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  • Standard for GeotechnicalSite and Route SurveysMinimum Requirements for the Foundationof Offshore Wind Turbines

  • Dr. Roland AtzlerNautik Nord GmbH, Pohnsdorf

    Dr. Alexander BartholomForschungsinstitut Senckenberg am Meer,Wilhelmshaven

    Prof. Dipl.-Ing. Horst BellmerProf. Bellmer Ingenieurgruppe GmbH, Bremen

    Dipl.-Ing. Jost BergmannDet Norske Veritas, Hamburg

    Dipl.-Ing. Ulrich HachmannGermanischer Lloyd Offshore and Industrial Services GmbH, Hamburg

    Prof. Dr.-Ing. Harry HarderInstitut fr Geotechnik, Hochschule Bremen

    Dipl.-Ing. Marcus KloseGermanischer Lloyd WindEnergie GmbH, Hamburg

    Dr. habil. Wolfram LemkeInstitut fr Ostseeforschung, Rostock-Warnemnde

    Dr.-Ing. Kerstin LesnyInstitut fr Grundbau und Bodenmechanik,Universitt Duisburg-Essen

    Dr. Klaus MichelsOSAE Offshore Survey and Engineering,Gesellschaft fr Seevermessung mbH, Bremen

    Peter Petersen, M.Sc.Det Norske Veritas, Hellerup (Dnemark)

    Dr. Lutz ReinhardBundesanstalt fr Geowissenschaften und Rohstoffe,Hannover

    Prof. Dr.-Ing. Werner RichwienInstitut fr Grundbau und Bodenmechanik,Universitt Duisburg-Essen

    Knut Ronold, Ph.D.Det Norske Veritas, Hvik (Norwegen)

    Dr. Klaus SchwarzerInstitut fr Geowissenschaften, Universitt Kiel

    Prof. Dr. Volkhard SpiessFachbereich Geowissenschaften, Universitt Bremen

    Dr. Hansjrg StreifNiederschsisches Landesamt fr Bodenforschung,Hannover

    Dipl.-Ing. Jrn UeckerIMS Ingenieurgesellschaft, Hamburg

    Standard for Geotechnical Site andRoute SurveysMinimum Requirements for the Foundation of Offshore WindTurbines and Power Cable Route Burial Assessments

    Status: August 1st, 2003

    Issued by Bundesamt fr Seeschifffahrt und Hydrographie (BSH) in co-operation with

  • Bundesamt fr Seeschifffahrt und Hydrographie (BSH)Hamburg und Rostock 2003www.bsh.de

    All rights reserved. No part of this publication may be reproduced, stored in a retrieval system,or transmitted in any form or by any means, electronic, mechanical, photocopying, recording orotherwise, without the prior written permission of the BSH.

  • C o n t e n t s 3

    Section A Introduction 7

    A.1 Preface 7

    A.2 General 7

    A.3 Standard for Geotechnical Site Surveys for Offshore Wind Turbines 8

    A.4 Deviation from the Standard 11

    A.5 Updating 11

    Section B Minimum Requirements forthe Geological-GeophysicalSurveying 12

    B.1 General 12

    B.2 Quality Assurance 12

    B.3 Time Schedule 13

    B.4 Objectives 13

    B.4.1 Preliminary Reconnaissance Phase 13

    B.4.2 Planning Phase 13

    B.4.3 Monitoring Phase 14

    B.5 Technical Instructions 14

    B.6 Power Cable Route Burial Assessment 20

    B.7 Geological Report 20

    B.7.1 Purpose 20

    B.7.2 Contents 20

    B.7.3 Requirements 20

    C o n t e n t s

  • C o n t e n t s4

    Section C Minimum Requirements for theGeotechnical Site Surveying as aBasis for Planning/Designing theOffshore Wind Farm 21

    C.1 General 21

    C.2 Quality Assurance 21

    C.3 Requirements for Field Testing 21

    C.3.1 Drilling and Penetration Tests 21

    C.3.2 Field Testing for Pile Foundations 22

    C.3.3 Field Testing for Gravity Foundations 23

    C.3.4 Requirements for Soil Sampling 24

    C.4 Requirements for Laboratory Tests 25

    C.4.1 Laboratory Tests on Board Survey Vessels 25

    C.4.2 Onshore Laboratory Tests 25

    C.5 Geotechnical Site Survey Report 28

    C.5.1 Contents of the Geotechnical Site Survey Report 28

    C.5.2 Presentation of the Results 28

    C.5.2.1 Field Tests 28

    C.5.2.2 Laboratory Tests 29

    C.5.3 Geotechnical Site Description 29

    C.5.4 General Geotechnical Site Assessment 29

    C.6 Soil and Foundation Expertise 29

    C.6.1 Contents of the Soil and Foundation Expertise 29

    C.6.2 Information Provided by the Soil and Foundation Expertise 29

    C.7 Supplementary Investigation (Tendering Phase) 30

    C.8 Monitoring in the Construction Phase 30

    C.9 Monitoring in the Operation Phase 30

    C.10 Literature 31

  • C o n t e n t s 5

    Figures

    Fig. A.1 Overview of the Individual Phases and ReconnaissanceMeasures 9

    Fig. C.1 Cyclic Direct Shear Test 27

    Fig. C.2 Cyclic Compression Test 27

    Fig. C.3 Cyclic Triaxial Test 28

    Tables

    Table A.1 Type and Minimum Scope of Reconnaissance for the Planning,Construction, and Operation Phases 10

    Table B.1 Requirements for Echosounder Surveys (Single-Beam andMultibeam Echosounding) 15

    Table B.2 Requirements for Side Scan Sonar (SSS) Surveys 16

    Table B.3 Requirements for Seismic Surveys 17

    Table B.4 Requirements for Borehole Drilling 18

    Table B.5 Requirements for Magnetometers and Active Metal Detection Systems(Recommended) 18

    Table B.6 Requirements for the Power Cable Route Survey 19

    Table C.1 Field Testing for Pile Foundation 22

    Table C.2 Exploration Depth and Sampling in Field Testing for Pile Foundations(Minimum Requirements) 23

    Table C.3 Exploration Depth and Sampling in Field Testing for Gravity Foundations(Minimum Requirements) 24

    Table C.4 Laboratory Tests for Evaluation of Cohesionless Soils 26

    Table C.5 Laboratory Tests for Evaluation of Cohesive Soils 26

  • S e c t i o n A 7

    Section A Introduction

    A.1 Preface

    Within the framework of the approval procedures foroffshore wind farms in the Exclusive Economic Zone(EEZ), developers submitting applications are re-quired to furnish proof of the installations' structuralsafety and use of approved state-of-the-art technolo-gy (cf. Art. 5, para. 2, Marine Ordance Facility(Seeanlagenverordnung, SeeAnlV)). Up to now, geo-technical site investigations as a technical prerequi-site to the construction of foundations for offshorewind turbines have been subject to standards issuedby classification companies and to a number of tech-nical instructions, which differed, however, at least inpart aspects. As, on the one hand, the high cost ofsuch installations had caused companies to discusspotential conflicts between safety requirements andeconomic viability and, on the other hand, the ap-proval authority considered the safety of some instal-lations unsatisfactory depending on the standardused the present standard has been developedunder the guidance of the BSH and introduced as atechnical standard according to Art. 4, para. 2(SeeAnlV). Compliance with the standard will berequired in the approvals granted under SeeAnlV. Thepurpose of the standardisation of requirements is toensure legal certainty and investment security, and toprovide the approval authority being committed to theprinciple of equal treatment, with a valuable tool in itsdecisionmaking process regarding offshore wind farmapprovals.

    The present Standard for Geotechnical Site Surveysfor offshore windfarms, which has been issued by theapproval authority, specifies the minimum require-ments to be met when performing the mandatory geo-logical and geophysical site investigations. The stand-ard is based on a well-researched proposal made bya group of engineers and geoscience experts fromvarious universities, authorities, engineering andclassification companies. It takes into account experi-ence that has been gained with offshore wind farmsin other European countries (Denmark and Sweden)as well as preliminary results of the offshore wind

    farm research group Gigawind, which has beenfunded by the Federal Ministry of Education andResearch (BMBF). It defines the thematic and techni-cal minimum requirements for geotechnical site inves-tigations prior to the erection of offshore wind turbinesand their foundations.

    It should be noted that this standard represents theoutcome of discussions characterised by a high levelof commitment and expertise. If individual opinionsand concepts discussed in the course of the deci-sionmaking process have not been taken into consid-eration, this does not imply any criticism of suchopinions but only means that the approval authority,after consultation with the experts, has chosen one ofseveral possible solutions or has allowed alternativesconsidered suitable for the procedure.

    The present standard is open to development, i.e. theconsiderable number of new experience and findingsexpected to be available in the near future will beevaluated and incorporated in the standard as requir-ed.

    A.2 General

    Offshore wind turbines are construction projects in-volving a high level of geotechnical difficulty. Apartfrom constructional aspects and load cases, the soilconditions play an crucial role. Contrary to commonopinion, the seabed in the EEZ in the North Sea andBaltic is not a homogeneous body of sediment butmay be highly heterogeneous on regional and localscales. Unlike the steel or concrete structure of theturbines, the soil properties of the seabed cannot bechanged to fit the structure. Therefore, exact knowl-edge of the geological and geotechnical properties ofthe seabed are indispensable if a wind farm project isto be successfully implemented.

    A geological model of the seabed structure has to bedeveloped as a basis for subsequent phases of siteinvestigation, planning, and project implementation. Acomprehensive geological/geophysical survey pro-gramme may allow reconnaissance measures in theplanning phase to be reduced and allows the identifi-

  • S e c t i o n A8

    cation of alternative sites for offshore turbines in seaareas with unfavourable soil properties.

    The design of offshore wind turbine foundations re-quires comprehensive knowledge of the seabed properties and geotechnical parameters at each indi-vidual site. It must be ensured by the scope of thegeotechnical site investigations that all soil propertiesrelevant to planning are determined well before instal-lation of the turbines. Therefore, a geotechnical siteinvestigation and evaluation by qualified experts ismandatory.

    The present standard prescribes a investigation pro-gramme for the planning and construction of offshorewind turbines which is graded in type and scope tomeet the requirements of the individual planning andconstruction phases and defines the minimum re-quirements.

    Fig. A.1 illustrates the integration of appropriatephases of the site survey and assessment into theplanning and approval process of offshore windturbines.

    The following phases are distinguished:

    Preliminary activities Preliminary Geological Report Preliminary reconnaissance Geological Report Site investigation Geotechnical Site Survey Report

    Update of Geological Report Foundation design Soil and Foundation Expertise Invitation to bid for foundation work Construction of foundations Records and evaluation Monitoring of foundation work Reports and evaluation Monitoring of plant operation Reports and evaluation

    A.3 Standard for Geotechnical Site Surveysfor Offshore Wind Turbines

    The Standard is based on geological, geophysical,and geotechnical field and laboratory studies. It has tobe adjusted to the planned foundation concept, withadequate consideration of the difficulties of foundationdesign, on the one hand, and soil properties andother conditions, on the other hand. The scope ofgeotechnical site surveys shall be such that all soilproperties that are relevant to planning are deter-mined well before the turbine installation. The investi-gation methods used comprise:

    geophysical methods and geotechnical methods.

    Geophysical methods comprise indirect methods(sonar, seismics, echosounding, etc). Geotechnicalmethods comprise direct exploration methods, mainly

    bottom sampling (drilling) and indirect explorationmethods (penetration tests), field testing such as thevane tests, borehole pressiometer tests as well aspile-driving tests or pile test loading.

    The soil characteristics are generally determined inlaboratory tests, rarely in field testing. The type andscope of investigations to be carried out in the indi-vidual planning and construction phases have beencompiled in Table A.1.

    The concept presented in this guideline defines theminimum scope of investigations that is normallyrequired.

    Requirements for geotechnical site investigations foroffshore windfarms have been defined in DIN 4020(drafted in 2002). Besides, requirements for geotech-nical site investigations are part of the relevant stand-ards for offshore structures (Det Norske Veritas

  • S e c t i o n A 9

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  • S e c t i o n A10

    Table A.1: Type and Minimum Scope of Reconnaissance for the Planning, Constructionand Operation Phases

    Phase Purpose of reconnaissance Type of reconnaissance

    1 Preliminaryreconnaissanceof the area

    Site selection and preliminary planningof the structures

    The purpose of preliminaryreconnaissance is decisionmaking onwhether or not the soil properties in aparticular area are suitable for theerection of the planned wind farmstructures, determination of generalrequirements for the foundationconcepts, design and construction, andof the required site reconnaissancemeasures

    Survey and evaluation of availabledocumentation

    Geophysical reconnaissance in theentire wind farm area

    Spot reconnaissance through direct andindirect exploration (rough grid coveringthe wind farm area)

    Spot checks of relevant foundation soilcharacteristics and properties

    2 Sitereconnaissance

    The scope of soil reconnaissance andinvestigation and the methods to beused depend on the type, size, andimportance of wind turbine design,homogeneity of the foundation soil,morphology of the seabed andsediment types encountered

    When investigating the area, possiblealterations of site planning must betaken into account

    The soil composition and sedimentcharacteristics must be determined ateach site

    Survey and evaluation of availabledocumentation

    Geophysical site reconnaissance Direct exploration by drilling boreholes

    at the turbine sites...

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