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ISV design is customised to work with LNG and FLNG carriers

Thu 31 Aug 2017

ISV design is customised to work with LNG and FLNG carriers
KT Maritime’s ISVs can justifiably claim to be the first of a new breed

July 2017 saw the first examples of a new class of support ship – an infield support vessel or ISV – mobilise to go to work with Shell’s Prelude floating liquefied natural gas unit

 

Perth-based KT Maritime Services Australia, a joint venture between Kotug International and Teekay Shipping Australia, is providing the Prelude floating liquefied natural gas (FLNG) unit with three 42 m, 100-tonne bollard pull ISVs. The tugs are of Robert Allan’s advanced rotor tug ART 100-42 design and were built by ASL Marine Holdings. The ISVs have Robert Allan’s RAstar hullform and three separate azimuth propulsion units – two astern and one amidships – that comprise the rotor tug power system technology pioneered by Kotug.

The primary roles assigned to KT Maritime’s ISVs include assisting LNG carriers approaching the Prelude FLNG and when LNG and LPG is offloaded via a side-by-side vessel configuration using specially designed cryogenic loading arms. Although this is an operation that is similar in some respects to normal berthing of LNG tankers, the unsheltered environmental conditions and impact of waves at the FLNG location can be a challenge. Apart from berthing and assisting LNG tankers, the ISVs also have a number of secondary roles for which they are responsible, including safety standby, evacuation, personnel transfer and surveillance.

Speaking to OSJ earlier this year, Osman Munir, KT Maritime’s director commercial/Kotug’s chief commercial officer, explained that LNG tankers will have to berth alongside the FLNG’s manifolds to take on the LNG produced on board. Although this is an operation similar to normal berthing of LNG tankers, this is usually carried out in sheltered waters.

In the procedure developed by KT Maritime for this kind of operation, an amended push-pull method has been developed. Two of the ISVs, sailing stern first, establish a towline connection, one at the bow and one at the stern. The tanker is brought alongside the FLNG, and the tugs move in to the side of the tanker in order to push it against the side of the FLNG. Pushing against the side with standard stern drive tugs can only be done in good weather conditions.

Maintaining an acceptable footprint with a stern drive tug would be impossible in adverse conditions, with the increased risk of damage to the tug’s fender and the LNG carrier’s hull. In contrast, with the rotor tug, if the significant wave height increases, the unique propulsion configuration enables it to maintain position whilst applying force against the hull of the tanker in the designated area.

For work in increased wave heights, KT Maritime has also proposed an alternative procedure, known as ‘rotoring’ or indirect towing, which will be used when required on the Prelude contract. This particular technique sees the rotor tugs assisting the LNG carrier on a short wire length. The tugs stay connected centre forward and centre aft on a short wire, and the tugs then push against the short tow line. They do not need to be repositioned to transfer forces onto the tanker. This alternative procedure mitigates the risk of damaging the hull of the LNG carrier and is only possible with the unique rotor tug configuration.

In the event of an emergency on board the FLNG, personnel will be able to make their way safely to temporary refuge sites on the vessel via multiple escape routes forward and aft. They can then be evacuated from the facility in a controlled manner using helicopters, freefall lifeboats and integrated chute-based liferafts. Once evacuated, they can be recovered by the ISVs. Each of these tugs will be able to accommodate 85 people in such situations.

The ISVs are the most powerful and sophisticated rotor tugs in the world and were purpose built to support the Prelude FLNG facility, which is located in the Prelude and Concerto gas fields in the Browse LNG Basin 200 km off the coast of Australia.

Ard-Jan Kooren, director of KT Maritime and acting CEO of Kotug, described the introduction into service of the vessels as a historic moment for KT Maritime. “With the development of FLNGs, natural gas production is moving in a new direction, and our new ISVs are at the vanguard to support that development,” he said.

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