Section 5 Site-specific support structure design evaluation

5.1 Module requirements

5.1.1 The purpose of this evaluation is to confirm that the site-specific support structure design complies with the design basis. The support structure includes the tower, sub-structure, foundations and mooring systems (floating structures only).

5.1.2 This module cannot be completed successfully without prior acceptance of the design basis evaluation and ILA (see Ch 2, 2 Design basis evaluation and Ch 2, 3 Integrated loads analysis evaluation).

5.2 Guidance on input requirements

5.2.1 The applicant is expected to provide documentation fully detailing the design of all parts of the site-specific support structure including drawings, calculations and manufacturing specifications. The documentation submission for the below site-specific support structure items should include amongst others:

Tower:

tower design evaluation in relation to ILA results, including design calculation reports (these are to include secondary structures critical to personnel safety, e.g. tower ladders);
tower stiffness and damping comparison with ILA;
impact of tower internals on structural integrity of the tower primary structure;
manufacturing, transportation and installation, and maintenance plans in relation to tower in-place structural integrity;
material selection and corrosion protection system, based on design basis (see Ch 2, 2 Design basis evaluation); and
design drawings.

Sub-structure:

sub-structure design evaluation in relation to ILA results, including design calculation reports (these are to include secondary structures critical to personnel safety, e.g. boat landings);
sub-structure stiffness and damping comparison with ILA;
impact of secondary structures (e.g. boat landings) on structural integrity of the primary structure;
manufacturing, transportation and installation, and maintenance plans in relation to sub-structure in-place structural integrity; and
material selection and corrosion protection system, based on design basis (see Ch 2, 2 Design basis evaluation); and
design drawings.

Foundation:

foundation design evaluation in relation to ILA results (for floating WT, this should include mooring lines and other project-specific components as applicable);
foundation stiffness and damping comparison with ILA;
manufacturing, transportation and installation, and maintenance plans in relation to foundation in-place structural integrity; and
material selection and corrosion protection system, based on design basis (see Ch 2, 2 Design basis evaluation); and
design drawings.

The above documentation should incorporate the guidance provided in the following paragraphs as applicable to the type of support structure.

5.2.2 Fatigue is likely to be the dominant degradation mechanism for the support structure of an offshore WT. Fatigue damage should be minimised by:

ensuring that the structural natural frequencies do not coincide with blade-passing or WT rotational frequencies or with frequencies containing significant wave energy except where a system exists to sufficiently damp or otherwise mitigate the oscillations;
avoidance of dynamic response to wind, including vortex-induced vibration (VIV) except where a system exists to sufficiently damp or otherwise mitigate the oscillations;
designing structural details to minimise stress concentration factors;
controlling manufacturing precision and quality to achieve the tolerances specified in the selected code of practice for construction; and
prohibiting temporary or permanent unauthorised attachments, modifications or repairs.

The process of fatigue is aggravated by corrosion and therefore appropriate corrosion protection should be applied. Corrosion protection in the splash zone is of particular importance because cathodic protection will be less effective and in-service replacement of coatings will be difficult.

5.2.3 The air gap should be calculated based on deck elevation requirements within IEC 61400-3-1 Wind energy generation systems – Part 3-1: Design requirements for fixed offshore wind turbines. The air gap is defined as the vertical gap between a highest wave crest elevation including the effects of highest astronomical tide, positive storm surge, motion of the support structure and the lower of either the tip of the WT blade or lowest point (beam, equipment or fixing) of the lowest deck of the support structure. The air gap should be greater than or equal to 0,2H_s50, where H_s50 is the 50-year return value of significant wave height, with a minimum air gap value of 1 m for fixed structures. In addition, for floating platforms, the strength with respect to loads resulting from wave impacts, including slamming, sloshing and green water should be confirmed in accordance with ISO 19904-1: Petroleum and natural gas industries – Floating offshore structures – Part 1: Monohulls, semi-submersibles and spars.

5.2.4 If the air gap for the abnormal condition is assessed based on a favourable parked blade position, then the air gap must also be assessed for the operating wave crest elevation. The possibility of contact between a wind-turbine blade and a vessel should be considered; there should either be a means of remotely stopping the turbine blades or the air gap should be set such that there can be no clash in the limiting sea-state for service vessels. For other vessels, a risk assessment should be undertaken to determine policy.

5.2.5 The design of a fixed steel WT support structure should generally meet the requirements of IEC 61400-3-1 Wind energy generation systems – Part 3-1: Design requirements for fixed offshore wind turbines, supplemented by the applicable parts of other recognised industry standards:

ISO 19902 Petroleum and natural gas industries – Fixed steel offshore structures should be followed for partial factor design and API RP 2A-WSD Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms – Working Stress Design for allowable stress design;
the post-impact condition can be regarded as an abnormal load-case. Environmental conditions with a one-year return period should be considered.

5.2.6 Design of foundations for fixed structures should consider the effects of accumulated deformation in the soil. This may be especially significant for monopile and gravity-base type structures that are subject to dynamic cyclic loading. Special attention should be given to the effect of foundation stiffness upon the behaviour of the support structure and WT/RNA.

5.2.7 To allow for corrosion, design of fixed structures should generally meet the requirements of Annex G, IEC 61400-3-1 Wind energy generation systems – Part 3-1: Design requirements for fixed offshore wind turbines, and take into account the following:

for design strength calculations, the design thickness should be reduced by the full corrosion margin;
for accidental vessel impact and post-impact conditions, the design thickness should be reduced by up to the full corrosion margin; and
for design fatigue calculations, the design thickness should be reduced by 50 per cent of the corrosion margin.

5.2.8 The design of floating steel WT support structures should generally meet the requirements of IEC TS 61400-3-2 Wind energy generation systems – Part 3-2: Design requirements for floating offshore wind turbines, and take into account the following:

using a WSD approach, Pt 4, Ch 3, 4 Structural design loads of LR’s Rules and Regulations for the Classification of Ships,July 2019 or equivalent should be applied; and
for IEC 61400-3-1 Wind energy generation systems – Part 3-1: Design requirements for fixed offshore wind turbines, the safety factors in Pt 4, Ch 5, 2.1 General 2.1.1 of LR’s Rules and Regulations for the Classification of Ships, July 2019 or equivalent should be applied.

5.2.9 To allow for corrosion, design of floating steel structures should generally meet the requirements of Annex G, IEC TS 61400-3-2 Wind energy generation systems – Part 3-2: Design requirements for floating offshore wind turbines, and take into account the following:

for design strength calculations, the design thickness should be reduced by the full corrosion margin for local strength assessment (e.g. panel buckling) and by 50 per cent of the corrosion margin for global strength assessment;
for design fatigue calculations, the design thickness should be reduced by 50 per cent of the corrosion margin for local stress assessment and 25 per cent of the corrosion margin for global stress assessment; and
the corrosion rates in Table 2.5.1 Recommended corrosion rates for one side of structural members, are provided as guidance. For a full range of corrosion rates refer to Pt 4, Ch 3, 7.4 Scantling compliance 7.4.6 of LR’s Rules and Regulationsfor the Classification of Ships, July 2019.

Table 2.5.1 Recommended corrosion rates for one side of structural members

Environment	Position	Corrosion rate on each exposed surface (mm/year)
ballast water	within 3 m below the top of the tank	0,15
ballast water	elsewhere	0,1 (see Note 1)
exposed to atmosphere	weather deck plating	0,1
exposed to atmosphere	elsewhere	0,075
exposed to seawater	all	0,075
other tanks and spaces	all	0,05
Note 1 - 50 per cent of the corrosion rate may be used for permanently filled ballast tanks.

5.2.10 The design of floating concrete WT wind turbine hull structures should generally meet the requirements of IEC TS 61400-3-2 Wind energy generation systems – Part 3-2: Design requirements for floating offshore wind turbines; and follow Pt 9 Concrete Unit Structures of LR’s Rules and Regulations for the Classification of Ships, July 2022 or equivalent.

5.2.11 Mooring system loading conditions for floating WTs should generally meet the requirements of IEC TS 61400-3-2 Wind energy generation systems – Part 3-2: Design requirements for floating offshore wind turbines, and take into account the following:

if using a WSD approach, safety factors defined in Pt 3, Ch 10, 6 Anchor lines of LR’s Rules and Regulations for the Classification of Ships, July 2022 or equivalent should be used, based on equivalence as shown in Table 2.5.2 Applicable factors of safety for mooring; and
fatigue life of the mooring components should be calculated in accordance with API RP 2SK Design and Analysis of Station-keeping Systems for Floating Structures following the safety factors defined in LR’s Rules and Regulations for the Classification of Ships, July 2022; if the design is unable to tolerate the loss of any single mooring line, line tensions and stresses should be increased by a factor of 1,5 before calculating the fatigue damage.

Table 2.5.2 Applicable factors of safety for mooring

IEC 61400-3 Design situation	Load combination per LR Rules	Factor of safety (break strength)
Normal (N)	Extreme storm or maximum environment with floating unit attached (intact)	1,67
Abnormal (A)	Extreme storm or maximum environment with floating unit attached (one line damaged)	1,25

5.2.12 Standards for subdivision, stability and freeboard prescribed by the National Administration for the planned location of a floating WT should be followed; if no such standards exist, the relevant parts of the IMO Code for the Construction and Equipment of Mobile Offshore Drilling Units (MODU Code), LR’s Rules and Regulations for the Classification of Ships, July 2022 or other suitable international conventions should be applied as applicable for the type of WT unit under consideration. It may be necessary to conduct a practical test to support the buoyancy and stability of a complex floating structure or one of irregular shape.

5.3 Evaluation methodology

5.3.1 LR’s evaluation of the support structure design will include:

confirmation that the support structure design has been carried out in accordance with the design basis;
review of primary structure design reports, including the ULS, SLS, FLS and ALS conditions;
review of the impact of secondary structures (e.g. boat landings, tower ladders) on the primary structure integrity;
review of secondary structure critical to personnel safety (e.g. boat landings, tower ladders) design reports, including the ULS, SLS, FLS and ALS conditions;
confirmation that the structural design reports incorporate the results of the ILA, including the assumptions made in the analysis (e.g. support structure stiffness and damping calculations);
assessment of the stability of a floating unit based on the design basis;
assessment of the geotechnical design documentation based on the design basis;
review of the plans for construction, transportation, installation and maintenance; and
review of the structural corrosion protection system based on the design basis.

5.3.2 Independent analysis is not required as part of LR’s evaluation. However, simplified parallel calculations (spot checks) may be undertaken for critical areas identified in the review of the design reports, and depending on the experience of the designer and the complexity of the design.

5.3.3 Special attention will be paid to the transition piece and the grouted and/or bolted connections at the interfaces between the WT tower and the substructure, or between the substructure and foundations (as applicable to the site-specific design). LR may undertake finite element analysis of critical areas identified in the review of the design reports, and depending on the experience of the designer and the complexity of the design and the quality of the submitted calculations.

5.3.4 The evaluation of the support structure will consider all project phases including loadout, sea-fastening, transportation and installation, as applicable. Where the client or applicant has appointed a Marine Warranty Surveyor (MWS) to assess the transportation and installation phases of the assets, LR may review the MWS’s reports and take credit for their reviews as part of the evaluation, provided the MWS reviews are considered to directly contribute to the certification to IEC 61400-22 Wind turbines – Part 22: Conformity testing and certification and IECRE OD-502 Project Certification Scheme.

5.3.5 LR’s review will be consider the impact of the load transfer from the secondary structures to the primary structure.