Thruster manufacturers are employing seawater-lubrication and non-oil-to-water interfaces, among other technology, to comply with US regulations
US regulations mandating the use of environmentally acceptable lubricants (EALs) in all oil-to-water interfaces on vessels has prompted propulsion system suppliers to develop new lines of thrusters, incorporating several design innovations.
Working with EAL manufacturers, in 2013 Wärtsilä began to develop propulsion systems to use biodegradable lubricants instead of mineral oils to comply with the US Vessel General Permit 2013 (VGP-2013) issued by the US Environmental Protection Agency (EPA). The regulations apply to all vessels operating in US coastal waters.
Undertaking the R&D at the Wärtsilä Propulsion Test Centre in Tuusula, Finland, Wärtsilä tested a range of EALs in its steerable thrusters. As a result of the testing, several design changes were made to the seals, hydraulics and oil monitoring systems in its propulsion systems.
Among the first vessels to be fitted with Wärtsilä’s VGP-2013-compliant propulsion systems were four MC Class module carriers ordered by BigRoll Shipping, a joint venture between Dutch shipowners BigLift Shipping and RollDock Shipping. When the joint venture co-operation ended in 2018, two of the heavy transport vessels, BigLift Baffin and BigLift Barentsz, were added to the BigLift fleet, and two others, BigRoll Bering and BigRoll Beaufort, joined the RollDock fleet.
Built by COSCO shipyard in Dalian, China, the four MC class vessels are dynamic positioning class 2-capable (DP 2), with flush decks and 1A ice class notions. Each of the vessels has Wärtsilä main engines, Wärtsilä tunnel thrusters (WTT) and Wärtsilä controllable pitch propeller systems (WCPs).
With an overall length of 173 m, beam of 42 m and deadweight of 20,675 tonnes, the module carriers have performed heavy-lift transport for several high-profile energy projects. BigLift Baffin and BigLift Barentsz, for example, carried large LNG modules in for Russia’s Yamal LNG project in the Arctic circle. BigLift Baffin transported two luffing frames, weighing 1,170 tonnes each, from the Huisman fabrication yard in Xiamen, China to Sembcorp Marine Shipyard in Singapore for the construction of Heerema Marine Contractors’ semi-submersible crane vessel Sleipner.
“Since the launch of EAL compliant products in 2015 we have been very satisfied with the performance of the solutions developed," says Wärtsilä director, thrusters and propulsion controls, marine business Lauri Tiainen. "Being more environmentally friendly does not mean giving up any other good features or performance of our thrusters,” adds Mr Tiainen.
A further development of Wärtsilä’s WTT transverse thruster line has been the WTT-40, initially launched for the cruise market, but applicable to large offshore construction vessels.
Specifically designed for the 4,000-kW power segment and to comply with EAL usage, the WTT-40 is the first Wärtsilä transverse thruster available with either a controllable pitch (CP) or fixed-pitch propeller.
A key improvement of the WTT-40 is its integrated lubrication system. Wärtsilä has designed a new hydraulic system that combines lubrication of the thruster’s gears and bearings with the hydraulics for the pitch actuation of the propeller. As a result of this, the conventional hydraulic power pack – consisting of a tank, with pumps and valves mounted on top of the tank – is no longer needed.
Wärtsilä combined the lubrication systems so that the hydraulic setting for the propeller is built directly on the steel construction of the thruster itself. Now, when Wärtsilä delivers a part of the ship’s tunnel in which the thruster is installed, it also delivers the hydraulic system as part of an integrated system. This saves space in the vessel and makes the transverse thruster easier to install at the shipyards, says the company. Another benefit is simplified maintenance, with access to all of the components, filter changes or inspections in one location.
Prior to extending the integrated lubrication system in its larger transverse thruster models, Wärtsilä first proved the technology in its 400-kW thrusters, used in cargo vessels, in 2014.
Wärtsilä has designed all of its new thrusters, including the WTT-40, in such a way that they can be optionally equipped with EALs, instead of the mineral oil normally used in thrusters.
Another VGP-2013-compliant alternative to using EALs is to employ a non-oil-to-water interface, such as the Schottel Leacon sealing system. Dutch shipowner Jan De Nul’s offshore jack-up installation vessel Voltaire, under construction in China, is incorporating the technology as part of its propulsion system. Due for delivery in 2022, the 170-m installation vessel will be powered by electric motors driving the Schottel azimuth thrusters. Four SRP 610 FP Schottel Rudderpropellers, each with input power of 3,000 kW, will be installed at the stern. When combined with the two 2,600-kW Schottel STT 7 FP transverse thrusters forward, Voltaire will have full DP 2 capability.
Equipped with the Schottel Leacon sealing system, Voltaire will comply with VGP- 2013 regulations. Separate seals on the seawater side and on the gearbox side ensure that any seawater entering, or gear oil escaping, from the system are collected in an intermediate Leacon chamber. This prevents water from entering the gearbox and, more importantly, oil from escaping into the seawater.
Seawater lubrication for thrusters
A third alternative to comply with US EPA VGP-2013 regulations is to lubricate the thruster with seawater. This is the approach used by the Voith Inline Thruster (VIT) from Germany’s Voith Turbo. With the capability to be used either as a transverse thruster or main propulsion, the VIT is available in a power range from 50 to 1,500 kW and combines electric, mechanic and hydrodynamic elements. Requiring neither shafting nor gearing, the rim-driven propeller is powered by a permanent-magnetic electric motor. The propeller is supported by a seawater-lubricated slide bearing patented by Voith. A centre roller bearing is utilised for units with a power range of 500 kW or higher. This technology ensures low vibrations, low noise emissions and a compact design. The specially developed blade geometry reduces cavitation and contributes to vibration and noise reduction, says the company.
As units up to 500 kW do not require oil, the VIT does not require dynamic seals, and there is no gearbox or propeller hub. Additionally, the carbon-fibre reinforced plastic propeller blades can be replaced individually and even underwater, simplifying maintenance.
Owned by Dutch shipping company Royal Wagenborg, the DP class 2-capable walk-to-work vessel Kroonborg has two 1,000-kW VIT rim drives and two 1,850-kW Voith Schneider Propeller (VSP) stern drives. It is the first offshore vessel in the world to burn Shell gas-to-liquid (GTL) fuel, which is made using natural gas instead of crude oil as a means of lowering greenhouse emissions.
Operating around oil and gas platforms, Kroonborg requires precise manoeuvring and position holding, relying on its VSP stern drives and VIT rim drives for quick, precise response in DP mode to the changing wind, waves and currents.
The option of active roll stabilization by the VSP creates a more stable working platform for crane operations and helicopter deployments. Additionally, the reduced roll motion of the ship increases on-board comfort.
Voith also notes that the low noise level of the VIT allows accommodation to be sited near the bow thruster room.