The application of the Control of Major Accident Hazards Regulations 2015 (COMAH) and the Pipelines Safety Regulations 1996 (PSR) to the storage of natural gas at depleted reservoir and salt cavity sites

Introduction

Great Britain's gas supply infrastructure includes facilities for natural gas storage in salt cavities and depleted hydrocarbon reservoirs. In both cases, gas is taken from the national transmission system (NTS) via a connecting pipeline and injected into storage at pressures up to 300 barg and returned to the NTS at a later date, typically at times of high gas demand.

Depleted reservoirs are underground layers of porous rock previously used for hydrocarbon extraction. The term 'depleted' refers to both fully or partially depleted reservoirs.

Salt cavities are used for high pressure gas storage because their physical properties mean that they are particularly well suited for this purpose.  

These sites normally consist of:

• a single depleted reservoir or cluster of salt cavities and associated wellheads,
• interconnecting pipework and
• an import and export processing facility connected by pipelines to the NTS.  

The processing facilities and wellheads associated with these types of gas storage may be separated by land not owned or controlled by the storage site operator and, overall, installations may cover an area several kilometres in diameter. This guidance helps site operators by describing the criteria that should be used to determine how COMAH and PSR apply.

How COMAH applies

'Installation' - Regulation 2(1) of COMAH defines an installation as: 'a unit in which dangerous substances present are, or are intended to be, produced, used, handled or stored, and it includes (a) equipment, structures, pipework, machinery and tools...'

Natural gas is listed as a dangerous substance under COMAH. The salt cavities or depleted reservoir, import and export processing facility and pipework of the storage site are all integrated with each other. Depleted reservoir or salt cavity storage sites meet the definition of an installation under COMAH.

'Establishment' - COMAH Regulation 2(1) defines an establishment as: 'the whole area under the control of the same person where dangerous substances are present in one or more installations...'

The operator of the salt cavity or depleted field storage facility may not have control of the land between the wellheads. However, the land upon which each wellhead is located will be an 'area under the control of the same person'. All such areas are under the control of the operator and dangerous substances are present at the installation, meeting the definition of 'establishment'.

Where the processing plant is integral to the storage operation, this also forms part of the establishment. Gas pipes connecting the cavities and processing facility become 'pipework' within the definition of installation and are therefore also part of the COMAH establishment.

Treating the gas storage facilities, wellheads, interconnecting pipework and import and export processing facility as one COMAH establishment means that:

• the operator provides a single, integrated COMAH Safety Report (SR) covering all of their major hazard operations
• HSE's specialist risk assessment teams (CEMHD5) can give Local Authorities advice on Hazardous Substances Consents and land use planning issues that covers the entire sit
• the Hazardous Substances Authority has to consider only one application for Hazardous Substances Consent
• the COMAH Competent Authority (CA) can prepare intervention plans taking into consideration the assessment findings for a single SR

How PSR applies

PSR Schedule 1 states that PSR does not apply to 'a pipeline contained wholly within the premises occupied by a single undertaking'. PSR does not define 'premises', however the Health and Safety at Work Etc Act 1974 definition includes 'any installation on land'. This means that the whole site can be considered to be one installation occupied by a single undertaking, and PSR does not apply to the interconnecting pipework.

The pipeline connecting the establishment to the NTS will be subject to PSR and not COMAH. This pipeline ends at the emergency control valve (ECV) situated at the import and export processing plant. COMAH applies downstream from this point.

Enhanced oil recovery (EOR) at depleted reservoirs

The storage of natural gas in a depleted reservoir may lead to EOR, and include the following elements:

• oil production plant (usually located separately from the gas processing plant),
• oil recovery wellheads (possibly remote from the gas storage wellheads),
• small volumes of natural gas recovery which is injected back into the gas storage reservoirs without further processing.

Under COMAH Regulation 3(3)(a), on-shore mineral exploration, extraction and exploitation (including oil and gas drilling and extraction) are excluded from the scope of COMAH. As such all EOR wellheads and pipelines are excluded from the COMAH establishment unless they are co-located with the gas storage wellheads, pipework or process plant. Where EOR pipelines are remote from gas storage wellheads, pipework and process plant they will be subject to PSR.

However, if the EOR and gas storage processing operations are physically close and undertaken by the same legal entity then COMAH will apply to the EOR process operation and plant.

Conversely, if the EOR and gas storage processing operations are physically separate and undertaken by different legal entities then COMAH will not apply to the EOR process operation and plant. Where this is the case, any pipeline used to export gas from the EOR process operation is subject to PSR.

COMAH Safety Report (SR) considerations

COMAH requires the operators of COMAH establishments to take all measures necessary to prevent major accidents and limit their consequences. SRs must consider all significant major accident hazards and risks affecting an establishment. SRs will be assessed by the CA against the criteria in the COMAH Safety Report Assessment Manual.

For depleted reservoir and salt cavity gas storage sites, the SR should consider risks associated with gas pipes running within the establishment and the interface between the site and external pipelines.

The SR should recognise that there are credible events that could affect pipes under land that is not under the operator's control. In order to demonstrate that all measures necessary have been taken, any pipes that run under land not in the control of the operator should comply with standards for the design and operation of pipelines as if subject to PSR.

The design and layout of each length of pipework connecting gas storage facilities and processing plants must enable it to be treated as a single pressure system and allow its integrity to be tested from end to end.

SRs are required to consider external events which could initiate a major accident at the establishment. Therefore incidents at remote enhanced oil recovery wellheads should be considered if they could affect safety at the COMAH establishment.

Summary

Description of plant/process and Comment on COMAH & PSR status

• Gas storage facilities (depleted reservoir or salt cavity), wellheads, interconnecting pipework and import and export processing facility
  These assets comprise the COMAH establishment. COMAH applies.
• Pipework connecting gas storage sites to import/export processing facility where it runs under land that is not under the control of the COMAH establishment operator
  In order to demonstrate all measures necessary are in place as required by COMAH Regulation 4 these should comply with PSR requirements for design and operation.
• Pipelines and process plant associated solely with enhanced oil recovery (including any pipelines conveying gas separated from recovered oil)
  Subject to PSR. These are not part of the COMAH establishment unless they are co-located and operated by same legal entity
  
• Enhanced oil recovery wellheads
  Only part of the COMAH establishment if they are co-located with the gas storage wellheads
• Pipeline connecting the establishment to the NTS
  Subject to PSR. This pipeline ends at the ECV at the import/export processing plant. COMAH applies from here.
  

Schematic

The schematic below shows how COMAH and PSR apply to a depleted reservoir or salt cavity storage site. The application of COMAH and PSR to depleted reservoir gas storage sites involving EOR should be decided on a case-by-case basis.