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DayPaper

ROTTERDAMOrganised by The ABR Company Ltd

TUGNOLOGY ’17 16

INTRODUCTIONThis paper addresses the value propositions of the thruster family for the tugboat business, and the key selections to tailor the thruster application to specifi c vessel requirements. This is followed by a more detailed technical explanation of the new family features, covering the main thruster systems and components. During the development of the thruster, Wärtsilä paid much attention to validation of systems and technologies. The last part of this paper addresses some of the validation outcomes with respect to the clutch and the electric steering system, two systems that have been thoroughly tested on full scale level, and aims to facilitate discussion with operators and ship design offi ces on real market needs.

In addition, we cover a number of specifi c topics of interest to the tugboat community:

• Thruster/vessel integration, and what benefi ts this brings to the operator.

• The advantages of advanced human machine interfacing (HMI). The new thruster family can be equipped with a Wärtsilä ProTouch bridge system and dedicated machinery room equipment.

• The advantages of a solid maintenance strategy, which clarifi es the maintenance levels, schedules and tasks by means of work cards, and contains a new inspection regime and condition-based service schemes.

• The fulfi lment of recent stringent requirements on environmental performance, including over releases of hazardous substances. These set additional requirements for the lubrication system and components. The new thruster series is compliant with US EPA VGP2013 regulations with

the use of environmentally acceptable lubricants (EAL) in the entire thruster.

• The validation process, especially full-scale thruster testing while simulating various operational conditions, demonstrated thruster performance under normal loading, overloading and during endurance tests. Advanced research is continuing, relating to operating under ice conditions.

The new Wärtsilä steerable thruster (WST) series covers a power range of 800-3,200kW in eight different sizes. This thruster family is intended for tugboats, OSVs, river- and sea-going vessels and other vessel types requiring thrusters in the power range up to 3,200kW or 110 tonnes of bollard pull. The units are primarily intended for use in ships with dual propulsion. As the application varies, so do the requirements made of the propulsion unit, in this case the steerable thruster. Subsequently, the newly developed thruster series is highly versatile and can be tailored to comply with specifi c vessel requirements. This is done without compromising on safety, performance, cost, simplicity, reliability or effi ciency.

Figure 1, overleaf, gives an overview of the power range covered by different thruster sizes. The diesel-mechanic confi gurations come with an integrated power take off (PTO) for steering, lubrication, clutch and pitch hydraulics. The (diesel-) electric confi gurations have electric steering and auxiliary systems.

PROPULSIVE PERFORMANCETugs are capable of delivering the highest performance in assisting, towing or repositioning a vessel. This performance is commonly measured in tonnes of

A New Propulsion Family for Tugboat Applications

SYNOPSISThis paper introduces a new family of steerable thrusters to the tugboat business, bringing solutions to the current needs of the market and covering multiple applications. The key values of the new design are: high overall effi ciency, high manoeuvring capability, reliability and advanced system integration. The thruster family is designed and validated following a rigorous engineering process, including full scale testing under controlled conditions. The fi rst commercial applications in an LNG tug and an OSV have been installed at the shipyard.

Edgar Snelders, (co-author/speaker), Joost van Eijnatten (co-author), Wärtsilä Netherlands, the Netherlands

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bollard pull and is obtained by the effective performance of the propulsion components and their integration. The WST series of thrusters is designed for integration with any electric motor, diesel, dual fuel or gas engine in the speed range 720-1,800 rev/min. In Table 1, some examples of thruster engine combinations for the tug market are provided.

In Table 2, an overview is given of the ideal Wärtsilä engine and thruster combination for a given bollard pull class. The engine rating is either ‘standard rating’ or ‘tug rating’, which is 110 per cent of the maximum continuous engine rating (MCR).

Wärtsilä can also deliver a hybrid propulsion package based on the steerable thruster family with Wärtsilä engines, electrical and automation equipment and power management system for 40-80 tonne BP tugs. This hybrid package can be extended with batteries, Wärtsilä dual fuel engines and LNGPac to realise the most environmentally friendly and efficient solution for tugs.

HYDRODYNAMICSThe hydrodynamic design process of a propulsion unit aims to optimise the transfer of all engine power into the water and to convert it as efficiently as possible into a thrust force without compromising on cavitation, noise and vibrations. For the WST series, computational fluid

Bollard pull class [tBP]

Wärtsilä Thruster Type

Propeller diameter [mm]

Power level [kW]

Engine Input speed[rpm]

55 WST-16 FP 2,200 1,600 Wärtsilä 8L20 1,00065 WST-18 FP 2,400 1,838 Niigata 6L28HX 75070 WST-21 FP 2,600 1,920

1,960Caterpillar 3516CDaihatsu 6DKM-26e

1,600750

>85 WST-24 FP 2,800 2,525 Caterpillar 3516C/E 1,800

Table 1: Example solutions for different bollard pull classes with engines from various manufacturers

Bollard pull class and power are determined for two thrusters with FP propellers and an optimised hull design

Bollard pull class [tBP]

Wärtsilä Thruster Type

Propeller diameter [mm]

Power level [kW]

Wärtsilä engine Engine rating

30 WST-11 FP 1,800 800 Wärtsilä 4L20 Standard40 WST-14 FP 1,800 1,200 Wärtsilä 6L20 Standard45 WST-14 FP 2,000 1,320 Wärtsilä 6L20 Tug55 WST-16 FP 2,200 1,600 Wärtsilä 8L20 Standard60 WST-18 FP 2,200 1,800 Wärtsilä 9L20 Standard65 WST-18 FP 2,400 1,864* Wärtsilä 9L20 Tug70 WST-21 FP 2,600 2,040 Wärtsilä 6L26 Standard80 WST-24 FP 2,800 2,244 Wärtsilä 6L26 Tug85 WST-24 FP 2,800 2,400* Wärtsilä 8L26 Standard90 WST-28 CP 2,800 2,720 Wärtsilä 8L26 Standard95 WST-28 CP 3,000 2,800* Wärtsilä 8L26 Tug100 WST-32 CP 3,000 3,060 Wärtsilä 9L26 Standard

Table 2: Ideal Wärtsilä engine and thruster combinations for a given bollard pull class

Figure 1: The Wärtsilä Steerable Thruster (diesel-mechanic and diesel-electric version), together with the power range of the family

Bollard pull class and power are determined for two thrusters with FP propellers and an optimised hull design* = power limit on WST; ‘Tug’ engine rating = maximum 110 per cent

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dynamics (CFD) analyses have been extensively used to align the design of the propeller, the thruster housing, the nozzle and its connections to the housing in order to achieve optimum performance. The optimisation takes into account the design constraints on the propeller power density and the propeller maximum tip speed.

As has been demonstrated in recent research1, CFD simulation is a reliable method to calculate the full scale performance of thruster units with pushing ducted propellers. CFD eliminates the need to quantify scaling effects, which need to be taken into account when determining full scale performance by means of model tests. In the development of the WST series, detailed analyses were made to determine the performance of thrusters with ducted controllable pitch (CP) propellers compared to ducted fi xed pitch (FP) propellers.

PROPELLER CONFIGURATIONSThe steerable thrusters can be equipped with either an FP or a CP propeller, according the needs of the application. Propeller diameters range from 1.6m up to 3.2m, and for each size a minimum of two propeller diameters is available, covering most of the needs of vessel installations in this market sector.

Based on its physical properties, the hydrodynamic effi ciency of an FP will be the highest. The design point of an FP propeller determines the relationship between the power and rev/min for any given vessel speed. The design can be optimised for bollard pull or for a specifi c vessel speed. In case both bollard pull and free sailing performance are important, a CP propeller can be a good choice to cover multiple operational conditions. Numerical analyses show that the difference in effi ciency between an FP and a CP propeller is less than 2 per cent, which is more than suffi ciently compensated for by operational gains.

CP propeller hubs are identical to the hubs as they are applied in the Wärtsilä Controllable Pitch Propeller (WCP) system. With almost 3,500 hubs commissioned in CP installations, tunnel thrusters and steerable thrusters between 1997 and 2017, the CP hub of the WST series has proven to be durable and reliable.

Designers or shipyards can also select a thruster with a CP propeller for vessels with PTO-driven fi re-fi ghting (fi -fi ) pumps. A CP propeller allows the fi -fi pump to be operated at nominal engine speed while maintaining full manoeuvrability and without compromising the effi ciency and reliability of the thruster. A thruster with a CP propeller provides a cost effective alternative to the heavy duty slipping clutch needed to provide similar functionality in a driveline with an FP thruster.

NOZZLE CONFIGURATIONSA nozzle is a key element for a thruster as it maximises the thrust at lower vessel speeds. For the WST series of thrusters, a choice can be made between a bollard pull or a free sailing nozzle. The bollard pull nozzle (WTN-BP) is half the length of the propeller diameter (L/D = 0.5) and has a specifi cally designed nozzle exit area.

This nozzle is most effective at low ship speeds, while at the same time allowing effi cient sailing at speeds of 12-14 knots. For free sailing applications, the dedicated WTN-FS nozzle has improved performance. This shorter nozzle, with L/D = 0.4, effectively contributes thrust up to vessel speeds of 16 knots by reducing drag. The choice of nozzle depends on the desired operational profi le of the vessel. In general, the bollard pull nozzle is more applicable to tugs and workboats while the free-sailing nozzle is more suitable for vessels operating in transit conditions, such as PSVs and inland waterway cargo vessels.

ICE CAPABILITIESThrusters from the WST series have been designed optionally to comply with Ice Class requirements. The two smallest types, WST-11 and WST-14 with FP propeller, are available for Russian and Russian river ice classes (RMRS up to Ice 3 and RRR up to Ice 40), as well as the Baltic Ice Class 1C. The larger WST sizes with FP propellers fulfi l the requirements of Baltic Ice Class 1B. The ice class can be selected to fulfi l the vessel’s class notation, but also to provide robustness in demanding operating environments.

CONTROLSTogether with the new thruster family, 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) 7in touchscreen at the door of the cabinet. The HMI’s user friendly graphical interface supports local control of 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.

Through means of a smart instrumentation set-up, and with the use of fi eldbus technology, installation and commissioning times are signifi cantly reduced. The LMCS can interface with an external remote control system by means of the fi eldbus, as well as with the Wärtsilä ProTouch system, to enable remote control of the thruster from the bridge and engine control room.

Figure 2: The ProTouch system with its levers, touchscreen displays and indicators

The state-of-the-art ProTouch system (Figure 2), with its levers, touchscreen displays and indicators,

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can easily be fi tted 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 standardised interfaces to those systems and with clear visual information of the selected mode for the operator on the bridge.

VESSEL INTEGRATIONTugs are commonly extremely compact and agile vessels, where the equipment installed onboard should occupy the minimum space required without compromising the reliability and safety of the vessel. During design of the WST, special attention was given to system integration in order to minimise the required space and physical thruster dimensions. This was achieved by building up all the hydraulics on to 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 (PTO). The stem section volume is used as a pressureless sump through which both the propeller gearbox and the upper gearbox circulate their lubrication oil. As a result, there is no need for any fi tting of hydraulic systems after the thruster has been installed in the vessel. Also, no separate header tank is necessary. Thruster connections onboard the vessel are limited to:

• drive shaft• electrical connection for the clutch cooling pump• cooling water• pressurised air • controls (CAN-open)

The thruster can be mounted from below, from above or split mounted (Figure 3). The fi xing of the thruster can either be a bolted or a welded construction. To accommodate different vessel dimensions there are two propeller arm lengths (PAL) available for each size, a short and a long variant.

SERVICEToday’s marine market expects insight into long term operational costs even before purchasing equipment for newbuilds. The need for a solid maintenance strategy follows directly from that expectation. Our maintenance strategy has been developed alongside the thruster. It contains a new inspection regime and condition-based service schemes.

When defi ning the maintenance strategy, we used the concept of reliability-centred maintenance, resulting in a strategy structure comprising four main components:

• Maintenance levels. (‘Who?’) This component outlines a clear distinction of tasks that need to be executed at an organisational level by the vessel’s crew, on an intermediate level with special tools

and detailed system knowledge (most likely by a Wärtsilä service engineer), or at a depot level, preferably a Wärtsilä service station.

• Maintenance schedules. (‘What and when?’) Each thruster will be accompanied by a specifi c maintenance schedule describing all tasks to be performed for every maintenance level.

• Work cards. (‘How?’) For every task on an organisational level, a work card is available, detailing the required actions to take, step by step. The cards specify the required parts, tools, consumables, and duration, as well as inspection criteria and limits.

• Measurement records. Some work cards are accompanied by measurement records, giving the customer the opportunity to record trends in the performance of an installation.

Operator and maintenance manuals for all new products will contain details of this new maintenance strategy. Based on the maintenance schedule and work cards, Propulsion Services can support customers with a life-cycle calculation tool which can be used to determine in advance what the overhaul and maintenance cost for a certain operating period will be.

INTEGRATED SLIPPING CLUTCHThe WST for diesel-mechanical drives is standardly equipped with an integrated clutch. The clutch is a hydraulic operated and cooled on-off clutch (in the case of a CP propeller) and an oil-lubricated medium

Figure 3: Mounting methods (above, below and split)

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duty slipping clutch (in the case of an FP propeller). Alternatively, a standalone heavy duty clutch can be selected as an option.

With the integrated slipping clutch and FP propeller, the propeller speed can be precisely controlled within the range of 0-50 per cent of the nominal diesel engine rev/min. This allows the vessel to operate at slow sailing speeds. Figure 4 illustrates this slipping clutch operation in more detail. With a constant engine speed, the propeller shaft speed can be varied by changing the clutch actuation pressure, effectively controlling the slip. Full attention is given to the clutch control system to ensure a smooth synchronisation phase and to correct for speed and temperature variations. The clutch is cooled with oil from the conical stem of the thruster (the oil sump). An electric driven gear pump circulates oil from the sump through a fi lter and freshwater tube cooler to the clutch plates. The cooling system capacity is designed to keep the clutch oil temperature under control in any given slipping condition. The thruster can therefore be continuously used at low vessel speeds during operations in narrow sailing areas, or operations resembling dynamic positioning (DP).

DRIVE-TRAIN ANDSUPPORTING STRUCTUREThe drive-train of the thrusters has been designed based on experience gathered from the fi eld and the latest insights in gear and bearing design. Design

tools used for the gear teeth fl ank topology take into account gear misalignments caused by operational conditions, loads, temperature expansion, bearing preloads and clearances. By means of loaded tooth contact analysis, it is possible to compensate for these displacements and determine a good initial contact position that will lead to a well-centred, full torque contact even in the most severe loading conditions. This systematic approach leads to lower tooth stresses and, as a consequence, increased reliability. Together with selected bearings, the WST series has a single maximum rating, allowing up to 8,000 running hours per year with a standard mission profi le.

LUBRICATIONThe fl ow for the propeller gear box lubrication is provided by a robust axial hydrodynamic pump (the ‘impeller’), which is driven directly by the vertical shaft. The upper gearbox lubrication, fi ltering and cooling system is a conventional system, in which the fl ow is realised by a PTO-driven gear pump.

ELECTRIC AND HYDRAULICSTEERING SYSTEMSAgility and manoeuvrability are key for tugs. The steering system of the WST series thrusters is capable of rotating the thruster 360 degrees in both directions (clockwise and counter-clockwise) with a maximum speed of 2.5 rev/min. The installed power is such

Figure 4: Operation of an integrated slipping clutch for a WST-14 FP confi guration

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that the steering system can bring the thruster back to neutral position from any allowable angle at any propeller and vessel speed. Furthermore, the steering system has the ability to hold the thruster in a fi xed position under all conditions.

The steering system consists of two steering units with pinions that drive one common slewing gear via three-stage planetary gear boxes. The required redundancy for manoeuvrability of the ship is normally provided by a setup of two thrusters per vessel. The standard steering system of the WST series is a hydraulic closed-loop steering system. This system comprises a PTO driven variable displacement pump, driving axial piston hydrostatic motors. The pump has an integrated low pressure make-up pump. Apart from the pump, the hydraulic steering system contains a tank, fi lters and a tube-type freshwater cooler. Manual override in case of an emergency is done directly on the pump.

Alternatively, Wärtsilä has also developed an electric steering system. The electric system is more energy effi cient, holds fewer components and has hardly any wear parts (no pump, fi lters, valves or seals). Relative to the hydraulic variant, the electric steering system provides equal or even greater performance levels for positioning accuracy and system loads.

Similar to the hydraulic variant, the electric system consists of two electric motors. These motors are controlled by one single variable frequency drive (VFD) using a high resolution pulse encoder for closed loop speed control. Energy fed back to the VFD is converted into heat by a brake resistor.

The electric motors are equipped with active cooling fans in order to provide suffi cient air cooling even at low motor speeds. Each motor has a holding brake that is used to freeze the steering position when the VFD system is not in operation. These brakes are spring applied and electrically released. The VFD is mounted in a dedicated, fully enclosed cabinet that serves all automated functions for the electric steering system. The VFD has an internal freshwater cooler. Push buttons for manual override in case of an emergency are positioned on the VFD cabinet.

The steering system (hydraulic or electric) is controlled by a separate standard propulsion control unit (PCU) system with a main and a backup controller.

SEALS AND SEAL-MONITORING SYSTEMThe propeller shaft seal package and steering seal package form a barrier system between oil and sea water. In the WST design, both barrier systems are connected to each other, and subsequently provide a basic seal monitoring functionality. This is standard for the entire WST series. The non-pressurised stem section of the thruster acts as an oil sump. The fl ow for the propeller gearbox (PGB) lubrication is provided by a robust axial hydrodynamic pump (the ‘impeller’), which is driven directly by the vertical shaft. As the hydrodynamic impeller requires a low counter pressure,

the pressure in the PGB is only slightly higher than the sump pressure, and as a consequence lower than the pressure in the adjacent water. By keeping the barrier system pressure higher than the water pressure, the barrier function is combined with a basic seal-monitoring function. Ingress of sea water or oil leakage can be easily monitored by means of visual inspection of the separate seal monitoring tank level.

The propeller shaft seal package consists of four seals (Figure 5). The seals are protected by a labyrinth between the propeller gearbox and the propeller hub. Standard net cutters are mounted to safeguard the unit even more from ropes and wires. The steering seal package consists of fi ve seals mounted in a cartridge. The cartridge can be dismounted without removing the thruster unit, enabling easy replacement of individual seals.

Figure 5: Barrier system (propeller shaft and steering seal package)

ENVIRONMENTALLYACCEPTABLE LUBRICANTSThe WST series of thrusters has been designed to comply with the latest developments and environmental legislation. The series not only has an advanced

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propeller shaft seal and steering seal system with monitored seal compartments, but the entire thruster is also prepared for the use of environmentally acceptable lubricants (EALs). Due to this preparation, the WST complies with the requirements set by the US Environmental Protection Agency (EPA) in its Vessel General Permit 2013 (VGP2013) regulations and class regulations (such as the rules for DNV-GL CLEAN DESIGN notation). WST propulsors can be operated in any ECA or other environmentally controlled area.

THRUSTER VALIDATION PROCESSTo validate thruster systems and new technologies, we tested various prototypes (an LCT CS250 reference thruster, a WTT-11 and a WST-14) on a full scale test rig. The test rig enabled us to verify the performance of the thruster and its systems under controlled conditions. The rig is capable of running a thruster at any given combination of speed and torque. In addition, external forces can be applied by a dynamic loading system, allowing us to simulate thrust forces, steering forces

Figure 6: Torque transfer during clutch synchronisation

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and impact loading. The prototypes were all tested with different loading cases to mimic operational conditions in the fi eld. In addition, multiple endurance runs under maximum load and even overload were executed as part of the design assurance. An advanced measurement system with a grid of over 140 sensors, outside and inside the thruster, recorded the exact test conditions and thruster behaviour under those conditions.

Slipping clutchAmong other validation objectives, specifi c attention was given to the slipping clutch. Three main issues were verifi ed in order to demonstrate its performance and reliability: the torque transfer during synchronisation, the clutch oil temperatures in continuous slipping mode, and the ability to cope with torque variations that occur in practice due to, say, steering actions.

A smooth torque transfer during the synchronisation phase in both an on-off clutch and a slipping clutch improves system reliability, hence torque spikes should be avoided by regulating the clutch actuation pressure precisely. Figure 6, previous page, shows the two-stage clutch actuation for an on-off clutch, where in the fi rst stage the torque transfer, as a consequence of mass moment of inertia, is built up gradually as the clutch

closes. Once the clutch is fully closed, the pressure will increase to the holding pressure in the second stage. This allows higher torque transfer and prevents an unexpected release due to torque variations in the system, allowing safe vessel operation. With the hydraulic system layout and component selection, combined with the newly developed clutch control, a much smoother clutch operation is achieved.

Another key element in addressing the reliability of the slipping clutch is the ability to cool the clutch oil during continuous slipping mode. In a continuous slipping test at 40 per cent, the oil temperature of the clutch increased by 20 degrees C to an acceptable level of 55 degrees C before stabilising. Figure 7 shows the stabilisation of the clutch oil temperature after around 20 minutes in a test sequence of more than two hours, demonstrating that the clutch cooling capability is suffi cient.

In slipping mode, the clutch control system corrects for sudden variations in propeller torque due to specifi c vessel operations, such as manoeuvring or operating conditions. Figure 8, overleaf, illustrates how variation in torque, induced by the system, leads to pressure adaptations, keeping the propeller shaft speed at the desired level.

Figure 7: Clutch oil temperature development at 40 per cent slipping

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Electric steering systemAlongside the full scale thruster tests, dedicated system validation tests were designed and executed as part of the design assurance process. Figure 9 shows the specifi c test set-up for the electric steering system. An electric steering cabinet with one VFD inside controls two electric motors mounted to a steering gearbox. On the opposite side, a load motor is mounted, driven by another VFD. Both VFDs are controlled by one PCU system. The load motor enables mimicking of steering forces, based on a realistic load model calibrated to fi eld measurements. The following load components were considered in the load model: vertical shaft torque, hydrodynamic load, drag load, acceleration compensation load (due to differences in inertia between load motor and thruster) and friction losses in the gearbox. The outcome of this system validation test was that the electric steering system was capable of executing all predefi ned steering actions against predefi ned loads without overheating. The system was also able to accurately position the thruster at any azimuth angle and without overshoot.

Figure 9: System validation test set-up for electric steering

Figure 8: Ability of the clutch to correct for torque variations. After every torque change, the clutch output speed returns to its original target speed, without problems

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Torque synchronicity between the two electric motors was proven by comparing the individual current consumption of each motor. Figure 10 illustrates the torque synchronicity between the two electric motors during a 30 degree zigzag test, since both electric motors show the same behaviour with only small differences in current consumption.

A 180-degree crash stop manoeuvre executed at full ship speed without reducing power is the most severe steering load. The steering system is designed for this load, although in practice a crash stop is most probably not executed with full power on the propeller for vessel safety reasons. Figure 11 illustrates a 180-degree rotation of the thruster under this maximum

Figure 11: Crash stop test

Figure 10: Torque synchronicity between two electric motors during a 30 degree zigzag test. During all manoeuvres, the current offset is less than 10 per cent continuously

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load, simulating a crash stop. During this crash stop the steering speed is not affected by the load, nor the electric motor temperature, demonstrating that the steering system is perfectly capable of handling a crash stop.

PROJECT REFERENCESSeveral vessels with thrusters from the WST series are currently being built. The first vessel to be delivered with the new thrusters is a 55 tonne BP eco-tug for Dubai Dry Docks World (Figure 12). The vessel is equipped with two WST-18 CP thrusters driven by Wärtsilä 9L20DF dual fuel engines and a Wärtsilä LNGPac.

A series of 79m PSVs for Nam Cheong International Ltd is equipped with WST-18 FP units with electric steering in a diesel-electric (DE) propulsion layout. The third reference is for a research vessel for the Guangzhou Marine Geological Survey Bureau. The geological exploration vessel is equipped with two WST-21 FP units as part of a DE propulsion system.

Figure 12: 55 tonne BP eco-tug for Dubai Dry Docks World, equipped with two WST-18 CP thrusters

CONCLUSIONThe Wärtsilä Steerable Thruster (WST) series is a wide product family which is fully configurable to customer needs. Several options and features allow the thrusters to be integrated into any vessel design in combination with a wide range of diesel engines and electric motors. The thrusters are also available with Ice Class. The thruster has excellent bollard pull performance and hydrodynamic efficiency. Bollard pull or vessel speed requirements can be fulfilled for each application and operational profile via the selection of appropriate nozzle types, as well as FP and CP propellers.

The thrusters are equipped with a control system consisting of redundant controllers, modern levers and intuitive, graphical touchscreens and interfaces to other navigation systems. The systems can be fitted in even the most compact bridge configuration while allowing access to and control over all propulsion system parameters and alarms.

In response to the demands for insight into long term operational cost, maintenance strategies have been developed and are embedded in operator and maintenance manuals. The maintenance strategy for the WST thrusters contains different maintenance levels, schedules, work cards and measurement records.

The thrusters have been developed and validated following an extensive design assurance process. This included building several prototypes and testing those in a dedicated full scale test facility. Among the tests carried out were tests with different loading conditions resembling operational circumstances for tugboats. Extensive system validation tests, such as clutch performance and electric steering, were carried out successfully.

REFERENCES1 Bulten, N, Stoltenkamp, P, Full scale Thruster-hull Interaction Improvement Revealed with CFD Analysis, OMAE2013 Conference, Nantes, 2013

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