Camera Observation on Abrasive Well Cutting
EV Offshore Ltd supply camera monitoring equipment and control instrumentation for use in an offshore decommissioning operation.
March 2002, North Sea, Northern Sector
Abrasive well cutting systems have been developed for both platform and subsea well abandonment operations. Generally the systems are capable of simultaneously severing casings in a typical well through one rotation. This can be achieved regardless of casing loading, eccentricity and contents.
 |
A
severance system comprises of typically the following sub-systems: Cutting head - the assembly that concentrates the high pressure abrasive jet; Manipulator - the chassis that incorporates the cutting head, hydraulic locking mechanism, rotational drive and control system; Umbilical and handling system - composite umbilical providing hydraulic control lines for providing rotational drive, locking and transport of abrasive medium and also instrumentation cable for control systems and monitoring; Control system and data monitoring system - control system including manipulator controls and monitoring system including depth monitoring, rotational speed, direction and position and visual reference by camera. Abrasive mixing, filling, storage and delivery - system that handles the garnet and water solution that is delivered at high pressure to the cutting nozzle that actually initiates and completes the cut. |
 |
One of the most important parts of the system is the control system and data monitoring. The casing needs to be cut at the correct depth, be maintained at that depth, and the manipulator controlled and monitored in such a way that the operators can be assured that the cut has been achieved as required prior to the casing being retrieved. The hydraulic clamp mechanism ensures that the manipulator is maintained at the correct cutting height, however constant depth monitoring is required to ensure that this position is maintained. |
For visual monitoring, a camera is mounted at 90º to the cutting nozzle; the picture shows the camera cutout to the left of the nozzle. The camera is used as a visual reference to ensure that the casing has been breached and the cut has penetrated the casing. In the event that a cut has not been achieved, the operator can note the rotation position and return to attempt a second cut. The speed, direction and position of the nozzle are achieved by quadrature encoder feedback from the rotation of the motor from a datum set at the start of the cut. A digital readout provides the operator with the rotation in degrees and direction clockwise or counter clockwise.
The camera used was a monochrome camera, with self contained internal LED's to provide sufficient illumination in the pitch-black environment.
A challenge instantly highlighted in the design stage was the effect that the high pressure abrasive mix would have both on visibility due to turbulence in the water, and also the abrasive effect on the camera itself. These two potential problems had to be addressed. Simplicity was the key; materials were sourced that provided intrinsic protection - a stainless steel housing with a sapphire glass viewing port. To minimalise the effect of the turbulence, a simple deflector shield was found to be adequate. First designs attempted to incorporate a servo controlled shield that was retracted when viewing was required - it was found however that dynamic seals were breached by the high pressure back-splatter of the abrasive mix which then led to the camera being flooded.
 |
The picture shows the camera with the sapphire crystal view port in its stainless steel housing. Two 3mm ultra bright LED's were sufficient to provide illumination for the low light sensitive monochrome camera. The simple shield can be seen on the front face of the camera. Actual size is 90mm (l) x 50 (w) x 36 (d). This specific unit depth rated to 1000m |
The visual monitoring gives invaluable information that cannot be achieved by instrumentation alone. By allowing the operator to assess whether a full cut has been completed, ensures that when the casing is ready for lifting, it is lifted clear from the remaining structure. In the event that a lift is attempted with out a complete cut, the casing would have to be re-cut or alternative methods of decommissioning may be required such as explosives. If a second cut were attempted, it is unlikely that the same cut could be re-entered, thus a new cut would have to be made thus repeating the entire process again. This obviously leads to project delays, which in turn cost a great deal of time and hence money. The potential for environmental impact also increases.
Developments of the visual monitoring system include sonar and other visual methods of determining cut completion and depth to allow a 3 dimensional image to be presented that can also determine how deep the actual cut is to ensue that all casings have been cut in multiple casing structures.
For more information on this application and other camera system and instrumentation please contact the following:
EV
Offshore Ltd
Alexander House
7 - 13 Rose Lane
Norwich
NR1 1PL
ENGLAND
Tel:
+44 (0) 1603 630555
Fax: +44 (0) 1603 762533
Email: info@evoffshore.com
Internet: www.evoffshore.com
Contact: David Clover