This article is taken from the January/February issue of Renewable Energy Focus magazine. To register to receive a digital copy click here.

Active positioning

A number of vessel-mounted mechatronic (mechanical/electronic) solutions have been developed that are essentially bow-mounted hydraulically operated gangways. A boarding platform at the far end of the gangway is positioned by hydraulic servos that respond to signals from an electronic control system which generates these according to the responses of accelerometer motion sensors. Essentially, signals representing bow motions are reversed and the opposite-sense signals are amplified and fed to the servo actuators that move the platform.

Under these ‘inverse kinematics’ as the boat's bow rises, the gangway end falls. Similar compensation occurs in all the axes being catered for by the particular system.

One of the solutions currently being marketed is the Turbine Access System (TAS) developed by Houlder Limited in partnership with naval architectural consultancy BMT Nigel Gee Associates. Much of the capability of this device is due to bespoke control software developed by Integrated Systems and Control (ISC) Ltd, a spin-off from Strathclyde University.

ISC's Dr Andy Clegg explains the software was designed using LabVIEW, a graphical system design tool. First, dynamic simulation was used to establish that the control requirement could actually be met. Subsequently the development team was gratified when the performance of an actual prototype system matched closely that predicted by the simulations.

Thanks to this way of developing the necessary algorithms, the inverse kinematics can calculate the lengths of hydraulic stroke and application rates needed to compensate for vessel heave, pitch and roll with reasonable accuracy. Vessel motions are sensed by a motion reference unit. This needs a five-minute warm-up period after switch-on during which time it models the sea conditions and prepares compensation signals.

These are filtered to remove noise and spurious influences, and then fed to a series of cascade control loops. Smoothing is applied so peak demands for hydraulic and electric power from the vessel are not excessive. Feedback, together with predictive elements, is used to ensure accurate closed-loop servo control. A surge cycle compensator adjusts for fore and aft (surge) movement. Conditioned signals are fed to the hydraulic actuators that position the platform (human control of the TAS is enable via a pair of touch-panel screens).

The TAS is mounted on a rigid metal frame bolted to the vessel's foredeck. It has robust handrails along each side of all walkways. The structure is made from marine-grade aluminium to keep its weight down. Other features include an anti-slip gangway coating and an access traffic light system.

Rollers at the stabilised end of the boarding gangway bear against the turbine tower's friction bars and serve to dampen any residual motions the system has not dealt with. Houlder says the TAS, which already features in several proposed workboat designs, provides a stable platform without the need for dynamic positioning or other complex vessel systems. Compensation can be provided for up to +/− 1.5m vertically, with bow angulation of up to +/− 25 degrees. The first operational TAS made its public debut at the Seawork Exhibition, Southampton, in the summer of 2012.

Alternatives

A less sophisticated device with a simpler electronic control system is the Autobrow, being developed by UK-based Otso Ltd, with design input from Ad Hoc Marine Design Ltd and experience garnered by WFSV manufacturer South Boats. Compensation is provided, via hydraulic actuators, chiefly in the vertical direction where the main motions are experienced, though with some lateral accommodation also. A telescopic stage in the gangway/bridge caters for surge and a horizontal force is applied to maintain contact between the ladder and the turbine tower. Designed for both new-build craft and retrofit, the unit weighs less than a tonne and can be installed on smaller, lighter service vessels.

At the other end of the scale, the Momac Offshore Transfer System (MOTS) from German firm Momac GmbH relies on a turret-mounted articulating arm that carries an aerial lift cage at its far end. The arm with its end cage is controlled electro-hydraulically so turbine technicians can be positioned precisely, with their equipment, ready for transfer to or from the turbine ladder.

The MOTS 5000 system allows for up to 3.2m of vertical movement and is claimed to provide safe transfers in sea conditions of up to 2.5m Hs. It utilises proven robotic control elements within a sophisticated servo control system, Momac says. A vessel movement reference unit is complemented by a laser sensor. The sensor registers precise separation from the turbine, enhancing positional accuracy. Multiple fail-safety features are also included to ensure safe operation.

MOTS 5000 weighs some four tonnes and is suitable for retrofit as well as incorporation at new-build. In addition to transferring technicians to and from turbines, it is able to perform ship-to-ship transfers, for instance from offshore accommodation ships onto service vessels. MOTS 5000 has already been put through a programme of sea trials, said to have been successful, but further trials are due on a vessel equipped with a dynamic positioning (DP) system.

Meantime, a larger version, the MOTS 1000, weighs seven tonnes and is suitable for installation on bigger vessels. Both versions are available for use with vessels fitted with DP systems as well as non-DP craft.

Part 3 of this series of articles will look at Passive Solutions.

About: George Marsh Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.