More and more owners of offshore support vessels are opting for liquefied natural gas (LNG) propulsion in order to take advantage of its reduced emissions and the cost benefits that can accrue from bunkering with LNG rather than fuel oil. Almost all modern offshore vessels are also dynamic positioning (DP) ships.
LNG-fuelled DP vessels have been in operation for several years and have a good operational and safety track record, but some owners have reservations about ordering them because of concerns about loss of DP in a safety-critical situation. However, a presentation by Terje Nordtun, a project manager at Wärtsilä Ship Design, at the 2015 European Dynamic Positioning Conference should put their minds at rest.
A platform supply vessel (PSV) is used to transport deck cargo, various bulk cargoes and fluids (such as liquid mud, dry bulk mud, fuel and methanol) to and from offshore installations. Cargo is offloaded with the vessel either ‘backing in’ or alongside the rig in order to be dynamically positioned while cranes on the rig offload the cargo. This being the case, dynamic positioning is critical in keeping the PSV close to the rig – and avoiding a potential collision. The minimum DP notation for redundancy to ensure safety in close proximity to rigs is DP2.
Mr Nordtun’s presentation at the conference was based on a review of more than a decade of experience with LNG-fuelled DP vessels, with a focus on Rem Eir, an LNG-fuelled PSV managed by Remøy Shipping. At the time that it was delivered, it was the largest LNG-fuelled PSV in the world.
The first LNG-fuelled DP vessel was Viking Energy, which was delivered to Norwegian shipowner Eidesvik in 2003. This DP2, LNG-fuelled vessel is a diesel-electric ship with a quartet of Wärtsilä 6L32DF engines, each of 2,020kW, forming the basis of the electrical powergenerating machinery. However, at the time that the conference took place in June 2015, there were 24 LNG-fuelled PSVs in operation or on order, of which 22 had dual-fuel engines. Among them is the diesel-electrically driven Rem Eir, which was delivered in November 2014. With a length overall of 92.50m and breath of 20.00m, the vessel has a deck area of 1,090m2 and is a DP2 class vessel with an environmental regularity number (ERN), which describes the position-keeping ability of a vessel, of 99,99,99,99. This particular vessel makes use of Wärtsilä’s Low Loss Concept and combines the use of Wärtsilä 6L34 and 6L20 dual-fuel engines providing a total installed power of 7,350kW.
Mr Nordtun explained that, prior to Viking Energy entering service and more recently, concerns had been expressed about whether LNG-fuelled vessels might be less capable or reliable in DP mode. These concerns included classification requirements for such an innovative vessel, including the requirement for an emergency shut-down concept, the bunkering procedure for the vessel, the need for redundant enginerooms and the need for a vessel hazard identification study (HAZID).
One key question that arises about any LNG-fuelled vessel is what would happen in the event of a collapse of LNG tank pressure or any other event leading to an interruption in the operation of the vessel’s propulsion and manoeuvring systems. Viking Energy was also intended for operation in very harsh conditions, which could lead to mixing of gas with liquid phase condensed gas and the potential for pressure drops. What would be the effect on dynamic positioning integrity were such a phenomenon to occur with the vessel station keeping close to an offshore installation in harsh conditions, and what could the potential consequences be of slow response (load acceptance) of engines in gas mode?
By more recent standards, this first ever LNG-fuelled PSV had a fairly basic LNG fuel supply arrangement. More recently, Wärtsilä has introduced its LNGPac concept – a complete gas-handling system for LNG-fuelled ships with LNG tank and related process equipment as well as the control and monitoring system. As Mr Nordtun explained, LNGPac also maximises LNG storage volume, makes the most efficient use of available space on a vessel, reduces the number of interfaces and enhances reliability whilst driving down costs.
As Mr Nordtun further explained, the key to maintaining redundancy in the fuel system on a DP ship with LNG propulsion is to ensure that the dual-fuel engine onboard can transfer instantaneously to diesel mode – at any load and at any time. “Diesel mode will act as a redundant fuel system for DP2 class requirements,” he explained. “A sudden failure in the gas supply during full power ramp-up does not compromise safety.”
“LNG as fuel is feasible for DP vessels,” Mr Nordtun concluded. “It is a proven solution with a 12-year history encompassing 24 vessels. Adopting LNG does not mean accepting any limitations in DP capability and performance. It is a perfect solution for PSVs making regular port calls, and vessel owners with LNG experience have not expressed any concerns related to vessel operation.
He said the future development of LNG-fuelled offshore vessels depends not on any concerns about DP performance but could be influenced by the additional investment cost for LNG ships and price development of LNG and MGO and by the development of emission control regulations – which provide an incentive for using LNG – and the need for more shore-based LNG infrastructure to be developed.
Article published in the Guide to OSV Propulsion, a supplement to Offshore Support Journal
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