RM4: Ocean Current Turbine

Project Summary

The Reference Model Project was a partnered effort to develop open-source marine hydrokinetic (MHK) point designs as reference models to benchmark MHK technology performance and costs, and an open-source methodology for design and analysis of MHK technologies, including models for estimating their capital costs, operational costs, and levelized costs of energy.

• Reference Model 1: Tidal Current Turbine
• Reference Model 2: River Current Turbine
• Reference Model 3: Wave Point Absorber
• Reference Model 4: Ocean Current Turbine
• Reference Model 5: Oscillating Surge Flap
• Reference Model 6: Oscillating Water Column

Reference Model 4

Reference Model 4 (RM4) is a “flying-wing” ocean current turbine concept intended for deployment in the Gulf Stream off the southeast coast of Florida. The RM4 device has four rotors, with a rotorless center nacelle housing the power electronics, attached on a straight wing 120 m long. The device is designed to be submerged ~50 m below the surface and is moored to the seabed. The RM4 uses buoyancy within the wing and the five nacelles to maintain its position in the water column. Each rotor has a diameter of 33 m and has a 1-MW power rating, yielding a total device rated power of 4 MW. The rotors on the left and right side of the wing rotate in opposite directions in order to balance the torque applied to the device. The rotorless center nacelle housing the power electronics serves to condition the power generated by the rotors before it is delivered to the grid.

The two-point mooring system consists of a tension (or buoyancy) and thrust mooring lines that are secured to the seafloor using a suction pile and a drag embedment anchor (DEA), respectively. As the name indicates, the thrust mooring line supports the thrust loads produced by the turbine in operational conditions. The buoyancy tanks and tension mooring provide the necessary buoyancy to keep the device at an approximately stationary position in the water column, for the range of thrust values that the turbine will produce during operation.

The blades designed for the RM4 device leveraged the blade design developed for the RM1 device.

The wing design uses two side-by-side circular pipes that span the length of the wing. These members act as the primary load-carrying member of the wing. Both pipes are connected together along the wing with diagonal pipe trusses and perpendicular plate stiffeners so that they form a rigid structure. To design the wing structure, each wing pipe was assumed to support 50% of the structural loads. Therefore, each pipe acts independently and was designed with enough strength to transfer half of the rotor thrust, rotor shear, and wing drag loads from the device to the mooring system.

The support structure for the turbine was designed to house the drive components and to act as the means of connection to the wing spar. The center support housing of each nacelle provides the main support structure with attachment to the wing spar. It is also designed to provide access to the gearbox. The center support housing was designed with cylindrical end flanges to mate with the forward and aft nacelles.

The nacelle was designed for easy system assembly and access for maintenance. The generator and gearbox assembly is mounted to a sliding rail system permitting easy access to these components through removal of the forward conical section.

The buoyancy tank was designed to withstand the hydrostatic pressure at a water depth of 100 m. The main support of the wing base is a dual cylindrical spar system and an emergency recovery system was designed into the device in the case of loss of control or persistent descent. The spars serve as: 1) a primary support structure for the nacelles and the wing fairings, 2) a space for dry storage (aft spar), and 3) a floodable forward spar for buoyancy control and recovery. The sealed aft spar will house power cables and transmission to the central power conditioning pod, and storage for the buoyancy control auxiliary equipment (logic controller and air supply). The buoyancy control is based on controlling the floodable volume in the forward spar. System components would include pumps and piping to expel water, valves to selectively flood or isolate the wet spar sections, distributed pressure sensors for depth sensing and a programmable logic control system.

All drive train components would be housed within a watertight nacelle to maintain overall system buoyancy and provide a closed seawater-tight system to permit use of standard industrial or wind turbine components. The watertight nacelle design eliminates the need to seal each component individually in a watertight, seaworthy environment.

The RM4’s PCC assembly consists of four major systems:

• Bearing and seal assembly
• Gearbox and coupling section
• Generator section
• Nacelle body

The mooring design consists of a thrust and buoyancy mooring line. The buoyancy tank provides sufficient up-force to always keep the buoyancy mooring line in tension and provide an attachment point for the wing structure that will remain at a fixed water depth while the rotors induce a variable drag-force on the mooring system.