VESSEL REVIEW: Dux – Ground-breaking dual fuel escort tug

The first of three unique dual fuel RAstar 4000-DF-class extreme escort tugs, Dux, has been delivered to Østensjø Rederi by Spanish shipbuilder Astilleros Gondán.

The 40.20-metre vessel is the first of a three-boat order from Østensjø for severe weather operations at Statoil's Melkøya LNG Gas terminal at Hammerfest in the extreme north of Norway.

Statoil's requirements for these vessels were challenging: extremely high escort performance, high free running speed, low flash point oil recovery and storage, towing from both ends, and compliance with IMO III emissions standards, all under harsh environmental conditions.

These escort tugs will conduct approximately 300 LNG ship escorts annually, assist with berthing operations and will be maintained in readiness for emergency services such as long line towing, fire-fighting, and oil spill response.

Østensjø proposed a dual-fuel solution (LNG and diesel) to address the charterer's requirement to comply with IMO Tier III emissions standards. The gas fuel installation consists of an "LNG Pac" (IMO type-C vacuum-insulated LNG tank with tank connection space (TCS)) located in the LNG tank hold, and two dual fuel main engines.

Each TCS contains the LNG tank connections (liquid and vapour), a pressure build-up evaporator, a main gas evaporator and the gas valve unit for each engine. The tank hold is designated as a non-hazardous space accessed from the engine room by means of an A-60 rated watertight door. The TCS is accessed from within the tank hold by an air lock, which is integrated with the TCS and is supplied as part of the LNGPac system.

The engines and engine room are configured to be "gas safe". All gas supply piping is double wall. The outer annulus of gas supply piping is under-pressure ventilated at 30 air changes per hour and is equipped with gas detection and flow sensing.

The LNG bunker station is on the main deck. LNG bunkering is possible either from a tank truck or a bunkering barge. A foldable drip tray at the bunker station and provisions for water flushing protect the hull from gas spills.

A vent mast serves as an outlet for small quantities of gas released by pressure relief valves and "block and bleed" valves in piping. Although the outlet is normally required to be at least 10 metres away from potential sources of ignition such as engine exhaust outlets, the relatively small size of the vessel and need to keep the mast low for ship handling operations made this impractical.

Instead, to keep the mast as compact as possible, Robert Allan Ltd. developed a unique double wall vent mast. Ventilation air forced through the outer annulus mixes with any vented gas carried by the inner pipe at the outlet. The resulting mixture is carried safely away by the ventilation air flow. To prove the effectiveness and safety of this arrangement, Robert Allan Ltd. used its powerful CFD capability to study the gas dispersion around the vessel for a range of gas release scenarios and wind conditions. The results of this study were applied in the risk assessment process conducted with the classification society, Bureau Veritas.

To connect to the relatively fast moving incoming tankers, Dux is capable of a free running speed of more than 15 knots. Schottel's VarioDuct SDV 45 nozzle, which provides improved efficiency at higher speeds, was under development at the same time and Dux ended up being the first vessel to be fitted with this new nozzle profile.

Dux is designed to generate exceptionally high escort forces, and has been BV approved for a rated Fs of 167 tonnes indirect steering, and more than 200 tonnes braking at 10 knots. To achieve this, Robert Allan Ltd. applied BV-approved CFD based prediction methodologies during design, and verified the predictions through extensive self-propelled model testing at the Vienna Model Basin.

To operate under harsh winter weather and sea conditions and low ambient air and water temperatures, the RAstar 4000-DF has a number of special features. Its hull form is optimised for excellent sea-keeping. Both fore and aft winches are enclosed in insulated and heated winch rooms. Exposed working decks have heat tracing to reduce ice build-up. The wheelhouse is fitted with heated windows and an efficient combination electric/oil fired boiler provides hot water for domestic heating, domestic hot water supply, pre-heating for engines and heating for recovered oil tanks.

A foam-capable external fire-fighting system with water spray protection is fitted with capacity to deliver streams of water totalling 2,400m³/hour at ranges in excess of 120 metres (Class Fi-Fi 1 standard). Two pumps of 1,500m³/hour pumps are driven through step-up Kumera gear boxes driven off the front of the main engines. The monitors are rated for 1,200m³/hour of water and 300m³/hour foam. Additionally four DN100 NOR couplings are located port and starboard for delivery of fire-fighting water to the terminal.

The tug is heavily fendered for the expected high swells. Bow fendering consists of a 1,000mm diameter cylindrical fender extending well aft along with a lower course of 400mm thick W-fender specially designed to limit fender contact pressures to 20t/m². Sheer fendering is 400mm by 400mm D-fender. Stern fendering is 400mm thick W-fender.

The accommodations are arranged to be generous and comfortable for a crew of eight. The main deck features a generous sized mess and galley, the master's cabin, a large office, and a wet gear/locker room with toilet space. The lower deck includes four MLC-compliant cabins, each with en suite toilet, for up to seven crew. The lower deck also contains a large gym, laundry and galley store. The wheelhouse is fitted with a single control console with winch operator position, and provides the master with excellent all-around visibility and clear sightlines from the console.

On trials, the vessel exceeded all performance expectations achieving an ahead bollard pull of 107.5 tonnes, astern bollard pull of 103 tonnes, and a free running speed ahead in excess of 15 knots.