The monitoring of the touchdown point (TDP) during pipelay operations is generally considered compulsary by most oil and gas companies. The aim of this monitoring is to check the actual portion of the pipe on the seabed, in its theoretical corridor, to detect unexpected obstacles during the pipelay, and to verify the integrity of the pipe before it is too late to recover it.
Traditionally, this monitoring is carried out by an ROV (remotely operated vehicle) supported by a dedicated support vessel, sailing at the stern of the laybarge, unless the distance to the touchdown point is so short (flexible lay, J-lay, shallow waters) that the ROV can be supported directly from the vessel.
If a dedicated vessel is required, and this is the most common case, the vessel cost alone may represent up to 90% of the monitoring cost, especially in deepwater where a DP (dynamically positioned) vessel would be required.
Cybernetix, a French based research and development company involved in the design and fabrication of advanced subsea robotics, has launched the development of an auto nomous vehicle which can carry out this TDP monitoring without a dedicated support vessel and regardless of the water depth or distance to TDP.
System architecture
The vehicle is an underwater crawler on which tracks have been especially designed for optimal compromise between mobility and ground pressure (the vehicle design is compatible with ground pressure as low as 3 kPa).
The communication between the vehicle and the pipelay vessel on which the control station is installed, is provided by two acoustic links:
- Bi-directional low-speed link for the commands and the data exchanged between the control station and the vehicle
- High speed link for the video transmission from the bottom monitoring unit.
The power is supplied by a high energy lead acid battery pack installed in the vehicle. Cybernetix's sister company, Comex, has used such battery packs on its autonomous two-seater mini-submarine, Remora 2000.
The monitoring tool consists of a telescopic pole mounted on a pan and tilt tower, with a pan and tilt camera standing at its extremity. This system allows a full set of views around the pipe avoiding the necessity for the vehicle to cross over it.
The control system, split up into a surface and a bottom unit, is designed for a semi-automatic pipe following and monitoring, and provide navigational aids.
Once the video contact is established with the pipe, the tracks and the monitoring tool can be both controlled by the command system embedded. This uses a kind of pipe tracker consisting of a video camera, coupled to a video treatment system which offers many servo modes as two axes scrolling or automatic pipe following.
An automatic ballast system returns the vehicle back to the surface, whatever the pitch and roll may be (even if the vehicle is flipped over), when the battery power pack is dis charged or in case of emergency.
The free spans are measured with an acoustic profiler with up to 50 meters range.
A dedicated or on-site acoustic base coupled with a differential global positioning system (DGPS) provides the vehicle absolute coordinates, and hence, the pipe absolute position.
Operation
The overall system is integrated in a 20-ft container with two vehicles (back to back).
The low weight of the vehicle (less than 2 tons) allows it to be launched directly from the container with a small winch and a hydraulically deployed A-frame.
Once the vehicle has reached the bottom, the lowering cable is released, and the operator, thanks to the graphical navigation interface, brings the vehicle nearby the TDP. The monitoring camera, at the extremity of the pole, between 10 cm and 1 meter over the pipe, then is free to switch on the automatic pipe-following system.
The vehicle has been designed for an autonomy of more than 48 hours or 15 km at a nominal speed of 0.1 meter/sec. This performance is obtained, thanks to energy consumption optimization and high power density batteries.
When the vehicle batteries are low, it is replaced by the second vehicle, while reaching the surface using its ballast system. This back-to-back arrangement with two vehicles allows a continuous mission, whatever the duration, and provides a spare vehicle in case of breakdown. A first prototype is now under fabrication and should be ready for qualification trials by the end of the year.
