Fixed gas detection systems are coming for many

A key outcome of recent IMO deliberations has been the formation of a correspondence group, which will develop Solas amendments to make fixed hydrocarbon gas detection systems mandatory on double hull tankers of 20,000 dwt and above

The merits of fixed hydrocarbon gas detection systems on double-hull oil tankers were reviewed at the latest meeting of IMO’s Sub-Committee on Fire Protection (FP), which sat in London during January.

The Maritime Safety Committee (MSC) designated the issue ‘a high-priority item’ for completion by 2009 (two sessions of the FP subcommittee, which is responsible for progressing the matter, are needed to complete this item), during discussions at its 82nd session in November/December 2006. The FP Sub-Committee has convened a dedicated correspondence group and is working closely with the Bulk Liquids and Gas Sub-Committee.

Discussions were triggered by a document– MSC 82/21/12 – submitted to MSC 82, supported by a mix of EU member states, the European Commission and industry non-governmental organisations (NGOs). This calls for all tankers of 20,000 dwt and above to be fitted with a fixed hydrocarbon gas detection system that is compliant with the fire safety system (FSS) code for measuring flammable vapour concentration in all ballast (and any other) tanks and void spaces adjacent to cargo tanks.

In support, MSC 82/21/12 proposes detailed amendments to Solas regulation II-2/4.5.7 and a new chapter (16) to the FSS code. The latter would amend gas sampling rules, stating that a sampling system has to allow for a minimum of two hydrocarbon gas sampling points in each space, one located at the lower and one at the upper part where sampling is required. This would not apply to ships of less than 20,000 dwt, with the proposal stating that only one sampling point is required.

At the FP Sub-Committee in January, as part of its assessment of the proposed chapter 16, the South Korean delegation presented the results of a computational fluid dynamics (CFD) study that it had sponsored to assess how a fixed gas measuring system in the L-type ballast tanks (50.7m long/39.6m deep/34m breadth) of a 300,000 dwt VLCC compares with periodical measurements using portable instruments.

Through the stresses placed on a vessel during the course of its operational life, cracks can occur in a tanker’s cargo shell, resulting in gases from the oil cargo migrating into ballast tanks. The study modelled how gas expands, and how quickly and efficiently the two types of detector register the gas when it leaks through the structure. Six key recommendations were made “that need to be considered in order for fixed systems to be considered more effective then periodical measurements using portable instruments”. These are:

• since the detection capability of the fixed gas measurement system is greatly influenced by the extraction velocity (ie, pump capacity and size of detection pipe), this needs to be adequately considered at the design stage, especially when the fixed gas sampling line is also used as a gas sampling line for portable instruments

• measures to prevent or monitor pipe clogging, which will reduce the internal diameter or block the gas sampling pipe, need to be considered

• since gas freeing of ballast tanks during the operation of a gas detection system may reduce detection capability, clear instruction on the relationship between the procedure for inert gas filling of ballast tanks, fixed gas measurement system or instructions on gas freeing for ballast tanks needs to be provided to operators

• because the position of the detection point is very important, it is best to set that point at a position below an area where any oil leakage is most likely to occur. As for the detection opening, it is preferable to have it positioned at a point higher than the height of girders of bottom shell plating

• the delay in detection depends on the quantity and spilled area of any leaked oil

• in the case of minor leakage, it is likely that there will not be much difference in gas detection capability between the fixed gas detection system and a portable instrument. Therefore, an assessment using a failure mode analysis is necessary.

A related issue considered by the sub-committee was a French delegation request that issues surrounding inerting double-hull spaces after the detection of hydrocarbon gas be considered in parallel. It was decided that such an undertaking would take the group beyond its original remit and would necessitate experts in this field being co-opted on to the committee. The French delegation was therefore invited to submit a proposal to MSC 84 which meets between 7 and 16 May this year to expand the scope of the committee’s brief.

The session concluded with an agreement to create a correspondence group, to be chaired by Intertanko’s technical director, Dragos Rauta, to revisit the proposed draft amendments to the Solas regulation II-2/4.5.7 and review the plans for chapter 16 of the FSS Code to detail the specifications for such systems. A report will be submitted to the next IMO FP Sub-Committee, which has been tentatively scheduled to take place from 19 to 23 January 2009.

A likely discussion point will be whether, instead of having gas detection, it is better to keep these spaces continuously inerted. Others in the industry say that keeping inert gas derived from flue gas in empty ballast tanks increases the risk of corrosion at a high rate (ie, high humidity of salt water vapours plus sulphur and other ingredients combine to create a highly corrosive atmosphere in the tanks).

Momentum for fixed gas detection systems to be an industry standard has been building for quite a number of years as part of the International Safety Guide for Oil Tankers & Terminals guidelines. Nevertheless, the quest for a mandatory requirement has arrived since publication of the European Maritime Safety Agency’s Double Hull Tankers: High Level Panel of Experts Report in June 2005.

Many oil majors consider fixed hydrocarbon gas systems as a principle of best practice. In parallel, an increasing proportion of double hull tankers are equipped with fixed gas detection systems as an important element in securing both future charters as well as crew welfare. The build up in flammable vapours in ballast tanks can result in an explosion should a spark occur. In addition, these vapours cause a severe toxic risk to crew members entering a tank, including asphyxiation as a result of low oxygen levels from oxidisation. TST