DNV Standard for Certification No. 2.6, certification of ... · CERTIFICATION OF OFFSHORE MOORING...

CERTIFICATION NOTES No.2.6

DET NORSKE VERITAS CLASSIFICATION AS Veritasveien 1, N-1322 Høvik, Norway Tel.: +47 67 57 99 00 Fax: +47 67 57 99 11

CERTIFICATION OF OFFSHORE MOORING CHAIN

AUGUST 1995

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© Det Norske Veritas 1995 Data processed and typeset by Division Ship and Offshore, Det Norske Veritas Classification AS Printed in Norway by Det Norske Veritas 8.95.2000

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FOREWORD DET NORSKE VERITAS (DNV) is an autonomous and independent Foundation with the object of safeguarding life, property and the environment at sea and ashore.

DET NORSKE VERITAS CLASSIFICATION AS (DNVC), a fully owned subsidiary Society of the Foundation, undertakes classification and certification and ensures the quality of ships, mobile offshore units, fixed offshore structures, facilities and systems, and carries out research in connection with these functions. The Society operates a world-wide network of survey stations and is authorised by more than 120 national administrations to carry out surveys and, in most cases, issue certificates on their behalf.

Certification Notes are publications which contain principles, acceptance criteria and practical information related to the Society's consideration of objects, personnel, organizations, services andoperations, in connection with issuance of certificates or declarations, which are not necessarily related to classification.

An updated list of Certification Notes is available on request. The list is also given in the latest edition of the Introduction-booklets to the”Rules for Classification of Ships”, the”Rules for Classification of Mobile Offshore Units” and the”Rules for Classification of High Speed and Light Craft”.

In “Rules for Classification of Fixed Offshore Installations”, only those Certification Notes which are relevant for this type of structure have been listed.

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CONTENTS

1. General requirements......................................................4 1.1 Scope ...............................................................................4 1.2 Chain grades ....................................................................4 1.3 Approval of chain manufacturers ....................................4 1.4 Approval of quality system at chain manufacturers ........4 1.5 Approval of steel mills; rolled bar ...................................4 1.6 Approval of forges and foundries; accessories................4 2. Materials...........................................................................4 2.1 Scope ...............................................................................4 2.2 Rolled steel bars...............................................................5 2.3 Forged steel .....................................................................6 2.4 Cast steel..........................................................................7 2.5 Materials for studs ...........................................................8 3. Design and chain manufacture.......................................8 3.1 Design..............................................................................8 3.2 Manufacturing process ....................................................8 3.3 Manufacturing process records........................................8 3.4 Workmanship ..................................................................9 3.5 Dimensions and dimensional tolerances..........................9 3.6 Welding of studs............................................................10 3.7 Connecting common links .............................................10 4. Tests and inspection of finished chain .........................10 4.1 General ..........................................................................10 4.2 Proof and break load tests..............................................11 4.3 Dimensions and dimensional tolerance .........................11 4.4 Mechanical tests ............................................................11 4.5 Non-destructive testing..................................................11 4.6 Retest, rejection and repair criteria ................................12 4.7 Marking .........................................................................13 4.8 Documentation and Certification...................................13 5. Tests and inspection of accessories...............................13 5.1 General ..........................................................................13 5.2 Proof and break load tests..............................................14

5.3 Dimensions and dimensional tolerances ....................... 14 5.4 Non-destructive testing ................................................. 14 5.5 Test failures................................................................... 14 5.6 Marking......................................................................... 14 5.7 Documentation and Certification .................................. 14 6. APPENDIX A APPROVAL SCHEME FOR CHAIN

MANUFACTURERS .................................................. 15 6.1 Scope............................................................................. 15 6.2 Approval Scheme.......................................................... 15 6.3 Approval Procedure ...................................................... 15 6.4 Documentation of Technical Capabilities ..................... 15 6.5 Documentation of Manufacturing Processes and Process

Controls ......................................................................... 15 6.6 Documentation of product quality; approval testing..... 16 6.7 Extension of Approved Diameter Range ...................... 17 7. APPENDIX B APPROVAL SCHEME FOR STEEL

MILLS.......................................................................... 18 7.1 Scope............................................................................. 18 7.2 Approval Scheme.......................................................... 18 7.3 Approval procedure ...................................................... 18 7.4 Documentation of Technical Capabilities ..................... 18 7.5 Documentation of Manufacturing Processes and Process

Controls ......................................................................... 18 7.6 Documentation of Product Quality; Approval Testing . 18 7.7 Extension of Approved Diameter Range ...................... 19 8. APPENDIX C TENTATIVE SPECIFICATION FOR

STUDLESS CHAIN .................................................... 20 8.1 General Requirements................................................... 20 8.2 Design and Chain Manufacture..................................... 20 8.3 Testing and Inspection of Finished Chain..................... 20 9. FIGURES....................................................................... 21 10. TABLES....................................................................... 23

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1. General requirements

1.1 Scope

1.1.1 This Certification Note specifies requirements for the materials, design, manufacture and testing of offshore mooring chain and accessories, and which are subjected to classification or certification by DNV. Material furnished per this Certification Note shall conform to the applicable general requirements of DNV Classification Rules Pt.2 Ch.1 unless otherwise provided herein.

1.1.2 The equipment specified is intended for use in applications such as: mooring of mobile offshore units, mooring of floating production units, mooring of offshore loading systems, and mooring of gravity base structures during fabrication. Mooring equipment covered are common links, connecting common links (splice links), enlarged links, end links, detachable connecting links (shackles), end shackles, swivels and swivel shackles.

1.1.3 A tentative specification for studless mooring chain (open common links) is included in 8 Appendix C.

1.1.4 Manufacturers and purchasers are adviced that the concepts of classification and certification, respectively, are significantly different concerning DNV’s formal role and involvement as a third party. Equipment purchased per this Certification Note may be intended for DNV classified vessels, in which case DNV will make decisions in cases such as: proposed equivalent solutions, non-conformities, proposed repairs, deviations, or disputes relevant to the applicability of the requirements. Conversely, equipment may be intended for other vessels or structures not subject to classification, in which case DNV will certify conformance. Circumstances quoted above are then formally to be resolved subject to agreement between the manufacturer and the purchaser, unless DNV is expressly authorized to represent the purchaser. It is in the interst of all parties to agree on scope of work and clarify lines of communication at the earliest opportunity.

1.2 Chain grades Depending on the nominal tensile strenght of the steels used for manufacture, chains are to be subdivided into three grades, i.e.: NV R3, NV R3S and NV R4.

1.3 Approval of chain manufacturers Offshore mooring chain are to be manufactured by works approved by DNV. The approval scheme is detailed in 6 Appendix A. A register of approved manufacturers is published separately by DNV.

1.4 Approval of quality system at chain manufacturers

1.4.1 Chain manufacturers are required to implement a quality system ISO 9002, or equivalent, certified by DNV or an accredited body. The provision of such a quality system is required in addition to, and not in lieu of, the witnessing of tests by the Surveyor as specified in Sections 2 to 5.

1.4.2 1While DNV recognizes accredited certification by others, and consequently will not duplicate such certification unless requested to, DNV will utilize its experience in certification of offhore mooring chain and exercise the right to conduct audits, as and when found appropriate.

1.5 Approval of steel mills; rolled bar Bar material intended for chain and accessories are to be manufactured by works approved by DNV. The approval scheme is detailed in 7 Appendix B. A register of approved manufacturers is published separately by DNV.

1.6 Approval of forges and foundries; accessories

1.6.1 Forges and foundries intending to supply finished or semi-finished accessories are to be approved by DNV.

1.6.2 The manufacturer is required to submit information on organisation, manufacturing and testing fascilities and equipment, capacity and past experience. A flow diagram is to be submitted showing the principal process steps and the associated inspection points involved in converting materials into finished accessories. Where any operation is sub-contracted (e.g. heat treatment, machining, inspection) the name of the company is to be given. The manufacturer is also required to submit a complete specification of the chemical composition as determined by ladle analysis and detailed procedure(s) for heat treatment and non-destructive testing. The scope of approval testing will be determined after review of the above information.

1.6.3 Manufacturers intending to supply accessories in machined condition (e.g. Kenter type shackles) are to submit detailed drawings for approval.

2. Materials

2.1 Scope These requirements apply to rolled, forged, and cast steels used for the manufacture of mooring chain and accessories.

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2.2 Rolled steel bars

2.2.1 Steel manufacture The steels are to be manufactured by basic oxygen, electric furnace or such other process as may be specially approved. All steels are to be killed and fine grain treated. The hot work reduction ratio is to be minimum 7 : 1.

2.2.2 Chemical composition For acceptance tests, the chemical composition of ladle samples of each heat is to be determined by the steel maker and is to comply with the approved specification, see 7.6.2.

2.2.3 Mechanical tests

2.2.3.1 Bars of the same nominal diameter are to be presented in test units of 50 tonnes or fraction thereof from the same heat. Test pieces are to be taken from material simulated heat treated, see also 7.6.3.

2.2.3.2 Each heat of NV R3S and NV R4 steel is to be tested for hydrogen embrittlement. In case of continuous casting, test samples representing both the beginning and the end of the charge shall be taken. In case of ingot casting, test samples representing two ingots, one from the first and one from the last groups cast, shall be taken. The samples are to be taken from material representing the top end of the ingot after removing the head.

Two (2) tensile test pieces shall be taken from the central region of bar material. The test pieces shall preferably have a diameter of 20 mm, alternatively 14 mm. One test piece is to be tested within max. 3 hours after machining. For a 14 mm test piece, the time limit is 1,5 hours. (Alternatively, the test piece may be cooled to -60oC immediately after machining and kept at that temperature for a period of max. 5 days). The other test piece is to be tested after baking at 250oC for 4 hours, alternatively 2 hours for 14 mm Ø test piece.

A slow strain rate < 0,0003 s-1 must be used during the entire test, until fracture occurs. (This means approx. 10 minutes for a 20 mm Ø test piece). Tensile strength, elongation, and reduction of area are to be reported. The requirement for the test is:

Z1/Z2 > 0,85

Z1 = Reduction of area without baking Z2 = Reduction of area after baking

If the requirement is not met, the bar material may be subjected to a hydrogen diffusion treatment, and then tested.

2.2.3.3 For all grades, one tensile and three Charpy V-notch test pieces are to be taken from each sample selected. Preferably, middle of strand or ingot product lengths should be sampled. The test pieces are to be taken as shown in Figure 9-1. The results of all tests are to be in accordance with the appropriate requirements of Table 10-1.

2.2.3.4 If the tensile test requirements are not achieved, a retest of two further test pieces selected from the same sample shall be permissible. Failure to meet the specified requirements of either or both additional tests will result in rejection of the test unit represented unless it can be clearly attributable to improper simulated heat treatment.

2.2.3.5 If the impact test requirements are not met, three further test pieces selected from the same sample shall be tested. The results shall be added to those previously obtained to form a new average. This average shall comply with the requirements. No more than two individual results are to be lower than the required average and no more than one result is to be below the specified single value. Failure to meet the requirements will result in rejection of the test unit represented unless it can be clearly attributable to improper simulated heat treatment.

2.2.4 Dimensional tolerances The diameter and roundness shall be within the tolerances specified in Table 10-2, unless otherwise agreed.

2.2.5 Non-destructive testing and repair

2.2.5.1 All bar material is to be subjected to ultrasonic testing at an appropriate stage of the manufacture insofar as rolling operations are concerned, e.g. finished bars, billets, blooms. The bars shall be free of pipe, cracks, and flakes.

2.2.5.2 All finished bars are to be tested by magnetic particle or eddy current methods or such other methods as may be specially approved. The bars shall be free of injurious surface imperfections such as seams, laps, and rolled-in mill scale. Provided that their depth is not greater than 1 % of the bar diameter, longitudinal discontinuities may be removed by grinding and blending to a smooth contour.

2.2.6 Marking Each bar is to be stamped with the steel grade designation and the charge number (or a code indicating the charge number) on one of the end surfaces. Other marking methods may be accepted subject to agreement.

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2.3 Forged steel

2.3.1 General Where the forge produces an end product, i.e. a finished accesssory, the reader is additionally referred to section 5 for complete coverage of requirements to testing and inspection.

2.3.2 Materials and manufacture Forged steels used for the manufacture of accessories must be in compliance with specifications submitted and approved, see 1.6.2.

2.3.3 Chemical composition See 2.2.2.

2.3.4 Heat treatment Finished forgings are to be properly heat treated in compliance with specifications submitted and approved.

2.3.5 Mechanical properties The forgings must comply with the mechanical properties given in Table 10-1, when properly heat treated.

2.3.6 Mechanical tests For test sampling, forgings of the same nominal thickness originating from the same heat treatment charge and the same heat of steel are to be combined into one test unit. Each sample forging shall either be an individual product forging, or it shall be a representative separate forging made from the same heat of steel and shall receive substantially the same reduction and type of hot working and shall be of the same nominal thickness as the production forgings. The separate test forging shall be heat treated together with the production forgings. From each sample, one tensile and three impact test pieces are to be taken per Figure 9-1 and tested.

2.3.7 Non-destructive testing

2.3.7.1 Time of testing and surface preparation Non-destructive testing shall be carried out after heat treatment. Visual inspection and magnetic particle testing shall be performed following final machining by the manufacturer. Ultrasonic testing may be carried out prior to final machining. All non-machined surfaces are to be sand or shot blasted to permit a thorough inspection.

2.3.7.2 Visual inspection All forgings shall be visually inspected on accessible surfaces and shall be free from cracks, laps, seams, folds, and slivers. Other surface discontinuities shall meet the acceptance criteria specified in 2.3.7.3

2.3.7.3 Magnetic particle testing All forgings shall be magnetic particle tested on accessible surfaces. The surfaces shall be divided into high stress and low stress areas, repectively. All fillets, abrubt changes in section, and other areas of significant loading constitute high stress areas. The manufacturer is required to have a sketch or a purchaser supplied forging drawing showing the test locations.

Testing shall be performed in accordance with ASTM A275 or another recognized standard using wet magnetic particles and continuous magnetization method. All surfaces to be tested shall be free of:

a) linear indications exceeding 1,6 mm in high stress areas and 3,2 mm in low stress areas, respectively;

b) nonlinear indications exceeding 4,8 mm; c) four or more nonlinear indications exceeding 1,6 mm

that are aligned and separated by 1,6 mm or less end to end;

d) ten or more nonlinear indications exceeding 1,6 mm contained in any 40 cm2 of surface, with the major dimension of this area not to exceed 150 mm. The area shall be taken in the most unfavorable location relative to the indications being evaluated.

2.3.7.4 Ultrasonic testing Ultrasonic testing shall be performed on each of the first five forgings of a particular design, and on one additional forging for each additional five forgings produced. If any of these additional forgings proves to be unacceptable, each of the remaining four of that group shall be tested.

Testing shall be performed in accordance with ASTM A388 or another recognized standard. Reference blocks shall be manufactured from steel that is similar in chemistry and processing history to the production forging being tested. The blocks shall contain a 3 mm flat-bottom hole.

A forging containing indications equal to or larger in amplitude to that of the reference hole, when properly corrected for distance, is subject to rejection, or repair if applicable.

2.3.8 Repair When surface or subsurface defects are found, these are to be removed by grinding down no more than 5 % of the diameter in depth and streamlined to provide no sharp contours. Repair welding is not permitted. Complete removal of the defect is to be proved by magnetic particle testing.

2.3.9 Marking Marking is to be similar to that specified in 5.6.

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2.4 Cast steel

2.4.1 General Where the foundry produces an end product, i.e. a finished accessory, the reader is additionally referred to section 5 for complete coverage of requirements to testing and inspection.

2.4.2 Materials and manufacture Cast steels used for the manufacture of accessories must be in compliance with specifications submitted and approved, see 1.6.2.

2.4.3 Chemical composition See 2.2.2.

2.4.4 Heat treatment All castings are to be properly heat treated in compliance with specifications submitted and approved.

2.4.5 Mechanical properties The castings must comply with the mechanical properties given in Table 10-1. The requirement for Reduction of Area is, however, reduced to 40% for grades NV R3 and NV R3S, and reduced to 35% for grade NV R4.

2.4.6 Mechanical tests For test sampling, castings of the same nominal thickness originating from the same heat treatment charge and the same heat of steel are to be combined into one test unit. Each sample casting shall either be an individual product casting, or it shall be a test block cast integrally or as separate block made from the same heat of steel and shall be of the same nominal thickness as the production castings. Test blocks shall be heat treated together with the production castings. From each sample, one tensile and three impact test pieces are to be taken per Figure 9-1 and tested.

2.4.7 Non-destructive testing

2.4.7.1 Time of testing and surface preparation See Section 2.3.7.1.

2.4.7.2 Visual inspection All castings shall be visually inspected on accessible surfaces and shall be free from adhering sand, scale, cracks, and hot tears. Other surface discontinuities shall meet the acceptance criteria specified in 2.4.7.3.

2.4.7.3 Magnetic particle testing All castings shall be magnetic particle tested on accessible surfaces. The surfaces shall be divided into high stress and low stress areas, repectively. All fillets, abrubt changes in section, and other areas of significant loading constitute high stress areas. The manufacturer is required to have a sketch or a purchaser supplied casting drawing showing the test locations. Junctions of risers, gates, or feeders to the casting shall be indicated.

Testing shall be performed in accordance with ASTM E709 or another recognized standard using wet magnetic particles and continuous magnetization method. All surfaces to be tested shall meet the acceptance criteria specified in 2.3.7.3.

2.4.7.4 Ultrasonic testing Ultrasonic testing shall be performed on each of the first five castings of a particular design, and on one additional casting for each additional five castings produced. If any of these additional castings proves to be unacceptable, each of the remaining four of that group shall be tested.

Testing shall be performed in accordance with ASTM A609 or another recognized standard. Reference blocks shall be manufactured from steel that is similar in chemistry and processing history to the production casting being tested. The blocks shall contain a 3 mm flat-bottom hole for testing to a depth of 25 mm below surface and a 6 mm flat-bottom hole for testing the remaining volume.

A casting containing indications equal to or larger in amplitude to that of the reference holes, when properly corrected for distance, is subject to rejection, or repair if applicable.

2.4.8 Repair

2.4.8.1 When surface or subsurface defects are found, these are to be removed by grinding down no more than 5 % of the diameter in depth and streamlined to provide no sharp contours. Complete removal of the defective material is to be proved by magnetic particle testing.

2.4.8.2 Proposals to repair by welding are to be submitted for approval. Such proposals are to include details of the extent and position of all defects. Repairs shall be made using qualified procedures and welders.

2.4.8.3 If repair by welding is carried out after heat treatment, and the depth of the cavity prepared for welding exceeds 20 % of the section thickness or 25 mm, whichever is smaller, the casting shall be reheat treated completely. Smaller weld repairs are subject to thermal stress relieving.

On completion of heat treatment, the weld repairs and adjacent material are to be ground smooth and subjected to visual inspection, magnetic particle and ultrasonic testing.

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2.4.9 Marking Marking is to be similar to that specified in 5.6.

2.5 Materials for studs

2.5.1 The studs are to be made of steel having the same nominal composition as the chain, or in compliance with specifications submitted and approved. In the latter case, it is required that the steel’s heat treatment response is reasonably similar to that of the chain steel.

2.5.2 In general, the carbon content should not exceed 0,25%, alternatively the carbon equivalent (IIW) should not exceed 0,58%, if the studs are to be welded in place.

3. Design and chain manufacture

3.1 Design

3.1.1 Drawings giving detailed design of chain and accessories made by or supplied through the chain manufacturer are to be submitted for approval. Typical designs are given in ISO 1704:1991. Detailed design of Kenter shackle is given in API Spec 2F:1988. Components of unconventional design, i.e. not covered in recognized standards, shall have their drawings accompanied with calculatuations or design reports.

3.1.2 In addition, drawings showing the detailed design of the stud shall be submitted for information. The stud shall give an impression in the chain link which is sufficiently deep to secure the position of the stud, but the combined effect of shape and depth of the impression shall not cause any harmful notch effect or stress concentration in the chain link. The radius r, see Figure 9-4, shall be minimum 4 mm. Unless otherwise approved, the depth of the impression c, see Figure 9-4, shall be minimum 2 % and maximum 6 % of nominal diameter, respectively.

3.1.3 Machining of Kenter shackles shall result in fillet radius minimum 3% of nominal diameter.

3.2 Manufacturing process

3.2.1 Offshore mooring chains shall be manufactured in continuous lengths by flash butt welding and are to be heat treated in continuous furnace; batch heat treatment is not permitted.

3.2.2 Furnaces are to be fully stabilized before the production chain enters. The leading and trailing ends of the production chain shall be provided with sufficient scrap chain essentially identical to the production chain to ensure the establishment of uniform conditions during heat treatment.

3.2.3 Temperature uniformity of furnaces shall be checked at least annually during normal furnace operations and whenever production changes to a chain diameter which differs by more than 15% from the previous check.

3.2.4 Furnaces are checked by a monitoring link identical to production links instrumented with two thermocouples; one attached to the surface of the straigth portion and one imbedded in the center of the straigth portion. Temperature shall be monitored by a recorder with resolution sufficient to clearly define all aspects of the heating, holding and cooling process. From the recording of link temperatures a comparison is to be made with respect to recorded furnace temperature. The chart of time and temperature showing the output from thermocouples shall indicate that the chain temperature never exceeds the allowable maximum chain temperature specified after all thermocouples have reached the minimum chain temperature specified. The soaking time shall be within the specified limits.

Any other method for checking temperature uniformity is subject to special consideration and prior approval.

3.3 Manufacturing process records Records of bar heating, flash welding and heat treatment shall be made available for inspection by the Surveyor.

3.3.1 Bar heating

3.3.1.1 For electric resistance heating, the heating phase shall be controlled by an optical heat sensor. The controller to be checked at least once every 8 hours and records made.

3.3.1.2 For furnace heating, the heat shall be controlled and the temperature continuously recorded using thermocouples in close proximity to the bars. The controls shall be checked at least once every 8 hours and records made.

3.3.2 Flash welding The following welding parameters shall be controlled during welding of each link:

a) Platen motion; b) Current as a function of time; c) Hydraulic pressure.

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The controls shall be checked at least every 4 hours and records made.

3.3.3 Heat treatment

3.3.3.1 Chain shall be austenitized at a combination of temperature and time within the limits established. When chain is supplied quenched and tempered, chain shall be tempered above 570°C at a combination of temperature and time within the limits established.

3.3.3.2 Furnace zone temperatures, chain speed and quenching water temperature shall be controlled and continuously recorded. All charts, including monitoring link charts, are to be clearly identified with all pertinent information required for maintaining records.

3.3.3.3 To further control the heat treatment of production chain of grade NV R4 exceeding 700 meters continous length and the test links that represent them, hardness surveys along the length shall be made every 100 meters of continuous heat treatment provided that every cast is represented. Two consecutive links, alternatively two links with one link in-between, shall be tested at this frequency. Hardness surveys shall also be made on each link subjected to mechanical property tests.

The hardness tests are to be made at the same place on each link, preferably on the straight portion, after suitable surface preparation. A minimum of five indentations are made on each link to obtain an average hardness value.

Each link not tested for mechanical properties shall have an average value within 15 % of the link(s) from the same cast that has been satisfactorily tested for mechanical properties. The results shall be recorded. If results do not comply, the link with the largest deviation shall be cut out and subjected to mechanical testing. The hardness surveys shall not be cause for rejection, or reheat treatment, if the mechanical properties are met.

Note Hardness alone is not the sole factor determining the effect of heat treatment since excessively coarsened grains, overheated, or improperly tempered steels may show adequate hardness, but may be deficient in ductility and toughness. However, hardness is a physical property which can be readily determined along the length using portable equipment. Hence the control and monitoring of hardness is seen as a means of achieving consistent and reproducible heat treatment. However, it is recognized that the present experience with this method of process control is insufficient and, consequently, that it may be changed in future.

3.4 Workmanship

3.4.1 All chains are to have a workmanlike finish consistent with the method of manufacture and be free from defects prejudicial to its proper use. Each link is to be inspected in accordance with section 4.5 using approved procedures.

3.4.2 Excess flash weld material shall be removed. A clean fusion zone, including the zone where the stud is pressed into the link, free of foreign material such as scale, weld spatter, dirt, and slag, shall be maintained.

3.4.3 The trimming knives used for excess flash removal shall be systematically and periodically controlled in order to monitor the degree of deterioation. The knives shall be changed out at regular intervals as specified in the applicable work procedure.

3.5 Dimensions and dimensional tolerances

3.5.1 The shape and proportion of links and accessories must conform to ISO 1704:1991 or the designs specially approved.

3.5.2 The following tolerances are applicable to links:

a) The cross sectional area measured per Section 4.3.1 must have no negative tolerance compared with the theoretical area. As a result of being bent around the anvil, however, a particular diameter may be smaller than the nominal:

up to 40 mm diameter : - 1 mm over 40 up to 84 mm diameter : - 2 mm over 84 up to 122 mm diameter : - 3 mm over 122 mm : - 4 mm

The plus tolerance on diameter may be up to 5 % of the nominal diameter, except for the flash weld area where 15 % plus tolerance is permitted.

b) The manufacturing tolerance on a length of five links is + 2,5 %, but may not be negative. Subject to the purchaser’s concent, chain segments not intended to work in way of windlass and fairlead may have 1,5 % negative tolerance.

c) All other dimensions are subject to a manufacturing tolerance of ± 2,5 %, provided always that all parts fit together properly.

d) Studs must be located in the links centrally and at right angles to the sides of the link. The following tolerances are acceptable when measured in accordance with Figure 9-3 and provided that the stud fits snugly and its ends lie flush against the inside of the link:

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Maximum off-centre distance "X" is 10 % of the nominal diameter.

Maximum deviation "α" from the 90°-position is 4°.

3.5.3 The following tolerances are applicable to accessories:

a) Nominal diameter: + 5 %, - 0 % b) Other dimensions: ± 2,5 %

3.6 Welding of studs

3.6.1 A welded stud may be accepted for grade NV R3 and NV R3S chains. Welding of studs in grade NV R4 chain is not permitted unless specially approved, e.g. fatigue testing is required.

3.6.2 Where studs are welded into the links this is to be completed before the chain is heat treated.

3.6.3 The stud ends must be a good fit inside the link and the weld is to be confined to the stud end opposite the flash butt weld. The full periphery of the stud end is to be welded unless otherwise approved.

3.6.4 Welding of studs both ends is not permitted unless specially approved, e.g. fatigue testing is required.

3.6.5 The welds are to be made by qualified welders using an approved procedure and low-hydrogen approved consumables.

3.6.6 The size of the fillet weld shall as a minimum be as per API Specification 2F.

3.6.7 The welds are to be of good quality and free from defects such as cracks, lack of fusion and gross porosity. The toes of the fillets shall have a smooth transition to the link without undercut.

3.6.8 All stud welds shall be close visually inspected. At least 10 per cent of all stud welds within each length of chain shall be tested by dye penetrant or magnetic particles after proof load testing. If cracks or lack of fusion are found, all stud welds in that length are to be tested.

3.7 Connecting common links

3.7.1 Single links to substitute for test links or defective links without the necessity for re-heat treatment of the whole length are to be made in accordance with an approved procedure. Separate approvals are required for each grade of chain and the tests are to be made on the maximum size of chain for which approval is sought.

3.7.2 Manufacture and heat treatment of connecting common link is not to affect the properties of the adjoining links. The temperature reached by the adjoining links is nowhere to exceed 300°C unless proof is furnished to the effect that no embrittlement takes place at higher temperatures.

3.7.3 Each link is to be subjected to the appropriate proof load per Table 10-3, and non-destructive testing per Section 4.5. A second link shall be made identical to the connecting common link; the link shall be tested and inspected per Section 4.4 and 4.5.

3.7.4 Each connecting common link is to marked on the stud in accordance with Section 4.7 plus a unique number for the link. The adjoining links are also to be marked on the studs.

3.7.5 The use of connecting common links is restricted to 3 links, on average, in each 100m of chain.

4. Tests and inspection of finished chain

4.1 General

4.1.1 All chain is to be subjected to proof load tests, break load tests and mechanical tests after final heat treatment in the presence of a Surveyor. Where the manufacturer has a system to record proof loads and the Surveyor is satisfied with the adequacy of the recording system, he need not witness all proof load tests. The Surveyor is to satisfy himself that the testing machines are calibrated and maintained in a satisfactory condition.

4.1.2 During the manufacturing process sufficient links shall be produced to provide the required break test samples and mechanical test samples. These test links shall be suitably identified and attached evenly at both ends of the production chain to be heat treated. Where sub-lengths of production chain are temporarily joined for continuous passage through the furnace, test links shall also be attached in-between. Prior to heat treatment the Surveyor will check the positions of the test links.

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4.1.3 Prior to test and inspection the chain is to be free from scale, paint or other coating. The chain shall be sand- or shot blasted to meet this requirement.

4.2 Proof and break load tests

4.2.1 The entire length of chain shall withstand the proof load specified in Table 10-3 without fracture and shall not crack in the flash weld. The load applied shall not exceed the proof load by more than 10% when stretching the chain. Where plastic straining is used to set studs, the applied load is not to be greater than that qualified in approval tests.

4.2.2 A break-test sample consists of at least 3 links. For chain with nominal diameter 132 mm or above, the sample may consist of one link, provided that tools of similar size and geometry providing a good fit are used. The test frequency is to be based on sampling intervals according to Table 10-4 provided that every cast is represented. Each sample shall be capable of withstanding the break load specified in Table 10-3 without fracture and shall not crack in the flash weld. It shall be considered acceptable if the sample is loaded to the specified value and maintained at that load for 30 seconds.

4.2.3 If the break load capacity of the manufacturer’s testing machine is insufficient, the testing shall be carried out at another place recognized by the Society.

4.3 Dimensions and dimensional tolerance

4.3.1 After proof load testing, measurements are to be taken on at least 5 % of the links for conformance with the requirements of Section 3.5. The cross sectional area shall be determined based on the calculated average of four diameter measurements equally spaced around the circumference on the bent section of the link close to an adjacent link, but without the need for repositioning of the adjacent link.

4.3.2 The entire chain is to be checked for the length, five links at a time. By the five link check the first five links shall be measured. From the next set of five links, at least one link from the previous five links set shall be included. This procedure is to be followed for the entire chain length. The measurements are to be taken preferably while the chain is loaded to 5 - 10 % of the minimum proof load. The links held in the end blocks may be excluded from this measurement.

4.4 Mechanical tests

4.4.1 Links detached from finished, heat treated chain shall be tested for mechanical properties. End links and enlarged links heat treated with the chain need not be tested, provided that common links from the same charge are tested. Prior proof loading of the mechanical test links may be omitted, provided it is documented that the properties, when tested after proof loading, generally equal or exceed those of links without prior proof loading. Comparative data from at least 3 heats of a particular grade to be submitted.

4.4.2 A sample shall consist of one tensile and nine impact test pieces. The tensile test piece shall be taken in the side opposite the flash weld. Three impact test pieces shall be taken across the flash weld with the notch centered in the middle. The position of the weld shall be accurately identified by etching with a suitable reagent before cutting the notches. Three impact test pieces shall be taken across the unwelded straigth side and three impact test pieces shall be taken from the bend region.

4.4.3 The test frequency is to based on sampling intervals according to Table 10-4 provided that every cast is represented. For the location of test pieces see Figure 9-2. Mechanical properties shall be as specified in Table 10-1.

4.4.4 The frequency of impact testing in the bend may be reduced subject to prior approval, provided it is verified by statistical means that the requirements are consistently achieved, and that the energy values are reasonably consistent with those of the straigth side. Data from at least 5 heats of a particular grade to be submitted.

4.5 Non-destructive testing

4.5.1 After proof load testing, all accessible surfaces, including the outer bends, of every link shall be visually inspected. Links shall be free from burrs, rough edges, mill defects, surface cracks, dents, cuts, distinct circumferential trimming marks, and burn marks. Studs shall be correctly positioned and securely fastened.

4.5.2 All links shall be magnetic particle tested on the accessible circumference of the flash welded area and the area gripped by the clamping dies. Additionally, for chain with nominal diameter 132 mm or above, 10 % of the links shall be tested on all accessible surfaces.

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Testing shall be performed in accordance with practice ASTM E709 or another recognized standard using wet magnetic particles and continuous magnetization method. All surfaces to be tested shall be free of:

a) linear indications exceeding 1,6 mm in transverse direction and 3,2 mm in longitudinal direction, respectively

b) nonlinear indications exceeding 4,8 mm c) four or more nonlinear indications exceeding 1,6 mm

that are aligned and separated by 1,6 mm or less end to end.

d) ten or more nonlinear indications exceeding 1,6 mm contained in any 40 cm2 of surface, with the major dimension of this area not to exceed 150 mm. The area shall be taken in the most unfavorable location relative to the indications being evaluated.

4.5.3 Ultrasonic testing shall be performed on all links to test the entire flash weld area for proper fusion and upset.

Testing shall be performed in accordance with practice ASTM E587 or another recognized standard using angle-beam shear waves in the range from 45 to 70°, as appropriate.

Equipment shall be calibrated using IIW blocks. The search unit shall be checked for beam exit point and angle of reflection at least once per working shift or 8 hours, whichever comes first.

Reference blocks shall be made from a chain link that is similar in diameter, surface condition, chemistry, and processing history to the production links being tested.

The block shall contain two reference reflectors in the form of surface notches in the plane of the weld oriented 180° apart; one located on the inner surface adjacent to the stud, and one located on the outer surface. The notch shall be maximum 3 mm wide and cut to a depth 4 % of nominal diameter or 5 mm, whichever is smaller. The notch shall be cut circular with radius 15 mm. With the search unit positioned as appropriate, calibrate the instrument to obtain an indication amplitude from both reflectors of approximately 75 % full screen heigth. Repeat the procedure from the other side of the weld.

Perform the testing by scanning from both sides of the weld with the amplitude calibration increased by 6 dB. A link containing indications equal to or larger in amplitude to that of the reference notch, when properly corrected for distance, is subject to rejection, or repair if applicable.

4.6 Retest, rejection and repair criteria

4.6.1 If the length over 5 links is short, see 3.5.2, the chain may be stretched by loading above the proof test load specified provided that the applied load is not greater than that approved and that only random lengths of the chain need stretching. If the length exceeds the specified tolerance, the overlength chain links shall be cut out and 4.6.2 shall apply.

4.6.2 If single links are found to be defective or not to meet other applicable requirements, defective links may be cut out and a connecting common link inserted in their place. The individual heat treatment and inspection procedure of connecting common links is subject to the Society's approval.

Other methods for repair is subject to the written approval of the Society and the end purchaser.

The use of detachable joining shackles to replace defective links is subject to the written approval of the end purchaser in terms of the number and type permitted.

4.6.3 If a defect is found by visual inspection or magnetic particle testing, it shall be ground down no more than 5 % of the nominal link diameter in depth and streamlined to provide no sharp contours and then rechecked.

4.6.4 If indications of interior flash weld defects are detected during ultrasonic testing, 4.6.2 shall apply. Near surface breaking defects may be ground down per 4.6.3.

4.6.5 If link diameter, length, width, or stud alignment do not conform, see 3.5.2, these shall be compared to the particular dimensions of 40 more links; 20 on each side of the affected link. If a single particular dimension fails to meet the requirement in 2 or more of the extra links, all links shall be examined. Section 4.6.2 shall apply.

4.6.6 If a break load test fails, a thorough examination with the Surveyor informed in a timely manner is to carried out to identify the cause of failure. Two additional break test samples representing the same sampling length of chain are to be subjected to the break load test. Based upon satisfactory results of the additional tests, and the results of the failure investigation, it will be decided what lengths of chain can be accepted. Failure of either or both additional tests will result in rejection of the sampling length of chain represented and 4.6.2 shall apply.

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4.6.7 If a link fails during proof load testing, a thorough examination with the Surveyor informed in a timely manner is to be carried out to identify the probable cause of failure of the proof test. In the event that two or more links in the proof loaded length fail, that length is to be rejected. The above failure investigation is to be carried out especially with regard to the presence in other lengths of factors or conditions thought to be causal to failure.

Two additional break test samples, one from each side of the failed link, are to be subjected to the break load test. Based upon satisfactory results of the additional tests, and the results of the failure investigation, it will be decided what lengths of chain can be accepted. Failure of either or both additional tests will result in rejection of the sampling length of chain represented, and 4.6.2 shall apply.

4.6.8 If the tensile test fails to meet the requirements, a retest of two further test pieces selected from the same sample shall be permissible. Failure to meet the specified requirements of either or both additional tests will result in rejection of the sampling length of chain represented and 4.6.2 shall apply.

4.6.9 If the impact test requirements are not met, three further test pieces selected from the same sample shall be tested. The results shall be added to those previously obtained to form a new average. This average shall comply with the requirements. No more than two individual results are to be lower than the required average and no more than one result is to be below the specified single value. Failure to meet the requirements will result in rejection of the sampling length represented, and 4.6.2 shall apply.

4.7 Marking

4.7.1 Each length of chain shall be marked on the stud of links as follows:

a) At leading and tail end b) At intervals not exceeding those of Table 10-4 c) On connecting common links or shackles and the

immediately adjacent links d) On the first and last common link of each individual

charge used in the continuous length.

4.7.2 The links shall be marked with: a) Chain grade, including NV. b) Certificate number, or an abbreviation as identified in

the certificate. c) The Society’s stamp to be put on each end of the chain

to signify compliance with the requirements.

4.7.3 The identification marks shall be permanent and legible throughout the expected service life of the chain.

4.8 Documentation and Certification

4.8.1 A complete Chain Inspection and Testing Report shall be provided by the chain manufacturer for each order. This report shall include all dimensional checks, test and inspection reports, NDT reports, process records, photographs (if applicable), as well as discriptions of nonconformities, corrective actions and repair work. All accompanying documents, appendices and reports shall carry reference to the original certificate number.

4.8.2 The manufacturer will be responsible for storing, in a safe and retrievable manner, all documentation produced for a period of at least 10 years.

4.8.3 Individual chain certificates are to be issued for each continuous single length of chain.

4.8.4 The chain certificate is to include the following particulars:

a) Purchaser’s name, order number, and vessel identification, when known;

b) Manufacturer’s name and order number; c) Grade of chain and type; d) Chain description and dimensions; e) Identification mark; f) Reference to material certificate numbers; g) Details of heat treatment; h) Mechanical test results; i) Break test load; j) Proof test load; k) The number and locations of all connecting common

links and all marked links; l) Statement to the effect that the results of dimensional

measurements are satisfactory. m) Statement to the effect that the results of visual

inspection and non destructive tests are satisfactory.

5. Tests and inspection of accessories

5.1 General Additional to the requirements specified in Section 2.3 and 2.4, respectively, accessories are to be subjected to proof load tests and break load tests in the presence of a Surveyor. Where the manufacturer has a system to record proof loads and the Surveyor is satisfied with the adequacy of the recording system, he need not witness all proof load tests. The Surveyor is to satisfy himself that the testing machines are calibrated and maintained in a satisfactory condition.

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Prior to test and inspection the chain accessories are to be free from scale, paint or other coating.

5.2 Proof and break load tests

5.2.1 All accessories are to be tested to the proof load prescribed for the grade and size of chain for which they are intended.

5.2.2 At least one accessory out of every test unit or every 25 accessories, whichever is less, is to be tested to the break load prescribed for the grade and size of chain for which it is intended. Accessories which have been break load tested are to be scrapped.

For individually produced accessories or accessories produced in small numbers, alternative testing will be subject to special consideration.

5.3 Dimensions and dimensional tolerances At least one accessory (of the same type, size and grade) out of 25 is to be checked for dimensions after proof load testing, see 3.5.3. The manufacturer is to provide a statement indicating compliance with the purchaser's requirements.

5.4 Non-destructive testing After proof load testing, all chain accessories shall be subjected to close visual inspection and magnetic particle testing as specified in Sections 2.3.7 and 2.4.7, respectively.

5.5 Test failures In the event of a failure of any test, the entire test unit represented is to be rejected unless the cause of failure has been determined and it can be demonstrated to the Surveyor's satisfaction that the condition causing the failure is not present in any of the remaining accessories.

5.6 Marking

5.6.1 Each accessory is to be permanently marked in a low stress area with:

a) Chain grade, including NV; b) Certificate number, or an abbreviation as identified in

the certificate. c) The Society’s stamp to be put on each end of the chain

to signify compliance with the requirements. d) Heat number, or an abbreviation as identified in the

certificate; e) The Society’s stamp to signify compliance with the

requirements.

5.6.2 All detachable component parts shall be stamped with a serial number to avoid mixing components.

5.7 Documentation and Certification

5.7.1 A complete Inspection and Testing Report shall be provided by the manufacturer for each order. This report shall include all dimensional checks, test and inspection reports, NDT reports, process records, as well as discriptions of nonconformities, corrective actions and repair work.

All accompanying documents, appendices and reports shall carry reference to the original certificate number.

5.7.2 The manufacturer will be responsible for storing, in a safe and retrievable manner, all documentation produced for a period of at least 10 years.

5.7.3 Individual certificates are to be issued for each type of accessory.

5.7.4 The certificate is to include the following particulars:

a) Purchaser’s name, order number, and vessel identification, when known;

b) Manufacturer’s name and order number; c) Grade of chain; d) Accessory description and dimensions; e) Identification mark and position; f) Reference to material certificate numbers; g) Details of heat treatment; h) Mechanical test results; i) Break test load; j) Proof test load; k) Statement to the effect that the results of dimensional

measurements are satisfactory. l) Statement to the effect that the results of visual

inspection and non destructive tests are satisfactory.

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6. APPENDIX A APPROVAL SCHEME FOR CHAIN MANUFACTURERS

6.1 Scope This appendix describes the approval scheme and gives the procedure to be followed to obtain DNV’s approval for the manufacture of offshore mooring chain of the stud-link type and open common link type. Approval is confined to a single chain factory and is limited to one type and grade of chain made from bar from a nominated and approved steel works, see also 7 Appendix B. Separate approvals are required if steel bar is supplied from more than one works and for other types and grades of chain.

6.2 Approval Scheme The approval scheme involves the following principal features:

− Assessment of technical capabilities by evaluation of fascilities, processes and experience.

− Detailed evaluation of the manufacturering processes and associated process controls.

− Verification of product quality by approval testing.

6.3 Approval Procedure

6.3.1 General Evaluation of the manufacturer’s report and issuance of the approval certificate will be done at DNV’s office in Høvik, Norway. Approval tests will be witnessed, unless carried out at a recognized laboratory, and Works will be inspected by representatives of DNV.

6.3.2 Application Application for approval with information on type of chain, grade, diameter and delivery condition should be sent to the local DNV office prior to a visit by DNV’s representative.

6.3.3 Approval report On completion of approval testing, the manufacturer is to submit one signed and dated copy of the approval report containing all relevant documentation as specified herein. The report shall be endorsed by DNV’s Surveyor inspecting the works and witnessing the approval tests, confirming that the information is correct.

6.3.4 Approval of manufacturer certificate Manufacturers whose approval report has been satisfactorily reviewed will be issued with a Approval of Manufacturer Certificate and an entry made in DNV’s publication “Register of Approved Manufacturers”.

6.4 Documentation of Technical Capabilities A description of the chain factory is to be submitted with information on organisation, manufacturing and testing fascilities and equipment, capacity and past experience. A flow diagram is to be submitted showing the principal process steps and the associated inspection points involved in converting bar material into finished chain.

Copy of quality system certificate and copies of load testing machines’ calibration certificates to be submitted.

Where mechanical testing or non-destructive testing is sub-contracted, the name of the company is to be given.

6.5 Documentation of Manufacturing Processes and Process Controls Details of the following manufacturing processes and associated controls are to be submitted:

a) Bar heating and bending including method, temperatures, temperature control and recording.

b) Flash welding including current, force, time and dimensional variables as well as control and recording of parameters.

c) Flash removal incl method and inspection. d) Stud insertion method. e) Heat treatment incl. furnace types and profiles, position

of thermocouples, furnace temperature and chain speed control systems, allowable variations in chain temperature and speed, quenching bath temperature control and means of agitation.

f) Proof loading incl. method/machine, means of horizontal support, method of measurement and recording.

g) Non-destructive testing procedures; in-process and final inspection.

6.5.1 Temperature uniformity of furnaces An extensive survey of heat treatment furnaces is required in order to assure uniform temperature and reliable control and checking of temperature.

Temperature uniformity of furnaces shall be verified by two monitoring links identical to production links instrumented with two thermocouples each; one attached to the surface of the straigth portion, and one imbedded in the center of the straigth portion. One monitoring link is to be installed at the leading end and one link at approximately the mid section of the approval chain length. A sketch showing the total length of chain (including also scrap chain at leading and trailing ends) and the relative positions of monitoring links shall be submitted.

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Temperature shall be monitored by a recorder with resolution sufficient to clearly define all aspects of the heating, holding and cooling process. From the recording of link temperatures a comparison is to be made with respect to recorded furnace temperature. The chart of time and temperature showing the output from monitoring link thermocouples and furnace control thermocouples shall be submitted.

6.6 Documentation of product quality; approval testing

6.6.1 General For each type and grade subject to approval one chain length of the max. dimension intended delivered is to be tested. The chain length shall consist of a sufficient number of links to accomplish the testing prescribed below. The chain length is to be produced from bars originating from one charge and shall be heat treated according to the same procedure intended used in production. All proof load tests, breaking load tests and mechanical tests (tensile and impact) are to be witnessed by the Surveyor.

The approval will normally be limited to a chain diameter less than or equal to that of the chain tested.

6.6.2 Records of Manufacturing Processes For the chain length subject to product approval, records shall be submitted covering

a) bar heating b) flash welding c) heat treatment d) proof loading

6.6.3 Chemical composition Complete chemical composition as determined by ladle analysis shall be submitted if the chain manufacturer’s compositional limits in any way differ from those specified by the steel works.

6.6.4 Hardness measurements The hardness distribution after quenching and tempering shall be checked on one link by hardness traverses along two diameters clear of the weld. Vickers equipment with 10 kp load shall be used. Maximum spacing between impressions is 2,5mm.

Diagram showing the hardness distribution is to be submitted. For ordinary QT-steel, the hardness in the core region shall not be less than 85% of the hardness at 1/3 R position below the surface.

6.6.5 Proof load test The chain length shall be proof load tested and meet the applicable requirements in Section 4.2 for stud link chain and 8 Appendix C for open common link chain, respectively.

Where plastic straining is used to set studs, the increase in length as a function of applied load shall be determined and reported. For studless chain (open common link), the increase in length and reduction in width as a function of applied load shall be determined and reported.

The outside and inner length, the outside width and the diameter of three (3) links shall be measured before and after proof load testing and the results submitted. It must also be checked that the links fit each other and can be moved easily.

6.6.6 Examination of stud indentation After proof load testing, one link is to be split into two parts in order to allow examination of stud indentation on both weld side and opposite side of link body. After splitting, photographs of the two footprints shall be taken showing the entire area. Measurements according to Figure 9-4 are to be made and reported.

The two link parts are then to be split into longitudinal sections along the centreline of the footprint. Photographs of the longitudinal sections shall be taken, and measurements of depth and fillet radius as indicated on Figure 9-4 are to be made and reported.

6.6.7 Break Load Test Two (2) break-test samples shall be tested and meet the applicable requirements in Section 4.2. The results to be submitted.

6.6.8 Tensile testing Three (3) links shall be tested and meet the applicable requirements in Section 4.4. From each link, the following test pieces are to be taken 1/3 R below the surface:

− One test piece clear of the weld (opposite side to flash weld).

− One test piece across the flash weld (Location of fracture to be stated).

The results to be submitted.

6.6.9 Impact testing Eigth (8) links shall be tested at -20°C and meet the applicable requirements in Section 4.4. From each link, the following test pieces are to be taken 1/3 R below the surface:

− Three test pieces clear of the weld (opposite side to flash weld).

− Three test pieces across the flash weld − Three test pieces in the bend

The results to be submitted.

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6.6.10 Crack Tip Opening Displacement (CTOD) testing Eigth (8) links shall be tested in accordance with BS 7448, 1991. Samples for CTOD testing consist of the same eigth links as used for impact testing. The CTOD test piece is to be a standard 2 x 1 single edge notched bend piece. The minimum test piece size shall be 50 x 25 mm for chain diameter less than 120 mm. For diameter 120 mm and above, the test piece size shall be minimum 80 x 40 mm. From each link, the following test pieces are to be taken, see Figure 9-5:

− One test piece clear of the weld (opposite side to flash weld)

− One test piece across the flash weld with the crack tip accurately located in the weld seam

The second lowest CTOD value shall meet or exceed the specified requirements in Table 10-5 when tested at minus 20°C.

A complete laboratory test report to be submitted.

6.7 Extension of Approved Diameter Range Application for extension of an existing approval to a larger diameter chain will be considered in each separate case. A reduced test program may be accepted, taking into account the previously obtained test results, the actual increase in diameter and the chain’s delivery condition.

Provided that the increase is restricted to maximum 10 % for quenched and tempered chain, or restricted to 15 % for normalized chain, it will be considered satisfactory if the impact and CTOD testing is carried out on three (3) links. The lowest CTOD value shall then meet or exceed the applicable requirements.

Temperature uniformity of furnaces shall be verified and reported as per 6.5.1, and records of manufacturing processes submitted as per 6.6.2regardless of the increase in chain diameter.

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7. APPENDIX B APPROVAL SCHEME FOR STEEL MILLS

7.1 Scope This appendix describes the approval scheme and gives the procedure to be followed to obtain DNV’s approval for the manufacture of rolled steel bars for offshore mooring chain. Approval is confined to bar to be supplied to a nominated chain manufacturer and is limited to one grade. Separate approvals are required if steel bar is to be supplied to more than one chain manufacturer and for other grades of chain.

Approval will be given only after successful testing of the completed chain.

7.2 Approval Scheme The approval scheme involves the following principal features:

− Assessment of technical capabilities by evaluation of fascilities, processes and experience.

− Detailed evaluation of the manufacturing processes and associated process controls.

− Verification of product quality by approval testing.

7.3 Approval procedure

7.3.1 General Evaluation of the manufacturer’s report and issuance of the approval certificate will be done at DNV’s office in Høvik, Norway. Approval tests will be witnessed, unless carried out at a recognized laboratory, and Works will be inspected by representatives of DNV.

7.3.2 Application Application for approval with information on grade, diameter and delivery condition should be sent to the local DNV office prior to a visit by DNV’s representative.

7.3.3 Approval report On completion of approval testing, the manufacturer is to submit one signed and dated copy of the approval report containing all relevant documentation as specified herein. The report shall be endorsed by DNV’s Surveyor inspecting the works and witnessing the approval tests, confirming that the information is correct.

7.3.4 Approval of manufacturer certificate Manufacturers whose approval report has been satisfactorily reviewed will be issued with a Approval of Manufacturer Certificate and an entry made in DNV’s publication “Register of Approved Manufacturers”.

7.4 Documentation of Technical Capabilities A description of the steel works is to be submitted with information on organisation, manufacturing and testing fascilities and equipment, capacity and past experience. A flow diagram is to be submitted showing the principal process steps and the associated inspection points involved in converting raw materials into finished bar.

Where mechanical testing or non-destructive testing is sub-contracted, the name of the company is to be given.

7.5 Documentation of Manufacturing Processes and Process Controls Details of the following manufacturing processes and associated controls are to be submitted:

a) Steelmaking furnace and capacity of furnace. b) Hot metal ratio, type of scrap used, pig iron

composition. c) Deoxidation practice, alloying practice. d) Ladle treatment (vacuum degassing, desulphurization,

Ar or N2 rinsing) e) Casting method, type of casting machine (when

continuously cast) and teeming practice. Any special routine measure applied for prevention of re-oxidation/reduction of oxide inclusion (stream shrouding, submerged teeming, argon flushing in the mould via the stopper rod etc.) shall be stated.

f) Ingot or billet size and weight. The minimum hot work reduction ratio shall be stated.

g) Hydrogen diffusion treatment, if applied. h) Billet conditioning (hot scarfing, grinding etc.). i) Reheating furnace temperature. j) Rolling temperatures, last pass included. k) Descaling treatment during rolling. l) Surface conditioning and final inspection.

7.6 Documentation of Product Quality; Approval Testing

7.6.1 General For each grade subject to approval, bar of the max. diameter intended delivered originating from one charge is to be tested. The bar samples shall originate from the top of the ingot or, in case of continuous casting, preferably from the tail end of the billet string (after cutting of discards). The bars shall be of a sufficient number and length to accomplish the testing prescribed below.

The approval will normally be limited to a bar diameter less than or equal to that of the bar tested.

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7.6.2 Chemical composition The steelmaker is to submit a complete specification of the chemical composition as determined by ladle analysis. The specification shall give the permissible range or maximum value, as appropriate, for each of the following elements: C, Si, Mn, P, S, Cr, Ni, Mo, Cu, Sn, Sb, As, B, O and N. Elements which are intentionally added, but not listed here, must also be specified.

For Grade NV R4 chain the steel should contain a minimum of 0,20 per cent molybdenum.

The ladle analysis for the approval charge is to be submitted.

7.6.3 Heat treatment sensitivity analysis Samples of full bar diameter and approximately 200 mm long shall be used to determine the sensitivity to deviations from the assumed optimum heat treatment of the chain with respect to temperature and time. It is important that the assumed optimum heat treatment be performed in a manner to simulate realistic chain production conditions. Two different test programs are specified (dependent of the chain's delivery condition), and the results shall be reported.

7.6.3.1 Test program normalizing Four (4) samples shall be normalized at the temperatures given below. Unless otherwise directed by the chain manufacturer, the soaking time should be 30 minutes.

1: Ac3 + 30oC 2: Ac3 + 60oC 3: Ac3 + 90oC 4: Ac3 + 120oC

From each sample, one tensile and three Charpy V-notch test pieces shall be tested. Test piece sampling and orientation is given in Figure 9-1.

7.6.3.2 Test program quenching and tempering Nine (9) samples shall be quenched and tempered in accordance with the program stipulated below. The tempering temperature chosen, denoted T, is a matter of agreement between the steel works and chain manufacturer. However, it shall not be less than 570oC. The soaking times may be adjusted in order to simulate as close as possible the specific chain factory conditions.

1: Ac3 + 30°C / 30 minutes T / 60 minutes 2: Ac3 + 60°C / 30 minutes T / 60 minutes 3: Ac3 + 90°C / 30 minutes T / 60 minutes 4: Ac3 + 60°C / 30 minutes T / 30 minutes 5: Ac3 + 60°C / 30 minutes T / 90 minutes 6: Ac3 + 60°C / 30 minutes T-40°C / 60 minutes 7: Ac3 + 60°C / 30 minutes T-20°C / 60 minutes 8: Ac3 + 60°C / 30 minutes T+20°C / 60 minutes 9: Ac3 + 60°C / 30 minutes T+40°C / 60 minutes

From each sample, one tensile and three Charpy V-notch test pieces shall be tested. In addition, Jominy data or CCT diagram shall be provided for the candidate material.

7.6.4 Testing for temper embrittlement; QT steels In order to check the candidate material's susceptibility to temper embrittlement, the following heat treatment and testing is to be carried out:

Bar material shall be austenitized, quenched and then heated to the lowest allowable tempering temperature (as determined in 7.6.3.2). Two different cooling conditions after soaking shall be used:

− Rapid cooling, i.e. by water. − Slow cooling, i.e. a minimum of 40 minutes is required

for cooling down to 300oC.

For each cooling condition, Charpy V-notch impact testing shall be carried out at the following temperatures: 0, -20 and -40°C. Requirement for the test is that the specified Charpy impact energy at -20°C shall be met. Test results are to be presented.

7.6.5 Testing for strain aging The susceptibility to strain aging of the candidate material is to checked by Charpy V-notch impact testing after the following treatments:

− Simulated heat treated. − Simulated heat treated and deformed 5 %. − Simulated heat treated, deformed 5 % and aged 1 h at

100oC. (100oC is chosen since only nitrogen aging is considered relevant at the normal service temperature).

For each condition, Charpy V-notch impact testing shall be carried out at the following temperatures: 0, -20 and -40°C. Requirement for the test is that the specified Charpy impact energy at -20°C shall be met. Test results are to be presented.

7.6.6 Testing for hydrogen embrittlement; grade NV R3S and NV R4 The susceptibility to hydrogen embrittlement is to be checked by tensile testing in accordance with Section 2.2.3.2.

Test results are to be presented.

7.7 Extension of Approved Diameter Range Application for extension of an existing approval to a larger diameter bar will be considered in each separate case. A reduced test program may be accepted, taking into account the previously obtained test results, the actual increase in diameter, minimum hot work reduction ratio, and the material’s hardenability, when applicable.

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8. APPENDIX C TENTATIVE SPECIFICATION FOR STUDLESS CHAIN

8.1 General Requirements

8.1.1 Scope The following tentative requirements apply to the materials, design, manufacture and testing of studless mooring chain (open common link type) intended to be used for long term mooring applications. Such applications will generally mean that the mooring lines are deployed once for a pre-determined design life, and where the possibilities for periodic inspections may be restricted.

The specification is made tentative because of limited experience with this type of link. DNV therefore reserves the right to make adjustments during a period in order to obtain the intention reflected in the specification.

Studless chain furnished per this Certification Note shall conform to all the applicable requirements for stud link chain in Sections 1 to 5, and in 6 Appendix A and 7 Appendix B. Consequently, this appendix only provides the particular requirements relevant for studless chain, where the corresponding requirements in Sections 3.1, 4.2 and 4.7 are not directly applicable.

8.2 Design and Chain Manufacture

8.2.1 Design

8.2.1.1 Drawing(s) giving detailed design of the open common link is to be submitted for approval.

The nominal outside length shall be 6xd (d is nominal diameter), and the nominal outside width shall be in the range 3,3xd to 3,4xd. Links having different proportions, but provided always that the ratio outside length to outside width equals or exceeds 1,76, may be specially approved subject to submittal of calculatuations or design reports, including information on the link’s ability to accomodate the enlarged link and other relevant connectors.

Minimum proof test load to be applied shall be stated on the drawing together with outside length and outside width when measured after proof load testing.

8.3 Testing and Inspection of Finished Chain

8.3.1 Proof load test

8.3.1.1 All chains shall be tested to at least the proof load specified on the approved drawings, provided that the load complies with the following minimum requirements:

NV R3 proof test load (kN) : 0,0156d2(44 - 0,08d) NV R3S proof test load (kN) : 0,0174d2(44 - 0,08d) NV R4 proof test load (kN) : 0,0192d2(44 - 0,08d)

The actual load applied may exceed the minimum specified load provided that plastic straining in single links is restricted to 2,5% permanent elongation.

Note The above proof loads correspond to 70 % of the break loads. Compared with the proof loads for stud link chain of same diameter, the respective values in percentage are 100 (R3), 96,7 (R3S), and 88,9 (R4).

8.3.2 Marking

8.3.2.1 The method of marking shall result in legible identification and should preferably be permanent throughout the expected lifetime of the chain, however, marking by stainless steel welding consumables is not permitted.

8.3.2.2 The chain links shall be marked on the outer straigth part; non-welded side.

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9. FIGURES

Figure 9-1 Sampling of steel bars, forgings and castings

Figure 9-2 Sampling of chain links

Figure 9-3 Manufacturing tolerances

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Figure 9-4 Sectioning of link and measurements of stud footprint area.

Figure 9-5 Position of CTOD and Charpy test pieces. CTOD piece shown sectioned through notch.

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10. TABLES

Grade Min. Yield Stress

Min. Tensile Strength

Min. Elongation

Min. Reduction

of Area

Min. Charpy V-notch Energy

J

N/mm2 N/mm2 % % Temp. (1) Average Single

°C Base Weld Base Weld

NV R3 410 690 17 50 0

-20

60

40

50

30

45

30

38

23

NV R3S 490 770 15 50 0

-20

65

45

53

33

49

34

40

25

NV R4 580 860 12 50 0

-20

70

50

56

36

53

38

42

27

1) At the option of the purchaser, and when chain segments are intended to be permanently submerged, testing may be carried out at 0°C. Otherwise, testing to be carried out at -20°C.

Table 10-1 Mechanical properties of offshore mooring chain

Nominal diameter

mm

Tolerance on diameter

mm

Tolerance on roundness

dmax - dmin

mm

51 - 80 -0 +2,0 1,50

81 - 100 -0 +2,6 1,95

101 - 120 -0 +3,0 2,25

121 - 160 -0 +4,0 3,00

Table 10-2 Dimensional tolerances for rolled bars

Grade NV R3 Grade NV R3S Grade NV R4

Proof test load (kN) 0,0156d2(44 - 0,08d) 0,0180d2(44 - 0,08d) 0,0216d2(44 - 0,08d)

Break test load (kN) 0,0223d2(44 - 0,08d) 0,0249d2(44 - 0,08d) 0,0274d2(44 - 0,08d)

Chain weigth (kg/m) 0,0219d2

Length over 5 links (mm) Minimum 22d

Maximum 22,55d

Table 10-3 Formulas for proof and break test loads, weigth and length over 5 links

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Nominal

chain diameter

(mm)

Maximum

sampling interval

(m)

- 48 91

49 - 60 110

61 - 73 131

74 - 85 152

86 - 98 175

99 - 111 198

112 - 124 222

125 - 137 250

138 - 149 274

150 -162 297

163 - 175 322

Table 10-4 Frequency of break load and mechanical tests

Chain type Grade NV R3 Grade NV R3S Grade NV R4

Base Weld Base Weld Base Weld

Stud link 0,20 mm 0,10 mm 0,22 mm 0,11 mm 0,24 mm 0,12 mm

Studless 0,20 mm 0,14 mm 0,22 mm 0,15 mm 0,24 mm 0,16 mm

Table 10-5 CTOD requirements

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DNV Standard for Certification No. 2.6, certification of ... · CERTIFICATION OF OFFSHORE MOORING...

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