The new Wartsila family 1

The new Wärtsilä steerable thruster family

To provide the optimal solution for vessel applications where the steerable thruster requirements vary according to the vessel type, Wärtsilä has developed a new series of steerable thrusters with a power rating from 800 kW to 3 MW.

Text: ELIAS BOLETIS, DIRECTOR, RESEARCH & DEVELOPMENT NORBERT BULTEN, GENERAL MANAGER, HYDRODYNAMICS ALBERT DROST, PROGRAM MANAGER Photo: -

When it comes to steerable thrusters with a power rating of 3000 kW, applications vary and include vessels such as tugboats, OSV (Offshore Support Vessels), PSV (Platform Supply Vessels) and AHTS (Anchor Handling Tug Supply Vessels). As the application varies so do the requirements set towards the propulsion unit, in this case the steerable thruster. To provide the optimal solution for this market Wärtsilä has developed a new series of steerable thrusters, which is highly versatile and can be configured to comply with the specific vessel requirements. This is done without compromising on cost, simplicity, reliability or efficiency. On the contrary, as compared to available existing thrusters, the new Wärtsilä steerable thruster (WST) is more efficient, stronger and can comply as standard with ice-classes up to ice-1B. At the same time, the thruster can deal with various input speeds ranging from 750 to 1800 rpm, which means that it can be connected to more or less any diesel engine. The WST-18 is the first in this series to be introduced onto the market. The first field applications include the propulsion units for an offshore vessel (Nam Cheong Shipyard) and a harbour tug (Drydocks World Dubai).

Systems integration

By updating the systems and through extensive integrating, the physical dimensions of the thruster have remained small, thereby reducing the need for additional space in the machine room. This also allows the thrusters to be installed from either below, above, or split mounting. This was achieved by building-up all the hydraulics onto the top-plate and, where possible, integrating the oil conduits and the lubrication pump into the castings. Another feature contributing to the reduced space requirements is the integration of the clutch into the upper gearbox housing, which in addition serves as the support for the power take-off. The clutch itself was specifically developed for the WST thrusters in cooperation with one of the main clutch suppliers, and is available in either on/off or slipping execution, thus enabling optimal operation of the engine and thruster under all conditions. The clutch suits either a CPP (Controllable Pitch Propeller) or FPP (Fixed Pitch Propeller) thruster application.

Drive-train and supporting structure

The drive-train of the thrusters has been designed based upon the experience gathered from the field and the latest insights in gear and bearing design. The latest design tools allow any operational conditions caused by gear misalignments resulting from the accumulated effect of geometric tolerances, loads and temperature expansion, to be taken into account. This data is then used when calculating the gear-teeth flank topology.

The new Wärtsilä 2
Fig. 2 - Calculated velocities of the jet stream of the thruster in bollard pull condition.
The new Wärtsilä 3
Fig. 3 - Calculated streamlines and pressure distribution on the thruster unit in free sailing condition.

Propulsion controls

Together with the next generation of thrusters, a new machinery controls automation platform has been developed. The Local Machinery Control System (LMCS) contains redundant embedded controllers and has a full colour Human Machine Interface (HMI) 7” touch screen at the door of the cabinet. The HMI’s user friendly Graphical User Interface (GUI) supports local control of the steering and thrust, calibration and test modes, as well as trending and logging, since the thruster’s sensors and transmitters are all connected to the LMCS. By standardizing the instrumentation and with the use of fieldbus technology, engineering, assembly, test, installation and commissioning times are all reduced. The LMCS can interface with an external remote control system by means of fieldbus, as well as with the Wärtsilä ProTouch system to enable remote control of the thruster from the bridge and engine control room.

The state-of-the-art ProTouch system with its levers, touch screen displays and indicators, can easily be fitted into even the most compact bridge designs, while providing the user with full manual control under all circumstances. Other sailing modes, such as dynamic positioning, joystick and auto pilot, are supported by means of standardized interfaces to those systems and with clear visual information of the selected mode for the operator on the bridge.

Optimized hydrodynamic unit design

The propeller is obviously the most important and complex part in the design process, since it has to transfer all the engine power into the water and to convert it as efficiently as possible into a thrust force. Based on years of hydrodynamic research, it has been established that the cross-sectional shape of the nozzle, which is placed around the propeller, has an important impact on the occurring flow phenomena. The pressure distribution around the nozzle can easily produce about half of the total unit thrust in the so-called bollard pull condition. Therefore, the design of the nozzle and propeller need to be aligned in order to achieve optimum performance.

In the hydrodynamic design process of the lower gearbox housing and the vertical shank, the main focus is on reducing the drag of these components as much as possible. The main dimensions of these components are to a large extent, however, dictated by the interior mechanical parts.

Wärtsilä’s hydrodynamic experts make use of a variety of design tools to develop the optimum design of the units. This includes, to a large extent, the use of computational fluid dynamics.

Propeller variants

The WST-18 thruster units can be equipped with either FPP or CPP propellers, according to the need of the application. Based on the physics, the hydrodynamic efficiency of an FPP will be the highest. Nevertheless, the CPP can be a good alternative should multiple operational conditions need to be covered.

With an FPP there is a direct relationship between power and the RPM for any given ship speed. This might be a limiting factor where both bollard pull and free sailing performance are important.

Two diameter options are available with the WST-18 type (2200 mm and 2400 mm) covering most of the needs of ship installations in this market section.

Nozzle variants

The fact that steerable thrusters are being used for different operational profiles (bollard pull or free sailing) has resulted in the development of two different nozzles. The bollard pull nozzle has a length that is 0.5 of the propeller diameter (L/D=0.5) and a specifically designed nozzle exit area. The effectiveness of this nozzle is best at low ship speeds. At ship speeds in the range of 12 to 16 knots, the contribution to the total thrust is limited.

For free sailing applications, a dedicated nozzle design has been developed, which has improved performance in the free sailing speed range. The length of the nozzle has been reduced to 40% of the propeller diameter. This reduces the wetted area on the outside significantly, which is beneficial for drag reduction at transit speed.

Design tools & methods

The design of the propeller geometry has been carried out using various in-house developed propeller design software modules. In order to properly determine the impact of the thruster housing geometry on the performance, and to analyze the occurring flow phenomena, the commercial CFD (Computational Fluid Dynamics) software Star-CCM+ has been used. The viscous CFD calculation method has been validated extensively within the Hydrodynamics department. Nowadays, accurate predictions can be made not only for thruster performance, but also on the detailed transient blade loading fluctuations. These blade load fluctuations can be a result of the flow obstruction of the shank or, alternatively, due to oblique inflow at a given steering angle.

The thruster validation process

Wärtsilä follows a systematic approach to thruster validation. This includes the production of a prototype unit for the series (a WST-14 in this case), which is used for the validation of the manufacturing process and the design structural integrity. Testing of the latter is carried out at our full load test facility in Tuusula, Finland. The test facility allows the simulation of field loads as applicable in high ice classes.

 

Thrusters less than 3 MW

Middle Easts first LNG fuelled harbour tug

Leave a comment

Load more comments