Building a bridge for wind energy

Although offshore wind farms are not common in the U.S., they are increasingly being built overseas, because wind turbines can capture the more consistent winds off the coast while also reducing environmental and visual impacts. But like all wind turbines, offshore designs require vigilant maintenance. So installing them and maintaining them can be a challenge.

Imagine how difficult it is to climb and service a wind turbine tower on land, then add hostile weather conditions and choppy waves into the mix. Traditionally, maintenance crews using small boats had to wait for a window of good weather before undertaking both routine and unscheduled maintenance. This often meant a very long wait, as the optimum locations for wind turbines inherently experience extended periods of rough sea conditions.

Dutch company Ampelmann partnered with Moog to devise a new strategy. The company fixes an inverted 6-axis motion base onto a shipboard platform, to cancel out the wave motion. This technology is widely used in flight simulators to generate precise motion, whereas in this application, it is used to absorb motion.

Mounted on the stabilized platform is a telescopic walkway or bridge that can be extended to reach the wind turbine base. The movement of the bridge to the static structure is manually controlled, and once in contact a controlled pre-load pressure is applied to ensure contact is maintained.

Ampelmann decided to design a new motion base customized for the long strokes and high loads required for this application. The company chose Moog’s high flow servo valves and safety cartridge valves because it was easy to customize these control valves to achieve the speed, resolution and integrated safety features required.

According to Jörg Hätzel, Account Manager, Simulation at Moog, “We specialize in building blocks. We have the ability to change the valves to exactly what the customer wants. This is neither off-the-shelf nor custom. We find out exactly what their needs are and then meet them.”

The inverted motion base keeps the platform independent of the ship’s motion. To achieve this, a sophisticated gyro-based transducer or motion reference unit (MRU) is mounted on the platform to detect vertical and horizontal accelerations. The gyro output signals are processed by a custom designed controller that sends signals to the hydraulic actuators with the objective of producing zero acceleration in all axes.

Secondary position control loops tend to force the actuators to mid-stroke, ensuring that any inevitable small acceleration errors don’t accumulate and cause the actuators to extend or retract and hit the mechanical end-stops.

This motion base offers a long operating stroke (long heave compensation of 2.5 m) and offset asymmetric loads. The system also offers several levels of redundancy for safety reasons, including:

• Duplex motion sensor on platform
• Triplex position sensors in the hydraulic rams
• Duplex hydraulic system with ‘switchable’ control valves
• Control valves with integral ‘abort’ function as in a flight simulator
• Duplex control cabinets

Moog’s D663 valves control the hydraulic rams. They offer high flows of up to 645 lpm (170 gpm) at 70 bar (1000 psi) pressure drop, and feature a fast response of up to 90 Hz with 90° phase-lag at 25% signal. Resolution is extremely fine, with responses to very small command signal changes of less than 0.1%. In addition, they offer integrated control electronics with error monitoring and an integral abort function, which gives ‘soft’ failure mode in the event of a complete electrical failure.

In the event of a complete loss of electrical power to the system, the failsafe mechanism in the Moog valve mechanically produces a small pre-determined spool offset. This offset ensures that the actuators retract slowly to lower the motion base to the safe ‘home’ position, in exactly the same manner as a flight simulator.

Hätzel said the platform also uses a Moog RSE40HV6 Position Monitored Aktive Cartridge size NG40 (according to ISO 7368) valve, which features open position monitoring. These valves help ensure reliable switching between the duplex hydraulic circuits and incorporate a position monitoring system to provide an extra level of system integrity.

Ampelmann has produced a total of eight access platforms, leased to customers who operate them all over the world. In practice these platforms can be successfully deployed in sea states of up to ±3 m (±9.8 ft) depending on where the platform is mounted on the vessel. The most effective location is the center of the ship as this minimizes the influence of pitch and roll on the motion of the platform.

This new technology has also been applied to other applications such as transferring personnel and materials during the construction of offshore structures.

Ampelmann also introduced an even larger access platform, incorporating the same Moog technology. This unit—the ‘E-type— has an even higher payload capacity and works in sea conditions in excess of ±3 m (±9.8 ft) to enable operation in a wider spectrum of sea conditions.

Hätzel said that the new E type is 1.5 times bigger than the original A type of platform. It features a 3-m stroke cylinder and can accommodate a maximum 100-ton compensated load. With the added weight capability, the gangway bridge can be used to transfer cargo, equipment, tools and other such items. In addition, the new E type will eventually be fitted with a crane in the future.

Moog Inc.
www.moog.com