Riser emergency shut down valve (ESDV) leakage assessment

SPC/TECH/OSD/49

OG status:

Fully open

Author unit / section:

OSD3.2

Target audience:

All OSD Inspectors, SI3A and SI3B

Version No:

1

Purpose

To describe how HSE assesses the acceptability of riser emergency shutdown valve (ESDV) leakage rates. This SPC replaces SPC/Tech/OSD/26 which has been withdrawn.

Legal requirements

Duty holders are not required to meet fixed criteria for maximum riser ESDV leakage rates. The goal setting regulatory regime allows maximum rates to be set according to the installation's ability to safely control the hazards produced by such a leak.

The Pipeline Safety Regulation 1996 (PSR)1 states that an "emergency shut-down valve" means a valve which is capable of adequately blocking the flow of fluid within the pipeline at the point at which it is incorporated. In the absence of a definition, "adequate" is taken to mean sufficient for a particular purpose.

The guidance to PSR (Schedule 3 and regulation 19) indicates that an ESDV should be capable of stopping the flow of fluid within the pipeline. However, minor internal leakage past the ESDV may be accepted providing it does not represent a threat to safety. The rate of leakage should be based on the installation's ability to control safely the hazards produced by such a leak.

The duty holder assessment must be consequence based and can take some mitigation systems into account.

Mitigation measures that can be credited

• Automatic closure of wellhead, down hole, process isolation and ESD valves.
• Automatic closure of other import and export pipeline ESDVs,
• Blowdown of the process system (depending upon the level of ESD initiated).
• PFP applied to pipework, vessels, decks and structures etc.; provided it can be demonstrated to have been designed to provide the required level of protection and has been properly maintained.
• Bunding and drainage; provided it can be shown to be unobstructed and have the required capacity.
• Location; through physical separation, in well ventilated areas (with respect to gas releases), on grated decks or platforms (with respect to oil releases).

Mitigation measures that cannot be credited

• Active systems such as water or foam deluge.
  Manual intervention (ie manual application of water or foam).

To be credited for mitigation, all systems should be designed to operate automatically in a fail-safe manner, be capable of remaining effective in an emergency and must be demonstrated to meet their performance standards.

Specialist assessment

OSD's fire and explosion specialist inspectors (OSD 3.2) will assess analysis methodologies used by all duty holders. Where Inspectors find that ESDV leakage has risen above levels previously considered acceptable in accordance with the guidance to SI 1029 revoked (1sm3/min gas or 6kg/min oil) the issue can be forwarded to OSD 3.2 for assessment.

Hazard analysis issues

The following issues are expected to be included in the consequence based hazard analysis of ESDV leakage.

Issue

• Leakage Consequences
  Hazard analysis should consider consequence to personnel, impairment of SCEs and escalation to the TR.
• Process Plant Sections
  The first isolable process section inboard of the ESDV should be considered as the potential source of loss of containment. Where high process pressures, gassy condensates or spiked crudes are involved it may be appropriate in addition to consider the next section inboard.
• Separation distance to SCE/Systems/Structures
  The distances to relevant process plant and safety critical structures, systems, equipment and escape routes should be confirmed for jet, pool fire and explosion analysis.
• Blowdown
  Where the relevant isolable section is connected to the blowdown system, blowdown can be credited. The additional hazard arising from the contribution of ESDV leakage to a range of hydrocarbon release scenarios should include the blowdown period and subsequent steady state phase. A conservative approach to gas releases will assume that ESDV leakage is released entirely through the breach rather than vented via the blowdown connection.
• Multiple ESDVs
  Where several ESDVs are connected to a single isolable section the maximum acceptable leak rate should be proportioned between the valves.
  Where a number of similar riser ESDVs exist across a platform it may be appropriate to apply a single ESDV leakage rate across all valves on the platform, or across separate groups of valves having similar duties. In this case it should be ensured that the hazard analysis is appropriate to all of the valves. The rate should be set to the minimum acceptable leakage for the particular group of valves.
• Alarm and Action Levels
  A percentage of the maximum leakage level (say 25%) should be set as an alarm level to initiate further investigation. A higher level (eg50%) should be set as an action level beyond which repairs or maintenance must be carried out.

References

A guide to the Pipelines Safety Regulations 1996 Guidance on Regulations L82 HSE Books 1996 ISBN 978 0 7176 1182 9

Further information

Further information can be obtained from OSD3.2 Fire, Explosion and Risk Assessment 0151 951 3144