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New Wärtsilä thrusters – Keeping drilling vessels safely in place under harsh conditions

The new Wärtsilä LMT-3510 thruster provides more available force and excellent bollard pull performance, which are essential factors in offshore dynamic positioning applications.

Text: NORBERT BULTEN, GLOBAL MANAGER HYDRODYNAMIC ANALYSIS ALBERT DROST, PROGRAM MANAGER R&D | ELIAS BOLETIS, DIRECTOR PROPULSION R&D Photo: -

Wärtsilä is continuously improving its thruster portfolio in response to market demands, classification requirements, and feedback from thruster products in operation. In the coming years Wärtsilä will renew its entire thruster portfolio. The Wärtsilä LMT 3510 thruster is the first result of this extensive development programme that uses new insights and the latest technical and hydrodynamical knowledge.

The main objectives for the LMT-3510, as compared to the already existing LMT-3500, were:

  1. To improve the hydrodynamic efficiency of the thruster,
  2. Incorporate latest knowledge with regard to the propulsion driveline and structural strength of the thruster

The new thruster has found a broad market acceptance, as is manifested by the fact that, in a limited time, in excess of 100 units have already been sold. The new technology implemented in this development work provides very important input for the thrusters being developed for the new thruster portfolio

This article describes the background to how the design objectives were reached, and describes the main features of the LMT-3510.

As regards the hydrodynamic aspect of the design, the thruster was designed to address the issue of high interaction losses between the thruster and the adjacent hull. The basic idea is to deflect the jet from the steerable thruster sufficiently downwards to avoid interaction. The most efficient way to achieve this deflection, is to tilt the complete pod, shaft line, propeller, and nozzle by 8 degrees. It has been found that a geometrical tilt of 8 degrees is enough to deflect the jet sufficiently downwards without compromising the overall performance of the unit. The consequence of tilting the shaft, however, is a complete redesign of the underwater gearbox. A bevel gear transmission of 82 degrees was developed to accommodate this. Furthermore, the propeller diameter was slightly increased, which has a positive effect on the bollard pull performance of the unit. In the detailed design phase, hydrodynamic improvements to the nozzle shape, and connecting the nozzle with the rest of the unit, have also been implemented. This has resulted in an additional performance increase. Figure 2 shows a picture of the tilted unit with the 82 degree gearbox.

In the design process, the entire driveline, consisting of gears, shafts and bearings, was adapted to match the larger propeller. The resulting changes involve a larger gear-set and a different bearing configuration. Additionally, specific attention was given to the supporting structure and hydraulics. Last but not least, the LMT-3510 comes as standard with a torsional damper incorporated into the input shaft, and a PCMS (propulsion condition monitoring service) to monitor the condition of the system, thereby reducing the risk of downtime (off hire).

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Fig 1. – Artist impression of the LMT-3510 Underwater Mountable thruster.
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Fig 2. – Side view of tilted configuration.

Hydrodynamic aspects

In most marine applications, the propulsion system propels the vessel forward to move it from one location to another. For offshore 
drilling vessels, on the other hand, the main purpose of the propulsion units is to keep the rigs exactly in their place, independent of the weather conditions, waves and current. This function, known as dynamic positioning, is executed by a number of steerable thrusters to generate a net-force on the vessel, which counteracts the environmental forces acting on the vessel, such as wind forces and the effects of currents. 

In order to create as much thrust as possible in all directions, the thrusters are positioned at critical, pre-selected locations 
on the vessel. Typically, semi-sub(mersible) drill rigs with two pontoons have a total of 6 or 8 thruster units. The thrusters are placed at the front and aft of the pontoons, and sometimes also in the middle. This differs distinctly from main propulsion systems where the propeller is always located at the stern of the vessel.

A drill-rig with 8 thruster-units is shown in Figure 4. Each thruster unit has a range of steering angles in which the jet from the 
thruster may cause unwanted interaction effects, either with the pontoon or with  another thruster. This loss of performance 
is called a thrust-deduction factor. For thrusters on a drill rig, the thrust-deduction factor can be in the range of 15-50%, depending on location and steering angle.

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Fig. 3 – Assembled unit being transported to customer.
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Table 1 – Overview of different available LMT3510 configurations.

Numerical flow simulations

The understanding concerning the flow of water around a steerable thruster is obtained nowadays with the aid of Computational Fluid Dynamics (CFD). These numerical analysis methods have been in use within Wärtsilä for many years now. Simulations of the flow along the thruster are performed under full scale conditions. The analysis allows the hydrodynamic forces applied on the different components (propeller thrust, and torque and nozzle thrust) to be calculated. These component loads are used in the structural analyses to determine the stress levels that occur in the unit under different operating conditions.

In order to verify the analysis, the hydrodynamic characteristics of the new thruster design have been evaluated via scale-model testing in a towing tank. This is the conventional method to determine the performance of a propulsion unit. In this case, a model scale thruster unit of 250 mm has been made. The results from the model scale measurements are compared to the model scale CFD calculations. The performance of a thruster unit is then evaluated based on the required input torque and the delivered total thrust. In the measurements and in the simulations, the thrust on the propeller shaft is also determined separately. In Figure 5, the thrust and torque are presented in non-dimensional parameters for thrust, torque, and advance speed of the water. The agreement is good over  the complete range of operating velocities. This gives additional confidence in the accuracy of the numerical simulations. An example of the calculated pressure distribution on the unit is shown in Figure 6.

Thanks to modern computer power, it has become possible to simulate a steerable thruster in combination with the actual hull geometry. Two typical geometries have been simulated: a semi-submersible drill rig with two pontoons, and a drill-ship with a thruster in the bow. For both vessels the complete flow field (velocities in all 3 directions and pressure) has been analyzed. The calculations have been made for the conventional straight unit and for the tilted unit. The comparison  of the results fromboth geometries has revealed some very 
interesting phenomena.

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Fig. 5 – Performance chart of the thruster shows the thrust coefficient of the propeller (Ktp) and unit (Ktt), the torque-coefficient of the propeller (Kq), and the efficiency (Eta0) of the unit. The solid lines represent model-scale experimental data and the dotted lines show the results of the CFD analysis on model scale. The measurements and calculations are made for a single thruster-unit, thus without interaction with a hull shape.
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Fig. 6 – Example of a CFD result: the colours represent the pressure distribution on the LMT-3510 thruster unit, where blue indicates low pressure. The grey area at the propeller blade tip indicates a small region where the pressure is below the water vapour pressure

Example case 1: 


A drill rig in DP operational mode
The first example case is based on a drill-rig where the thruster jet is directed towards the second pontoon. This condition often gives such a large thrust-deduction that operation of the unit in this steering direction is excluded. The calculation for the straight unit gives a thrust-deduction factor of 50% at zero speed. This means that only half of the unit thrust is available at full power, which creates a dramatic drop in the effectiveness of the unit. In the top picture of figure 7, the streamlines out of the straight thruster are shown. It can be seen that the jet slowly diverges toward the second pontoon. At this pontoon, a region is found with increased surface pressure due to the impact of the jet on the hull surface. On the bottom side of the hull, additional friction is also found, which is related to the higher water speed in the jet. Due to the impact of the jet, a thrust-deduction factor of 50% is found in this condition.  

The new tilted thruster design gives a completely different picture. It has been found that the mean deflection of the jet is approximately 5 degrees downwards.  And, although there is a certain amount of divergence in the jet, there is still no impact on the second pontoon. The thrust-deduction in this case is limited to 5%. This value is very acceptable for operating in this steering direction.

Based on the results for the tilted unit, the conclusion has been drawn that the maximum thrust-deduction factor will be 5% in any steering direction in bollard pull condition. This is a remarkable achievement for DP-applications.

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Fig. 7 – analysis of streamlines from the thruster for a drill-rig in side-way operation based on full scale CFD-simulations. Conventional straight unit at the top and the tilted unit below.

Example case 2: 

A drill ship operating at 11 knots
In addition to drill rigs, drill-ships are also used in the offshore industry. With mono-hull drill-ships, it is common that a number of steerable thrusters are placed in the bow to create as large as possible a distance from the units in the stern. For DP-applications this distance is beneficial to keep the vessel in place. Conversely, in transit condition, when the ship sails from one location to another, the magnitude of the hull-interaction effect might become very significant. In this case, the overall effectiveness of the thrusters at the front of the vessel will be quite moderate.

Again, calculations have been made for the straight and tilted units, and in Figure 8, the streamlines out of the thruster are shown for both units. In accordance with expectations, a clear interaction has been found for the straight unit. The jet from the thruster interacts with the hull some distance away from the unit. This results in a thrust-deduction factor of around 20-25%

In the case of the tilted unit, the deflection of the jet is once again sufficient to keep the jet stream away from the hull. As a consequence a thrust-deduction factor of less than 3% is found.

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Fig. 8 – Analysis of streamlines out of the thruster for a drill-ship at 11 knots forward speed based on full scale CFD-simulations. Conventional straight unit at the top and the tilted unit below.

Consequences of improved hull-interaction 

The set target of reducing hull-interaction has been achieved. The bollard pull performance of the new thruster units is excellent, which means improved capability for dynamic positioning. The overall power requirements can be reduced, thus leading to more fuel efficient operations.

For drill rigs, this significant improvement in performance opens the door to reconsidering the strategy for Dynamic Positioning algorithms. The limited interaction losses in side-way operation mean that these units no longer need to be excluded from operation. Consequently, the generated side force can be almost doubled with the new LMT-3510-tilted thruster units.

In free sailing condition, there are also clear benefits to be found for drill ships. This is beyond expectations, since the design process was focussed on bollard pull performance.

Mechanical design

One of the focal points in the design of the LMT-3510 was structural strength reliability. The approach to this was to diminish the effect of load fluctuations on the internal elements of the thruster, in combination with a review of components inside the thruster.

To address the load fluctuations, the thruster has been fitted with a floating input shaft with a torsional damper incorporated, which safeguards against three phenomena:

  1. Load fluctuations and peak loads in the system caused by the propeller or the E-motor in response, are dampened.
  2. Electrical current going through the system from either the E-motor or the ICCP (Impressed current cathodic protection system) of the vessel, which may potentially damage the bearings and gears, is isolated by the rubber elements in the damper.
  3. Short circuit torques coming from the E-motor, which even though they are very short can be extremely high, are dampened before reaching the gear-set.

To make the intermediate shaft easier to install, composite shafts are applied. Especially for the longer shafts there is a significant weight reduction.

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Fig. 9 – LMT-3510 assembly in the delivery centre Trieste (Italy) factory.
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Fig. 10 – Calculation of misalignments.

Driveline design

The Spiral Bevel gear-set is the heart of the thruster, and is crucial to thruster operations. One of the possible failure issues with thruster gears that has attracted a lot of attention in recent years is called TIFF (tooth interior fatigue fracture). It manifests itself in the pinion of larger, heavily loaded thrusters. Peak loads initiate a crack just beneath the hardened layer of a tooth, which expands in time due to fatigue and eventually causes the tooth to shear off. In order to safeguard against this, the following measures have been taken in the design of the gears:

  • Raising of the Application and Safety factors,
  • Improvement of the material
  • Optimization of the tooth contact pattern

Firstly, the application factor and safety factors for Wärtsilä´s larger thrusters were re-examined and raised. For LMT-3510, this resulted in a larger and stronger gear-set.

Secondly, the gear material specifications were reviewed by our metallurgists and forge operators, which resulted in an overall improved material specification for the gear-set. In order for the gear-set to optimally fulfill its function, and to guarantee that it can bear peak-loads without any problem, the LMT-3510 gear-sets are designed in such a way that they can deal with misalignments caused by operational load, tolerances, and thermal expansion. Elaborate finite element calculations were carried out to determine the deformations due to load. The results were processed in special software programmes which are used by the gear manufacturer to design and calculate the tooth contact pattern of the gear-set under load. Finally, the tooth design was verified on the torque test-bench at the Wärtsilä factory in Trieste.

On the pinion shaft, a stiffer alignment of the pinion was realized through the implementation of an axial spherical roller bearing in combination with a tapered roller bearing above the pinion, and a so called PEX cylindrical roller bearing beneath the pinion.

True separation of the radial and axial loads on the propeller shaft is ensured by, (toroidal) CARB radial bearings, which besides having a higher load capacity, are able to accommodate expansion of the shaft by having considerable axial freedom.

When it comes to castings there are two requirements that need to be satisfied. The casting needs to provide the strength and stiffness to withstand external loads, and guarantee the alignment of the bearings and gears. At the same time the casting needs to be as light as possible. In order to meet both of these requirements, finite element calculations were used to reduce the weight. In short, the weight was reduced in places where loads would allow it, and the weight was increased by reinforcements where stiffness was required. Additionally, casting simulations were carried out to verify the cast ability of the material.

For the LMT-3510 a closed loop hydraulic steering system was implemented. For the large, high power LMT3510, the closed loop system is smaller than an open loop system, a feature that is especially important in drilling vessel applications. An additional benefit of the closed loop system is that steering operations are more efficient as less energy is consumed. 

The rate of re-circulation of the lubrication oil was raised to achieve cleaner oil and better maintainability of the oil temperature in the thruster itself.

In addition to structural strength, the LMT-3510 also addresses maintainability as it is supplied with PCMS standard. The PCMS system measures vibration and other parameters that determine the condition of components, such as the bearings, gears and seals. All information is monitored by Wärtsilä and reported back to the customer who can then make an informed decision as to whether and when to overhaul the thrusters, thus facilitating the maintenance planning. Based upon the condition of the total thruster, the overhaul interval can be extended beyond the conventional 5 year period.

Special attention is given to the sealing arrangement. The LMT-3510 is fitted with a 4BL seal, and the thruster design includes the possibility of seal monitoring. Alternatively, it is possible to fit the LMT-3510 with a face type seal as well.

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Fig. 11 – Measurement of the tooth contact pattern by torque testing and through comparison with theoretical results.
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Fig. 12 – A drill rig in operation.

CONCLUSIONS

The new LMT-3510 thruster unit has very good bollard pull performance, which is the key-issue in offshore dynamic positioning applications. The hydrodynamic design of the unit has led to a significantly reduced loss of thrust caused by thruster-hull interaction. This results in more available force to keep the drilling vessel in place under harsh operating conditions. The hydrodynamic development has been verified with the aid of state-of-the-art numerical simulation tools, which provided good insights into the occurring phenomena. This created a solid base for achieving significant improvements in the performance of the overall system.

The mechanical design has also been improved, thanks to the latest know-how and the extensive experience at Wärtsilä.

The LMT-3510 is, therefore, a notable step in the development of thrusters to serve the market with hydrodynamically efficient and highly reliable products.

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