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Drives help submersible to excavate efficiently

28 June 2011

A motor control system supplied by AC variable speed drive manufacturer, Vacon is central to the operation of the revolutionary UT-1 submersible ultra-trencher, which was built by underwater engineering specialist SMD and is owned and operated by CTC Marine.

The UT-1 uses hydraulic jets to cut trenches in the sea bed to enable the rapid and cost-effective installation of flexible flow lines and cables. The drive system, which uses active front-end (AFE) technology in a common DC bus configuration with modular inverters, controls the pumps that power the trenching jets.

The engineers at SMD chose these particular AC drive systems because of their versatility and proven reliability in marine applications. In addition, the intrinsically low level of harmonics associated with AFE drives was an important benefit as the UT-1 derives its supplies from a shipboard generator.

The jetting pump drive system developed by Vacon and SMD for the submersible comprises four drive panels, each equipped with a 750 A LCL filter and a 750 A AFE module to provide power for the common DC bus. The bus supplies four 460 A INU inverter modules, which deliver their output via sine filters to transformers which step up the voltage to the 3.3 kV needed by the two 375 kW medium voltage pump motors.

Despite their high current ratings, the power modules used in the jetting pump drive system for the submersible are of a compact construction, an important benefit given the restricted space available to accommodate them. All are also air-cooled, eliminating the additional complexity that would have been involved in implementing a water-cooled solution.

Monitoring and supervisory control of the AFE front-end modules and the INU inverter modules are provided via a common fieldbus system, making the drives easy to integrate with the other systems on board the UT-1.

Permanently installed on the multi-role subsea construction vessel Volantis, the UT-1 ultra-trencher ROV is currently being used in a variety of roles to service the offshore oil and gas, life-of-field seismic, telecommunications, e-field, power and renewables market sectors.
The innovative design of the UT-1 permits deployment and offers unparalleled flexibility in even the most severe weather conditions but, in spite of the huge demands operating in these conditions places on its onboard systems, it is delivering exceptional levels of availability, an achievement to which the Vacon drive system has made no small contribution.

The Vacon AFE units used in this project are regenerative power converters that have been specifically developed for common DC bus applications. In addition to their excellent harmonic performance, these modules can also boost the DC link voltage by up to 35% above nominal. The INU inverter units are also bidirectional, and are available with integral DC supply systems in ratings up to 75 kW. Higher ratings, such as those featured in this in project, are exclusively for use with external DC supply systems.


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