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Drive for improved deck machinery

Published:  25 June, 2015

Next-generation electric winches are being rapidly adopted on marine vessels, replacing the older hydraulic and early electric units. ABB and its marine & offshore Authorised Value Provider, MJR Power & Automation, show how the latest generation of variable-speed drives is bringing major advantages to the operation of shipboard winches. ODEE reports.

The growing size of cruise ships and container vessels, together with oil & gas exploration and production in ever more challenging environments, is driving a demand for improved deck machinery and handling system performance. Subsea operations lead to the use of very long wire lengths, requiring the use of high power winches and handling equipment with aggressive load cycles, as a cable can go from low load to having to deal with extremely high loads and torque in just a few seconds.

Furthermore, the increasing specification of more sophisticated controls such as constant tension and active heave compensation, in many subsea construction projects, demands a level of performance not previously required. As such, the search is on for a power source for winches that is efficient, compact, reliable and highly controllable. Until recently, the favoured power source was hydraulic. Although hydraulic controls, valves and motors have improved over the years, they exhibit some fundamental drawbacks that are causing winch designers to look at alternatives.

Among these drawbacks is the fact that hydraulic components suffer continuous wear, requiring a high degree of maintenance of filters, valves, pipework and seals to prevent failure. They are extremely noisy, take up a lot of the vessel’s volume and present a significant environmental pollution risk.

Commissioning is time consuming with tanks, pipework and hoses requiring very careful handling and flushing to remove debris introduced during the installation or mobilisation phase. In fact, achieving and maintaining the required level of oil cleanliness to ensure reliable operation is critical and often difficult. Many costly system failures have occurred during commissioning and operation due to contamination, resulting in expensive vessel downtime and severe equipment damage.

Low pressure hydraulic systems in particular have large piping systems, together with associated motors, pumps, filters, valves and coolers. Efficiency is poor as losses occur during energy conversion from electric to hydraulic and then to mechanical energy. High pressure hydraulic systems that use standard hydraulic components, offer a more compact and slightly more efficient alternative to low pressure systems, however they do not address the other more fundamental disadvantages.

Moving towards electric operation

Unlike their hydraulic equivalents, electric drive systems convert electrical energy directly into mechanical energy, offering a higher efficiency that results in 20-30% less installed power required. Electric drives are inherently quiet in operation and offer zero risk of pollution.

With the push towards ever more stringent environmental controls and efficiency standards and the trend towards diesel-electric and hybrid power systems on board the latest generation of commercial and offshore vessels, the case for direct electric drive has never been stronger.

In the early conversion to variable-speed electric drives (VSDs) for winch systems, external PLCs, encoders and load cells were necessary for all applications, adding complicated and sometimes unreliable systems. The relatively crude power conversion stages often had a negative impact on vessel power quality due to harmonic distortion of the network.

However major advances in power electronics and processor power, has resulted in the latest low harmonic drives with advanced control algorithms. These major developments have eliminated the first generation issues and enable a very robust, reliable and high performance electric drive system to be realised, even in the harshest environments.

Recent developments in ruggedised ‘environmentally hardened’ drives for portable equipment now enable ‘on skid’ mounting of drives in exposed weather deck locations and even in hazardous areas. Ideal for portable deck equipment that previously required the use of an ISO 20’ climate controlled containerised electrical power unit (EPU), these drives are set to revolutionise electric drive systems on portable offshore deck equipment.

VSD systems score highly over hydraulic systems when it comes to management of braking or regenerated energy when controlling loads. Lowering a load whilst controlling its speed means that the winch needs to absorb and dissipate mechanical energy. Hydraulic winches use the motor as a brake, ultimately converting the mechanical power into heat and dissipating this heat wastefully through large oil coolers.

VSDs can offer this facility, with the motor acting as a generator and the reverse power taken out through the brake chopper and external brake resistor, again dissipating as heat. However, this wastes an enormous amount of energy and in some cases, such as active heave compensation, it is beneficial to locally store and re-use this energy during the motoring phase. This dramatically reduces the power required from the vessel and offers serious efficiency and performance benefits.

With hydraulic systems, storage has traditionally been done via pressurisation of oil stored in nitrogen charged accumulators. These are very large, heavy and require frequent maintenance. Electric systems can control regeneration of high quality power back into the supply if required and/or locally store this energy very conveniently and flexibly in capacitors or batteries that require no maintenance to release regenerated power back into the system when required.

The cost of installation for electric and hydraulic systems and life cycle costs need to be considered. The installation for piping, fittings and hydraulic components compared to the running of cables and accessories for electric drive systems will yield significant installation savings. For hydraulic systems the total cost of ownership requires a high level of maintenance. However with an all-electric drive system, where the VSD has no moving parts, only periodic checks are needed to maintain maximum life of components. With the installation of a modern electric system, all parameters can be accessed locally or remotely to enable efficient diagnostics and trending of data for the full drive chain.

Achieving the ideal VSD application for marine handling applications was the aim of ABB. The company worked closely with winch OEMs to develop a VSD based winch control system that better meets their needs. Using DTC, ABB tailored the software and hardware of its drives to meet the needs of commercial and offshore vessels, producing a specific winch control firmware with tailor-made parameters for applications such as anchoring, auto-mooring and hand-mooring among others. These features are equally beneficial in offshore equipment handling applications such as constant tension ROV LARS, umbilical and survey winches.

The result says ABB, is precise VSD motor control and auto mooring/tension operations, without costly and troublesome encoders, load cells or external controllers at full torque, over the whole speed range, enabling the winch to be stopped and started smoothly.

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