Weldless repair of safety critical piping systems

Introduction

1. This notice provides advice on safe practice for the repair of piping used on offshore installations and highlights poor practice. It is possible during the design life of a topside plant of an offshore installation that a pressurised system will require either modification or repair. The modification of a pressurised system is generally covered by the dutyholder's change control procedures. However, maintenance activities such as patch repair or installation of mechanical clamps may not be controlled to the same degree, partly due to the absence of a clear policy for piping repair.

Background

2. A major hydrocarbon release incident occurred because an unreliable method of repair was applied to safety-critical pipework. This incident is an example of failure to select an appropriate repair technique for defects in a piping system.

Philosophy

3. For safety-critical pipework the repair philosophy should be:

• replace like for like;
• temporary repair until replacement can be carried out;
• permanent repair only where replacement is not practical.

This approach is in line with the principles of prevention as outlined in the ACOP and guidance to the Management of Health and Safety at Work Regulations 1999. In particular, paragraph 30 of the guidance advises dutyholders to 'control risks at source, rather than taking palliative measures'.

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Risk assessment

4. Before any repair is undertaken a risk assessment should be carried out to determine the suitability of using pipe repair technique. Factors that should be considered at this stage include:

• an understanding of the degradation mechanism leading to the requirement of pipe repair and evaluation of the current pipe condition;
• consideration of full duty conditions for the repair including external loading, vibration, environmental deterioration, fire performance etc;
• classification of repair as to whether it covers safety critical pipework and determination of suitable techniques;
• determining whether the repair will be temporary or permanent;
• the ongoing inspection requirements for the repair and suitable inspection techniques.

A suitable Technical Authority should approve these considerations before the repair proceeds.

Design of the repair

5. The basis of the design of repair should follow a recognised design code or demonstrate the equivalent. Most of the offshore topside piping systems are designed to ASME B31.3 Process Piping - ASME code for Pressure Piping. In the first instance ASME B31.3 is not intended to apply to the operation, maintenance or repair of piping systems. The common practice in the oil and gas industry is to use American Petroleum Institute (API) 570, the piping code for the inspection, repair, alteration and re-rating of in-service piping systems According to API 570, the principles of ASME B31.3 shall be followed for repair and alteration of piping system designed to ASME B31.3.

6. In order to follow the principles of ASME B31.3, the repair should meet the installation and operational requirements of piping systems. These requirements can be divided into two categories: structural and functional. The former requirement relates to static strength and fatigue performance, while later include assembly, installation and sealing.

7. For static strength, the repair should be designed to withstand design live loads as well as any cyclic loads imposed on the damaged pipework. To fulfil functional requirement the repair should be made (ie assemble or disassemble) in a reliable manner consistent with the assumptions made in the analysis to demonstrate compliance with both static and fatigue strength criteria. Where the repair has to contain internal or external pressure, seals should be provided that remain functional after installation and during operation.

8. The static strength and fatigue performance of a repair should be validated by recognised finite element or engineering analysis with some degree of experimental verification of the results.

9. Analytical modelling of variables, such as surface contact, surface roughness and friction is difficult and unreliable. These variables affect the functional performance of repair in terms of assembly and sealing. Tests shall be conducted to demonstrate that the repair meets the functional requirements. Testing has to include use of full-scale prototypes, under conditions equivalent to, or more stringent than, those expected during operation.

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Types of repairs

10. Common practice for non-welded pipe repairs employed offshore fall into five broad categories:

• wraparound, off-the-shelf, elastomer lined mechanical clamps;
• engineered clamps manufactured to order;
• off-the-shelf pipe connectors;
• engineered pipe connectors manufactured to order;
• composite overwrap repair, which basically comprises of a woven glass fibre or carbon fibre mat impregnated with a curable resin. This type of repair fall into two groups – off-the-shelf repair kit or engineered design to suit particular applications.

11. Many off-the-shelf clamps and connectors available for use offshore have type approval from a number of inspection authorities. A type approval is an approval of a representative item of equipment endorsed by an independent inspection authority such as Lloyds Register or DNV after appraisal/audit of design and test documentation. There may be limitation on the approval for material of construction, pressure and temperature. Before using such repairs, the dutyholder should make sure that all the structural and functional requirements have been considered in the approval process. For example, for off-the-shelf clamps, in addition to the usual design pressure and temperature loads, the design approval shall include control of axial thrust loads if the damaged piping being clamped has insufficient strength to control the pressure thrust. The effect of clamping and crushing forces on the damaged piping shall also be considered in the design approval.

12. Further advice on the use of repair clamps is provided in the HSE publication 'Leak Sealing Repair Clamps'.

13. For an engineered repair solution, it is common practice to carry out a 'fitness-for-purpose' engineering assessment. This includes: detailed design assessment of repair for process and mechanical requirements; conformance to code requirements; and evaluation of all necessary specifications, drawings and supporting documents.

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Installation of repair

13. For the installation of the off-the-shelf clamps or connectors manufacturer's instructions should be followed in full.

14. Even the simplest repair method, when applied without due attention to training, may result in integrity problems. It should be emphasised that only trained and competent personnel should be used for applying any off-the-shelf repair kit or engineered design repair on piping systems.

Inspection

15. All the weldless repair method utilised to repair a damaged pipe should be regarded as placing a pressure vessel around the damaged area (no welding is involved or substituted). In these situations the dutyholder must consider the life cycle of the repaired pipe system; will the internal metal loss of the pipe underneath the repair continue or will degradation of the repair occur? These considerations will dictate the need to undertake periodic inspection and/or testing of the repair.

16. For all pipe repairs, the operator needs to perform a structured risk assessment that includes consideration of all the potential future damage or deterioration mechanisms (see paragraph 4). The output from this risk assessment will typically be the specification of the necessary inspection and testing activities, and associated periodicities, to ensure continuing 'fitness-for-purpose'.

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Maintenance

17. It is recommended that dutyholders maintain a register of piping repairs to improve the ease of monitoring the integrity of the repair throughout its design life. This provides an immediate overview of the number and location of repairs and should also detail the installation date, inspection requirement and intended replacement date.

Management system

18. Dutyholders should review their safety management system for the non-welded repair of piping system to ensure that the management system recognises the specialist knowledge, skills and competence required for:

• design or selection of repairs;
• installation of a repair;
• inspections of a repair.

Relevant legal requirements

1.

• Health and Safety at Work etc Act 1974;
• the Provision and use of Work Equipment Regulation 1998 SI 1998/2306 The Stationery Office 1998 ISBN 0 11 085625 2;
• the Management of Health and Safety at Work Regulation 1999 SI 1999/3242 The Stationery Office 1999 ISBN 0 11 025051 6; and
• Offshore Installations and Wells (Design and Construction etc) Regulations 1996 SI 1996/913 The Stationery Office 1996 ISBN 0 11 054451 X

There are legal duties on both the owner/operator of the plant and on those who carry out the repair and/or modification

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References

19. You can find additional information in the following publications:

• Management of health and safety at work. Management of Health and Safety at Work Regulations 1999. Approved Code of Practice and guidance L21 (Second edition) HSE Books 2000 ISBN 0 7176 2488 9
• ASME B31.3 (ASME Code for pressure piping, An American National Standard)
• API 570 [Piping Inspection Code –Inspection, repair, alteration, and rerating of in service piping system]
• Leak sealing repair clamps

Further information

Any queries relating to this notice should be addressed to:

Health and Safety Executive
Hazardous Installations Directorate
Offshore Division
Lord Cullen House
Fraser Place
Aberdeen
AB25 3UB
Tel: 01224 252500
Fax: 01224 252615

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