An affordable and functional solution for Mid Water Station Keeping (MWSK) has been on the wish list of most ROV operators and manufacturers for some time now; the huge benefits in terms of operational efficiency and the future potential for pre-planned automated missions has made it a case of when, not if.
Against this backdrop and at the request of C-Innovation, CDL and Schilling Robotics engineers started work on a collaborative solution over 12 months ago, the results of that effort were demonstrated in March in the Gulf of Mexico following an extensive period of Factory Acceptance Tests.
Traditional ROV station keeping functions rely on a Doppler Velocity Log (DVL) system on the ROV which can only maintain bottom lock within a limited distance of the seafloor (typically 30m)with the added disadvantage of only providing relative position. USBL positioning systems, while providing absolute position are unable to support MWSK functionality due to inaccuracy (especially in deep water) and slow update rates, while inertial systems suffer from drift when unaided.
The solution centers around fusing these readily available positioning technologies (INS, USBL and DVL) and the integration of those technologies with Schilling's next generation closed loop control system.At the heart of the solution is the CDL MiniPOS3 INS (Inertial Navigation System)platform which uses the USBL position (and DVL when available) to produce a highly stable INS aided solution feeding precise position, acceleration and rotation information directly to the control system on the ROV at update rates of up to 50Hz.
During the trials carried out in March 2011 on the Chouest Holiday, a C-Innovation performed an extensive set of test procedures with a UHD ROV system to demonstrate the world's first practical ROV MWSK system utilizing inertial technology aided by a surface based USBL positioning system. MiniPOS3 is hardware independent and can work with any USBL or LBL system and a variety of topside software packages. The first software package to fully support the functionality is WinFrog by Fugro Pelagos which was used during testing.
The performance of the MiniPOS3 and the Schilling control system can be seen in the screenshot opposite and the results are remarkable. The absolute positioning accuracy of the ROV (without DVL) was noted at a standard deviation of less than 40cm during MWSK. With DVL lock this improved dramatically to approximately 3cm.
Commenting on this development, Neil Manning, President at CDL Inc said: "The operational advantage that this technology offers is the focus on reducing time and cost of both vessel and positional equipment. I am pleased with the results and the performance of the MiniPOS3 as a low cost solution to navigation in mid water or at depth. The MiniPOS3 is a large step towards true flyby wire functionality with real world coordinates instead of relative positioning, offering high positional repeatability to return to known locations".
Peter MacInnes, VP Sales and Marketing for Schilling Robotics commented: "Our customers routinely utilize the existing Stationkeep functionality due to the extremely high accuracy and stability that it offers for performing ROV intervention. This automated control mode greatly enhances operational efficiency, and extending this capability to include MWSK will result in similar efficiency improvements for complex ROV tasks conducted throughout the entire water column."
Steven Thrasher, Manager of Projects commented for C-Innovation: "We are extremely excited to bring this technology forth to our customers and operators as a standardized feature to improve efficiency and productivity through automated controls and better tracking capabilities. Schilling Robotics have raised the bar in ROV control systems and CDL have proven that they are willing to respond to its customers technical requirements. Many thanks to all of those involved."