Maintenance of diving bell hoist ropes

Introduction
Use of galvanised or bright (black) wire rope
Maintenance
Inspection and testing
Discard criteria
Rope records
Notes
Further reading

Introduction

1 This diving information sheet is part of a series of information sheets providing guidance on diving at work. It replaces the following diving safety memorandum: DSM 5/1991. The remaining DSMs have either been replaced by other diving information sheets, or will be cancelled when the Diving at Work Regulations come into force on 1 April 1998.

2 The sheet gives recommendations on the composition and maintenance of diving bell hoist ropes. It gives guidance on examining and testing the ropes, and on how to determine whether a rope should be taken out of use.

3 The recommendations set out in this information sheet have been prepared on the basis of earlier guidance and discussions with industry representatives.

4 The recommendations apply to all main hoist ropes used for systems which transport divers through the water, including diving bells, wet bells, baskets etc. (In this guidance, the term 'bell' should be taken to include all these types of system.)

5 Based on the results of fatigue testing and other investigations at British Ropes Ltd and Reading University,¹ the recommendations give particular emphasis to the special problems associated with spin resistant multistrand ropes. These ropes have a characteristic of developing large numbers of internal wire breaks, long before external signs of deterioration become apparent. This characteristic becomes even more pronounced when the rope is frequently immersed or sprayed with sea water, as this leads to severe internal fretting wear.

Use of galvanised or bright (black) wire rope

6 Evidence, from studies of the effects of sea water on the degradation of wire rope, shows that the performance of galvanised rope is considerably superior to that of rope manufactured from bright, ungalvanised wire. Not only does the zinc provide sacrificial protection to the steel from general corrosion, it also counteracts corrosion fatigue and corrosion fretting. So it is recommended that only galvanised wire ropes be used for bell hoist ropes.

7 With regard to the potential risk of hydrogen embrittlement resulting from the cleaning process, not only are there numerous factors that render this almost impossible (including the hot dip in the zinc bath) but rope manufacture from an embrittled wire would be impossible. Therefore any suggestion that galvanised wire has been embrittled in this way is wholly without basis.

8 Galvanised or bright (black) wire ropes may be used as long as the guidance in this information sheet is followed.

Maintenance

9 All sheaves and guide rollers in contact with the moving rope should be given regular visual checks for surface wear, and lubricated to make sure that they run freely. This operation should be carried out at appropriate intervals (generally not exceeding more than one week during diving operations).

10 The rope should be maintained in a well lubricated condition using appropriate marine grade rope lubricants (as recommended by the rope manufacturer). The maintenance of internal lubricant is especially important for multistrand ropes, due to the way they are constructed.

11 A surface dressing should be applied at appropriate intervals from the winch drum to the bell, commensurate with the depth to which the bell is to be deployed during diving operations. Apply the surface dressing by whatever application method is convenient.

12 In order to make sure that the inner layers of the rope remain well lubricated, use an effective pressure lubricator² from the winch drum to the bell, commensurate with the depth to which the bell is to be deployed. This should be carried out at intervals not exceeding six months, and particularly at the end of an operating season, or if the diving system is not to be deployed for a period. The lubricant should be an appropriate heavy marine grade, as recommended by the manufacturer of the lubricator, and approved by the rope manufacturer.

13 In systems that use multilayer winding onto the winch drum, there is the possibility that local damage can be caused to the rope in the regions where the rope crosses from one layer to the next, or where the rope crosses over the turns in the layer beneath. In systems where this form of damage occurs, it is possible to extend the life of the rope. This is done by pulling in the back end of the rope on the drum by a length sufficient to move all the layer crossover regions on the rope, and the Lebus crossovers, where this type of winding is used. This cut-and-slip of the drum end of the rope does not eliminate the crossover damage but distributes it more uniformly over the rope length. Guidance on whether this form of maintenance is necessary, and if so at what frequency, can only be given by an experienced rope examiner in relation to a specific installation. Where this kind of damage reaches a level which indicates that this form of maintenance should be carried out more frequently than every six months, then it is likely that either the wrong rope has been used or there is something fundamentally wrong with the reeving arrangement. In this case, take appropriate steps to reassess the installation.

14 When a new rope is installed, or when the end is pulled in (see paragraph 15), to avoid miscoiling and subsequent damage, it is important that the rope is tensioned as it is wound onto the drum. The weight of the loaded bell in water will indicate the correct magnitude of the load to apply for this operation. The chosen procedure should avoid damage to the rope and in particular should avoid rubbing contact with a hard object.

Inspection and testing

15 During diving operations, where practicable, all accessible rope and associated equipment should be checked visually once each day. Pay particular attention to the termination and the part of the rope entering the socket. On those systems where the termination is inaccessible, then the correct inspection arrangements should be agreed with the competent person.

16 A visual inspection of the rope, from the winch drum to the bell commensurate with the depth to which the bell is to be deployed, should be carried out at appropriate intervals during diving operations, as agreed with the competent person. At the same time, because multistrand rope tends to suffer internal damage, local reductions in diameter or lay distortions should be treated with the utmost suspicion as they are likely to indicate serious internal degradation. Any anomalies should be reported promptly to the diving supervisor who should record them and take appropriate action.

17 The rope and associated equipment must have been examined, tested and certified by a competent person³ (ie someone who, in the case of the rope examination, has had suitable training and experience of rope examination) not more than six-months prior to any use. As an integral part of the diving system, at six-month intervals, the rope should be subjected to a static test at 1.25 times the safe working load, a functional test at the safe working load, and a visual examination. In addition, an overload test (1.5 times the safe working load) should be performed at installation or after any modification which affects its safe working. The safe working load (SWL) is defined as the weight of the rope and bell when the bell is operationally manned. This can be either in air at the surface, or at maximum depth, whichever is the greater.

18 At intervals not exceeding 12 months, the rope should be cut and reterminated in order to:

remove that part adjacent to the termination which is subject to the greatest corrosion due to depth and pressure, and also the part subject to the most severe dynamic loading as the bell is deployed; and
permit an ultimate strength test to be carried out on a sample of the latter, ie that part subject to the most severe dynamic loading.

19 The rope should be cut back to just beyond the first sheave from the bell termination (with the bell fully submerged and allowing for swell). A length sufficient to provide two test samples should be removed from the sheave (in board) end of the cut section. In systems where there is a single vertical fall directly from the winch to the bell (where there is no multi-reeved cursor arrangement), it will be necessary to cut right back to the winch. Full details of the samples (identity and position) should be recorded. Should the first test prove unsatisfactory due to problems with testing procedures, a second test may be carried out. This alternative test should not be used as a way of avoiding discard where a valid test is performed which indicates low strength. (In certain circumstances the competent person may waive the recommendation to cut all the way back to the first sheave (see paragraph 30).

20 During retermination of the rope, a thorough internal examination of a short length of rope adjacent to the cut end should be conducted by a competent person. This is in order to assess:

internal lubrication;
internal wear;
level of residual zinc;
extent of corrosion.

21 After termination, the end fitting should be examined for good workmanship and general integrity. Pay particular attention to alignment, concentricity and lubrication at the socket entry. It is considered good practice to remove the serving at the socket entry, after completion of the termination, to facilitate inspection and effect any relubrication needed.

22 Where the competent person judges that it is highly likely the factor safety between rope strength and SWL will fall below 8:1 before the next examination, then he/she should request more frequent examination and testing.

23 When a rope is not in service, corrosion can take place, particularly within the rope where sea water may have been trapped by any external dressing. This means it is necessary to subject the rope to appropriate inspection and tests, as defined in the following paragraphs, before putting it back into service.

24 For installed equipment not in use, when the period of non-use exceeds one month, the wire rope and termination should be subjected to external visual inspection to identify any corrosion or damage over the length from the winch drum to the bell. This should be commensurate with the depth to which the bell is to deployed.

25 If the period of non-use exceeds six months, the load test specified in paragraph 17 should be carried out.

26 If the period of non-use exceeds 12 months, the rope should be cut back and reterminated, and tested in accordance with paragraphs 18-22.

27 For demobilised equipment that is about to be mobilised, as an integral part of the lifting system, the rope should be subjected to an overload test at 1.5 times SWL. If the rope needs retermination during installation, then consideration should be given to removing samples for destruct testing (see paragraphs 18-19), in order to give 12-month validity to the rope certification. In all other respects, the requirements of paragraphs 24-26 apply.

28 It is recommended that once a year the six-monthly examination is supplemented by the use of an electromagnetic non-destructive test (NDT) device appropriate for the rope concerned. The inspector should use the NDT device to supplement the visual inspection, and in particular to help focus attention on areas of damage. Sections of the rope, from the winch drum to the bell, commensurate with the depth to which the bell is to be deployed, which should be given special attention are:

the part immediately adjacent to the socket;
the part which is in contact with the sheave nearest the bell as the bell enters the water;
in systems which operate with heave compensation, and where there has been repeated operation at the same depth, the part or parts of rope which repeatedly move on and off sheaves or drum when the bell is at operating depth.

29 The NDT device used must be one which has been demonstrated to be suitable, and which the competent person considers acceptable, for the identification of internal damage in the type of rope being examined. The device should be used in accordance with the manufacturer's recommendations, with special regard to rope speed through the device, and diameter and construction of the rope. The device should be used in conjunction with a recorder which can provide a permanent record of the output of the device. This is so that, on successive examinations, a comparison can be made to help detect any degradation. Not only should the same or a similar instrument be used for successive examinations of a particular rope, but also careful records should be retained of any operating variables, instrument settings or calibration procedures, to ensure repeatability.

30 Where a NDT is performed as recommended here, then the recommendations (see paragraphs 18-19) for the annual removal of a length of rope from just beyond the first sheave from the bell termination (with the bell fully submerged and allowing for swell) may be set aside. Instead, a length sufficient to provide test samples for two ultimate tests (see paragraphs 18-19) should be cut from the bell end adjacent to the termination.

Discard criteria

31 Discard criteria, as specified in BS 6570, should be used. However, more specific recommendations are set out in paragraphs 32-35.

32 There should be an examination by a competent person who, when certifying the rope, should be satisfied from an interpretation of the examination that:

the minimum breaking load has not been reduced by 10% or more of its strength when new, as certified by the manufacturer;
the factor of safety between rope strength and SWL has not fallen below 8:1.

33 An ultimate strength test should be carried out and the rope discarded if:

the minimum breaking load measured in the sample taken for testing purposes shows a reduction of 10% or more of the breaking load, when new, as certified by the manufacturer;
the factor of safety between the measured rope strength and SWL has fallen below 8:1.

34 Alternatively, where either the examination or the test indicates a strength reduction as defined in paragraph 33, and the rope examiner is satisfied that the reduction is local to the more heavily loaded section of rope which carries the full weight of the bell in air then, provided the rope is cut back to remove all the doubtful parts, the rope may be retained in service. Where this procedure is followed, the security of the remainder should first be demonstrated by a further ultimate strength test of a sample taken from the end of the remaining rope.

35 If the discard criteria recommended in the preceeding paragraphs are met, then there need be no limitation on the life of the rope. The competent person may however feel that the frequency of testing should be reconsidered after a number of years' service.

Rope records

36 In the interests of good safe practice, and to help identify critical regions for inspection, it is recommended that full records should be kept for each diving bell hoist rope, from its first installation to discard. This rope record, which could generally be incorporated as part of the records of a planned maintenance programme, should record the information in paragraphs 37-40.

37 At installation, details of the installed rope should include:

date and name of manufacturer;
minimum breaking load and certificated test strength;
diameter, construction, wire grade and surface treatment;
date and length installed.

38 Confirmation (and a record of comments when appropriate) of daily and weekly inspections needs to be kept.

39 A record of lubrication (date, lubricant used and application method) needs to be kept.

40 A record of inspections, maintenance and testing, needs to include:

results of six monthly and annual visual inspection;
details of length removed;
location of test sample and strength obtained;
results of internal examination;
details of the NDT, if applicable;
details of pressure lubrication, if applicable;
details of retermination;
results of the functional tests.

41 Where possible, an identifying sleeve or tag should be used to indicate which record the rope relates to. The tag should also specify the maximum weight of the bell and rope.

42 None of the recommendations in this guidance are intended to conflict with or set aside any other recommendations, statutory or otherwise, which may relate to the inspection, maintenance and discard of diving bell hoist ropes or associated installations.

Notes

Maintenance, inspection and discard of diving bell hoist ropes OTH91 338 HSE Books 1993 ISBN 0 11 886387 8
An effective lubricator is one that will not only convey fresh lubricant to the core of the rope but will also displace entrained water. This generally requires high pressure application of a heavy grade of lubricant.
Sl 1976/1019 Offshore Installations (Operating Safety, Health & Welfare) Regulations require that the 'competent person' is neither the owner nor an employee of the installation.