Two breakthrough technologies bunkering vessels and floating liquefaction have added new dimensions to LNG ship-to-ship (STS) transfer operations. LNG bunkering will introduce a wider range of seafarers to the intricacies of transferring a cryogenic liquid at high flow rates between two floating vessels.
STS transfers:
STS transfers have always been crucial in the commercial LNG operations for the past 10 years, primarily for discharging cargoes from conventional carriers to floating storage and regasification units (FSRUs) moored at either jetties or offshore turret facilities.
Only a handful of FSRU-based import terminals are equipped with jetty-mounted marine loading arms (MLAs) to transfer LNG from the delivery tanker to the regasification vessel, where the two ships are usually moored together side-by-side and use composite hoses to initiate STS transfers.
In other cases, the FSRU’s are fitted with MLAs to enable side-by-side transfers. The FSRU are popular for fast tracking of LNG imports keeping in mind the low costs involved.
STS newcomers:
The lastest vessels to join the LNG bandwagon are Bunker tankers and floating production (FLNG) units where STS transfers are crucial to ensure smooth, day-to-day functioning of these vessels.
The first two purpose-built LNG bunker vessels – the 5,100m3 Engie Zeebrugge for an Engie/NYK-led group and the 6,500m3 Cardissa for Shell have recently been completed. They are about to be joined by a third, the 5,800m3 Coralius, which Skangas will charter. All three are set for busy operating lives, fuelling the growing fleet of LNG-powered ships serving in northwest Europe.
FLNG vessel:
The first FLNG vessel, PFLNG Satu, is currently stationed off Sarawak in Malaysia where it is processing gas from the Kanowit field on behalf of Petronas. The unit, which is designed to produce up to 1.2 million tonnes per annum (mta) of LNG, loaded its first cargo in March 2017.
A second FLNG vessel, Shell’s 3.7 mta Prelude, is due to commence commercial operations off the coast of northwestern Australia in 2018. The unit has recently departed Samsung, the Korean yard where it was built, and is en route to its turret mooring station under tow. The Prelude boasts itself as the largest floating structure yet built.
Both FLNG vessels have been provided with midships-mounted with MLAs for side-by-side STS transfers to conventional LNG carriers.
Duplicating convention:
It is crucial for all STS arrangements to incorporate all the safety features that characterise the transfer systems used to load or discharge LNG carriers berthed at a conventional shore-based terminal, irrespective of the application and whether they use hoses or MLAs. Thus, dry-break emergency release couplings (ERC), emergency shutdown (ESD) valves and control systems are integral to all STS arrangements.
Furthermore, STS operation include metocean conditions, relative vessel sizes, heading control and operating envelopes. Manifold elevation differences between the vessels involved in the STS transfer and the range of vessels to be handled also need to be borne in mind.
Guidelines for STS transfers:
The Society of International Gas Tanker and Terminal Operators (SIGTTO) produced best practice guidelines for STS transfers of LNG in 2013, which are now the accepted norm for the industry standard. Exmar and Excelerate, the trailblazers in the development of LNG STS operations, provided much of the input for the guidelines.
STS transfers of hydrocarbon liquids such as crude oil and LPG have been an established part of tanker shipping for much longer than for LNG. These are usually carried out to assist in delivery logistics, consolidating loads on a mothership or lightering from a larger ship to coastal tankers for onward delivery to local customers.
LNG is notorious for having more technically complex set of STS operational criteria such as to maintain LNG’s carriage temperature of -162˚C to facilitate the use of special materials throughout the transfer system while the risk of flammable vapours generated by cargo spills merits a thorough hazard analysis to ensure any detrimental impacts are minimised.
LNG transfers also require high flow rates to ensure the commercial viability of the operation. Hose design is a critical factor as the combination of high flow rates and the possibility of an ESD valve being quickly activated raises the spectre of high-amplitude shock waves affecting piping, seals and cryogenic hoses.
STS transfer systems are generally too short for classic surge wave generation, but vapour pockets may form if cargo pumps are stopped. This is most likely in transfers from a large to a small vessel, as the topsides pipework on the bigger ship will be at a much higher point in the transfer system than the small vessel’s manifold. Vapour pockets can contribute to unstable flow, noise and vibration.
Bunker boats:
There are now more than 100 LNG-fuelled ships in service and on order, and the number will grow rapidly in the years ahead. To date the in-service vessels have mainly been fuelled on the jetty, using truck-to-ship (TTS) transfers.
Seagas, a former ferry converted into a bunkering tanker in early 2013 with the addition of an insulated 187m3 pressure vessel tank leads the way for the new LNG bunker vessels. Seagas provides the 57,000gt ro-ro/passenger ferry Viking Grace with 60-70 tonnes of LNG, carrying out STS operations in Stockholm harbour five or six times a week.
The dual-fuel Viking Line ferry burns primarily LNG on its daily round trip to Helsinki and to date Seagas has carried out almost 1,300 fuelling operations, each lasting around 60 minutes.
The cause of global uniformity is supported by the harmonised bunkering guidelines developed by the International Association of Classification Societies and the Society for Gas as Marine Fuel (SGMF).
LNG-powered crews:
It is also necessary for crew on the gas-fuelled ships to be thoroughly familiar with LNG hazards and proper handling procedures. Here again, Viking Grace illustrates the measures being implemented by shipowners making a commitment to gas fuel.
Viking Line’s LNG Bunkering Operations Manual covers the Stockholm arrival, bunkering and departure procedures and protection measures for Viking Grace. The Viking Grace training regime specifies that its relevant officers complete a programme arranged according to IMO Resolution MSC.285 (86). The gas-related part of the package includes basic and supplementary training for deck and engineer officers while the systems-related section deals with equipment functions and arrangements and the ESD concept.
There is also an engine-related training element developed by Wärtsilä, the supplier of Viking Grace’s dual-fuel diesel-electric propulsion system, for engineers and electricians, which includes simulator sessions. Viking Line’s training manager coordinated the required project-specific learning rotas during the construction of the ship.
Viking Grace’s crew completes a report for each STS bunkering operation, listing the times at which the 15 separate steps in the procedure are taken within the hour available and recording the status of the ship’s two bunker tanks at the beginning and end of the filling.
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Source: LNG World Shipping
