DNV-CG-0456 Repair guidance for vessels in operation

CLASS GUIDELINE

DNV-CG-0456 Edition September 2021

Repair guidance for vessels in operation

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DNV AS

FOREWORD

DNV class guidelines contain methods, technical requirements, principles and acceptance criteriarelated to classed objects as referred to from the rules.

© DNV AS September 2021

Any comments may be sent by e-mail to [email protected]

This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of thisdocument. The use of this document by other parties than DNV is at the user's sole risk. Unless otherwise stated in an applicable contract,or following from mandatory law, the liability of DNV AS, its parent companies and subsidiaries as well as their officers, directors andemployees (“DNV”) for proved loss or damage arising from or in connection with any act or omission of DNV, whether in contract or in tort(including negligence), shall be limited to direct losses and under any circumstance be limited to 300,000 USD.

CHANGES – CURRENT

This document supersedes the September 2020 edition of DNVGL-CG-0456.The numbering and/or title of items containing changes is highlighted in red.

Changes September 2021

Topic Reference Description

Rebranding to DNV All This document has been revised due to the rebranding of DNVGL to DNV. The following have been updated: the companyname, material and certificate designations, and references toother documents in the DNV portfolio. Some of the documentsreferred to may not yet have been rebranded. If so, please seethe relevant DNV GL document. No technical content has beenchanged.

Editorial correctionsIn addition to the above stated changes, editorial corrections may have been made.

Changes - current

Class guideline — DNV-CG-0456. Edition September 2021 Repair guidance for vessels in operation

DNV AS

CONTENTS

Changes – current.................................................................................................. 3

Section 1 General....................................................................................................51 General................................................................................................ 5

Section 2 Damage and repair process - general principles..................................... 81 Owner’s obligations............................................................................. 82 Damage and repair survey follow up...................................................83 Planning a repair............................................................................... 114 Survey of repair.................................................................................13

Section 3 Major conditions - critical damage requiring prompt and thoroughrepair.................................................................................................................... 14

1 General.............................................................................................. 142 Hull structures...................................................................................143 Machinery and systems..................................................................... 144 Statutory related equipment..............................................................14

Section 4 Underwater and in-water repairs.......................................................... 151 Dry welding on hull with water on opposite side...............................152 Underwater wet welding................................................................... 153 In-water non-destructive testing – magnetic particle testing............ 15

Section 5 Pressure vessels....................................................................................171 General.............................................................................................. 172 Repair guidance.................................................................................173 Prefabricated elements......................................................................174 Post weld heat treatment..................................................................175 Non-destructive testing..................................................................... 176 Hydrostatic testing............................................................................ 18

Section 6 Repair by build-up by metal coating and clad welding.......................... 191 General.............................................................................................. 192 Repair by build-up by metal coating and clad welding.......................193 Qualification for thermal spraying..................................................... 19

Changes – historic................................................................................................20

Contents


DNV AS

SECTION 1 GENERAL

1 General

1.1 ObjectiveThe objective of this class guideline (CG) is to highlight the principles and provide guidance for planning andhandling of repairs of damages and defects for vessels in operation, classed by the Society.

1.2 ScopeThe class guideline will detail the principles for the damage and repair follow up process:

— communication with the Society— survey follow up— certification and replacement of components and material— requirements for workmanship— preparation of the repair plans prompt and thorough repair— prompt and thorough repair.

It also includes description of selected repair related methods and testing for vessels in operation.

1.3 ApplicationThese guidelines are applicable to all vessels classed with the Society.

1.4 ReferencesFor relevant rule references, see Table 1. Relevant IACS documents are listed in Table 2. Rule reference forother regimes than DNV rules for classification of ships may not have been mentioned in this class guideline,but the respective rules and regulations shall nevertheless be applied as relevant.

Table 1 DNV documents

Document code Title

DNV-RU-SHIP Pt.1 Ch.1 General regulations

DNV-RU-SHIP Pt.7 Ch.1 Survey requirements for fleet in service

DNV-CG-0044 Metal coating and clad welding

DNV-CG-0182 Allowable thickness diminution for hull structure

DNV-CG-0051 Non-destructive testing

DNV-CP-0484 App.B [14] Service suppliers engaged in alignment of propulsion shafting and installation of rotatingmachinery

DNV-CG-0550 Maritime services

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Table 2 External documents

Document code Title

IACS Rec. No.47, Part B Shipbuilding and Repair Quality Standard, Repair Quality Standard for Existing Ships

IACS UR Z7, UR Z10 Hull classification surveys, (1.3 repair)

IACS UR Z13 Voyage Repairs and Maintenance

1.5 Definitions and abbreviations1.5.1 Definition of verbal formsVerbal forms used in this document are defined in Table 3.

Table 3 Definition of verbal forms

Term Description

shall verbal form used to indicate requirements strictly to be followed in order to conform to thedocument

should verbal form used to indicate that among several possibilities one is recommended asparticularly suitable, without mentioning or excluding others

may verbal form used to indicate a course of action permissible within the limits of the document

1.5.2 Definition of termsTerms used in this document are defined in Table 4.

Table 4 Definition of terms

Term Definition

applicable rules see DNV-RU-SHIP Pt.1 Ch.1 Sec.2

current rules rules in force at the time of the repair

emergency repair repairs performed by crew or 3rd party in order to retain the vessel's manoeuverability,stability, weather tightness strength and safe operation in order to reach safe- or scheduledport

material certificate see DNV-CG-0550

material declaration see DNV-CG-0550

occasional damage survey non-scheduled survey with the purpose of either establishing the extent of damage to thevessel or its equipment or confirming the condition of the vessel or its equipment afterrepairs have been conducted

owner see DNV-RU-SHIP Pt.1 Ch.1 Sec.1 [1.2]

plan approval see DNV-RU-SHIP Pt.1 Ch.1 Sec.1 [1.2]

product certificate see DNV-CG-0550

product declaration see DNV-CG-0550

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Term Definition

repair restore an object that is damaged or broken to a condition that complies with the rulesIn general, this implies that the original design is restored. Repair shall be agreed andsurveyed by the Society.

temporary repair remedial measure for assuring the vessel's fitness for continued service for a specific timelimitA temporary repair shall be followed up with a condition of class.

type approval certificate see DNV-CG-0550

voyage repair repair performed by riding crew where a surveyor is not attending during the voyage

1.5.3 AbbreviationsAbbreviations used in this document are defined in Table 5.

Table 5 Abbreviations

Abbreviation Description

AoSS approval of service supplier

AWS American Welding Society

CG class guideline

DATE direct access to technical expert

EU MR European Union mutual recognition

ITP inspection and test procedure

MED marine equipment directive

MPI magnetic particle inspection

MT magnetic testing

NDT non-destructive testing

TAC type approval certificate

WPQR welding procedure qualification record

WPQT welding procedure qualification test

WPS welding procedure specification

WWA welding workshop approval

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DNV AS

SECTION 2 DAMAGE AND REPAIR PROCESS - GENERAL PRINCIPLES

1 Owner’s obligationsIt is the owner's obligation to inform the Society about damage presumed to lead to a condition of class, seeDNV-RU-SHIP Pt.1 Ch.1 Sec.3.Damage to be considered are those relating to hull structure and components serving main and auxiliaryfunctions and relevant notations as described in DNV-RU-SHIP Pt.3, DNV-RU-SHIP Pt.4, DNV-RU-SHIP Pt.5and DNV-RU-SHIP Pt.6.This also includes damage related to equipment required under statutory regulations, or covered by astatutory certificate where the flag has delegated authority to DNV.

2 Damage and repair survey follow up

2.1 SystematicsAfter informing the Society of a damage, the Society will inform the owner about the need for an occasionalsurvey of the damage. This occasional damage survey serves the purpose of identifying the extent ofthe damage and its impact on the safety of the vessel, and/or crew or the environment. Subsequently,the attending surveyor will issue a report describing the extent of the damage and the time frame of therectification of the deficiency (condition of class) as well as any mitigating actions or remedial measuresrequired until completion of the repair. This may involve temporary repairs.Owners shall then present a repair and test plan for acceptance by the Society. Once accepted, the repairsmay be conducted. Any agreed and/or required intermediate surveys of the ongoing repairs shall be carriedout by the Society.Once the repairs are completed, a final survey shall be conducted to examine and test the quality of therepairs as well as the final condition and functionality of the vessel or equipment.

2.2 Applicable rules, regulations and supporting documentsRepairs shall be performed according applicable rules and shall restore the vessel or its equipment to itsoriginal condition. Original approved drawings shall form the basis for the repair.Recognised repair recommendations and guidelines may be used for preparation of repair plan proposals, seeSec.1 [1.4].

2.3 Replacement of materials and componentsThe governing principle for the acceptance of materials and components for vessels in operation is thatreplacement material and components shall be documented to fulfil the applicable rules and regulationsat the time of newbuilding. This means that where the rules require a material or product declaration, themanufacturer shall be approved by DNV GL, and where the rules require a certificate issued by the Society,this shall be a DNV product or material certificate.Certification according to the current rules is acceptable, also in cases where these certification requirementsare less stringent than the applicable rules. However, where the same is relevant for certification according tostatutory requirements, flag acceptance may be required prior to applying a less stringent requirement.

2.4 Material and component certificates issued by other societiesSince there is no mutual recognition of product or material certificates within the IACS organisation,certificates issued by other IACS societies are not accepted, neither are certificates issued by other non-IACSsocieties.

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The only certificates accepted issued by other class societies are type approval certificates (TAC) issuedunder the European Union mutual recognition (EU MR) agreement. This shall be clearly stated on the TAC.Replacement material and components delivered with or without a non-DNV certificate, when the applicablerules require a DNV certificate, shall not be accepted for installation on board. However, depending on severalfactors, amongst others available documentation, manufacturer, material supplier, re-testing, it may bepossible to qualify (i.e. re-certify) the part to DNV standard. This in turn may make it possible to accept thematerial or components for installation on board.

2.5 Equivalency of material gradesReplacement material should be of the same grade as the original approved material. Alternatively, materialgrades complying with recognised national or international standards may be accepted by the Societyprovided such standards fulfil or provide equivalence to the rule requirements of the original grade.Where the same grade of material as the originally approved one is not available, the Society may acceptreplacement materials with equivalent or better properties subject to case-by-case evaluation. Assessment ofequivalency between material grades should at least include the following aspects:

— mechanical properties, including yield and tensile strength, ductility, and Charpy V-notch impacttoughness (test temperature and obtained value), and test unit size

— chemical composition with focus on weldability (carbon equivalent, P, S, Cu, Ni, Cr and Mo content, finegrain treatment and deoxidation)

— heat treatment/delivery condition— thickness tolerances (plates: max. -0.3 mm, sections: according to a recognized standard).

Where DNV certified materials are available, or the material has been re-tested and witnessed by theSociety, the evaluation may be based on the obtained test values for the material in question instead of thestandardized specified minimum values. For such cases, the Society may request an adequate safety factor,e.g. for strength and toughness values.

2.6 Welding procedures and consumablesWelding procedure specifications (WPS) shall be approved and accompanied by the approved weldingprocedure qualification record (WPQR) which is based on a welding procedure qualification test (WPQT).Approval by other certifying bodies than DNV may be accepted on a case-by-case basis.The WPS shall always be relevant for the material, position and type of weld required by the repair proposal.Welding consumables shall be type approved by DNV.Applicable rules and standards are listed in Table 1.

Table 1 Welding procedure and consumables requirements

Item Standard Additional requirement from DNV

WPS Applicable rules or DNV-RU-SHIP Pt.2Ch.4 Sec.5

WPS qualified to recognized standardsmay be evaluated case-by-case forpossible acceptance by the Society.The WPS and supporting WPQR shall inthis case comply reasonably with therequirements of the applicable rules.

WPQR Applicable rules or DNV-RU-SHIP Pt.2Ch.4 Sec.5

WPQT according to a recognizedstandard may be evaluated case-by-case for possible acceptance bythe Society. The WPQR shall in thiscase comply reasonably with therequirements of the applicable rules.

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Item Standard Additional requirement from DNV

WPS qualification for clad welding ISO 156 14-7 and ASME IX DNV-CG-0044, see also Sec.4 [3].Additional requirement on testing willinclude micro examination to check thefusion line bonding.

Welding consumables DNV-CP-0069 Non-DNV type approved consumablesmay be accepted provided an all-metalweld test is conducted.

2.7 Qualification of workshops and personnelClass related repair on vessels in operation and requirements for qualification of personnel shall followcurrent rules or statutory regulations, see Table 2.Any workshop conducting repairs to machinery equipment shall have access to the relevant data, for exampledimensions, tolerances and clearances.Service suppliers and workshops shall be qualified as required by the applicable statutory, class or IACSrequirements, see DNV-CP-0484. In case no specific requirements exist for repair by an approved servicesupplier, the maker should be consulted prior to repairs to get advice on appropriate method. Repairs byothers than manufacturer or manufacturer’s representative may, depending on scope of repair, affect the TA/MED approval, and in worst case invalidate it.

Table 2 Certification requirements for welding workshops

Object/item Certificate type Issued by Certification standard Additional description

Welder Welder certificate/qualification

Society or recognizedbody

DNV-RU-SHIP Pt.2Ch.4 Sec.3 [2.1.1]ISO 9606

ASME BPVC IX

ANSI/AWS D1.1

IACS UR W32

Where certificationis performed byanother recognizedclassification societyor independentorganizations,recognition of suchcertification will beevaluated on a case-by-case basis.

Welding workshop Welding workshopapproval *) (WWA)

Society*) DNV-CP-0352 *) Recommendedbut not required forrepairs for vessels inoperation.

NDT operator Level 2 NDT operatorcertificate

Society or recognizedbody

DNV-RU-SHIP Pt.2Ch.4 Sec.7 [3.1.1]ISO 9712

ASNT (AmericanSociety for Non -destructive Testing) orequal

For SNT-TC-1A seeDNV-CG-0051.

NDT service supplier Approval of servicesupplier *) (AoSS)

Society*) DNV-CP-0484 App.B[4]

*) Recommendedbut not required forrepairs for vessels inoperation.

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Object/item Certificate type Issued by Certification standard Additional description

Statutory equipment -work shops

AoSS Society or recognizedbody

DNV-CP-0484

3 Planning a repair

3.1 Repair plan acceptanceIt is the owners responsibility to prepare the repair and test plan which shall be agreed and accepted by theattending surveyor before any repair commences.For some repairs where the survey station or attending surveyor is not known the repair plan proposal maybe sent to 'direct access to technical expert' (DATE) for a review. For statutory items, normally the maker'sadvice should be sought. It will, however, always be the attending surveyor who will formally accept the planand may request changes to the repair plan reviewed by DATE as the repair scope and conditions may havechanged.Repair plan proposals is not required to undergo a formal plan approval by class, the assumption being thatwhen restoring a vessel or its equipment to its original design and function, re-approval of the originallyapproved design is not required.However, when it is either evident or suspected that the damage/failure and a repair to the vessel orequipment adversely affects its design, safety, and/or function, a new approval shall be conducted.Also, design modifications in relation to repair, such as redesign and manufacturing of parts of a component,addition of structural brackets shall be approved by the Society.

3.2 Repair plan proposalA repair plan proposal shall be described and detailed according to Table 3.

Table 3 Repair plan content

Repair plan

Procedure

Is a damage survey carried out? If it is not possible to call in a class surveyor, the owneris responsible to submit a clear description of the damagescope in the repair proposal. The damage shall bedocumented with photos and drawings/sketches detailingthe damage.

Are the original design drawings available? Details of the original design are required in order torestore to original approved design.

Does the planned repair address the cause of the defect? Evaluate if the cause of the repair is addressed andconsider if any modification are necessary to avoid arepeat.

Are any modification planned? Minor modifications may be evaluated and accepted on acase-by-case basis by surveyor otherwise the alteration/modifications shall be submitted for approval.

How will the repair be followed up? Surveyors attendance shall be planned: during repair,voyage repair, with surveyor attendance and remotesurvey.

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Repair plan

Information to be included

Description of defect — Crack: exact location, length, orientation, evidence ofprevious repair.

— Deformations/indents: dimensions, depth, evidencesthat may determine it to be results of mechanicalforces, overheat, creep etc.

— General corrosion/pitting: location, thicknessmeasurement.

Any emergency repair conducted shall be described.

Details of original design — General arrangement.— Details of the relevant welds.— Material specifications.— Tolerances.— Properties specified, e.g. roughness, hardness.

Repair method and processes — Drawing showing cut outs material, including accessopenings, preparation of joints including NDT.

— Repair method description, i.e. use of spare parts,inserts, welding, straightening, grinding, etc.

— Pre-fabrication of components and material.— Arrangement (method) for control of heating/cooling

rate and temperature recording for pre-heating or postweld heat treatment.

Material and component documentation/certification Replacement material and components shall bedocumented/certified as in the original design. Currentrules may be applied, also if these requirements are morelenient.

Weld procedure specifications (WPS) See [2.6]. It shall be ensured that the WPS is relevant forthe type of the joint, type of the weld, position of the weld,parent materials, type of electrodes, stress relieving, i.e.PWHT, etc.

Welder’s certificate See [2.7]. Shall be checked for the type and position of theweld and material.

Welding consumables, i.e. electrodes See [2.6].

NDT operator qualification See [2.7].

Testing and inspection plan

Inspection and test procedure (ITP) The hold points for surveyor’s visit (e.g. fit-up inspections,witnessing of tests) shall be agreed before the repair iscommenced.

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Repair plan

Final testing The type and extent of the final testing shall be included inthe repair proposal, e.g.:

— NDT— hydrostatic testing— leakage tests— sea trials: if significant repairs are carried out to main

or auxiliary machinery or steering gear, a dock and/or sea trial shall be carried out as required by theattending surveyor.

4 Survey of repair

4.1 RepairsThe reviewed and accepted repair proposal shall be discussed with the attending surveyor prior to the startof the repairs. The surveyor shall indicate how the repair progress should be documented and at which pointsin the repair process class shall be invited to witness for example fit up, alignment or intermediate testing.These so-called holding points are normally limited to those parts of the repair procedure essential for thefinal quality of the repair.In addition, the scope of the final survey and testing shall be agreed. Final survey may for example includepressure testing, dock or sea trial or load test.

4.2 Voyage repairsThe same principles as for regular repairs apply, however, it will normally be necessary for the vessel toprovide documentation concerning relevant certification and qualification upfront, in order for class to verifythat these are correct and avoid unnecessary long delays due to e.g. incorrectly certified parts.It is also essential to establish, prior to commencement, that at no point during the repair process, the repairnegatively affects the effectiveness of the structure, including the longitudinal strength and the watertight/weathertight integrity of the vessel. Or that for repairs of machinery, systems, components and equipment,the remaining arrangements meet necessary requirements and intended functionality.The scope of the final survey, testing and attendance by class, shall be agreed upfront.

4.3 Remote surveyFor some damage it might be possible to accept the repairs through a remote survey. This should be agreedupfront with class together with the documentation requirements.

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SECTION 3 MAJOR CONDITIONS - CRITICAL DAMAGE REQUIRINGPROMPT AND THOROUGH REPAIR

1 GeneralSome defects are considered more critical and requiring prompt and permanent repair before continuedoperation and trading. The condition endangers the safety of people on-board, the stability or integrity of thevessel, the cargo, or is likely to cause pollution or presents a threat or harm to the environment.Another guidance for deficiencies normally requiring prompt repair are given in IMO guidelines on port statedetentions A.1119(30) Appendix 2.

2 Hull structuresAny damage in association with wastage over the allowable limits (including buckling, grooving, detachmentor fracture) or extensive areas of wastage over the allowable limits, which affect or in the opinion of theSociety will affect the vessel’s structural, watertight or weathertight integrity and safe operation. Areas to beconsidered include:

— bottom structure and bottom plating— side structure and side plating— deck structure and deck plating— watertight and oil tight bulkheads— hatch covers and hatch coamings— air pipes on the exposed decks including heads and weld connection between air pipes and deck plating— vent piping systems including ventilators and closing devices— for bulk carriers, additional areas to be considered include:

— inner bottom structure and inner bottom plating— inner side structure and inner side plating— bunker piping systems.

3 Machinery and systemsAny damage to main and auxiliary machinery and systems serving main functions, such as malfunction of:

— propulsion system— rudder and steering gear— emergency generator— control system, shut down alarms— excessive oil spills/leakages in engine room/pump room indicating high fire hazard.Some of these components and systems are also related to statutory requirements.

4 Statutory related equipmentDamage to equipment required under statutory regulations or covered by a statutory certificate where flaghas delegated authority to DNV, such as:

— malfunctioning emergency fire pumps— missing/defect lifeboats and rafts.

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SECTION 4 UNDERWATER AND IN-WATER REPAIRS

1 Dry welding on hull with water on opposite sideWelding on hull plating below the waterline of vessels afloat is acceptable only on normal and high strengthhull structural steels with specified yield strength not exceeding 355 MPa and only for local repairs. Weldinginvolving other steels or more extensive repairs against 'water backing' is subject to special considerationand approval by the Society of the welding procedure.Low-hydrogen electrodes or welding processes shall be used when welding on hull plating against 'waterbacking'. Coated low-hydrogen electrodes used for manual metal arc welding shall be properly conditioned toensure a minimum of moisture content.In order to ensure dryness and to reduce the cooling rate, the structure shall be preheated by a torch orsimilar simultaneously with the welding, to a temperature of minimum 5°C, or higher where specified in thewelding procedure.

2 Underwater wet weldingGenerally, DNV is restrictive to underwater wet welding for repairs and in particular for permanent repairs.The detailed repair procedure (method statement) shall be submitted to the Society and is subject to a DNVacceptance in each individual case.Permanent repairs may only be accepted on a case-by-case approval and if the contractor has class approvedwelding procedures qualified for the material, positions, and joint geometry to be welded. The contractorshall also have a DNV workshop approval for underwater wet welding in place and shall have a weldingengineer on staff.The acceptance of temporary underwater wet welding is based on qualified welders, approved weldingprocedures and post-weld inspection procedures. A relevant specification in this respect is AWS D3.6M ClassA: Underwater Welding Code. AWS D3.6M also has stipulations for non-destructive testing to be performedunderwater.Welding performed underwater does not achieve the same mechanical properties as equivalent dry welding.If underwater wet welding shall be considered for permanent repair, the reduced mechanical properties shallbe subject to evaluation and approval by the Society.The AWS D3.6M:2017 Underwater Welding Code  defines important variables associated with underwaterwelding, e.g. metal transfer  characteristics, solidification behaviour, weld appearance, mechanical properties,and describes welding and inspection procedures so that work of a known quality level may be achieved.Three weld classes, A, B and O, are specified in the AWS D3.6M standard. Each weld class defines a set ofcriteria for weldment properties that shall be established during qualification.  The WPS and WPQR shall meetclass A weld requirements in accordance with AWS D3.6.  The welder qualification requirements shall be basedon class A welds.

3 In-water non-destructive testing – magnetic particle testingFor in-water MPI DNV does not have a specific approval programme which may be applied. However, thesame approval principles as outlined in DNV-CG-0051 may be used for verification of the process prior tobeing used in agreement with the attending surveyor.In theory, the in-water magnetic particle testing (MT) should be an effective means of detecting defects asthe apparent weight of the magnetic particles is less and they are more mobile in water. Also, since magneticforces are unaffected by water and gravitational forces are reduced, leakage fields are more effectivelyholding the particles. See examples in Figure 1 and Figure 2.However, when MPI is applied in-water for a vessel, there are conditions that may make MPI in-water difficultto use:

— There shall be no current or movement in water around the inspection areas as this will wash away theparticles.

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— The water shall have good visibility.— The detection medium shall be approved for under water use.— For underwater MPI, the continues magnetization method shall be used.

For in-water NDT, the following applies:

— The NDT operator shall be qualified for in-water NDT inspection to a recognised standard.— NDT shall be carried out in accordance with an agreed written NDT procedure.

Figure 1 Underwater MPI - courtesy Subsea Global Solutions

Figure 2 Fracture indication - courtesy Subsea Global SolutionsSection 4


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SECTION 5 PRESSURE VESSELS

1 GeneralRepair of pressure vessels requires special consideration and good planning due to the criticality of the designand safety concerns. This section describes specific repair guidance for auxiliary and propulsion boilers, andthe principles are equally applicable to pressure vessels in general. Definition of pressure vessels are given inDNV-RU-SHIP Pt.4 Ch.7 Sec.1 [1.1.2].

2 Repair guidanceThe repair plan shall cover permanent repairs/repair solutions which shall be finalised prior to taken intooperation.Any emergency repair or unauthorized repair performed shall be permanently repaired.

— Emergency plugging of boiler tubes initially constitutes a temporary measure only. Depending on theextent of plugging and possible acceptance as permanent repair, it is subject to assessment by theSociety.

— Cracked steel boiler shell plating, tube plates, corroded welding and pitting shall be repaired through platereplacement.  Gauge and weld of some cracks may in some cases be accepted with specified monitoringfollow up requirements.

— Heat affected zones of insert welding shall not interfere with heat affected zones of original welding andprevious repairs.

— Weld joints for replacement material/tubes shall be as detailed in the approved drawing.— Longitudinal and circumferential main joints on boiler shells/drums shall be full penetration butt welds.— Joints shall be welded from both sides of the plates unless otherwise accepted. Circumferential jointswhere dimensions do not allow welding from both sides in headers, pipes and tubes may be welded fromone side only, with or without backing strip.

— No attachments shall be welded on or in the immediate vicinity of a welded joint.— Ends made of welded plates shall be so arranged that they are exposed to the least possible stress.Welded joints passing through flanged curvatures shall be at right angles to these.

3 Prefabricated elementsPrefabricated replacement component elements, i.e. pin element assemblies, tube stacks, formed/weldedplates and bended pipes shall be produced by an approved manufacturer and shall come with a DNV productcertificate, i.e. a material product certificate is not sufficient.

4 Post weld heat treatmentPressure vessels including boilers shall be thermally stress relieved after welding when thematerial thicknesses at any welded connection exceed 20 mm. Material grades NV 1Cr0.5Mo and NV2.25Cr1Mo shall be subject to heat treatment regardless of material thickness.Alternatively, if the manufacturer can document that work procedures control or even eliminate formation ofundue stresses, the requirement for additional stress relieving may be omitted by the Society.

5 Non-destructive testingNon-destructive testing shall be carried out as follows:

— All longitudinal butt weld joints in drums, shells and headers shall be subject to 100% radiographic testing.— 25% of the length of circumferential butt weld joints in drums, shells and headers shall be subjectto radiographic testing.

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— 10% of the total number of circumferential butt weld joints in pipes and tubes shall be subjectto radiographic testing.

— For set-in flat plates the cylindrical shell shall be ultrasonically tested for lamellar tearing in way ofthe circumferential weld to the flat plate. For shell plate thickness 15 mm and less the extent of testingshall be at least 10% and for greater thickness at least 20% of the total length of the weld. The internalfillet weld shall be 100% magnetic particle tested for surface flaws.

— For standpipes and branches with outside diameter exceeding 100 mm, all weld connections to shelland reinforcement rings shall be subject to magnetic particle testing. For outside diameters 100 mmand less, spot testing is considered a satisfactory extent. The magnetic particle testing shall also coverweld joints between reinforcement rings and shell.

— For furnaces and fireboxes non-destructive testing shall as a minimum be carried out as follows, andbe witnessed by the Society:

— 10% of the length of longitudinal  butt weld joints subject to radiographic examination— 10% of the length of circumferential butt weld  joints subject to radiographic examination— 25% of the length of T-welds and corner welds subject to ultrasonic examination.

6 Hydrostatic testingHydraulic pressure testing shall be carried out at 1.5 times the design pressure and be verified by a surveyor.For boilers with downgraded working pressure, the hydrostatic test may be 1.5 times the maximum workingpressure.In cases of a minor repair on an internal component, a leakage test at normal working pressure may be analternative to the hydraulic pressure test, provided the general condition of the boiler both waterside andfireside is examined by the Society and found satisfactory.

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SECTION 6 REPAIR BY BUILD-UP BY METAL COATING AND CLADWELDING

1 GeneralMetal build-up by clad welding or metal spraying is not considered equivalent to what it is supposed toreplace, i.e. solid metal. Thus, special case-by-case considerations shall be taken before using this as a repairmethod.DNV-CG-0044 describes what to consider.For each application location a given coat type, coating procedure and application shall consider:

— thermal, contact and bending loads— stress evaluation, i.e. reduction in fatigue strength— corrosion.

2 Repair by build-up by metal coating and clad weldingWPS for overlay/clad welding shall be qualified according to ISO 15614-7, ASME IX or another recognisedstandard. See DNV-CG-0044.

3 Qualification for thermal sprayingProcedures for the application of thermally sprayed coatings shall record all parameters as required by ISO14921. DVS work sheet 2301 and 2304 may be used as the basis for performing of the thermal spraying.The testing of thermal spraying layers should be performed according to ISO 14923.The responsible personnel shall be checked according to ISO 14918. See DNV-CG-0044See DNV-CG-0044.

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CHANGES – HISTORIC

September 2020 editionThis is a new document.

Changes – historic


DNV AS

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Whether assessing a new ship design, optimizing the performance of a wind farm, analyzing sensordata from a gas pipeline or certifying a food company’s supply chain, DNV enables its customers andtheir stakeholders to make critical decisions with confidence.

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DNV-CG-0456 Repair guidance for vessels in operation

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Transcript of DNV-CG-0456 Repair guidance for vessels in operation