Marginal Column

The great ascent

November 2011

 

An innovative, continuous jacking system developed by Rexroth makes installing and maintaining offshore wind farms even safer, more reliable and more efficient.

 
 
Photo | MPI Resolution

Working on stilts: The MPI Resolution at a wind turbine in the Thanet Wind Farm.
Photo: MPI Resolution

Weighing around 25,000 tons, the jack-up rig slowly rises out of the sea – as if by magic – and comes to a stop only when high above the waves. These so-called installation platforms or vessels serve the booming offshore wind farm market and are absolutely indispensable since they stand firmly on their legs on the seabed, even in rough seas. Work can be carried out safely and quickly and the operations window is extended significantly.

High degree of reliability

The practical design requires dependable jacking systems and Rexroth has supplied them since the early 1960s. Its many years of experience led to the development of hydraulic jacking systems specially designed for leading companies in the offshore industry and generating jacking forces of 25,000 tons and upward. Following evolutionary development of the original concepts to create semi-automatic, double-yoke systems, Rexroth set a new standard at the end of 2009: the continuous jacking system. Great demands are placed on these powerful jacking systems. Customers expect service lives of twenty-five years and more, in spite of wind, weather, seawater and enormous loads. To enhance reliability and operational safety, the engineers had to eliminate fatigue risks.

To do so they developed special designs for the cylinders and piping. A special ceramic finish particularly suitable for the harsh environment protects the piston rods. The jacking cylinders are equipped with the Ceramax Integrated Measuring System (CIMS). It ensures precision, accurate movement and exact leveling of the installation platform during operation. This magnetic measuring system transmits high-resolution data to the control system, indicating the cylinder’s position.

A question of precision

The software specially developed for the jacking system provides for the smoothest possible transitions between the various operating modes, pressures and loads – thus helping to reduce wear. It guarantees exact control of all the acceleration and deceleration phases, parallel and opposing cylinder strokes, and smooth transfer of massive loads. In addition, it enables automatic transition between the four operating modes, i.e. leg lowering, platform lifting, platform lowering and leg lifting.

Especially the transition between setting down the legs on the seafloor and lifting the platform requires superb accuracy and speed. In this transitional phase the platform follows the rise and fall of the ocean. As a result, enormous forces act on the entire system. The continuous jacking system hoists the platform out of this critical situation faster than a lifting system that operates intermittently.

Working together to achieve savings

Responsibility lies within the control software and the drive system. They not only provide for higher speed during the lowering and lifting operations, but also economize on power consumption. The use of HS4P hydraulic controls supports this feature. They implement flow control for the jacking system, pressure regulation for the crane or winch, and power control to minimize the two pumps’ energy consumption. The owner also profits from integrating the various hydraulic functions.

That boosts redundancy and reliability – and lowers the investment costs for the ship, since it combines higher performance with lower consumption, reduced space needs and lower weight by means of secondary-controlled drives for the crane. All in all, this new hydraulic continuous jacking system supports the current tremendous rise in demand for installation platforms. Only a very small fleet of platforms is available to handle the present boom in offshore wind farm construction. The EU Commission anticipates that investments of 200 billion euros across Europe will be devoted to the expansion of the sector by 2030. So there’s still a lot of room to grow for jack-up rigs.

 

How the hydraulic jacking system works

Illustration | Bosch Rexroth AG / design hoch drei GmbH & Co. KG Magnifier

The functional principle is simple: The jacking system climbs up or down the platform’s legs, much like a monkey. Two yokes surrounding each leg are fitted with sets of holding pins.

The first such yoke snaps its pins in place in holes along the leg. The cylinders of this first yoke then lift the platform.

 

By the time the first yoke’s cylinders have nearly reached the end of their strokes, the second yoke has moved into position to insert its set of pins in the mating holes. Those pins at the second yoke now take over the load. The pins at the first yoke retract and the cylinders raise the first yoke so that its pins can again assume the load in the next position.

This new continuous jacking system eliminates any interruptions during the lifting or lowering operation. It thus attains a constant jacking speed of one meter a minute or more.