Efficient Subsea Operation Through Innovation

How Copenhagen Subsea Redefines Reliability and Performance

Author: Isaac Appelquist L?ge, Technical University of Denmark

Abstract

Subsea operations are both challenging and costly. When underwater, every second counts. Therefore, for subsea exploration and operation, selecting the right equipment is crucial for success. Functional thrusters are the key to optimizing underwater propulsion and maneuverability. However, traditional shaft thrusters are prone to breakdown in complex marine environments. Copenhagen Subsea A/S (CS), a leading technology developer within its field, has developed, tested, and commercialized a novel type of thruster. They provide innovative solutions meeting the demands of several industries such as offshore installations for maintenance and inspection, subsea defense systems, and maritime shipping.

This article describes what separates rim-driven thrusters from shaft-driven and gives key insight into the proven usability and reliability of the Copenhagen Subsea thruster.

Subsea Propulsion

Operating the subsea requires precise control of position, high stability, and good maneuverability. Here, thrusters are crucial, and traditionally shaft-driven thrusters are used. Shaft-driven thrusters consist of a propeller mounted on a shaft that connects to a gearbox. If a shaft-driven thruster is to be mounted on a subsea vehicle, the hull of the vessel must be breached and will take up precious space in vehicles. Furthermore, propellers are sensitive to complex environments, and if they get entangled with debris, it will be fatal for operation. An optimized solution has been proposed in the 1940s, namely rim-driven thrusters. A system where no gearbox is needed. The rim-driven thruster creates torques from blades mounted on a rim of a casing that is turned through an electromagnetic stator. The concept is simple, but execution has been lacking for many years. Copenhagen Subsea A/S (CS) has developed a product capable of delivering all the benefits of rim-driven thrusters.

Shaft- or rim-driven thrusters

Firstly, the rim-driven thruster delivers a higher torque compared to the shaft-driven thruster, due to the positioning of the driving force related to the blade. As the driving force is placed on the outside of the blades, the rotation uses Archimedes’ principle to drive the blades, also known as the lever principle. The lever arm is rotated at the full circumference of the rim, whereas in the shaft-driven thrusters, the rotation occurs from the center. For the rim-driven thrusters, the lever arm is significantly longer than the shaft-driven, giving it the option of achieving more torque. This allows rim-driven thrusters to obtain both higher control and absolute torque. Such control is essential in strong seas where stability is needed.

Furthermore, for certain subsea applications, operating with a low sound-print can be essential. Sound waves travel both farther and faster underwater, and sometimes neither foreign agents nor fish should be alarmed of a vessel’s presence. Here, the rim-driven thrusters also offer a superior solution compared to the shaft-driven thruster. The rim-driven thrusters reduce noise and vibration due to flow in front of the propeller and elimination of tip vortices in the wake of propeller blades. A large source of noise from traditional thrusters comes from the cavitation and collapse of bubbles that arise on the wake side of the propeller. However, as the rim-driven thruster has a more uniform thrust distribution and operates with lower tip speeds, they have reduced cavitation. In addition to lower cavitation, mechanical noise is reduced, as the motor is integrated into the rim of the thruster. This integration eliminates the need for a central shaft and gears, which are common sources of mechanical noise in traditional thrusters.

The CS-thruster: Reliable and customizable

The CS-thruster is designed with the end customer in mind. Customers in the maritime and subsea sectors face multiple challenges when selecting thrusters. Key issues include choosing a thruster that can achieve the necessary thrust force for demanding tasks, that performs uninterrupted for long periods, that has high compatibility with available power supplies, and that is customizable to the project at hand.

The CS-thruster is designed with compatibility in mind. For example, the stator design can be adjusted in the production stage, so that the thrusters can adapt to a supply voltage level between 24-800 VDC. This allows the system to be incorporated with a wide range of power sources. In addition, the motor controller is a commercially available shelf product, ensuring high compatibility with custom electronics. Lastly, thrusters can be incorporated as a propulsion system for both remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), and manned submersible vehicles. Depending on the application, Copenhagen Subsea is ready with their large expertise to propose how the thrusters can be integrated in various vehicles.

Second, the CS thrusters are engineered for reliability in harsh environments. Rim-driven thrusters have a built-in cooling mechanism, where the surrounding water is used as a cooling medium. However, in the case where high temperatures should arise, the CS-thrusters temperature measurement is incorporated in the design. Furthermore, the thrusters which integrate high torque and a robust stator contribute significantly to their stability and reliability, even under strenuous operational conditions. CS knows how important reliability is. Therefore, they have conducted extensive testing in saline and sediment-rich environments to demonstrate the durability of the thrusters. Their tests demonstrate operational integrity without degradation over extended periods. This resilience is crucial for subsea operations, where equipment failure can lead to significant operational and financial setbacks. To further increase the robustness of the CS solution, their thrusters are designed to fit with commercially available sensor-less motor control.

The lack of physical feedback sensors in thrusters enhances their reliability by reducing potential points of failure, increasing their robustness in harsh marine conditions.

Conclusion

In conclusion, the rim-driven thruster offers a significant step forward for thruster technology. Here, the CS thruster stands out as their thrusters are designed to be reliable, powerful, and silent. CS is achieved using cutting-edge materials and innovative design principles. They have extended operational capabilities and reduce the risk of mission failure. Therefore, the CS thruster should be the preferred choice for industries that rely heavily on propulsion in maritime and subsea environments. As the demand for more efficient and robust underwater vehicles grows, CS Thrusters continues to lead with solutions that not only meet but exceed customer expectations.

In conclusion, for those engaged in subsea or maritime operations, the acceptance of the CS thrusters could be the key to achieving new levels of operational efficiency and reliability.