NB/T 11374-2023Guide for suction pile jacket foundation design of offshore wind power project (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 Preparation and Revision of Sector Standards in the Energy Field and for the Translation and Publication of their English Versions (Guo Neng Zong Tong Ke Ji [2020] No. 106).

The drafting group carried out extensive investigation and research, made a careful summary of the practical experience gained in recent years in the planning and design of offshore wind power projects, and prepared this guide on the basis of a wide solicitation of comments.

This guide is divided into 8 chapters and is accompanied by an appendix and an explanation of provisions.

The main technical contents of this guide are: general provisions; terms; basic requirements; engineering geology; foundation design; penetration installation design; in-situ bearing capacity analysis; and transportation and installation scheme.

This guide is administered by the National Energy Administration.

China Renewable Energy Engineering Institute (the General Institute of Hydropower and Water Resources Planning and Design) put forward this guide and is responsible for its routine management.

The Sub-committee on Wind Farm Planning and Design of the Wind Power Standardization Technical Committee of the energy sector (NEA/TC1/SC1) is responsible for the interpretation of the specific technical contents of this guide.

Comments and suggestions arising in the course of implementation should be sent to China Renewable Energy Engineering Institute, at Jia No. 57 and Yi No. 57, Andingmenwai Street, Dongcheng District, Beijing, postcode 100011.

Chief drafting organization: Fujian Yongfu Power Engineering Co., Ltd.

Participating drafting organizations: PowerChina Huadong Engineering Corporation Limited; Shanghai Investigation, Design and Research Institute Co., Ltd.; CGN New Energy Holdings Co., Ltd.

Main drafters: twenty-seven specialists are named in the foreword, beginning with Liu Wei, Song Qiming, Fan Xialing and You Xianhui, and continuing with Li Wei, Xiao Jiandong, Wang Bin, Lin Yifeng, Chen Zhibing, Wu Yun, He Ben, Jiang Juan, Ren Yanzhong, Cai Tianqing, Wang Kuanjun, Wang Liji, Chen Jisheng, Li Xianghui, Zhu Shengdong, Lin Peng, Liu Bingwen, Zeng Bao, Li Aixin, Zhong Linyue, Jin Fan, Huang Haihan and Li Jialong.

Main reviewers: thirty-six specialists are named, beginning with Xie Hongwen, Zhao Shengxiao, Hao Jungang and Yue Lei, and continuing with Guo Zhenni, Liu Wei, Tian Weihui, Wang Dandi, Zhong Yao, Tang Dongsheng, He Jiangfei, Luo Lunbo, Qiu Xu, Li Sen, Li Weichao, Zhang Yukun, Xu Pu, Zhang Puyang, Wu Hehe, Xu Yaobing, Wang Zhaohui, Feng Xuepei, Tian Jingkui, Liu Congzhu, Li Hongyou, Liu Lei, Qi Jun, Qi Zhicheng, Liu Donghua, Zhu Xuemin, Li Ning, Yang Jing, Zhang Yunjie, Jiang Song, Xi Jing and Li Fagui.

Publication and issuance data

Designation of the standard as printed on the cover: NB/T 11374-2023.

Classification data printed on the cover: ICS 27.180 and CCS P 61; the cover also carries the filing classification letter P and the NB emblem of the energy sector standards of the People's Republic of China.

Chinese title on the cover: offshore wind farm engineering, design guide for suction pile jacket foundations; official English title printed on the cover: Guide for suction pile jacket foundation design of offshore wind power project.

Date of issue: 28 December 2023. Date of implementation: 28 June 2024. No superseded standard is indicated on the cover or in the sector standard catalogue.

Issuing authority: National Energy Administration of the People's Republic of China.

Title page data: chief editing organization, China Renewable Energy Engineering Institute; approving department, National Energy Administration; date of entry into force, 28 June 2024.

Publisher: China Electric Power Press, Beijing, 2024.

The standard was released by Announcement No. 8 of 2023 of the National Energy Administration, dated 28 December 2023, which approved 281 energy sector standards headed by the Basic Technical Specification for the Intelligent Construction of New Energy Stations (Annex 1), 33 English versions of energy sector standards headed by the Specification for Preparation of Special Geological Report on Impoundment-Affected Area for Hydropower Projects (Annex 2), and 3 amendment notices to energy sector standards (Annex 3).

In the sector standard catalogue attached to that announcement, this guide appears as serial number 39, with the standard number NB/T 11374-2023, approval date 28 December 2023 and implementation date 28 June 2024; the columns for the superseded standard and for the adopted international standard are both blank.

Structure of the document

The structure set out below is taken word for word from the official English contents printed in the standard itself, on pages VII and VIII.

Chapter 1, General provisions, begins on page 1.

Chapter 2, Terms, begins on page 2.

Chapter 3, Basic requirements, begins on page 3 and contains 3.1 Design principles, page 3, and 3.2 Design requirements, page 3.

Chapter 4, Engineering geology, begins on page 5 and contains 4.1 Geotechnical investigation, page 5, and 4.2 Geotechnical parameters for design and installation, page 8.

Chapter 5, Foundation design, begins on page 10 and contains 5.1 Overall structure design, page 10; 5.2 Jacket structural design, page 10; 5.3 Side wall of suction pile structural design, page 11; and 5.4 Transitional section and bucket lid of suction pile structural design, page 25.

Chapter 6, Penetration installation design, begins on page 27 and contains 6.1 General requirements, page 27; 6.2 Penetration resistance calculation, page 28; and 6.3 Allowable differential pressure, page 30.

Chapter 7, In-situ bearing capacity analysis, begins on page 32 and contains 7.1 General requirements, page 32; 7.2 Geotechnical engineering analysis, page 32; 7.3 Bearing capacity calculation, page 33; and 7.4 Deformation calculation, page 33.

Chapter 8, Transportation and installation scheme, begins on page 34 and contains 8.1 Transportation scheme, page 34, and 8.2 Installation scheme, page 34.

Appendix A, Design tolerances for the cylindrical shell members, begins on page 36.

Explanation of wording in this guide begins on page 42.

List of quoted standards begins on page 43.

Addition: Explanation of provisions begins on page 45.

The body of the guide therefore runs to 35 pages, the appendix occupies pages 36 to 41, and the explanation of provisions, which is printed as an addition, starts at page 45.

Section 5.3, on the structural design of the side wall of the suction pile, is by far the longest section of the guide, running from page 11 to page 24.

Standards quoted in the readable part of the guide

GB 51395, Standard for Investigation and Survey of Offshore Wind Farms, quoted in 4.1.1 for the grading of the engineering investigation.

GB 50021, Code for Investigation of Geotechnical Engineering, quoted in 4.1.2 for the classification of soils and rocks.

NB/T 10105, Code for Design of Wind Turbine Foundations of Offshore Wind Farm Projects, quoted in 3.1.1, 3.2.2, 3.2.3 and 3.2.9.

NB/T 10106, Code for Drilling of Offshore Wind Farm Projects, quoted in 4.1.3 for the drilling platform, equipment, technology and samplers.

NB/T 10107, Code for Geotechnical Tests of Offshore Wind Farm Projects, quoted in 4.1.2 and 4.1.5 for laboratory and in-situ geotechnical testing.

NB/T 10626, Code for Design of Corrosion Protection of Offshore Wind Farm Projects, quoted in 3.2.3 for the corrosion protection design.

The complete list of quoted standards is printed on page 43 of the guide, outside the extract read here.

1 Scope

NB/T 11374-2023 is the Chinese design guide for suction pile jacket foundations on offshore wind farms. A suction pile is a large steel cylinder open at the bottom and closed at the top: it is lowered to the seabed, and pumping water out of the sealed interior creates the pressure difference that pushes it into the soil - no pile driving, no noise, and installation in hours rather than days. Used as the legs of a jacket, suction piles suit the soft clays and loose sands of much of the Chinese coast. The guide sets the general provisions and the defined terms and symbols, then the basic requirements: the design service life, the limit states to be checked, the partial factors, and the site data needed on bathymetry, seabed soils, waves, current, wind and seismicity. Design of the foundation follows - the vertical, horizontal and moment capacity of the suction caisson in clay and in sand, the penetration resistance and the suction required to install it without piping or plug heave, the pull-out capacity under uplift, the settlement and the cyclic degradation under storm loading, and the scour protection. The jacket structure itself is covered with its joints, fatigue and corrosion protection, together with the interface to the tower and the transition piece. Installation, removal, monitoring and inspection close the guide. It applies to offshore wind projects in China.

Chapter 1 of the guide sets out its purpose, its field of application and its relationship with the other standards in force. It occupies page 1 of the document.

1.0.1 This guide is formulated in order to standardize the design of suction pile jacket foundations for offshore wind farm projects.

1.0.2 This guide is applicable to the design of suction pile jacket foundations for offshore wind farm projects.

1.0.3 In addition to complying with this guide, the design of suction pile jacket foundations for offshore wind farm projects shall also comply with the provisions of the relevant national standards currently in force.

2 Terms

Chapter 2 establishes eight terms with their official English equivalents and their definitions. It occupies page 2 of the document.

2.0.1 Suction pile jacket foundation.

A foundation composed jointly of a jacket and suction piles, which supports the upper structure.

2.0.2 Suction pile.

A cylindrical structure closed at the top end and open at the bottom end, which is inserted into the seabed by means of its own weight together with the pressure difference between the inside and the outside, thus forming a pile foundation.

2.0.3 Suction pile bucket lid.

The closed structure at the top end of the suction pile, comprising the structural top plate, the stiffening ribs and the connecting plate to the jacket, used to connect the suction pile with the jacket.

2.0.4 Suction pile side wall.

The shell structure located below the top plate of the suction pile.

2.0.5 Suction penetration.

The operation of expelling the air and the water inside the barrel of the suction pile, so that the pressure inside the barrel becomes lower than the external pressure and a pressure difference between inside and outside is generated, thereby sinking the suction pile into the soil mass.

2.0.6 Suction-up.

The operation of forcing sea water into the interior of the suction pile, so that the pressure inside the barrel becomes higher than the external pressure and a pressure difference between inside and outside is generated, thereby lifting the suction pile out of the soil mass.

2.0.7 Penetration resistance.

The resistance of the soil mass encountered by the suction pile during the penetration process.

2.0.8 Plug failure.

The instability failure of the soil core inside the barrel caused by an excessive pressure difference between the inside and the outside of the barrel during the penetration and installation of the suction pile.

3 Basic requirements

Chapter 3 is divided into two sections, design principles and design requirements, and occupies pages 3 and 4 of the document.

Section 3.1, Design principles, contains four provisions.

3.1.1 The design of the suction pile jacket foundation shall comply with the relevant provisions of the current sector standard Code for Design of Wind Turbine Foundations of Offshore Wind Farm Projects, NB/T 10105.

3.1.2 For the ultimate limit state of bearing capacity, checking calculations shall be carried out for the bearing capacity of the seabed subsoil and foundation, for the stability of the foundation structure or of its members, for the strength of the structural members or connecting members, for the fatigue of the structural members and for other specific states that determine whether the structure remains fit to carry load; calculations of the structural strength connected with the installation and with the lifting-up of the suction pile foundation shall also be carried out.

3.1.3 For the serviceability limit state, deformation checking calculations shall be carried out, and the effects of scour and of the installation tolerances shall be taken into account.

3.1.4 Where the jacket structure and the suction pile structure are calculated and analysed separately, it is advisable to perform a coupled analysis in which the lower subsoil and the suction pile foundation are made equivalent to a stiffness matrix at the interface that has been set. The interface and the load reference point should be located at the connection point between the jacket structure and the top plate of the suction pile.

Section 3.2, Design requirements, contains nine provisions.

3.2.1 The design of the suction pile jacket foundation shall adopt the limit state design method based on probability theory. The suction pile jacket foundation shall be designed according to the ultimate limit state of bearing capacity and the serviceability limit state.

3.2.2 The loads and the combinations of operating conditions adopted in the design of the suction pile jacket foundation, and the design safety standards, shall comply with the relevant provisions of the current sector standard Code for Design of Wind Turbine Foundations of Offshore Wind Farm Projects, NB/T 10105.

3.2.3 The corrosion protection design of the suction pile jacket foundation shall comply with the relevant provisions of the current sector standards Code for Design of Wind Turbine Foundations of Offshore Wind Farm Projects, NB/T 10105, and Code for Design of Corrosion Protection of Offshore Wind Farm Projects, NB/T 10626.

3.2.4 The design reference period and the design service life of the structure of the suction pile jacket foundation shall match those of the upper structure.

3.2.5 The structural design and the installation analysis of the suction pile jacket foundation shall take into account the complexity of the pile-soil interaction mechanism and the differences in the engineering characteristics of the seabed soils of different areas, and it is advisable that they be carried out in combination with the results of laboratory tests and field tests.

3.2.6 A penetration analysis for the installation of the suction pile jacket foundation shall be carried out; the level of the construction technology and the feasibility under the actual engineering conditions shall be considered, and the corresponding construction requirements shall be put forward.

3.2.7 The design of the suction pile shall satisfy the safety requirements of the structure with regard to manufacture, transportation, installation, operation and removal.

3.2.8 The suction pile jacket foundation shall be provided with seismic protection according to the basic seismic intensity of the project site.

3.2.9 The determination of the bottom elevation of the platform of the suction pile jacket foundation shall comply with the relevant provisions of the current sector standard Code for Design of Wind Turbine Foundations of Offshore Wind Farm Projects, NB/T 10105; the value adopted for the bottom elevation of the platform of the suction pile jacket foundation shall be taken in combination with the arrangement of the transitional section.

4 Engineering geology

Chapter 4 is divided into two sections, geotechnical investigation and geotechnical parameters for design and installation, and runs from page 5 to page 9 of the document. The extract read here covers Section 4.1 up to provision 4.1.6.

Section 4.1, Geotechnical investigation, opens on page 5.

4.1.1 The grade of the engineering investigation shall be divided into the three grades jia, yi and bing according to the grade of the wind farm project and the degree of complexity of the project site, and shall comply with the relevant provisions of the current national standard Standard for Investigation and Survey of Offshore Wind Farms, GB 51395.

4.1.2 The classification of the soils and rocks of offshore wind farm projects shall comply with the relevant provisions of the current national standard Code for Investigation of Geotechnical Engineering, GB 50021. The items and methods of the geotechnical tests and of the in-situ tests shall be determined according to the properties of the soils and rocks, the design requirements and the applicability of the test methods; the selection of the tests for the physical and mechanical property indexes of the soils and rocks should comply with the relevant provisions of the current sector standard Code for Geotechnical Tests of Offshore Wind Farm Projects, NB/T 10107.

4.1.3 The offshore drilling working platform, the drilling equipment, the drilling technology and the samplers shall comply with the relevant provisions of the current sector standard Code for Drilling of Offshore Wind Farm Projects, NB/T 10106.

4.1.4 The investigation of each wind turbine position shall comply with the following provisions.

4.1.4, item 1 There shall be not less than 2 investigation boreholes for each wind turbine position, and the investigation shall adopt a method combining drilling and static cone penetration testing. At least 1 borehole and 1 piezocone penetration test hole shall be guaranteed for each turbine position. Where the geological conditions are complex, it is advisable to add investigation holes at each suction pile position.

4.1.4, item 2 The depth of the boreholes shall reach from 1.5 times to 2 times the pile diameter below the anticipated tip of the suction pile, and shall not be less than 10 m; the hole depth of the piezocone penetration test hole should not be less than 40 m and should enter from 1.0 times to 1.5 times the pile diameter below the anticipated tip of the suction pile. Where the investigation hole is still in a soft and weak layer at the anticipated depth, it is advisable to pass through the soft and weak layer and to enter the underlying better bearing stratum by at least 5 m; where a stable, firm and thick soil or rock stratum is encountered within the depth of the investigation hole, the depth of the investigation hole may be reduced appropriately.

4.1.4, item 3 For each geological unit, the investigation holes arranged for the purpose of liquefaction assessment shall be not less than 3 in number, and the depth of the investigation holes shall be greater than the depth of the liquefaction assessment. Where the standard penetration test is used for the assessment of liquefaction, the assessment shall be based on the measured blow counts of each test hole; in the soil layers for which an assessment is required, the vertical spacing of the test points should be from 1.0 m to 1.5 m, and the number of test points in each soil layer should not be less than 6.

4.1.4, item 4 Where soft and weak soils are present in the subsoil, an assessment of the seismic subsidence of the soft soils shall be carried out. For each geological unit, the investigation points arranged for the assessment of the possibility of seismic subsidence of soft soils shall be not less than 3 in number, and in the soil layers for which an assessment is required the number of sampling test groups for each soil layer should not be less than 6.

4.1.5 The marine geotechnical in-situ tests mainly include the static cone penetration test, the standard penetration test, the vane shear test, the dynamic cone penetration test, the wave velocity test and the stratum resistivity test; the specifications of the test equipment and the test methods shall comply with the relevant provisions of the current sector standard Code for Geotechnical Tests of Offshore Wind Farm Projects, NB/T 10107.

4.1.6 The marine static cone penetration test should be carried out with a probe provided with pore water pressure measurement, and shall comply with the following provisions.

4.1.6, item 1 The corrected value of the cone tip resistance may be calculated with the following formulae.

Formula (4.1.6-1): q sub t equals q sub c plus u sub 2 multiplied by the quantity one minus a.

Formula (4.1.6-2): a equals A sub a divided by A sub c.

Symbol q sub t is the corrected value of the cone tip resistance, in kPa.

Symbol q sub c is the measured value of the cone tip resistance, in kPa.

Symbol u sub 2 is the pore water pressure at the cone shoulder, in kPa.

Symbol a is the effective area ratio, which is related to the actual specification of the probe; for most probes it is from 0.55 to 0.90.

Symbol A sub a is the cross-sectional area of the top column, in square centimetres.

Symbol A sub c is the cross-sectional area of the cone base, in square centimetres.

4.1.6, item 2 The friction ratio may be calculated with the following formula.

Formula (4.1.6-3): R sub f equals the ratio of f sub s to q sub t, multiplied by 100 percent.

Symbol R sub f is the friction ratio, expressed as a percentage; the list of symbols for this formula continues on the following page of the standard, which lies outside the extract read here.