NB/T 11378-2023Guide for foundation and mooring system design of floating offshore wind turbines (English PDF)

Foreword

This document was issued on 28 December 2023 by the National Energy Administration of the PRC and takes effect on 28 June 2024.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 27.180, Chinese classification P 61.

This Guide has been prepared in accordance with the requirements of the Notice of the General Department of the National Energy Administration on Issuing the 2020 Plan for the Development and Revision of Industry Standards in the Energy Field and the Plan for the Translation of Foreign-language Versions (Guo Neng Zong Tong Ke Ji [2020] No. 106). The drafting group carried out extensive investigation and research, carefully summarized practical experience, referred to relevant domestic and international standards, and prepared this Guide on the basis of a wide solicitation of comments.

The main technical contents of this Guide are: general provisions; terms and symbols; basic requirements; materials; loads and load combinations; stability analysis; motion response analysis; structural design of the foundation; mooring system design; corrosion and dirty prevention design; accessory equipment and systems of the foundation; construction organization design; and monitoring design.

This Guide is under the administration of the National Energy Administration. China Renewable Energy Engineering Institute (Hydropower and Water Resources Planning and Design General Institute) proposed this Guide and is responsible for its routine administration. The Sub-Technical Committee on Wind Farm Planning and Design of the Technical Committee for Standardization of the Wind Power Industry in the Energy Sector (NEA/TC 1/SC 1) is responsible for the interpretation of its specific technical contents.

Comments or suggestions arising in the course of implementation should be sent to China Renewable Energy Engineering Institute (Hydropower and Water Resources Planning and Design General Institute), at No. Jia 57 and No. Yi 57, Andingmenwai Street, Dongcheng District, Beijing, postcode 100011.

Chief development organizations of this Guide: Shanghai Investigation, Design and Research Institute Co., Ltd.; China Three Gorges Corporation; China Three Gorges Renewables (Group) Co., Ltd.

Participating development organizations of this Guide: Hydropower and Water Resources Planning and Design General Institute; Fujian Yongfu Power Engineering Co., Ltd.; Shanghai Green Environmental Protection Energy Co., Ltd.

Chief drafters of this Guide: Lin Yifeng, Fan Ke, Nie Yan, Chen Pengfei, Yuan Yibo, Liu Yunzhi, Lu Zhongmin, Wu Xinyi, Geng Dazhou, Liu Aihua, Zhang Kaihua, Liu Wei, Zhou Lili, Zhang Dailong, Tian Hongji, Ke Keteng, Xu Bing, Yang Xin, Ren Yajun, Jiang Juan, Xiao Jiandong, Wu Caihong, Wang Yibing, Chen Junyou, Lin Yu, Song Qiming, Huang Chen, Xu Jianwen, Xiong Wenliang, Zhang Zhiwei, Cheng Haifeng, Lin Lin, Huang Jun, Peng Qian, Jiang Shuzhou, Fan Rongshan, Zhang Xiao, Pan Zhixuan, Miao Zhixin, Huang Yi and Zhang Yang.

Chief reviewers of this Guide: Xie Hongwen, Hao Jungang, Zhao Shengxiao, Guo Zhenni, Zhong Yao, Zhang Xinyu, Tian Weihui, Li Hongyou, Yang Mindong, Wang Chaohui, Liang Huarong, Ding Ling, Wang Jiaxin, Zeng Fanquan, Liu Mingyue, Zhou Yeming, Fan Tianhui, Liu Wei, Xu Yaobing, Feng Xuepei, Tian Jingkui, Liu Congzhu, Liu Lei, Qi Jun, Qi Zhicheng, Liu Donghua, Zhu Xuemin, Li Ning, Yang Jing, Zhang Yunjie, Jiang Song, Xi Jing and Li Fagui.

The Guide was released by Announcement No. 8 of 2023 of the National Energy Administration, dated 28 December 2023, by which 281 energy industry standards were approved, this Guide being listed as item 43 of the catalogue of industry standards, approved on 2023-12-28 and put into effect on 2024-06-28.

Publication data: the chief editing department is the Hydropower and Water Resources Planning and Design General Institute; the approving department is the National Energy Administration; the date of implementation is 28 June 2024. Published by China Electric Power Press, Beijing, 2024 (book number 155198.6240).

1 Scope

NB/T 11378-2023 is the Chinese design guide for the floating foundations and mooring systems of offshore wind turbines. A floating turbine is a different engineering problem from a fixed one: the platform moves in six degrees of freedom, the turbine control interacts with that motion, and the whole assembly is held on station by a mooring system whose failure mode is drift rather than collapse. The guide covers the main platform concepts - semi-submersible, spar, tension leg and barge - and sets the design basis for each: the environmental conditions of wind, wave, current, tide, sea ice and typhoon; the design load cases combining turbine operating states with sea states and with fault and parked conditions; and the coupled aero-hydro-servo-elastic analysis by which the response is determined. It then treats the platform structure - global strength, fatigue, local scantlings, watertight subdivision, stability both intact and damaged, and the ballast system - and the interface with the tower. The mooring system has its own chapters: line configuration in catenary, taut and tension leg arrangements, line components and their strength and fatigue, anchors of drag, suction and pile type with their holding capacity, and the analysis of line tension and platform offset including the one-line-failed condition. Corrosion protection, the dynamic power cable interface, marine operations for tow-out and hook-up, and the inspection and monitoring expected in service close the guide.

1.0.1 This Guide is formulated in order to standardize the design of foundations and mooring systems of floating offshore wind turbines, so that such design is safe and applicable, technically reliable and environmentally friendly.

1.0.2 This Guide is applicable to the design of foundations and mooring systems of floating offshore wind turbines.

1.0.3 In addition to complying with this Guide, the design of foundations and mooring systems of floating offshore wind turbines shall also comply with the provisions of the relevant current national standards.

2 Terms and Symbols

2.1 Terms. The following terms and definitions apply in this Guide; the English equivalents printed alongside each Chinese term are those given in the standard itself.

2.1.1 Floating offshore wind turbine foundation: a foundation that floats on the sea surface and is anchored to the seabed by means of a mooring system, used to support a floating wind turbine generator unit.

2.1.2 Spar foundation: a floating foundation having a single vertical cylindrical column body.

2.1.3 Semi-submersible foundation: a column-stabilized foundation made up of columns together with submerged buoyancy bodies or braces connected to those columns.

2.1.4 Barge foundation: a floating foundation whose structural form is similar to that of a ship and which has no self-propulsion capability.

2.1.5 Mooring system: a passive anchoring system used to restrain the motion of a floating foundation, comprising mooring lines or tendons, the anchor foundations and the associated equipment.

2.1.6 Anchor foundation: the foundation that fixes the mooring lines or tendons to the seabed.

2.1.7 Tension leg platform foundation: a floating foundation that is connected to the anchor foundations by means of tendons.

2.1.8 Ballast system: a system that adjusts the draught and the floating attitude of a floating offshore wind turbine foundation by regulating the quantity of water in the ballast tanks; it consists of ballast pumps, ballast water tanks, piping, associated valves and other equipment.

2.1.9 Righting moment: the moment that restores a floating foundation to a level floating attitude, generated because the points of application of its gravity force and of its buoyancy force are not on the same vertical line when the foundation is inclined by the action of external forces.

2.1.10 Wind heeling moment: the moment induced by the wind load that causes the floating foundation to incline.

2.1.11 Watertight: the ability of a structure to prevent the passage of water from any direction when the pressure reaches the design water pressure of the surrounding structure.

2.1.12 Weathertight: the ability to prevent water from penetrating into the structure under any sea condition.

2.1.13 Stability: the ability of a floating foundation, having deviated from its equilibrium position and inclined under the action of external forces, to return by itself to its original equilibrium position once the external forces cease.

2.1.14 Intact stability: the ability of a floating foundation in the intact condition to resist an applied wind heeling moment by means of its own righting moment after inclination.

2.1.15 Damage stability: the ability of a floating foundation after damage, relying on its own righting moment after inclination, still to remain free from further flooding under the action of wind force.

2.1.16 Permeability: the proportion of the volume of a space, or of a part of a space, that can be occupied by liquid.

2.1.17 Wave frequency motions: motions induced by first-order wave forces.

2.1.18 Low frequency motions: motions induced by the low-frequency components of waves and wind.

2.1.19 Mean offset: the offset induced by the steady components of the wind, current and wave forces.

2.1.20 High frequency response: the response induced by high-frequency wave excitation.

2.1.21 Air gap: the clearance between the water surface and the lowest point of those parts of the structure which, by design, are not to be subjected to wave impact.

2.1.22 Temporary anchoring equipment: the general designation of the components and mechanical equipment used for the temporary anchoring or mooring of a floating foundation in an anchorage, at a quay, inside a harbour or in sheltered waters.

2.2 Symbols. The following abbreviations and symbols are used in this Guide.

COD: codirectional, that is, acting in the same direction.

DLC: design load case.

ECD: extreme coherent gust with direction change.

ECM: extreme current model.

EDC: extreme direction change of wind.

EOG: extreme operating gust.

ESS: extreme sea state.

ETM: extreme turbulence model.

EWLR: extreme water level range.

EWM: extreme wind speed model.

EWS: extreme wind shear.

Hs: significant (characteristic) wave height.

MIS: misaligned, that is, acting in directions that are offset from one another.

MSL: mean sea level.

MUL: multi-directional.

NCM: normal current model.

NSS: normal sea state.

NTM: normal turbulence model.

NWLR: normal water level range.

NWP: normal wind profile model.

SSS: severe sea state.

The list of symbols continues on the following pages of the printed standard with further abbreviations and with the algebraic symbols used in the load, stability, motion response, structural and mooring formulae.

3 Basic Requirements

Chapter 3 sets out the basic requirements governing the design of the foundation and mooring system and is divided into 3.1 Design Requirements, 3.2 Design Safety Standards and 3.3 Design Principles, occupying pages 6 to 7 of the standard.

4 Materials

Chapter 4 covers the materials used for the floating foundation and the mooring system and is divided into 4.1 Structural Steel, 4.2 Reinforcement Steel, 4.3 Concrete and 4.4 Mooring Materials, occupying pages 8 to 10 of the standard.

5 Loads and Load Combinations

Chapter 5 deals with the loads acting on a floating offshore wind turbine foundation and with the way those loads are combined; it is divided into 5.1 General Requirements, 5.2 Load Categorization, 5.3 Design Loads and 5.4 Load Combinations and Load Factors, occupying pages 11 to 19.

The design load cases of this chapter are expressed by means of the abbreviations defined in 2.2, so that each case is identified by its wind model (NTM, ETM, EWM, EOG, ECD, EDC, EWS), its sea state (NSS, SSS, ESS), its current model (NCM, ECM), its water level range (NWLR, EWLR) and the directional relationship between the environmental actions (COD, MIS, MUL).

6 Stability Analysis

Chapter 6 covers the stability of the floating foundation and is divided into 6.1 General Requirements, 6.2 Intact Stability and 6.3 Damaged Stability, occupying pages 20 to 29.

The analysis of this chapter rests on the concepts defined in 2.1: the righting moment and the wind heeling moment, intact stability as the ability of the undamaged foundation to resist the applied wind heeling moment, and damage stability as the ability of the damaged foundation to remain free from further flooding under wind action, with the permeability of the flooded spaces taken into account.

7 Motion Response Analysis

Chapter 7 covers the analysis of the motion response of the floating foundation and is divided into 7.1 General Requirements, 7.2 Analysis Methodologies and 7.3 Motion Response, occupying pages 30 to 32. The motions considered are those defined in 2.1, namely wave frequency motions, low frequency motions, the mean offset and the high frequency response.

8 Structural Design of Foundation

Chapter 8 covers the structural design of the floating foundation and is divided into 8.1 General Requirements and 8.2 Steel Structures Design, occupying pages 33 to 39.

9 Mooring System Design

Chapter 9 covers the design of the mooring system and is divided into 9.1 General Requirements, 9.2 Mooring System Design and Analysis, 9.3 Selection of Mooring Equipments and 9.4 Anchor Foundation Design, occupying pages 40 to 44.

The mooring system addressed by this chapter is the passive anchoring system defined in 2.1.5, comprising the mooring lines or tendons, the anchor foundations that fix those lines or tendons to the seabed, and the associated equipment; for a tension leg platform foundation the connection to the anchor foundations is made by tendons.

10 Corrosion and Dirty Prevention Design

Chapter 10 covers the protection of the foundation and mooring system against corrosion and marine fouling and is divided into 10.1 Corrosion Prevention Design and 10.2 Dirty Prevention Design, occupying pages 45 to 46.

11 Accessory Equipments and System

Chapter 11 covers the accessory equipment and systems of the foundation and is divided into 11.1 General Requirements, 11.2 Outfitting Equipments, 11.3 Ventilation system, 11.4 Ballast System, 11.5 Electrical System and 11.6 Life-saving and Fire Fighting Equipments, occupying pages 47 to 50.

The ballast system treated in 11.4 is the system defined in 2.1.8, which adjusts the draught and the floating attitude of the foundation by regulating the quantity of water in the ballast tanks and which consists of ballast pumps, ballast water tanks, piping and the associated valves; the outfitting equipment of 11.2 includes the temporary anchoring equipment defined in 2.1.22.

12 Construction Organization Design

Chapter 12 covers the construction organization design and is divided into 12.1 General Requirements, 12.2 Construction Conditions, 12.3 Construction of Foundation and Mooring Equipments, 12.4 Foundation Launching and Transportation, 12.5 Offshore Installation, 12.6 General Construction Layout and 12.7 Master Construction Schedule, occupying pages 51 to 55.

This chapter therefore follows the whole sequence of realization of a floating unit, from the fabrication of the foundation and of the mooring equipment, through launching and towing to the site, to installation offshore, and it fixes the general layout of the construction works and the master construction schedule.

13 Monitoring Design

Chapter 13 covers the monitoring design and is divided into 13.1 General Requirements, 13.2 Monitoring Principles and 13.3 Monitoring Scheme, occupying pages 56 to 58. The standard closes with the Explanation of Wording in this Guide on page 59, the List of Normative Standards on page 60 and, as an addition, the Explanation of Provisions beginning on page 62.