On Glutra, LNG is vaporised by the engine coolant and supplied to four 675kW ultra lean burn natural gas engines placed above deck in four separate and well ventilated engine rooms. Each engine is coupled to a 720 kVA generator supplying electric power through frequency converters to asynchronous 1,000kW motors coupled to twin steerable propellers at each end of the ferry. The Glutra solution has proved to be just one of a wide range of gas-powered propulsion system arrangements now being used by shipowners.
In addition to the IMO Resolution
MSC.285(86) interim guidelines and the DNV rules, most of the other major class societies have also published rules or guidelines for gas-fuelled engine installations. These standards align closely with the IMO interim guidelines and in some cases provide more comprehensive requirements. In working with this regime to assess the viability of vessel designs that incorporate gas- fuelled propulsion systems, flag administrations are developing their own levels of expertise with the technologies involved. They, in turn, utilise the guidance as a baseline standard in developing their own set of design criteria for gas-fuelled vessels. Adherence to such criteria, with any additional
requirements they may
contain, is intended to provide a level of safety in line with that inherent in compliance with the original MSC.285(86) provisions. In these early days for gas-fuelled ships the only viable approach for flag administrations is to consider applications for a design review on a case-by- case basis.
Under this permitting process, the prospective owner of an LNG-powered ship provides the regulatory authority with documentation such as the vessel’s general arrangement, a layout of the gas-fuelled system components and a list of standards proposed for the system’s design. Details also need to be supplied of how each provision of the MSC.285(86) interim guidelines is to be met and how any deviations are to be addressed. This approach enables significant issues to be identified early in the design phase and facilitates the plan approval and vessel certification processes. During construction, the administration’s marine inspectors are on hand to ensure that the ship is built in line with the approved plans. Two of the contentious ship design issues that have occupied those charged with drafting the IGF Code relate to the design concepts for ensuring machinery space safety and the placement of LNG bunker tanks.
The IMO interim guidelines provide two basic design concepts for running a natural gas-based fuel feed system in an engineroom. These are that they should be inherently gas-safe or there should be emergency shutdown (ESD) arrangements. The machinery spaces of ships designed to
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the inherently gas-safe concept are considered to be gas-safe under all conditions. Natural gas fuel piping within engineroom boundaries on such ships is fitted in a gas-tight enclosure by means of either double-walled pipe or single- walled piping within a gas-tight duct. The space between the inner and outer pipe/duct must be either pressurised with inert gas or ventilated. The machinery space is considered a non- hazardous area and there are no restrictions on electrical equipment installations.
On ships constructed to the ESD design concept machinery spaces are considered gas- safe under normal conditions but have the potential to become gas-dangerous spaces under
certain abnormal conditions. This
concept allows single-walled piping inside the engineroom without an external gas-tight enclosure. Extraction ventilation, at the rate of 30 air changes per hour, is used to prevent the accumulation of flammable vapours within the space. Should gas be detected at levels,
low
electrical equipment not certified safe for hazardous locations is automatically shut down.
all
The ESD concept was developed when engine manufacturers had not yet engineered a proper solution for fitting double-walled piping to the fuel manifolds on internal combustion engines. Technology improvements in more recent years have ensured that this is no longer an issue for the majority of engine sizes.
One of the challenges of the ESD concept is that the approach relies heavily on active safety measures such as gas detection sensors and automation systems that translate sensor signals into alarms and shutdowns. All these components require monitoring, maintenance and testing to ensure continuous efficacy. To date, the availability of a double level
of protection for gas transmission systems in machinery spaces has carried the day. All the systems
that have been accepted so far as providing a level of safety equivalent to that given by the existing regulations are of the inherently gas-safe type. The LNG bunker tank location debate revolves around whether or not the placement of such tanks below accommodation spaces, service spaces and control stations should be permitted. The issue is at its most divisive when passenger vessels are under consideration. IMO’s interim guidelines acknowledge the fact that design constraints for certain types of ship may not allow a well-defined area between transverse watertight bulkheads to be set aside for the exclusive use of LNG bunker tanks and gas transfer equipment. MSC.285(86) does this by providing several
layers of protection to
further reduce the risk of fuel system failure and to mitigate the hazards caused by a leak or rupture in the fuel system. These include gas
Natural gas-based fuel feed systems in LNG- powered vessels use the gas-safe concept
Marine Propulsion I April/May 2014 I 51
detection with associated alarms and shutdowns, continuous negative-pressure ventilation of the tank room at 30 air changes per hour and liquid level and temperature monitoring systems in the tank room bilge.
In addition, by prohibiting the installation of non-certified electrical equipment, the tank room’s designation as a zone 1 hazardous space is ensured. The use of cold-resistant material for the tank room boundaries provides further protection as does the thermal insulation separating the room from the hull structure. A number of class societies have considered additional requirements for tanks under accommodation areas on passenger vessels. These include providing a cofferdam between the tank compartment and adjacent machinery or accommodation space and placing the fuel tanks at a distance of B/5 from the hull, where B is the vessel’s beam. Bunker tank placement relative to other areas on gas-fuelled ships will be an issue requiring close scrutiny for future designs of such vessels. Owners and regulators will need to not only weigh up the various risks to the tank and their consequences but also give consideration to the measures taken to prevent or mitigate these consequences. Another aspect that needs to be considered in this respect is the design of the vessel’s LNG bunker tank or tanks. Amongst the other issues being addressed by IMO delegates, including flag administration representatives, during the finalising of the IGF Code’s provisions are hazardous area classifications, gas detection system certifications and fire protection arrangements. The task of developing a mandatory regulatory regime for LNG-powered ships will be accompanied by the equally rigorous work of ensuring its proper implementation. Both shipowners and regulators will derive benefit from beginning their cooperation on a proposed design concept at the earliest possible time. MP
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