A smooth approach to safer offshore turbine access

Published:  01 February, 2014

Increasingly, wind turbines are being sited offshore, where strong winds are more consistent and the visual and environmental impact is reduced. However, there are numerous logistical difficulties in installing and maintaining these turbines in such a remote and hostile environment.

Consider for instance the difficulty of deploying maintenance staff onto the turbine in poor weather conditions and rough seas. On most fixed offshore structures this can be achieved with the use of a helicopter, but obviously this isn’t feasible with a wind turbine. Traditionally, maintenance crews using small boats had to wait for a ‘window’ of good weather before undertaking both routine and unscheduled maintenance. Ironically, this often meant a very long wait, as the optimum locations for Wind Turbines inherently experience extended periods of rough sea conditions.

Today, there is another alternative thanks to Moog hydraulics technology and the innovative Dutch company Ampelmann.

A New Kind of Bridge

Back in 2007, Jan van der Tempel, now Ampelmann’s CEO, hit upon the idea of using a shipboard platform mounted on an inverted 6-axis motion base, to cancel out the wave motion. This technology is widely used in flight simulators, but for generating rather than absorbing precise motion.

Mounted on the stabilized platform is a telescopic walkway or bridge which 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.

Because of the exceptional loads and speeds required, it was decided to design a new motion base customised specifically for the long strokes and high loads required for this application. Moog was selected to supply the high flow servo valves and safety cartridge valves crucial for the success of the project. Key to this decision was Moog’s ability to customise these control valves to achieve the speed, resolution and incorporate the integrated safety features required.

Ensuring Stability at Sea

The inverted motion base is employed to produce a platform which is stationary in space independent of the ship’s motion. (This type of control is often referred to as a ‘sky-hook’ system). To achieve this result, 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 which 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 differs from established flight simulator technology in a number of areas, such as the long operating stroke and offset asymmetric loads. Another notable difference is the level of system redundancy, essential for reasons of safety, namely:

• 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

Absolutely crucial to the system’s ability to operate in extreme sea states, is the performance of the servo valves used to control the hydraulic rams. The Moog D663 Valves selected for this demanding application have the following characteristics:

• High Flow: Flows of up to 645 lpm (170 gpm) at 70 bar (1000 psi) pressure drop
• Fast Response : Up to 90 Hz with 90 degrees phase-lag at 25% signal
• Fine Resolution: Responds to very small command signal changes: < 0.1%
• Integrated Control Electronics: With error monitoring function
• Integral ‘Abort’ Function: Gives a ‘soft’ failure mode in the event of a complete electrical failure.

In this application, 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.

The latest generation of Ampelmann platform also utilises a Moog Active Cartridge Valve to ensure reliable switching between the duplex hydraulic circuits. This unit incorporates a position monitoring system to provide an extra level of system integrity.

A Bright Future for the Technology

To date, Ampelmann has produced a total of eight access platforms, leased to customers who operate them all over the world. It has been proven in practice that 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 centre of the ship as this minimizes the influence of pitch and roll on the motion of the platform.

This new technology permits the servicing of wind turbines in all but the most extreme conditions, increasing the efficiency and attractiveness of these installations. Also, these access platforms have been applied to other applications such as transferring personnel and materials during the construction of offshore structures.

Ampelmann is about to introduce an even larger access platform, incorporating similar Moog technology. This unit, - the ‘E-type’ will have an even higher payload capacity and will work in sea conditions in excess of +/- 3 m (9.8 ft) to enable operation in a wider spectrum of sea conditions.

For further information contact Moog +44 1684 858000, email info.uk@moog.com or visit www.ideasinmotioncontrol.com