VLT® drives deliver fast and accurate control from the ship to the sea floor

Thursday, 20 August 2015
VLT® drives deliver fast and accurate control from the ship to the sea floor

Laying HVAC subsea cables is simpler, safer and more accurate than ever, with the remote operated vehicle (ROV) trencher from CT Offshore A/S. Danfoss VLT® drives are on board the mother ship, controlling high and low pressure ROV seawater pumps on the sea bed via an umbilical cable, which powers and controls the ROV at underwater depths up to 100 m. In the demanding offshore wind industry where the stakes are high, the investment in high power drives pays off in just days – but the benefits are lasting, delivering safe and sustainable power from the high seas, for decades ahead.

Laying the subsea cables safely
With a six-month timeframe, CT Offshore had the task of developing and building a new generation ROV trencher. Ambitions were high despite the short time frame, and targets included 820 kW (1100 HP) capacity for propulsion and pumping and a new DOBI (Depth Of Burial Indication) tool, PLC control with a range of finesses, ergonomic work environment. 

A new technical challenge was to relocate the pumps from ship deck to sea floor to reduce losses and increase operation behavior of the ROV. With the pumps mounted on the ROV, high-precision variable speed pump control is crucial for success.

The solution: the ROV is fitted with a high and low-pressure system delivering jetting and back wash water through an advanced hydraulic operated manifold. This provides the unique opportunity to always maintain the optimum jetting pressure, combined with a controllable backwash system. 

Senior Manager, Head of Subsea at CT Offshore, Mads Carstens, explains the pump function: “The high pressure pump supplies a water jet to the sword, which undercuts the cable, creating a trench into which the cable gently settles under gravity. At the same time the low-pressure pumpblows the trenched material out backwards, to bury the cable just settled. It is important for us to keep the sea bed material flowing so the cable can be laid down unhindered”. 

Steps to a solution – engineering support all the way
Variable speed control of high and low pressure seawater pumps mounted at the ROV, requires a special solution. Mads Carstens therefore calls on Danfoss along with competing suppliers to find a good solution for controlling the sea floor pumps.