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Assessment of the adequacy of venting arrangements for cargo oil tanks on FPSO and FSU installations

Health and Safety Executive - Safety Notice
Department Name: Offshore Division
Bulletin No: OSD 5-2010
Issue Date: 19 May 2010
Target Audience: Duty Holders/Operators of FPSO/FSU Installation, Offshore Industry
Key Issues: Cargo Oil Tank Venting Systems

Introduction:

This safety notice highlights the need to ensure suitable and sufficient assessment of the adequacy of venting arrangements for cargo oil tanks on FPSO and FSU installations.

The risks from hydrocarbon vapours or harmful gases resulting from cargo tank venting must be adequately controlled. The Offshore Installations (Safety Case) Regulations 2005 [SCR 2005] Regulation 12 requires that all hazards with the potential to cause a major accident have been identified; and all major accident risks have been evaluated and measures have been, or will be, taken to control those risks to ensure that the relevant statutory provisions will be complied with.

Under the provisions of The Offshore Installations (Prevention of Fire and Explosion and Emergency Response) Regulations 1995 [PFEER] Regulation 4 requires duty holders to take appropriate measures with a view to protect persons on the installation from fire and explosion. Regulation 5 requires duty holders to perform, and thereafter repeat as often as may be appropriate a process (in this regulation called "an assessment") described in paragraph (2) of the regulations and Regulation 9 requires duty holders to take appropriate measures with a view to preventing fire and explosion.

Background:

A fire has recently occurred as the result of the ignition of flammable vapours from an FPSO cargo tank vent. The incident occurred during calm low wind speed (0-2 knots) stable type weather conditions, which were considered to be a contributing factor to the event. Such weather conditions reduce the effective dispersion of flammable or toxic emissions, and are significant factors for inclusion in the evaluation of the major accident hazard potential for fires, explosions or toxic exposure from such releases.

It appears that the composition of the vapours emanating from the cargo tank vent have been incorrectly evaluated as being a slightly buoyant inert gas (i.e. less dense than atmospheric air) rather than a passive or dense hydrocarbon vapour mixture. Under the release conditions the vapour dispersion may also have been significantly affected by thermal or ventilation effects caused by other equipment in the vicinity of the vent. In particular the thermal currents resulting from flares, hot exhausts and machinery/HVAC intakes should be considered.

FPSO and FSU installations generally follow the rules of the classification societies which tend to follow SOLAS1 Reg II/11.6 and 5 for guidance regarding the arrangements for the venting, purging, gas-freeing and ventilation of cargo oil tanks. However the operational activities of FPSO and FSU type installations require further considerations due to their static positioning, processing activities of the crude oil and the loading/discharge operational cycles. Additionally FPSO's have a live flare sometimes in reasonably close proximity to the cold vents.

Further information regarding the management of the atmosphere in the ullage space of cargo oil tanks and hence the composition of the vapours emanating from the venting system may also be found in documents such as the International Safety Guide for Oil Tankers and Terminals (ISGOTT)2 and documents produced by the International Maritime Organisation (IMO)3. Also DNV have produced an offshore standard DNV-OS-A101 'Safety Principles and Arrangements'4 which also indicates that the cargo oil tank venting system will contain vapours of a hazardous nature. Hazardous area classification guidance may be found in the Energy Institute Model Code of Practice Part 15: Area Classification Code for Installations Handling Flammable Fluids5 and BS IEC 60092-502 Electrical installations in ships Part 502: Tankers - Special features6 which is referenced in the IP15.

In certain normal operations a degree of hydrocarbon content will be co-mingled with the inert gas that is used to blanket the ullage spaces of cargo oil tanks, but vapour density differences can result in an increasing concentration of hydrocarbons closer to the liquid surface. Consequently the composition of the vapours vented during the process of loading will vary from a potential 100% inert gas [IG] (typical composition 83% N2 12-14% CO2 2-4% O2 plus traces of NOx & SOx) through a mixture of IG and hydrocarbons to 100% hydrocarbons depending on the operations and loading phase.

In pressurised systems the concentration of flammable component increases as the cargo tank is filled, so that towards the end of filling the vapours vented may be approaching 90-100% hydrocarbons. Pressurised systems are not usually re-inerted part way through tank filling operations.

The vented vapour composition will also be affected by the nature of the crude oil in the tanks and the processes bring undertaken. IMO circular 680, primarily for the development of Volatile Organic Compound (VOC) management plans for crude oil carriers, does contain information indicating the increase in hydrocarbon vapour concentration during loading in the vapour space of cargo oil tanks.

Typically during a cargo loading cycle the cargo oil tank vent is open and IG/Hydrocarbon vapour is vented to atmosphere through the cold un-ignited vent, unless a vapour emission control system is in operation. During a cargo discharge the vent is usually closed and inert gas is used to replace the tank atmosphere.

In accordance with SOLAS the tank venting system should normally be self draining for all normal trim conditions or if this is not possible permanent arrangements are to be in place to drain the vent lines to a cargo tank. This is intended to reduce the likelihood of condensed hydrocarbon to be ejected from the vent.

Action required:

FPSO and FSU installation duty holders should ensure their risk assessments adequately address the potential of flammable or harmful vapours to emanate from the cargo oil tank venting system and the potential for ignition or hazardous exposure. This should include all potential hazards and modes of operation and weather possibilities, particularly calm and low wind speed weather conditions. Dispersion to potential sources of ignition or vulnerable equipment/areas (e.g. machinery intakes) should be evaluated. Where assessments indicate that flammable or harmful vapours are likely to present a significant hazard appropriate controls should be identified and implemented. Such measures may include, but are not limited to cessation of operations in certain weather conditions, re-inerting cargo tanks part way through filling operations or physical modifications to vent locations and design to minimise the hazard of the emitted vapours.

The risk assessment should include due consideration of the hydrocarbon content of the vented vapours (may be as high as 100%) under operating conditions and the subsequent dispersion of such potentially flammable, explosive or otherwise harmful vapours. It may be necessary to include the influence of other operational equipment in the vicinity of the vent e.g. process flares, equipment intakes and exhausts.

The hazardous area formed by such dispersions should be correctly identified and hazard prevention measures verified to be effective for all reasonably foreseeable scenarios.

Should a risk of potential hazard, ignition and fire be identified then duty holders must put into place mitigating actions.

Relevant legal documents:

 The main relevant legal requirements are:

References:

  1. SOLAS 1974 and its amendments Reg II-2/11.6 and 5 (governing the design of cargo tank venting and inert gas systems).
  2. (ISGOTT) International Safety Guide for Oil Tankers and Terminals ISBN 1 85609 291 7.
  3. IMO MEPC.1Circ680 'Technical Information on Systems and Operation to Assist Development of VOC Management Plans' 27 July 2009.
  4. DNV Offshore Standard DNV-OS-A101 'Safety Principles and Arrangements'.
  5. Energy Institute Model Code of Practice Part 15: Area Classification Code for Installations Handling Flammable Fluids.
  6. BS IEC 60092-502 Electrical installations in ships Part 502: Tankers - Special features.

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

This information contains notes on good practice which are not compulsory but which you may find helpful in considering what you need to do.

General note:

Please pass this information to a colleague who may have this Product/ Equipment or operate this type of system/process.

2011-07-28