Østensjø Rederi’s new LNG-fuelled escort tugs operating at Statoil’s Melkøya LNG production terminal near Hammerfest, Norway, are jointly Tug Technology & Business’ Tug of the Year 2017
A trio of gas-fuelled vessels has been selected as Tug Technology & Business’ Tug of the Year 2017. Østensjø Rederi’s three tugs, powered by liquefied natural gas (LNG) – named Dux, Pax and Audax – have several innovations that single them out for the accolade. Not only are they some of the first tugs to be built with Wärtsilä dual-fuel engines, but they also operate in extreme weather and sea conditions within the Arctic Circle in temperatures down to -20˚C.
Østensjø’s tugs provide escort services to LNG carriers that are used for exporting LNG from Statoil’s production terminal at Melkøya, near Hammerfest. They will also be maintained in readiness for emergency services such as long-line towing, fire-fighting and oil spill response.
These 40.2 m escort tugs were designed by Robert Allan as RAstar 4000-DF class tugs. They comply with IMO Tier III emissions standards. They can achieve a maximum free-running speed of 15 knots and can produce up to 107 tonnes of bollard pull.
They were built by Astilleros Gondán in Spain, classed by Bureau Veritas and officially christened in Hammerfest in August. Gondán commercial manager Daniel Scavuzzo told Tug Technology & Business that their construction was challenging because of the lack of rules and regulations concerning LNG-fuelled tugs, especially for their venting systems and location of their LNG bunkers. “It was a steep learning curve for us,” he said.
It was also a steep learning curve for Robert Allan and Bureau Veritas, not only because of the same lack of rules to guide the tug design but also because of a shortage in relevant experience. Robert Allan project manager Mike Phillips explained that a refined RAstar tug hullform was chosen after extensive hydrodynamic simulations, hours of computational fluid dynamics and model testing.
He said the biggest challenge was fitting the LNG systems into a 40 m tug design. “The LNG tank is orientated in the longitudinal direction to minimise any risk of a pressure breakdown in the tank due to excessive sloshing and mixing of the liquid and gas,” he explained.
The design had to follow specific code and class requirements for safety and gas systems that cover the ventilation of the LNG system’s fuel tank, its connection spaces, gas regulation units, engines, double walled piping systems, bunker stations and airlocks. Those codes define the inlets and outlets of the ventilation systems for all these components as hazardous zones, which “must be kept clear of potential sources of ignition, entrances to accommodation, control spaces, ventilation inlets to non-hazardous spaces and even some other hazardous zones,” said Mr Phillips.
“Complicating matters even more, this project required the tugs to have an oil recovery ship class notation with tankage for recovered oil, storage locations for containment booms and a skimmer.” Each of these features has associated hazardous zones of their own that need to be considered in conjunction with the LNG system operation.
The IMO Type C insulated LNG tank and regasification system is located below deck, with the dual-fuel main engines in a gas-safe (non-hazardous) engineroom. Østensjø’s tugs each have two Wärtsilä 6L34DF main engines with a power rating of 3,000 kW each, which drive transverse thrusters.
Schottel developed a specific type of thruster for these three dual-fuel tugs and six units with the new VarioDuct SDV45 nozzle were supplied to their Spanish builder. On each tug, Gondán installed two Schottel SRP 630 CP azimuthing propellers and an STT 170 fixed pitch thruster rated at 250 kW in the bow.
According to the shipbuilder, these tugs are able to obtain an indirect steering force of 167 tonnes at 10 knots and a braking force exceeding 200 tonnes. These tugs also have FiFi 1 fire-fighting systems that can deliver streams of water at rates of 2,400 m3/hr to a range in excess of 120 m. Karmøy supplied fore and aft winches and hydraulic towing pins.
These escort tugs are heavily fendered to protect them and the gas carriers even in extreme weather and sea conditions. They have bow fendering consisting of a 1,000 mm diameter cylindrical fender extending well aft along with a lower course of 400 mm thick W-shaped fender, which was specially designed to limit fender contact pressures to 20 t/m2.
These innovative tugs have been working at the Melkøya LNG production terminal since the end of August. Østensjø expects they will become intricate parts of the terminal’s operations for years to come and will conduct around 300 LNG ship escorts annually.
Dux, Pax and Audax particulars
Owner: Østensjø Rederi
Type: Escort tugs
Builder: Astilleros Gondán
Designer: Robert Allan
Design: RAstar 4000-DF
Operating: Statoil’s Melkøya LNG terminal, Norway
Length, oa: 40.2 m
Beam, mld: 16 m
Draught: 6.7 m
Max speed: 15 knots
Bollard pull: 107 tonnes
Engine: 2x Wärtsilä 6L34DF
Propulsion: 2x Schottel SRP 3030CP
Bow thruster: Schottel STT 170 T-FP