Standards for design of offshore wind farm structures and their foundations

Transcription

1 Standards for design of offshore wind farm structures and their foundations An overview Knut O. Ronold, DNV GL

2 Overview of presentation Standards for design of offshore wind farm structures - What is available of wind farm specific standards for structural and geotechnical design - Origins: IEC, classification societies, national requirements Focus thereafter will be on DNV s standards for support structures for offshore wind turbines - Bottom-fixed structures - Floating structures For either, some highlights will be addressed - e.g. geotechnical issues A few words about the future 2

3 Standards for design of offshore wind farm structures IEC (Offshore wind turbines) - Refers to IEC (Wind turbines, general) - Detailed on load side - Insufficient on capacity side DNV - DNV-OS-J101 (Offshore wind turbine structures) - DNV-OS-J103 (Floating wind turbine structures) - Refers to DNV-OS-J101 - DNV-OS-J201 (Offshore substations) GL IV Part 2 (Offshore wind turbines) ABS - ABS #176 (Bottom-fixed wind turbines) - ABS #195 (Floating wind turbines) NKK (Floating wind turbine structures) 3

4 National requirements Some countries have national requirements in place for offshore wind turbines, which have to be fulfilled in addition to requirements set forth by IEC and/or classification societies Norway: none Germany; Bundesamt für Seeschifffahrt und Hydrographie (BSH) - Standard: Design of Offshore Wind Turbine Structures (2007) - Standard for Geotechnical Site and Route Surveys (2008) Denmark; Danish Energy Agency (Energistyrelsen) - Recommendation for technical approval of offshore wind turbines (2001) - makes reference to the following document: Recommendation to comply with the requirements in the technical criteria for the Danish Approval Scheme for Wind Turbines: Foundations 4

5 IEC ongoing work Technical specification for floating wind turbines is under development; will supplement IEC

6 DNV project certification of wind farms Technical requirements are given in three standards - DNV-OS-J101 (Offshore wind turbine structures) - DNV-OS-J103 (Floating wind turbine structures) - DNV-OS-J201 (Offshore substations) Certification services are described in two service specifications - DNV-OSS-901 (Project certification of offshore wind farms) - DNV-DSS-904 (Type certification of wind turbines) 6

7 DNV-OS-J101 Offshore wind turbine structures General design code for offshore wind turbine structures, but in practice much focused on bottom-fixed structures Major contents: - Safety philosophy and design principles - Site conditions, loads and response (load and response synchronized with IEC ) - Safety factor requirements (synchronized with IEC ) - Material selection - Structural design - steel structures - concrete structures - grouted connections - Foundation design - Corrosion protection - Transport and installation - In-service inspection and maintenance - Appendices with guidance (fatigue, geotechnical design, ice loads) 7

8 Geotechnical issues in DNV-OS-J101 Requirements and guidance for soil investigations Design rules for various foundation types - Jacket piles - Monopiles - Gravity base foundations Appendices with guidance for modelling and analysis of - Piles - Gravity base foundations - Scour 8

9 DNV-OS-J103 Floating wind turbine structures Capitalizes much on DNV-OS-J101 by reference Gives provisions regarding floater-specific technical issues not covered by DNV-OS-J101, including - Site conditions, load and response - Structural design in the ULS and the FLS - Semisubmersibles (mooring lines) - TLPs (tendons) - Spars - Structural design in the ALS - Station keeping - Design of anchor foundations - Floating stability - Control system 9

10 Station keeping Two types are foreseen: - Catenary or taut systems of chain, wire or fibre ropes for compliant systems such as DDFs - Tendon systems of tethers for restrained systems such as TLPs Various issues for catenary and taut moorings: - Mooring system is vital for keeping wind turbine in position - Optimization of mooring systems may lead to non-redundant systems where a mooring failure may lead to loss of position and conflict with adjacent wind turbines - Sufficient yaw stiffness of the floater must be ensured Various issues for tendon systems: - Systems with only one tendon will be compliant in roll and pitch - Floaters with restrained modes will typically experience responses in three ranges of frequencies - High frequency, wave frequency, low frequency - More complex to analyse than other structures Requirements are given for design of mooring lines and tendons Requirement for design to a higher safety class when SK system is nonredundant 10

11 Design of anchor foundations Anchor types covered - Pile anchors - Gravity anchors - Suction anchors in clay - Free-fall anchors in clay (torpedo piles) - Fluke anchors in clay - Plate anchors in clay - Pretensioned grouted rock anchors Design approach - Design by experience and testing for free-fall anchors and grouted rock anchors - Limit state design for the other anchor types Some anchor types are only suitable for single moorings, while others can be used as shared anchors for support of many floating units in a grid - Shared anchors pose some challenges in design (complex load pattern) 11

12 Floating stability For manned structures, floating stability is a requirement both in the intact condition and in the damaged condition For unmanned structures, floating stability is required in the intact condition For unmanned structures, floating stability is not required in the damaged condition, but may be considered as an option 12

13 Floater motion controller A floater motion control system is required The purpose is to minimize excitation in the pitch mode (for TLPs in the surge mode) and thereby minimize structural responses The control system provides intervention against amplification of motion and response and can, for example, be based on - General conventional, constant-power control system for the wind turbine, e.g. collective blade pitch control system - Pumping ballast water back and forth 13

14 DNV GL merger DNV and GL merged September 12, 2013 to form DNV GL Within wind energy, two parallel standard sets will be kept for two years - Legacy DNV OS-Jxxx series - Legacy GL IV Part 2 Work to form one common set of DNV GL standards for wind energy applications for use in the future has been initiated 14

15 Thank you Thank you for your attention 15