New-generation propulsors enhance performance of offshore wind ships

Research published by the Offshore Renewable Energy Catapult in the UK suggests that by 2050, more than 400 service operation vessels (SOVs) of various types could be in service in the offshore wind industry. Large numbers of SOVs have already been delivered into the market or are on order, the operating profile of which demands extremely high levels of availability and capability.

As the market evolves and new designs come to market, so SOV operators need to be able to predict as precisely as possible whether a given vessel will be able to execute its intended operation safely and efficiently, within the desired time and weather window. Responding to this requirement, and the fast growth in the size of the market, propulsion system specialists have introduced new, more capable thrusters for dynamic positioning systems for SOVs and related vessels, such as offshore energy support vessels, of which a growing number have recently been ordered.

One such propulsor is the Schottel’s RudderPropeller Dynamic SRP-D, the capability of which, says Schottel, was confirmed by independent tests conducted on behalf of Damen Group at Maritime Research Institute Netherlands (MARIN). The aim of the tests at MARIN was to demonstrate the ‘added value’ of the SRP-D for commissioning service operation vessels (CSOVs), compared with other types of propulsor.

Schottel says the results of the research project demonstrated a significant increase in thrust yield and reduced ‘forbidden zones’ (that is, zones in which thruster-thruster interaction is detrimental to performance) compared with conventional rudder propellers. The results of the project demonstrated what Schottel described as “a significant increase in thrust yield as well as reduced forbidden zones with the SRP-D compared with conventional rudder propellers.” An earlier study had already demonstrated a reduced DP footprint and fuel savings with the RudderPropeller Dynamic SRP-D, whereas the Damen-commissioned research focused particularly on propulsion efficiency during thruster interactions. Some of these benefits have already been described by OWJ, in the Q3 2024 issue.

Damen product portfolio manager SOVs Mark Couwenberg describes the performance of the thruster as “a great step.” He says RudderPropeller Dynamic SRP-D thrusters mounted on a hullform optimised by Damen resulted in reduced fuel consumption, greater comfort for those on board and a reduced thruster footprint. “Combined with Schottel’s robust methods of design, engineering and construction we are convinced that our clients will have life-long benefits if they select this combination,” says Mr Couwenberg. 

When it was developing the new thruster, Schottel studied how tilting the gearbox and propellers could optimise performance and minimise thruster-hull and thruster-thruster interactions. It found a gearbox inclination of 8° would redirect propeller wash downwards and improve performance, whereas tilted nozzles would result in increased cavitation, noise and vibration. Hence the tilted gearbox concept is superior to a tilted nozzle.

Schottel further states that, in combination with a high-speed azimuth steering system, the latest version of the SRP-D thruster enables faster allocation of thrust than is possible with a conventional rudder propeller. The benefit is that shorter response times enable a vessel to react more quickly to external forces such as wind and currents, thus increasing positional accuracy.

The research commissioned by the Dutch shipbuilder was carried out on a model of a CSOV equipped with a Schottel SRP-D to starboard and a conventional 90° thruster on the portside. The first part of the study measured the interaction between the propulsion unit and the hull. It confirmed the additional 8° downward tilt of the SRP-D’s propeller shaft significantly reduced thrust losses.

“What this means in practice is that a CSOV with 98° thrusters would experience only 10% thrust losses in the transverse direction, compared with 35% with 90° thrusters,” says Schottel. “The study shows that a key factor for the improved system performance is a reduced Coandă effect with the SRP-D.”

The second part of the study focussed on propeller flow interactions. Once again, significantly lower thrust losses occurred when operating a CSOV with the SRP-D: 50% losses were observed when the 90° thruster was directing its wake into the second thruster (in-line thrust losses), whereas only 20% losses occurred with the 98° thruster. 

When the 90° thruster was directing its wake into the second one while perpendicular to it, thrust in the transverse direction was reduced by approximately 40%, compared with 20% with the 98° thruster. In both cases, the 98° variant induced lower losses from the other thruster than the 90° variant did and ensured greater remaining force during DP operations.

“The research shows the additional 8° tilt of the SRP-D gearbox significantly optimises thrust yield and thrust distribution, minimising thruster-hull and thruster-thruster interactions,” says Schottel. 

“This means the forbidden zones of the thrusters can be reduced, which leads to enhanced DP performance for the vessel. An CSOV with a RudderPropeller Dynamic SRP-D will have significantly reduced fuel consumption and operate much more efficiently and safely, extending vessel operating in the process.”