Deep control for submersible trencher
The powerful UT-1 being lowered into position from the Volantis.
A sophisticated motor control system is central to the operation of the UT-1 submersible ultra-trencher, the world’s most powerful jetting ROV.
The ROV is out in all weathers from its base aboard the CTC Marine’s subsea construction vessel Volantis, and it needs both power and flexibility to deal with severe sea conditions. Of course, it is a balancing act as being an ROV, the component footprint has to be extremely compact, despite the relative size of the UT-1 to other ROVs.
The UT-1 thus hydraulic jets to cut trenches in the seabed to allow the rapid and cost-effective installation of, for example, flexible flow lines and cables.
The jetting pump drive system, developed by Vacon and shipbuilding company SMD, comprises four drive panels, each equipped with a 750A LCL filter and a 750A active front-end (AFE) module. The Vacon AFE units are regenerative power converters that have been specifically developed for common DC bus applications. These modules can also boost the DC link voltage by up to 35% above nominal.
The common DC bus configuration is used with modular inverters to control the pumps that power the trenching jets. The bus supplies four 460A INU inverter modules, which deliver their output via sine filters to transformers which step up the voltage to the 3.3kV needed by the two 375kW medium voltage pump motors.
Monitoring and supervisory control of the AFE modules and the INU inverter units are also provided via this common fieldbus system, making the drives easy to integrate with the other systems on board the UT-1. The INU inverter units are also bi-directional, and are available with integral DC supply systems in ratings up to 75kW.
SMD were particularly interested in the intrinsically low level of harmonics associated with AFE drives, especially since the UT-1 derives its supplies from onboard the host ship and harmonic interference on a line linked to generator is, to say the least, undesirable.
Despite their high current ratings, the power modules used in the jetting pump drive system have a very compact construction, an important benefit given the restricted space on the ROV. Further, all are air-cooled, eliminating the additional complexity that would have been involved in implementing a water-cooled solution.
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