Purpose

This SPC provides guidance for Inspectors on the application of COMAH and PSR to the development and operation of sites at which natural gas is stored in depleted (or partially depleted) reservoirs. It applies to establishments consisting of a single depleted reservoir, import and export process plant and interconnecting pipework (i.e. the pipeline(s) within the COMAH establishment that connect the depleted reservoir and the process plant).

Background

Depleted and partially depleted reservoirs are volumes of underground porous strata which are no longer used for hydrocarbon production. The term 'depleted reservoir' is used in this guidance to refer to both fully depleted and partially depleted reservoirs. Where a depleted reservoir has been previously used for oil production the process of storing natural gas may lead to enhanced oil recovery.

There are a small number of depleted reservoir natural gas storage sites operating in Great Britain. More are planned in the near future. Gas stored in depleted reservoirs is injected from the National Transmission System (NTS), stored at high pressure and returned to the NTS when required.

The sites normally consist of a single depleted reservoir, interconnecting pipework and an import and export processing facility connected via pipelines to the NTS. The depleted reservoir may be separated from the processing facility by land that may or may not be owned and/or controlled by the site operator.

The layout of depleted reservoir storage facilities may present some complications when determining how COMAH and PSR apply.

Application of COMAH and PSR

The COMAH regulations state that 'installation' means a unit in which dangerous substances present are used, handled or stored. This includes equipment, structures, pipework and machinery. The depleted reservoir, import and export processing facility, and pipes of the storage site are integrally related to each other and can therefore be considered a single installation.

COMAH also states that 'establishment means the whole area under the control of the same person where dangerous substances are present in one or more installation'. The operator may not control the land between the wellhead(s) and the process plant. However, the piece of land upon which each wellhead rests should be considered as an 'area under the control of the same person'. Since all pieces of the land upon which the wellhead(s) rest are under the control of the operator and dangerous substances are present in the installation, the definition of an establishment is met. The processing plant is integral to the storage operation and so forms part of the establishment.

Pipelines connecting the depleted reservoir and processing facility are 'pipework' within the definition of installation and therefore are part of the COMAH establishment.

PSR Schedule 1 states that PSR does not apply to 'a pipeline contained wholly within the premises occupied by a single undertaking.' 'Premises' is not defined under PSR. However the definition of 'premises' under HSWA 1974 'includes any place and, in particular, includes 'any installation on land.' Since the whole site can be considered as one installation which is occupied by a single undertaking (i.e. the operator's business) PSR is does not apply to the interconnecting pipework.

Enhanced oil recovery at depleted reservoirs

Where the storage of natural gas in a depleted reservoir leads to enhanced oil recovery any plant associated with the enhanced oil production will typically be separate from the gas processing plant. Sites where enhanced oil recovery takes place without significant storage capacity are unlikely to be COMAH establishments.

Oil recovery wellheads may be located at the same site as the gas storage wellheads and/or at other dedicated sites. Where oil recovery wellhead sites are remote from the gas storage wellheads then they, the associated pipelines and the processing plant should not be considered part of the COMAH establishment. However, where oil recovery wellheads are located at the same site at the gas storage wellheads then the combined wellhead site should be considered part of the COMAH establishment. In both cases the Borehole Sites and Operations Regulations 1995 (BSOR) continues to apply.

Extraction of oil by enhanced recovery may also produce small volumes of natural gas. This is normally be injected into the gas storage reservoir without further processing. Any pipeline used for this purpose should not be considered part of the COMAH installation and instead will be subject to PSR.

Although enhanced oil recovery wellhead sites which are remote from the gas storage wellheads are not considered part of the COMAH establishment they must be taken into account if they could cause or exacerbate a major accident. Safety reports are required to take account of external events which could lead to a major accident at the establishment. An incident at an enhanced oil recovery wellhead site which is remote from the gas storage wellheads must be taken into account in the safety report to the extent that it could affect safety at the establishment.

Further considerations

COMAH requires operators to take all measures necessary to prevent major accidents and limit their consequences. An operator's safety report must include consideration of the risks to the installation from interconnecting pipework. Inspectors will assess the safety arrangements in accordance with the criteria in the COMAH Safety Report Assessment Manual.

The operator must conduct an accidental risks analysis to identify what events could lead to the failure of the interconnecting pipework on land not under the operator's control. This is above and beyond those risks considered for the pipework contained within fenced areas of the establishment. It is reasonably practicable for the operator to control this increased risk by using the standards which are adopted for cross-country pipelines. In order to demonstrate that 'all measures necessary' have been taken HSE expects standards used for the design and operation of pipelines subject to PSR to be applied to the pipework connecting the depleted reservoir and processing plant.

The design and layout of each length of pipework connecting the depleted reservoir and processing plant must allow for its treatment as a single pressure system and provide for testing its integrity from end to end.

The depleted reservoir and associated process plant should be regarded as single installation. It follows that the depleted reservoir, the process plant and the interconnecting pipework are operated as a single establishment under COMAH.

This treatment of the whole storage site as a single establishment is consistent with the way that HSE's Risk Assessment and Process Integrity Unit (CI5) provides Hazardous Substances Consent and planning advice to local authorities.

• Considering the depleted reservoir, wellheads, interconnecting pipework and the import and export processing facility as one COMAH establishment means that:
• The operator provides a single, integrated COMAH report which covers processing, control systems, storage, pipework and emergency arrangements for the whole site
• Land Use Planning advice can be given for the whole site
• The Hazardous Substances Authority considers one application from one operator
• HSE prepares intervention plans on the basis of one safety report.

Conclusions

The depleted reservoir, wellheads, interconnecting pipework and the import and export processing facility should be considered as one establishment under COMAH. Operators and Local Authorities will be expected to comply with the requirements of COMAH in full.

Standards covering the design and operation of cross-country pipelines (where PSR applies) will apply to pipework connecting the depleted reservoir to the import and export processing facility running under land not in control of the operator.

Enhanced oil recovery wellheads should be considered part of the COMAH establishment only where they share the same site as the gas storage wellheads. COMAH will not apply to pipelines and process plant associated solely with enhanced oil recovery. Any pipelines conveying gas separated from recovered oil are be subject to PSR and not COMAH.

The pipeline connecting the establishment to the NTS will be subject to PSR. This pipeline ends at the emergency control valve situated at the import and export processing plant. COMAH applies from this point.

Annex 1 shows schematically where COMAH and PSR apply to a depleted reservoir gas storage site.

Annex 2 shows schematically where COMAH and PSR apply to a depleted reservoir gas storage site with additional enhanced oil recovery.

Action by staff

SI3 staff should apply this guidance to all depleted reservoir gas storage sites.

Further information

For further information contact the Gas & Pipelines National Inspection and Operational Support Team, City Gate West, Toll House Hill, Nottingham, (VPN 513 2911).

Annex 1 - Schematic layout showing the application of COMAH and PSR to natural gas storage in a depleted reservoir

Annex 2 - Schematic layout showing the application of COMAH and PSR to natural gas storage in a depleted reservoir with enhanced oil recovery