Pipelines and gas supply industry: FAQs

Introduction

The Health and Safety Executive [HSE] receives a number of enquiries each year about the minimum depth at which gas mains and services should be laid. Often the enquiries are prompted when members of the public damage pipes at home (for example when gardening) or contractors damage pipes when carrying out work at domestic premises (for example when relaying driveways).

This information document summarises the legal requirements concerning how deep gas mains and services should be laid and the precautions required to minimise the risk of damage to pipes from third party activities. The guidance should be considered as minimum good practice standards.

Minimum depth requirements

Gas mains and service pipes should be designed and installed in accordance with the requirements of the Pipelines Safety Regulations 1996 (PSR). PSR does not specify how deep mains and services should be laid.

However, the Regulations are supported by HSE guidance 'A guide to the Pipelines Safety Regulations 1996' and there is also a Health & Safety Commission [HSC] Approved Code of Practice and Guidance 'Design, construction and installation of gas service pipes'.

These publications refer to the Institution of Gas Engineers' guidance 'IGE/TD/3 Distribution Mains' and 'IGE/TD/4: Gas services' which specify the minimum depth of cover which gas mains and services should be laid in order to minimise the risk of accidental third party damage.

HSE expects mains and services to be laid at the depths specified in these publications unless other effective precautions are taken to minimise the risk of third party damage.

• A gas main should normally be laid with a minimum depth of cover of 750 mm in a road or verge and 600 mm in a footpath.
• A gas service pipe should normally be laid with a minimum depth of cover of 375 mm in private ground and 450 mm in footpaths and highways.

However, these depths are only a guide and should not be relied on when carrying out work near gas services or mains. For example, road levelling, landscaping and other changes to ground conditions after a gas main or service has been laid (often decades before) can result in the depth of the ground cover changing over time. Also, gas pipes may have projections coming from them, such as valves, which are not shown on plans and may have less depth of cover than the pipe.

Gas mains and services are at risk of being damaged by third party interference during construction activities. PSR, regulation 15 requires that no person should cause damage to a 'pipeline and is aimed at third parties carrying out work around pipelines.

HSE publication HSG47 'Avoiding danger from underground services' gives detailed guidance to contractors on avoiding damage to gas mains and services, including information on detecting underground services and safe digging practices.

The key precautions include:

• Obtain plans of the gas pipes from the pipeline operator;
• Locate the line of the pipes using suitable locating devices;
• If pipes operating at pressures of 2 bar and above are indicated, contact the pipeline operator before work begins;
• Ensure site workers are briefed on the location of the pipes and the precautions required;
• Adopt safe digging practices. For example, mechanical excavators should not be used within 500mm of a gas pipe and excavations should be done using hand tools.

If a gas main or service is damaged, it is very important that it is reported to the gas distribution company.

They can be contacted on emergency number 0800 111 999.

• HSE is satisfied that providing the above standards are met, then the risk of damage to gas mains and services is low.
• HSE continues to work with pipeline operators to ensure that mains and services are laid at the depths outlined above.
• HSE expects contractors to meet their obligations to prevent damage to gas mains and services, which includes compliance with the requirements in HSG47.

Under the Pipelines Safety Regulation [PSR] 1996, regulation 13, gas conveyors are required to maintain their networks in a safe condition.

In respect of iron gas mains, there is currently no feasible alternative to maintaining the network other than to decommission it and replace it with a more suitable material, usually polyethylene. This is the basis of HSE's enforcement policy, which requires iron gas mains within 30m of property to be decommissioned and replaced at the latest by March 2032.

In support of that policy PSR was amended in 2003 to allow HSE to approve a programme submitted by a pipeline operator for the decommissioning of iron pipes. This includes cast and ductile iron pipes but not steel or pipes made from other materials. The intention of PSR regulation 13A was to give legal underpinning to HSE's Enforcement Policy and was principally aimed at major distribution networks which, because of their size, have practicable constraints on how much main can be replaced each year.

There is no legal requirement for pipeline operators to have an approved replacement programme. However, if they submit a programme which is suitable and sufficient, HSE must approve it. HSE may also prepare and approve a programme for which no suitable and sufficient programme has been prepared by a pipeline operator.

If pipeline operators have an approved programme, they have a defence from prosecution if they are complying with it and a failure occurred on a pipe which was not yet due for replacement under the programme. However, the defence would not apply if the operator had knowledge which would indicate that the particular pipe was likely to fail.

It is important to note that the defence provided by regulation 13A does not give operators total immunity from prosecution. They still have an overarching duty under the Health and Safety at Work Act 1974 to do everything reasonably practicable to ensure the health and safety of both their employees and non-employees (which includes the general public), and the defence provided by the new regulation 13A does not remove that duty. It is only in the circumstances set out in PSR paragraph 13A(6) that the operator is provided with a defence.

Details of the approved programmes can be found on the Information page.

Introduction

The answers to these Frequently Asked Questions address some of the more common enquiries which have been made about the safety aspects of the new high pressure natural gas pipeline which is being built by National Grid Gas to connect the new Liquefied Natural Gas (LNG) terminals at Milford Haven to the National Gas Transmission System (NTS). The FAQs cover the role HSE has in enforcing health and safety law in relation to the pipeline and in giving advice to planning authorities.

What is HSE's role in ensuring the safety of the Milford Haven pipeline?

The operator of the pipeline, National Grid Gas, has a duty to ensure that the pipeline is designed, constructed and operated safely. HSE's role is to enforce health and safety law and to ensure that the risks are properly controlled. We do this by setting land use planning zones around the pipeline and associated above ground installations (AGIs), assessing the pipeline design, and undertaking inspections during construction and operation.

Under The Pipelines Safety Regulations 1996, the pipeline operator must notify HSE of any new cross-country pipeline which is to be constructed as part of the National Gas Transmission System (NTS) - this is known as the pre-construction notification. The operator must send a further notification 14 days before gas is introduced into the pipeline - the pre-operation notification.

When HSE received the Milford Haven Pipeline pre-construction notification we advised local authorities of appropriate land use planning controls along the route of the pipeline and HSE's specialist pipeline inspectors assessed the design. They determined whether the design of the pipeline, pressure regulating installations and other associated AGIs met the required codes and standards and, so far as is reasonably practicable, protected people from health and safety risks.

HSE's specialist pipelines inspectors inspect the pipeline and AGIs during the construction phase to make sure that the materials and techniques being used are in accordance with the design. They will also carry out inspections during commissioning to confirm that the test procedures are being followed.

HSE will also inspect the pipeline during its operation as part of HSE's existing NTS intervention plan. This intervention plan covers all aspects of the operation of the national transmission system and is the basis of HSE's inspections for all 6,800km of high pressure gas pipelines in the NTS.

HSE provides each local planning authority (LPA) along the pipeline route with 3 zone distances which are used to generate HSE's land-use planning advice in the vicinity of the pipeline. The 3 distances are based on the results of a quantified risk assessment (QRA) of each notified pipeline. The QRA uses the information in the notification, namely: pipeline diameter, wall thickness, maximum operating pressure, type of steel and its depth of burial. For natural gas pipelines, the 3 zones calculated are the distances at which there are 0.3, 1 and 10 chances per million per year (cpm) of receiving HSE's dangerous dose of thermal radiation. Where the risk of receiving 10 cpm is not achieved at the pipeline, the Building Proximity Distance calculated from the Institution of Gas Engineer's IGE/TD/1 code is used for the inner zone distance instead.

HSE's QRA methodology accounts for incidents at the pipeline which result in both complete rupture and two different sized punctures, their ignition and the thermal radiation from the resulting trench and jetfires. The likelihood of such events is taken from both operational experience and predictive failure frequency methodologies.

There may be locations along the pipeline route which have been constructed with additional protection, for example when passing through a built-up area. This protection is usually obtained by laying the pipeline in thicker wall sections as necessary. When applying HSE's PADHI+ land use planning methodology, if an 'advise against' response is obtained for a particular development, the LPA is encouraged to ask the pipeline operator whether such protection exists at the location in question. If the LPA sends the relevant information to HSE, a further QRA is undertaken to see if the additional protection would alter HSE's advice.

The advice outlined above relates to the land use planning process and is not the principal means to achieve safety when the pipeline starts to operate. That is achieved through the Pipelines Safety Regulations 1996.

We know that if the pipeline is properly designed and constructed then the likelihood of failure when it is brought into service will be remote. With adequate standards of inspection, monitoring and maintenance during its life the risks from the pipeline will be kept to a very low level. HSE's pipelines inspectors will assess the adequacy of the operator's design and inspect the construction phase of the project. Further inspections will follow during the pipeline's operational life.

The risks from the pipeline will be very low and comparable to many other risks which we experience in our everyday lives. There is no guarantee of absolute safety in anything that we do, even crossing the road and everyday activities around the home carry elements of risk. In comparison to other risks we experience in normal life the pipeline will be safe.

The Milford Haven pipeline is part of a category of some 22,000km of pipelines designated, as "major accident hazard pipelines". There is a requirement under Land Use Planning (LUP) for the area surrounding these pipelines to be zoned to indicate the level of risk more explicitly. The Inner Zone generally coincides with a surrounding area within which there is a 10 chances per million (cpm) per year risk to a typical householder of exposure to a 'dangerous dose'. The 'dangerous dose' level is taken to be equivalent to a 1% chance of fatality when a healthy person receives the dose.

However for the Milford Haven pipeline which carries natural gas, the risk is much lower approximately 2 cpm directly on top of the pipeline itself. In such cases HSE pragmatically sets the Inner Zone by using the building proximity distance as defined in the Institution of Gas Engineers and Managers design code "Recommendations on Transmission and Distribution Practice" (IGE/TD/1). This proximity distance is the minimum distance recommended between the pipeline and any normally occupied building.

The following table summarises the Land Use Planning zones for the Milford Haven Pipeline.

The following table compares the calculated risks from the pipeline with risks that are experienced in normal life.

Under The Pipelines Safety Regulations 1996, the pipeline operator has a duty to notify HSE of any new cross-country pipeline which is to be constructed as part of the National Gas Transmission System (NTS). The operator must send a further notification 14 days before gas is introduced into the pipeline.

HSE's specialist pipelines inspectors will inspect the pipeline and above ground installations (AGIs) during the construction phase to make sure that the materials and techniques being used are in accordance with the design. They will also carry out inspections during commissioning to confirm that the test procedures are being followed.

HSE will also inspect the pipeline during its operation as part of HSE's existing NTS intervention plan. This intervention plan covers all aspects of the operation of the national transmission system and is the basis of HSE's inspections for all 6,800km of the NTS.

The Milford Haven pipeline will operate between 70 – 94 bar. The National Gas Transmission System (NTS) currently operates pipelines at up to 85 bar in other parts of Great Britain. The NTS is made up of some 6,800 km of high pressure gas pipelines. Work on this system began in 1966 following the discovery of natural gas in the southern North Sea. The early network was largely constructed from 36ins diameter pipe running at 70 bar.

In the 1970s major gas finds were discovered in the northern North Sea. This led to the construction of an onshore gas terminal at St. Fergus, north of Aberdeen, and two pipeline feeders to take gas south. As gas input from the north increased extra feeders were built: another 36ins pipeline, followed by a 42ins pipeline. In 1999 the existing lines and these new lines were uprated to 85 bar. Finally, a 48ins feeder was added running as far south as Montrose. This is designed for 94 bar operation but currently runs at 85 bar due to supply constraints.

There are other onshore British pipelines which transport flammable substances at pressures higher than 94 bar.

Pipelines associated with gas terminals operate at up to 148 bar.

In Scotland and northern England the 1,200km ethylene pipeline network operates at 95 -100 bar. And offshore pipelines may operate at much higher pressures – some at over 300 bar.

The European standard for gas pipelines is EN 1594-2000 "Gas supply systems - Pipelines for maximum operating pressure over 16 bar - Functional requirements". This standard applies across the EU and in some other European countries such as Switzerland; it may be supplemented by national standards.

Other European countries either permit or already operate high pressure pipelines. For example:

• In Germany Ruhrgas has operated some 250km of 100bar pipeline since 1993
• In Ireland pipelines are built in accordance with I.S. 328 : 2003 Code of Practice for Gas Transmission Pipelines and Pipeline Installations. This allows pipelines to operate at up to 100 bar, though none currently operate at this pressure.
• The Dutch standard for pipelines is NEN 3650(2003). The standard does not specify operational pressure limitations but specifies that pipelines should be designed so that additional risks are tolerable. A 100 bar onshore gas pipeline is currently being designed.

The European standard for gas pipelines is implemented in Britain as BS EN 1594-2000 "Gas supply systems - Pipelines for maximum operating pressure over 16 bar - Functional requirements". This standard applies across the EU and some other European countries such as Switzerland.

However, the code normally used in Britain is an Institution of Gas Engineers document IGE/TD/1 – 'Steel Pipelines for High Pressure Gas Transmission' which covers pipelines operating at up to 100 bar. This code complies with the requirements of BS EN 1594 but is stricter in some areas such as built up areas or at road crossings where it requires thicker pipe than the European standard.

Pressure Regulating Installations (PRIs) are deemed to be part of the pipeline by the Pipelines Safety Regulations 1996. They are designed, constructed and operated to the Institution of Gas Engineers' technical document IGE/TD/13 – 'Pressure regulating installations for transmission and distribution systems'.

The recommendations contained in the IGE documents have been built up over years of operational experience and represent best practice. Use of the codes helps the pipeline operator to demonstrate that the risks from the pipeline and pressure regulating installations will be as low as reasonably practicable.

The design of the Milford Haven pipeline and its above ground installations (AGIs) meets the industry standards.

Before the pipeline is tested all welds will have been inspected for defects by ultrasonic or radiographic techniques.

The Pipeline Safety Regulations 1996 require National Grid to maintain the pipeline in good repair. The operator will carry out an on line inspection using an inspection tool, known as a pig, which is propelled inside the pipeline by the gas flow. The first inspection will take place early in the life of the pipeline and at scheduled intervals afterwards. Inspection pigs at the size of the Milford Haven pipeline (48 ins) are in use on existing NTS pipelines.

The pipeline operator has a duty to prepare and test emergency procedures for dealing with the consequences of a major accident involving a pipeline.

The local authority must prepare an emergency plan which details how an emergency relating to a possible major accident in its area will be dealt with. There is no legal requirement for information about a major accident hazard pipeline to be supplied to the public by the local authority or pipeline operator. However, local authorities and/or pipeline operators may elect to supply information in some cases – eg to those who have responsibility for vulnerable groups such as schools, hospitals and homes for the aged. The emergency plan should specify how warnings will be given to members of the public who may be affected by an incident or accident.

Emergency procedures and plans are already in place for the existing 6,800km of pipelines in the national gas transmission system.

The security services are involved in deciding on the appropriate measures which need to be taken to combat terrorism. HSE looks at the work related risks.