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Reducing the risks of hazardous accumulations of flammable/toxic gases in tanks and voids adjacent to cargo tanks on FPSO and FSU installations

Offshore Information Sheet No. 2/2010

(Issued January 2010)



This information sheet highlights the need for systems to monitor and protect against the accumulation of flammable/toxic gases in ballast tanks and voids adjacent to cargo tanks on FPSO and FSU installations.

The provision of suitable systems for gas detection and advanced warning of hazardous accumulations is considered to represent good practice in reducing these risks to as low as reasonably practicable. These systems also give a warning of structural defects on the bulkhead between ballast and cargo tanks.


The danger of cargo leakage into the ballast tanks with an associated build up of flammable/toxic gases is well known. Such accumulations may give rise to potentially explosive atmospheres developing in the adjacent spaces to the cargo tanks. Advanced warning of such a situation developing will allow operators to take preventative action and thus avoid such an occurrence.

FPSO and FSU installations generally follow the rules of the classification societies but depending on operating location may be subject to greater loadings than conventional hulls particularly in the harsh conditions found in the UKCS. As such, greater strength and fatigue performance may be required. As field life may be extended the design life of the installation may be exceeded with associated structural implications. These gas detection systems will also provide an indication that there is a defect within a bulkhead which is allowing the passage of cargo into the adjacent space and thus act as an ‘early warning’ system for structural defects.

Such gas detection systems therefore are also able to form a key element of the structural integrity management system for the hull structure as well as being able to give warning of potentially hazardous gas accumulations.

IMO have recognised the risks associated with the loss of containment from cargo tanks into adjacent tank areas and have via the sub Committee on Fire Protection dated 16-20th February 2009 recommended amendments to SOLAS chapter II-2 and the International Fire Safety System (FSS) Code be drafted and submitted to the Maritime Safety Committee (MSC) for approval and adoption

The recommendations issued by the sub committee are as follows:-

“SOLAS regulation II-2/4.5.7 to require fixed hydrocarbon gas detection systems to be installed in ballast tanks and void spaces adjacent to cargo tanks located outside the oil tanker’s block area, such as fore peak tanks and a new draft chapter 16 to the FSS code, to give the specifications for fixed hydrocarbon gas detection systems.”

Some dutyholders have already equipped their installations in line with these recommendations. SOLAS II-2 already requires for the continuous monitoring of the concentration of hydrocarbon gases within cargo pump rooms of tankers from 1 July 2005 (Regulation 1.6.7).


Owners and operators of FPSO and FSU installations should review their risk assessments for the potential of flammable gas to accumulate in adjacent tanks to the cargo tanks and ensure that effective means are provided for the detection and control of such potential accumulations. This could include providing a system which can also monitor for other gases such as hydrogen sulphide and oxygen and thus form part of atmosphere monitoring during tank entry operations. Systems should provide suitable warning to operators once a pre-set level has been acquired.

Suitable arrangements for monitoring the atmosphere within these areas, which are compliant to recognised industry standards and requirements are recommended to be installed by FPSO and FSU owners and operators. 

Relevant legal requirements

The main relevant legal requirements are:


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