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Active heave

Published:  15 October, 2014

With oil becoming harder to exploit and prices increasing, operations are moving into deeper waters. Active heave users need to move towards more energy efficient solutions as subsea activity needs more power to deploy the equipment to reach these hidden depths. Steve Smith, marine and offshore sector manager at Bosch Rexroth, says by innovative system design and integrating accumulators into the active heave compensation control process, end users could achieve energy savings of up to seventy four per cent.

With the days of easily accessible oil and gas fields becoming numbered, offshore operations are being forced to operate at increasing depths and harsher environments, putting new challenges on the equipment used. Crane and winch drives are having to position loads of more than 250 tonnes on the ocean floor, stretched to their limits at depths below 2500m.

Vessels at sea are continuously moving with the ocean swell and vertical motion in particular has significant effects on subsea lifting operations. This makes it difficult to build and maintain offshore wind turbines, deploy and place loads on the ocean floor, installing subassemblies on drilling rigs and fitting and removing various machines and devices on the seabed; all the while trying to protect vital umbilical cables and the equipment itself from damage during harsh seas conditions. All of these activities would be exceptionally hazardous to people, machines and the environment were it not possible to compensate for ship movements caused by wave action.

Ideally, subsea lifting operations would be completed at times when sea levels and weather patterns are placid. But, as the demand for energy increases, companies are increasingly being forced to operate at lower depths and in harsher sea conditions, putting a huge strain on energy resources due to the demands for much higher powered equipment to operate during these conditions. For example, a typical 50 tonne knuckle boom crane operating in waves of over three metres, will consume two megawatts of electricity per hour, a very costly operation.

This has led to an increased demand for fast acting, dynamic and responsive active heave compensation systems, which not only compensates for the motion, but also works as a form of energy recovery, reducing the energy usage.

System choice

Energy consumption remains a concern for machinery operators, meaning energy recovery systems, where possible, are of great interest and active heave systems present one of the best opportunities for energy recovery.

There are three active heave compensation systems which are used throughout the offshore and marine industry; primary, rotary and linear. All of which have different characteristics.

Primary active heave compensation systems use a classic closed loop flow coupled system generally best suited to a single winch type application of up to 30 tonnes. Energy recovery is possible electronically via the electric motors back into the ship’s system. The main advantages of this primary system are lower investment costs and the compact construction that takes up minimum deck space. Moreover, the concept is so simple that its benefits extend to installation and maintenance. The system has standard functionalities that make it possible to position the load at the required destination safely via different modes.

Linear active heave compensation systems use hydraulic cylinders and are most often used for large engineered lifts for subsea fabrications. They are commonly used with an existing standard winch to create an active heave system, however, it is not possible to recover energy with this system.

The most widely used system, and which can generate the most energy savings, is the rotary active heave compensation system. This system uses a pressure coupled system in conjunction with secondary controlled drive units, which work as both a motor and generator. These components power the active heave system so that the load hangs idle to the drilling rig or the seabed. So when the vessel moves upwards, the cable is paid out and when it comes down again, the cable is hauled in. This makes it possible to position loads accurately and safely and gives the ship personnel the ability to continue working safely, even in difficult sea conditions.

This is achieved by measuring the movement with motion reference unit acceleration sensors, load cells and position control devices such as encoders. The data is relayed to the main controller that uses the information to accurately control hydraulic motors or hydraulic cylinders and usually using a variable swash plate piston pump or proportional valves.

Driving out cost

The secondary-controlled drive, mentioned above, senses the torque at the winch, either positive or negative, and controls the system in conjunction with a large auxiliary hydraulic system with a bank of accumulators, producing a highly responsive system using the minimum of power. These very successful and efficient secondary control solutions, which compensate up to 95% of vessel movement, have been supplied by Bosch Rexroth to many companies worldwide.

Hydraulic accumulators effectively store energy in a nitrogen filled bladder within a steel shell. When fluid under pressure fills the accumulator it compresses the gas in the bladder and when the pressure in the system falls the oil from the accumulator is available to provide pressure and high flow rates back into the system. They could be considered as a very large hydraulic spring and follows the basic Boyles law (P1.V1=P2.V2).

Active heave compensation applications such as cranes and ROV deployment systems are ideal for energy recovering secondary control because of the significant amounts of energy that can be captured in the accumulators and released back to the system, resulting in substantial energy savings.

As the operation of support vessels occurs in a wave state, the vessel is moving up and down, creating energy during the upward motion of the vessel. This energy is absorbed by the hydraulic accumulators and then fed back into the hydraulic system during the vessel’s next downward motion, essentially working as a hydraulic spring. This allows for an extremely accurate, dynamic system which can recover up to 74% of the energy when in active heave mode.

For example, for a 50 tonne winch to achieve active heave at +/- 3m for a nine second wave period, you would need to use 1,800 kW of power when using a traditional primary active heave compensation system. With the rotary system, fitted with secondary controlled units and accumulators, the installed power could be significantly reduced to around 480kW. This system offers far more flexibility on larger systems, higher dynamic accuracy and increased energy recovery.

Active heave compensation integrated with secondary-controlled winch drives and accumulators have been developed by Bosch Rexroth to facilitate load placement on the ocean floor in most weather conditions. Dynamic, powerful drive systems play a decisive role in achieving a good compensation factor. With up to 95% movement compensation and up to 74% energy savings available, investing in Bosch Rexroth secondary-controlled winch drives is a sure and effective way of reducing energy consumption and costs.

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