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Wärtsilä’s next generation underwater mountable thrusters: efficient and easy to install

Wärtsilä’s next generation underwater mountable thrusters, with their 8° tilted propeller shaft, set a new standard in thruster propulsion performance and reliability.

Text: PIET VAN MIERLO, SALES APPLICATION ENGINEERING, WÄRTSILÄ SHIP POWER | JOOST VAN EIJNATTEN, SALES APPLICATION ENGINEERING, WÄRTSILÄ SHIP POWER | VINCENT BAST, SOLUTION OIL&GAS SYSTEM ENGINEERING, WÄRTSILÄ SHIP POWER Photo: Wärtsilä

Performance and reliability are understandably key considerations in the offshore drilling market, which is the main target market for the new Wärtsilä thrusters. A large number of design features have been incorporated that allow designers, builders, and operators to design drill ships or semi-submersible drilling rigs that are more reliable, that perform their operations with increased efficiency, carry more payload, and that are easier to build.

In the autumn of 2013, Wärtsilä released the first models of the next generation thruster series, which over the coming few years, will replace the existing Wärtsilä thruster product portfolio. The new thrusters have been developed in response to market demands and from the field experience of our extensive installed base of steerable thrusters. An important part of the new product range is the underwater mountable (UWM) thruster series. These thrusters can be exchanged without dry docking the vessel, which is an important advantage for large offshore vessels, such as drillships, semi-submersible drilling rigs or offshore construction vessels. Next to the exchangeability, the key requirements for these thrusters are reliability, performance, and ease of installation. This new UWM thruster series features an 8° tilted propeller shaft, which significantly boosts overall dynamic positioning (DP) performance, and in every respect fulfills these key market requirements.  

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Fig. 1 – The WST-45U as an example of the new UWM series thrusters.
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Fig. 2 – The Wärtsilä WST-45U with an 8° tilted propeller shaft and nozzle.
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Fig. 3 – The pinion bearing arrangement; bevel gear set with crown wheel and pinion, lower pinion bearing, upper pinion bearing arrangement, inspection conduit.

Reliable operations

With the new UWM thruster series, a new level of thruster reliability is provided.The latest model in the existing Wärtsilä thruster series, the LMT 3510 thruster with an 8° tilted propeller shaft, was introduced in 2011 since when more than 100 units have been sold. This model has served as the basis for the design improvements in the new series to which several design features have now been added.

Operational loads and load fluctuations associated with demanding operations are the most important elements in the lifetime and reliability of thrusters. Proper accommodation of these loads is ensured by all the elements in the driveline, i.e. the gears, shafts and bearings.

The spiral bevel gear-set, which is the heart of the thruster, has been thoroughly reviewed. Not only has the 8° gear angle been included in the gear geometry, the gear application and safety factors have been increased and a high quality gear material has been selected.

The pinion is supported on both sides with a new bearing layout that has an increased capacity to accommodate load fluctuations. This has been achieved by applying new roller bearing types in a pretensioned bearing configuration.

The propeller shaft is fitted with CARB radial bearings that provide a true separation of the radial and axial loads, and allow thermal expansion of the shaft. A torsional damper is part of the composite vertical floating shaft. This damper reduces torsional vibrations originating from the propeller - drive shaft - E-motor system, and isolates the thruster from electrical currents.

The slender nozzle connections and thruster housing design are beneficial to high hydrodynamic efficiency. In addition to this, by making the inflow to the propeller as smooth as possible, they also reduce thrust load fluctuations and the risk of unfavourable cavitation.

The life of the bearings and gears depends very much on the condition of the lubrication oil inside the thruster. The oil is filtered and cooled by the lubrication system, while a 4-lip redundant propeller shaft seal with Wärtsilä Unnet seal protection prevents water ingress.

The thruster’s lubrication system, its shaft seal, and a shaft earthening feature on the propeller shaft extend the lifetime of the bearings and gears. Optionally, seal monitoring, lubrication oil monitoring, and a water separator can be installed to further support reliability. A conduit provides the possibility to inspect the bevel gears while the thruster remains mounted on the vessel and without opening the thruster housing.

The new UWM thrusters are fully prepared for the Wärtsilä Propulsion Condition Monitoring Service (PCMS) system, which collects and provides real-time data from vibration sensors on the thruster, as well as operational parameters from the thruster steering system, the hydraulic auxiliary equipment, and the vessel parameters such as speed and heading. Wärtsilä can advise on maintenance planning and provide monthly thruster condition reports based on the detailed PCMS data analysis. This service helps to avoid costs and downtime resulting from unscheduled maintenance. As this service is recognized by the main marine class societies, the thruster overhaul intervals can be extended according to the condition of the equipment.

Ease of installation

For the yard, the new UWM thrusters bring a number of benefits as regards installation. The thruster scope of supply has become more clear and consists of four main groups; the outboard part, the thruster well, the vertical floating shaft, and the auxiliary systems.

The outboard part is the main delivery. It contains the thruster unit itself consisting of the nozzle, propeller, thruster housing, and the azimuthing system. The thruster well is a steel construction that forms the interface between the outboard part and the vessel. The thruster well is welded to the vessel, eliminating the need for surface and bolt hole machining, as well as final fixation by means of resin pouring. Prior to the start of the underwater mounting process, the well is kept watertight by bolted closing covers. The underwater mounting process follows a well documented step-by-step procedure. A mechanical sensor ensures that the thruster is in the correct position before the fixation bolts are mounted.

The azimuthing system is integrated into the outboard part and does not need to be installed separately. The auxiliary hydraulic steering and lubrication systems are connected to manifolds on the thruster by means of flexible hydraulic hoses. Once the vertical floating shaft with the torsional vibration damper, shaft brake, and electrical connections are installed, the thruster is ready for operation.

Thruster room dimensions

By applying a single slewing bearing, and by integrating the azimuthing system into the outboard part, the overall inboard height that is needed in the thruster room for the Wärtsilä Steerable Thruster (WST)-45U type is almost 1200 mm less than its predecessor, the Wärtsilä LMT FS3500NU. Similar space savings are achieved for the WST-55U at 5500 kW maximum power. At the same time, the weight of these new thrusters is significantly reduced; more than 20% when comparing the outboard part of the WST-45U type with its predecessor. By applying a closed loop hydraulic steering system, the space that is required for the auxiliary equipment is also reduced.

High performance

The Wärtsilä next generation UWM thrusters are designed with increased diameters, which have a positive effect on the bollard pull performance of the unit. In the detailed design phase, hydrodynamic improvements to the nozzle shape, propeller design, and connections of the nozzle to the thruster housing, have also been implemented. This has resulted in an additional performance increase for the thruster unit itself.

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Fig. 4 – Scope of supply overview.
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Table 1 – Technical overview of the WST-45/55/65.

Dynamic positioning

Propulsion units on offshore vessels are installed to keep the vessel in its position, independent of the weather conditions, waves and current. This functionality is known as dynamic positioning (DP). Typically, a number of steerable thrusters are installed to generate a sufficient total force on the vessel to counteract the environmental forces acting on the vessel, such as wind and currents.

To improve the vessel’s DP capability, these thrusters are designed with a downward tilt of the shaft line, propeller, and nozzle by 8°. The basic idea is to deflect the slipstream from the steerable thruster sufficiently downwards so as to minimize the unfavourable interaction effects between the thruster and its environment, such as the ship’s hull and/or other units.  

Effective thrust

In order to determine the DP capability of a vessel, it makes perfect sense to establish a footprint of the effective thrust of the vessel. Effective thrust is the total force required to keep the vessel in position to counteract the environmental forces. The effective thrust for the vessel can vary in every direction. The maximum value determines the environmental conditions under which the vessel can maintain its position. The required effective thrust is generated by means of thrusters and depends on various factors, such as the bollard pull performance of each thruster, the hull design, the positioning of each thruster and, most importantly, the thruster geometry.

Various studies have shown that there is a significant gain in effective thrust when applying thrusters designed with a downward tilt of 8°, compared to thrusters designed without a tilted gearbox. In order to quantify this benefit, Wärtsilä has performed comparative studies for non-tilted (straight) and 8° downward tilted thruster units by means of modern Computational Fluid Dynamics (CFD) methodology for a typical semi-submersible drilling rig application. Each thruster unit on the rig has a range of steering angles in which the slipstream from the thruster may cause interaction effects, thereby reducing the effective thrust. In establishing the most effective thrust footprint, a start is made by investigating the main thruster interaction effects that apply for each individual thruster:

  • Thruster operation with slipstream along the hull
  • Thruster operation with slipstream towards the second pontoon
  • Thruster-thruster interaction when two units are aligned.

These three phenomena and the computational results are shown in the sketch in Table 2, where an example case of a semi-submersible unit with 8 steerable thruster units is considered.

A 360° effective thrust plot has been derived for each individual thruster, depending on its location on the vessel and the type of thruster unit. One example is depicted in Figure 5. From this study it has been concluded that forbidden zones are reduced significantly, and the effective thrustfor all steering angles is largely improved by the 8° tilted solution.

The main types of interaction addressed are illustrated in the polar plot by coloured arrows, representing the slipstream. The interaction loss due to the thruster-thruster interaction (blue arrow) can be recognized by the small sections taken out of the polar plots, the so called forbidden zones. Operations with the slipstream along the hull are visualized by the red arrow. In case of sideways operation (green arrow), a significant interaction loss of about 50% for the straight unit is calculated. For the 8° tilted thruster, the most significant loss in thrust is found in the case of thruster-thruster interaction. In all other conditions, a maximum of 5% interaction loss of is found.

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Table 2 – A semi-submersible case with different types of interaction.
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Fig. 5 – Effective thrust of an individual thruster on a semi-submersible rig, for a straight and an 8° tilted thruster. The slipstream of the thruster is represented by the coloured arrows, for example the green arrow represents the slipstream when the thrust is generated towards 90°.

The derived thruster-interaction results for individual units have been combined to determine the overall effective thrust for a semi-submersible with 8 units. The results for the straight and 8° tilted units are presented in polar plots to indicate the clear benefits of the tilted thruster units, see Figure 7. The maximum increase of available thrust of the rig in sideways operation is about 35%. In forward operation the gain in available thrust is 9%. The average increase in available thrust over the complete 360° circle is about 20%.

This impressive increase of available thrust for 8° tilted thruster units is significant. Obviously, this is an important benefit that should be assessed already during the design of an offshore vessel with DP functionality. The increase in effective thrust can be utilised in the following ways:

Keeping the designed installation:

same power, more effective thrust

  • Less fuel consumption for the same weather conditions
  • Capable of handling more severe weather conditions

Redesigning the installation for

the same effective thrust, reduced installed power

  • Smaller/cheaper/fewer generator sets à more payload, improved machine room footprint, less fuel consumption
  • Fewer cylinders à reduced operational costs with respect to maintenance
  • Smaller thruster size à less weight, more payload
  • Smaller electrical and automation equipment à smaller E-motors, transformers, frequency drives
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Fig. 6 – The Wärtsilä LMT FS3510 NU, 8 degree tilted steerable thruster.
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Fig. 7 – Effective thrust of a semi-submersible, for a straight and an 8° tilted thruster.

Attractive business case

Where a vessel is designed specifically to meet its defined operating conditions, it is valid to design the vessel for the same effective thrust as defined in the traditional way, where the benefit of 8° tilted propeller shafts is not accounted for. In this section, an assessment is presented for actual semi-submersible drilling rigs foreseen as having Wärtsilä equipment, to highlight the benefits in a realistic fashion.

Three example cases have been defined. The first two cases are semi-submersible drilling rigs with 8 generator sets and 8 steerable thrusters. The third case is a semi-submersible with 6 generator sets and equipped with 6 steerable thrusters. The main particulars of the power plant and selected thrusters are listed in Table 3. The beneficial effect of tilted thrusters has to be taken into account in the DP capability analysis. For the base case of the three example cases, there is no tilted thruster benefit taken into account in the DP capability analysis. Alternative cases are composed for each base case where the benefit of a tilted thruster is accounted for. The alternatives are denoted by A, B and C in Table 3. In this assessment, the electrical losses in the generator, transformer, grid and E-motor are fixed at 9%. Furthermore, the required power for onboard electric consumers is considered to be the same for both the base case and the alternative cases.

In the base case of the first example, eight 12-cylinder Wärtsilä 32 (550 kW/cyl) generator sets in V-configuration and eight WST-55U (5.5 MW) thrusters have been selected. By taking the 20% tilt benefit on the effective thrust into account, it is calculated that in theory 35% too much power is installed for the vessel to meet its targeted DP capability. Accordingly, in alternative A, a lower power optimised 12-cylinder Wärtsilä 32 (480 kW/cyl) generator set in V-configuration and a one size smaller thruster, the WST-45U, operated at 4.5 MW, provides sufficient installed power for onboard operations, while generating the same effective thrust as the base case. In total, the installed power is reduced by 13%, and the power installed for propulsion is reduced by 18%. Capital expenses are compared based on realistic sales prices for the generators, thrusters, E-motors, transformers and variable frequency drives. For alternative A, the capital expenses are reduced by 6.9%. An additional benefit is the weight reduction of 10.3% for the various components in this alternative. In terms of additional payload for the vessel, this corresponds to 159 mt that can be utilized to allow, for example, more mud, more pipes, and more drilling hours without an external supply.

The advantages are even more pronounced for alternative C. Significantly smaller and lighter generator sets, the 16-cylinder Wärtsilä 26 in V-configuration, have been selected in combination with the same thruster size, operating at 4.2 MW instead of 5.5 MW. This results in a highly efficient thruster with an installed power reduction of 21% and reduced capital expenses of 8.5%. Furthermore, the payload is increased with an impressive 271 mt and the required space in the engine room is reduced significantly as well. This represents an attractive solution for rig designers that are looking for reduced equipment weights, dimensions, and investment costs.

The second example case has two attractive alternatives, namely a reduced thruster size and reduced generator types, the 9-cylinder in-line Wärtsilä 32 and the 12-cylinder Wärtsilä 26 in V-configuration. Capital costs are reduced by 6.7% and 7.4% respectively, while the installed weight is seriously reduced by 16.7% and 24.2%. Alternative B, where the 16-cylinder Wärtsilä 26 engine in V-configuration is selected, is again attractive for applications where cost, weight and space are considered to be important.

The third example case is a semi-submersible with 6 generators and 6 thrusters installed. One alternative is configured, based on the 12-cylinder Wärtsilä 32 in V-configuration low power optimised version, and a one size smaller thruster. The investment costs are reduced by almost 5%. Once more, it is demonstrated that including the benefit of tilted thrusters in a DP analysis makes a huge difference.

It is demonstrated by the alternative cases for the three semi-submersible rig designs, that a detailed input for DP analysis is required in order to optimize the design of the vessel in terms of capital expenditure (CAPEX), payload, and equipment space requirements. For example, the installed propulsion power and generator set power lead to a CAPEX reduction of 4.4 to 8.5%, depending on which alternative is considered. For a fictive scope of 25 to 35 M€, this could mean savings of 1 to 3 M€. The payload increase varies from 51 mt to 362 mt, depending on the chosen alternative. For a semi-submersible with a deck payload of 6000 mt, this means a capacity increase of 1 to 6%.

The above analysis focuses on the benefits for CAPEX, weight and space. Each of the alternatives also provides benefits to the operational costs, but a more detailed operating expenditure (OPEX) analysis falls outside the scope of this article.

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Fig. 8 – Comparison results for each example case.
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Table 3 – Three example cases and their alternatives.

CONCLUSION

Wärtsilä’s next generation UWM thrusters, which are available up to 6500 kW, provide a high level of reliability thanks to the large number of new design features.

The performance of the new UWM thrusters has increased as a result of applying larger propeller diameters, a new nozzle design, hydrodynamic optimization and, most importantly, the 8° tilted propeller shaft.

This high performance allows more effective DP operations, while allowing owners or operators to make their DP operations more efficient. In several business case examples it has been made clear that the high performance of the new thrusters with their 8° tilted propeller shaft provides the possibility for reducing the amount of installed propulsion power. In a few cases, a smaller propulsion unit can be selected without any reduction in the vessel’s overall DP capability. In all cases, the installed power of the onboard generator sets can be reduced. In some cases, generator sets with a lower number of cylinders can be chosen, which not only reduces fuel consumption but also lowers operational costs, weight and the dimensions

References
[1] Bulten, N., Stoltenkamp, P. (2013) “DP-capability of tilted thrusters”, Dynamic Positioning Conference, 2013, Houston
[2] Bulten, N., Drost, A., Boletis, E. (2013) “New Wärtsilä Thrusters - Keeping drilling vessels safely in place under harsh conditions”, Wärtsilä technical journal In Detail 1/2013

Press release: Wärtsilä announces the introduction of next generation thruster portfolio

Wärtsilä Propulsion Condition Monitoring Service

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