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

By Peter Pospiech at June 11, 2013 05:39
Filed Under: Azimuth pod, Company News, drive systems, Offshore

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

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. 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, 

Graph of the LMT-3510 Underwater Mountable Thruster

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 hydro dynamically efficient and highly reliable product thereby reducing the risk of downtime (off hire).  

Graph of a drill rig with 8 steerable thruster units

 

Graphs: courtesy of Wärtsilä

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