Schottel SRP-D: DP-Optimized Rudder Propeller for Offshore Ops

Due to the additional eight degrees downward tilt of the lower gearbox, the interactions between propulsion unit and hull are reduced. This results in increased thrust efficiency in DP operation and minimizes forbidden zones.

Image courtesy Schottel

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To ensure that wind turbines at sea operate safely, regular and effective maintenance is essential. Service operation vessels (SOVs) are used to secure this, while providing the crew on board with comfortable accommodation during multi-day operations. They are often equipped with motion-compensated gangways that allow safe crossing between the ship and the offshore platform. This places particular requirements on the propulsion systems: Not only do they have to maintain an exact position for hours on end, but it is also necessary to maneuver the vessel quickly and precisely, even in in extreme weather conditions. At the same time, decarbonization targets in the shipping industry are leading to increasing demands for emissions reduction and energy efficiency.

High Dynamic Thrust Allocation

SRP-D with a propeller shaft with an eight-degree tilt. Image courtesy Schottel

The German propulsion expert Schottel is meeting these growing requirements in the offshore sector with a new rudder propeller optimized for DP use. The SRP-D ("Dynamic") is a further improved variant for highly demanding DP operations in service operation vessels. The thruster is marked by an additional eight-degree tilt of the lower gearbox and improved propeller acceleration/deceleration times. In combination with a high-speed azimuth steering system with reinforced gear components, the SRP-D enables faster thrust allocation than conventional rudder propellers. Thanks to the shorter response times, it is possible to react faster and in a more targeted manner to external forces such as wind and currents, thus achieving a higher positional accuracy of the vessel.

In addition, the thruster is characterized by a vertically integrated electric drive motor (LE-Drive) which offers a number of significant advantages: the elimination of the upper gearbox increases mechanical efficiency, reduces fuel consumption and also minimizes vibration and noise. In terms of space savings, it scores with a lower installation height and minimal space requirements, as the L-drive does not require a separately arranged motor including its foundation.

Reduced DP Footprint & Fuel Savings

The dynamic properties of the SRP-D have been confirmed several times by external experts. A study examined the effects of thruster response on DP positioning. The simulation was performed on the numerical model of a real Service Operation Vessel (SOV) equipped with Schottel propulsion systems. Its position-holding capabilities were examined under extreme weather conditions: high winds, strong currents and effective wave heights of 2.5m and 3.2. The results of the extensive simulations at DNV showed a significantly improved positioning accuracy thus reducing the overall DP footprint. This permits successful gangway landing operations even in rougher conditions, thus increasing the operational window of the vessel throughout the year. Furthermore, the implemented optimizations are expected to result in additional fuel savings: Thanks to the faster response time of the propulsion, the vessel can be quickly held in position with only minor corrections being necessary. This prevents major corrections in position having to be made which would require more power.

The 98-degree variant induced lower losses at the other thruster than the 90-degree variant and therefore enabled a higher remaining force during DP. Image courtesy Schottel

98-Degree Tilted Propeller Shaft

Another independent study focused particularly on propulsion efficiency during thruster interactions. The research was carried out on a model of a CSOV equipped with a Schottel SRP-D 98-degree thruster on starboard and a regular 90-degree thruster on portside. The first part of the study measured the interactions between propulsion unit and hull. It was found that the additional eight-degree downward tilt of the SRP-D propeller shaft could significantly reduce thrust losses, both for azimuth variations and thrust variations. For example, CSOVs with 98-degree thrusters experience only 10 percent thrust losses in transverse direction, compared to 35 percent with 90-degree thrusters. The study showed that a key factor for the improved system performance is a reduced Coandă effect with the SRP-D. The second part of the research focused on propeller flow interactions. In this case, the additional eight-degree downward tilt is also beneficial: While 50 percent losses are observed when the 90-degree thruster is blowing the wake into the second thruster (in-line thrust losses), only 20 percent thrust losses occur with the 98-degree thruster, which therefore enabled a higher remaining force during DP.

Thorsten Tillack, head of hydrodynamics and propeller design at Schottel, summarizes the resulting advantages: “Overall, the SRP-D significantly optimizes thrust yield and thrust distribution, since thruster-hull- and thruster-thruster-interactions are minimized. Consequently, the forbidden zones of the thrusters may be reduced, which leads to an additional increase of the DP performance for the vessel. Furthermore, the optimized thrust output is expected to result in fuel savings. As a result, SOVs operate much more efficiently, extending their operating time throughout the year.”