the structural components in the station keeping system shall be designed to the safety class specified for the rest of the structure. Safety factor requirements for design of the station keeping system are a function of the applicable safety class. For mooring lines, requirements will be based on requirements given in the DNV position mooring rules, DNV-OS-E301 [9], with appropriate adjustment to reflect that in the wind industry characteristic environmental loads are defined by a 50-year return period. For steel tendons, requirements already given for steel design in DNV-OS-J101 will apply. For other components of the station keeping system separate requirements will apply such as those given in DNV-OS-E303 [19] for fibre ropes to be used as tendons.
STRUCTURAL DESIGN Floater specific issues related to structural design of steel will be covered in the new standard. The requirements for structural design given in DNV-OS-J101 apply to floating wind turbine structures with some exceptions, deviations and additional requirements which will be specified. Special provisions will be given for issues like Mathieu Instability (MI), Vortex Induced Motions (VIM) and P-delta effects. For concrete structures, reference will be made to DNV-OS-J101 [11] and DNV-OS-C502 [20]. Requirements for design fatigue factors (DFFs) for steel applicable for structural design of floating support structures and station keeping systems have been developed and will be included in the new standard. These requirements for DFFs depend on safety class (low, normal and high) and the level of accessibility for inspection and repair. A distinction is made between five different such levels. Special provisions will be given for semisubmersibles, TLPs and spars. Special provisions will also be given for structural design of anchors for transfer of loads to the seabed.
DESIGN OF ANCHOR FOUNDATIONS Anchors are used to transfer forces from the mooring lines and tendons of the station keeping system to the seabed soils and rocks. The new standard will give requirements for geotechnical design or rock-mechanical design, as applicable, of the following anchor types: • pile anchors • gravity anchors • suction anchors • free-fall anchors • fluke anchors • plate anchors • grouted rock anchors.
Special emphasis will be given to the complex load pattern that can be expected for those anchor types that have a potential for being used as shared anchors between several floating wind turbine units.
CABLE DESIGN For floating wind turbine structures, power cables are far more exposed than for bottom-fixed structures and hence more vulnerable, e.g. due to floater motions and the risk for dropped objects. There is an associated risk for cable failure and loss of grid connection. Cable design will be addressed to some extent in the standard. The following relevant structural cable design issues in the context of floating wind turbine structures will be covered: • Mapping of existing rules and standards relevant to dynamic power cables, including cable/floater interface • Requirements and guidance related to structural design of dynamic power cables and floater interface, mainly by reference to relevant rules and standards addressing: o environmental loads o floater motions, including coupled analyses o cable installation and corresponding loads o power cable structural design o structural design of cable/floater interface o corrosion
ANALYSIS GUIDANCE An appendix will be included in the standard, providing useful guidance for execution of coupled analysis of turbine and floating support structure including station keeping system. The following topics will be addressed in this appendix: • Comparison of different concepts in terms of importance aspects in coupled analysis • Collection of analysis experience and methodology from oil and gas semi-submersibles, deep draft structures and TLPs and bottom-fixed wind turbine structures • Guidance related to challenging issues such as cables and mooring systems • Input on how to utilize results from model experiments • Guidance on how to implement control system in the analysis • Guidance on which results that are important, load – response, section forces, structural analysis models etc.
CONTROL SYSTEMS Floater motion control for the purpose of limiting the excitation and associated responses is of utmost importance for floating wind turbine structures. This can be achieved in different ways, and one of the options is to capitalize on the capabilities of the control system of the wind turbine. The new standard will address the issue of control systems by requiring a floater motion controller to be in place. OTHER ISSUES Other issues such as requirements for mechanical systems, corrosion protection, transport and installation, inspection and maintenance will be addressed, but mainly by reference to other standards, first of all DNV-OS-J101.
DNV SERVICES – NOW AND IN THE NEAR FUTURE In June 2012, DNV issued a new Offshore Service Specification (OSS), DNV-OSS-901 Project Certification of Offshore Wind Farms [17]. This document outlines the project certification services that DNV offers for offshore wind farm projects. Project certification for offshore wind farms constitutes a robust means to provide, through independent verification, evidence to stakeholders (financiers, partners, utility companies, insurance companies and the public) that a set of requirements laid down in standards are met during design and construction and maintained during operation, of an offshore wind farm. The DNV certification service includes the verification and documentation of each of six project phases, viz. Phase I : Phase II : Phase III: Phase IV: Phase V : Phase VI:
Verification of Design Basis Verification of Design Manufacturing Survey Installation Survey Commissioning Survey In-Service
Each phase is completed with the issue of a statement of compliance and after completion of Phase I through V the DNV Project Certificate will be issued. The Project Certificate is valid at the time of commissioning of the offshore wind farm. For the In-service Phase, the Project Certificate can be validated based on the results of annual surveys and inspections. DNVs current project certification service is based on DNV-OS-J101. DNV is currently in the process of extending the project certification service to also cover floating wind farms. Floater specific issues like station keeping and stability will need to be included in the project certification scope when the service is expanded to cover floating wind farms. It may be foreseen that in order to assess the novelty in floating wind turbine concepts, some model testing and validation of software may be required before a project certificate can be issued for a floating wind farm. It is a prerequisite for project certification of wind farms that the wind turbines are type certified. DNV’s type certification service for wind turbines are described in DNV-DSS-904, [18]. It is foreseen that also the floating support structure itself can be subject to type certification in the future. Eventually, this will have to be type certification to meet a specific environmental class. The station keeping system will still have to be qualified to each specific site.
CONCLUSIONS Up until recently, all available standards for design of offshore wind turbine structures have been restricted to deal with design of bottom-fixed structures only. The lack of standards for design of floating wind turbine structures has prompted work for development of such standards to be initiated. Within the past year, two guidelines for design of floating wind turbine structures have seen daylight, and a third such document, a new DNV Offshore Standard for floating wind turbines, is currently being finalized. This paper deals briefly with the historical background for these developments and subsequently provides an overview of the contents of the upcoming new DNV standard, in particular highlighting the floater-specific issues that this standard will address.
REFERENCES 1. IEC61400-3 “Wind Turbines – Part 3: Design Requirements for Offshore Wind turbines”, International Electrotechnical Commission, Ed. 1, 2009. 2. DNV-OS-J101 “Design of Offshore Wind Turbine Structures”, Det Norske Veritas, Høvik, Norway, 2013. 3. GL, “Rules and Guidelines, IV Industrial Services, Part 2 Guideline for the Certification of Offshore Wind Turbines”, Germanischer Lloyd, Hamburg, Germany, 2005. 4. Ronold KO, Hansen V, Godvik M, Landet E, Jørgensen ER, Hopstad ALH. “Guideline for Offshore Floating Wind Turbine Structures”, Proceedings, ASME 29th International Conference on Ocean, Offshore and Arctic Engineering, OMAE2010, Shanghai, China, 2010. 5. IEC61400-1 “Wind Turbines – Part 1: Design Requirements”, International Electrotechnical Commission, Ed. 3, 2005. 6. DNV Guideline for Offshore Floating Wind Turbine Structures, Technical report, Det Norske Veritas, Høvik, Norway, December 2009. 7. DNV-RP-C205 “Environmental Conditions and Environmental Loads”, Det Norske Veritas, Høvik, Norway, 2010. 8. DNV-OS-C301 “Stability and Watertight Integrity”, Det Norske Veritas, Høvik, Norway, 2010. 9. DNV-OS-E301 “Position Mooring”, Det Norske Veritas, Høvik, Norway, 2010. 10. DNV-OS-C105 “Structural Design of TLPs (LRFD Method)”, Det Norske Veritas, Høvik, Norway, 2009. 11. DNV-OS-C101 “Design of Offshore Steel Structures. General (LRFD Method)”, Det Norske Veritas, Høvik, Norway, 2010. 12. ABS #176 Guide for Building and Classing Bottom-founded Offshore Wind Turbine Installations, January 2013 13. ABS #195 Guide for Building and Classing Floating Offshore Wind Turbine Installations, January 2013 14. Ronold KO, Landet E, Jørgensen ER, Sandberg J. “DESIGN STANDARDS FOR FLOATING WIND TURBINE STRUCTURES, Proceedings, European Wind Energy Association (EWEA) 2011, Brussel, Belgium. 15. Nippon Kaiji Kyokai, ClassNK Guidelines for Offshore Floating Wind Turbine Structures, July 2012. 16. GL, “Rules and Guidelines, IV Industrial Services, Part 2 Guideline for the Certification of Offshore Wind Turbines”, Germanischer Lloyd, Hamburg, Germany, 2012. 17. DNV-OSS-901 “Project Certification of Offshore Wind Farms”, Det Norske Veritas, Høvik, Norway, June 2012. 18. DNV-DSS-904 “Type Certification of Wind Turbines”, Det Norske Veritas, Høvik, Norway, January 2012. 19. DNV-OS-E303 “Offshore Fibre Ropes”, Det Norske Veritas, Høvik, Norway, 2013. 20. DNV-OS-E303 “Offshore Concrete Structures”, Det Norske Veritas, Høvik, Norway, 2012.
