NB/T 11371-2023Technical specification for foundation pile testing of offshore wind power projects (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 specification has been prepared in accordance with the requirements of the Notice of the General Affairs Department of the National Energy Administration on Issuing the 2020 Plan for the Formulation and Revision of Energy Sector Industry Standards and the Plan for Foreign Language Versions (Guo Neng Tong Ke Ji [2020] No. 106).

The drafting group carried out extensive investigation and research, conscientiously summarized practical engineering experience and prepared this specification on the basis of wide solicitation of opinions.

The main technical contents of this specification are: general provisions; terms; basic requirements; the high strain method; the axial compressive static load test; the axial uplift static load test; the lateral static load test; the low strain reflected wave method; the sonic transmission method; the core drilling method; the vibration test; and pile position survey.

This specification is administered by the National Energy Administration. It was proposed by, and is under the routine administration of, the China Renewable Energy Engineering Institute (General Institute of Hydropower and Water Resources Planning and Design).

The Sub-Technical Committee on Wind Farm Construction and Erection of the Energy Industry Wind Power Standardization Technical Committee (NEA/TC1/SC2) is responsible for the interpretation of the specific technical content.

Comments and suggestions arising during the implementation of this specification should be sent to the China Renewable Energy Engineering Institute, No. 57 Jia and No. 57 Yi, Andingmenwai Dajie, Dongcheng District, Beijing, postcode 100011.

The chief editing organizations are PowerChina Huadong Engineering Corporation Limited and CCCC Third Harbor Engineering Company Limited.

The participating organizations are Zhejiang Huadong Surveying, Mapping and Engineering Safety Technology Company Limited, the Institute of Rock and Soil Mechanics of the Chinese Academy of Sciences (Wuhan), Guangdong Tianxin Electric Power Engineering Testing Company Limited and the Ocean University of China.

The standard was approved by the National Energy Administration under Announcement No. 8 of 2023, dated 28 December 2023, and takes effect on 28 June 2024. It is published by China Electric Power Press.

The specification is listed as item 36 of the catalogue of industry standards attached to that announcement; no superseded standard is recorded against it, so this is a newly established specification rather than a revision.

1 Scope

NB/T 11371-2023 sets out how the foundation piles of an offshore wind farm are tested, from the trial piles driven before the works begin to the acceptance testing of the production piles. It covers the whole family of methods used offshore: static load testing in compression, tension and lateral loading; high-strain dynamic testing to establish bearing capacity and pile integrity; low-strain integrity testing; sonic logging of cast-in-place piles through pre-installed access tubes; core drilling; and the driving records and pile-driving analysis used to monitor installation of driven steel pipe piles. For each method the standard fixes the equipment and its calibration, the number and choice of test piles, the loading or excitation procedure, the data to be recorded, how the measured curves are interpreted, and how the result is judged and reported. It also addresses the conditions that make offshore testing different from testing on land - work from a jack-up platform or vessel, tide and wave windows, reaction systems bearing on the seabed or on neighbouring piles, corrosion protection and the safety rules that follow from all of this. The document applies to the design, construction and acceptance stages of offshore and intertidal wind power projects in China, and is the reference a Chinese reviewer uses when checking that a wind farm foundation has been verified properly.

Chapter 1 sets out the purpose of the specification, its field of application and the general principles governing the testing of foundation piles in offshore wind power projects.

The specification addresses the piles that carry offshore wind turbine generator foundations and offshore substation structures, where testing has to be performed in an open sea environment with tides, waves, currents and limited working windows.

It establishes that foundation pile testing shall verify the integrity of the pile shaft, the bearing capacity of the pile and the position in which the pile has been installed.

Testing is required to be planned and executed so that the results provide the evidence needed for construction acceptance and for the assessment of the completed foundations.

The chapter states the relationship between this specification and the other national and industry standards that apply to offshore wind power engineering, which remain in force alongside it.

The general provisions occupy page 1 of the standard and are followed immediately by the terminology chapter.

2 Terms

Chapter 2 defines the terminology used throughout the specification, covering pages 2 and 3.

The definitions relate to the foundation pile itself, to the quantities measured during testing and to the categories of test method applied offshore.

Terms are provided for pile integrity, for the ultimate bearing capacity determined by static and dynamic methods, and for the characteristic values derived from test data.

Further terms cover the instrumentation quantities recorded during dynamic testing, such as strain and acceleration signals measured at the pile head.

The terminology chapter provides the common vocabulary on which the acceptance criteria of Chapters 4 to 12 are based.

Where terms are also defined in the general standards for building foundation pile testing, the definitions given here apply for offshore wind power work.

3 Basic Requirements

Chapter 3 lays down the requirements common to all the test methods and runs from page 4 to page 10. It is divided into five sections.

Section 3.1, Testing Scheme, beginning on page 4, requires a testing scheme to be prepared before work starts, setting out the objectives, the methods selected, the piles to be tested and the arrangements for vessels, access and safety at sea.

The testing scheme has to take account of the pile type, the installation method, the ground conditions revealed by the site investigation and the design assumptions that the test is intended to verify.

Section 3.2, Number of Tests, beginning on page 7, prescribes how many piles are to be tested by each method, expressed in relation to the total number of piles in the project and to the role of the piles concerned.

Section 3.3, Timing of Testing, on page 8, governs when testing may be carried out, in particular the interval that must elapse after pile installation or after concrete placement before a valid test can be performed.

Section 3.4, Verification and Extended Testing, on page 8, sets out what has to be done when a test result is anomalous or inconclusive: the finding is to be verified, and the scope of testing extended to additional piles.

Section 3.5, Evaluation of Test Results and the Test Report, beginning on page 9, defines how results are classified and what the report must contain, including the project data, the method used, the instruments, the field records, the analysis and the conclusions.

The evaluation rules of Section 3.5 are the reference for the method-specific evaluation clauses that close each of Chapters 4 to 12.

4 High Strain Method

Chapter 4 covers the high strain dynamic method and is the longest of the method chapters, running from page 11 to page 21.

Section 4.1, General Requirements, on page 11, states the purposes for which the high strain method may be used, namely the assessment of the vertical compressive bearing capacity of a single pile and the assessment of pile shaft integrity.

Section 4.2, Instruments, Equipment and Installation, also beginning on page 11, deals with the drop hammer or driving system, the strain and acceleration transducers, the data acquisition unit and the way in which the transducers are mounted on the pile.

The requirements address the mass of the hammer in relation to the expected bearing capacity, the cushion arrangement and the symmetrical mounting of the sensors so that bending effects are cancelled.

Section 4.3, Field Testing, on page 13, prescribes the conduct of the test at sea, the number of blows to be recorded, the checks on signal quality and the conditions under which a record is rejected.

Section 4.4, Analysis and Evaluation of Test Data, from page 14 to page 21, is the most extensive section of the chapter and covers the processing of the measured force and velocity signals.

It sets out the analysis of the wave records for the determination of bearing capacity, including the fitting of the measured signals by signal matching analysis.

It also sets out how shaft defects are located and quantified from the reflections in the records, and how the resulting integrity category is assigned.

The length of Section 4.4 reflects the amount of tabulated and graphical material needed to interpret high strain records from large diameter offshore piles.

5 Axial Compressive Static Load Test

Chapter 5 covers the axial compressive static load test and runs from page 22 to page 29.

Section 5.1, General Requirements, on page 22, states that the test is used to determine the ultimate vertical compressive bearing capacity of a single pile and its load-settlement behaviour.

Section 5.2, Instruments, Equipment and Installation, also on page 22, deals with the reaction system, the jacks, the load measuring devices and the displacement gauges together with the reference beams.

The reaction arrangement has to be designed for the marine environment, where anchor piles, kentledge or a jack-up platform may provide the reaction and where the reference system must be isolated from wave-induced movement.

Section 5.3, Field Testing, on page 24, prescribes the loading procedure, the size of the load increments, the duration of each stage and the readings of settlement to be taken during loading and unloading.

It also defines the conditions under which loading is terminated, in terms of settlement, of rate of settlement and of the stability criteria for each load stage.

Section 5.4, Analysis and Evaluation of Test Data, from page 25 to page 29, describes the plotting of the load-settlement curve and the associated settlement-time curves.

The ultimate bearing capacity of the individual pile is determined from these curves, and the characteristic value for the group of tested piles is then derived and compared with the design value.

6 Axial Uplift Static Load Test

Chapter 6 covers the axial uplift static load test and runs from page 30 to page 32.

Section 6.1, General Requirements, on page 30, states that the test determines the ultimate uplift bearing capacity of a single pile, a quantity of particular importance for the jacket and multi-pile foundations used offshore.

Section 6.2, Instruments, Equipment and Installation, also on page 30, deals with the reaction frame, the jacks acting against it, the load cells and the uplift displacement gauges.

The connection between the jacking system and the pile head has to transfer the tensile load without damaging the pile and without introducing eccentricity.

Section 6.3, Field Testing, on page 30, prescribes the staged application of the uplift load, the observation intervals and the criteria for stabilization at each stage.

It also sets out the conditions for terminating the test, including excessive uplift displacement and rupture of the pile or of the connection.

Section 6.4, Analysis and Evaluation of Test Data, on pages 31 and 32, describes the load-uplift displacement curve and the determination of the ultimate uplift capacity from it.

7 Lateral Static Load Test

Chapter 7 covers the lateral static load test and runs from page 33 to page 39.

Section 7.1, General Requirements, on page 33, states that the test determines the lateral bearing behaviour of a single pile, which governs the design of offshore wind turbine foundations subjected to wind, wave and current loading.

Section 7.2, Instruments, Equipment and Installation, also on page 33, covers the reaction arrangement between two piles or against a reaction structure, the horizontal jacks, the load cells and the displacement gauges.

Where the deflected shape of the pile below the mudline is required, inclinometer casing or embedded strain gauges are installed in the pile in advance.

Section 7.3, Field Testing, from page 34, prescribes the loading procedures, which may be applied in a single cycle or in repeated cycles depending on the purpose of the test.

The readings of horizontal displacement and, where applicable, of rotation at the pile head are taken at defined intervals during each load stage.

Section 7.4, Analysis and Evaluation of Test Data, from page 36 to page 39, describes the load-displacement and load-gradient curves and the determination of the critical and ultimate lateral loads.

Where subsurface instrumentation has been installed, the section also covers the derivation of bending moment distribution and of the coefficient of horizontal subgrade reaction.

8 Low Strain Reflected Wave Method

Chapter 8 covers the low strain reflected wave method and runs from page 40 to page 44.

Section 8.1, General Requirements, on page 40, states that the method is used to assess pile shaft integrity and to identify the position of defects.

Section 8.2, Instruments, Equipment and Installation, also on page 40, deals with the hand hammers and other exciting devices, the receiving transducers and their coupling to the prepared pile head.

The pile head has to be clean, sound and level at the measuring points so that the impact produces a clear compressive wave.

Section 8.3, Field Testing, on page 41, prescribes the number and arrangement of measuring points, the repetition of blows and the acceptance of records in terms of signal consistency and noise.

Section 8.4, Analysis and Evaluation of Test Data, on pages 42 to 44, describes the interpretation of the reflected wave records in the time domain.

The location of a defect is calculated from the arrival time of its reflection and the wave velocity assumed or measured for the pile material.

The section defines the integrity categories into which the tested piles are classified and the treatment of piles whose records cannot be classified with confidence.

9 Sonic Transmission Method

Chapter 9 covers the sonic transmission method, in which ultrasonic signals are transmitted between tubes cast into the pile, and runs from page 45 to page 52.

Section 9.1, General Requirements, on page 45, states that the method is used to detect defects within the body of cast in place concrete piles and to assess their extent.

Section 9.2, Installation of Acoustic Access Tubes, also on page 45, prescribes the number, diameter, arrangement and fixing of the tubes within the reinforcement cage, and their sealing and filling with water.

The number of tubes is related to the pile diameter, and their parallelism has to be maintained over the full depth so that the measured travel distances remain valid.

Section 9.3, Instruments and Equipment, on page 46, deals with the transmitting and receiving probes, the depth measuring device and the acquisition unit.

Section 9.4, Field Testing, also on page 46, prescribes the plane scanning of all tube pairs, the sampling interval in depth and the supplementary oblique and fan scanning used to delimit a suspected defect.

Section 9.5, Analysis and Evaluation of Test Data, from page 48 to page 52, describes the calculation of acoustic velocity, wave amplitude and, where used, the received signal frequency and energy.

Judgment criteria are established from the statistical distribution of the measured parameters along the pile, and the defect zones so identified are used to assign the integrity category.

10 Core Drilling Method

Chapter 10 covers the core drilling method and runs from page 53 to page 59.

Section 10.1, General Requirements, on page 53, states that coring is used to examine the concrete of a cast in place pile, the condition of the pile base and the nature of the bearing stratum beneath it.

Section 10.2, Instruments, Equipment and Installation, also on page 53, deals with the drilling rig, the core barrels and bits, and the fixing of the rig so that a vertical hole is maintained from a floating or elevated platform.

Section 10.3, Field Testing, on page 54, prescribes the number and position of the boreholes, the recording of drilling depth and the continuous recovery, labelling and photography of the core.

The core is examined for concrete quality, for segregation, voids and inclusions, and for the interface between the pile base and the founding stratum.

Section 10.4, Core Sample Testing, on page 55, prescribes the preparation of specimens cut from the recovered core and their compressive strength testing.

Section 10.5, Analysis and Evaluation of Test Data, from page 56 to page 59, describes how the visual core log and the strength results are combined into an assessment of the pile.

The section defines the integrity categories assigned on the basis of the coring results and the conclusions to be drawn about the bearing stratum at the pile base.

11 Vibration Test

Chapter 11 covers the vibration test and runs from page 60 to page 63.

Section 11.1, General Requirements, on page 60, states the purposes of the test, which relate to the dynamic characteristics of the pile and of the pile-soil system.

Section 11.2, Instruments, Equipment and Installation, also on page 60, deals with the excitation device, the vibration transducers and their mounting on the structure.

Section 11.3, Field Testing, on page 61, prescribes the excitation procedure, the sampling parameters and the duration of the records to be acquired.

Section 11.4, Analysis and Evaluation of Test Data, on pages 61 to 63, describes the processing of the recorded signals in the frequency domain.

Natural frequencies and damping characteristics obtained from the records are compared with the values assumed in design, which for offshore wind turbine support structures govern the avoidance of resonance.

12 Pile Position Survey

Chapter 12 covers pile position survey, that is the verification of the as-installed position of the piles, and begins on page 64.

Section 12.1, General Requirements, on page 64, states that the survey establishes the plan position, the verticality or rake and the elevation of the installed pile, and compares them with the design values.

Positional accuracy is critical offshore because the piles of a jacket or multi-pile foundation must accept a prefabricated structure whose connection tolerances are small.

Section 12.2, Instruments and Equipment, also on page 64, deals with the surveying instruments used, including satellite positioning equipment and inclination measuring devices.

Section 12.3, Field Testing, on page 65, prescribes the procedures for taking the measurements from a vessel or platform and the corrections to be applied.

The chapter continues beyond page 65 in the printed standard; the pages of the table of contents examined here end at Section 12.3.