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 five 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 2,500m.
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 deploy and place loads on the ocean floor whilst protecting vital umbilical cables and the equipment itself from damage during harsh seas conditions.
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 the motion, but also works as a form of energy recovery, reducing the energy usage.
One way of reducing energy consumption is to install a secondary-controlled drive which senses the torque at the winch either positive or negative associated 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 that compensate for 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)
Applications such as knuckle boom cranes and ROV deployment systems are ideal for accumulators as they are subjected to large flows at high speeds, generating significant amounts of energy that can be stored in the accumulator. When required, the accumulator releases the stored energy back in to the hydraulic system resulting in substantial energy savings and an increased system lifespan.
An accumulator is most effective when integrated with a secondary-controlled drive. Normal active heave systems are controlled via the pump or motor and can only react to energy, not recover it. A secondary-controlled drive performs as both a motor and pump, directly on the cranes winch, which results in a compact, accurate and dynamic response system, able to compensate up to 95 per cent of the vessels movement.
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. Through this process, the drive system recovers up to 75 per cent of the energy, significantly reducing necessary installed power.
Driving out cost
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.
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 all weather conditions. Dynamic, powerful drive systems play a decisive role in achieving a good compensation factor. With up to 95 per cent movement compensation and up to 75 per cent energy savings available, Bosch Rexroth secondary-controlled winch drives integrated is a sure and effective way of reducing energy consumption and costs.
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