With built up experience of more than 50,000 hours on the seabed, it's for a fact not only seabed material present, but also various objects and obstacles. Surveys may help planning these operations, but often multiple toolings are brought offshore to prepare for everything.
Below we have focused on some key factors/challenges:
- It's important to prepare and consider the amount of debris during project planning as this will affect the dredging capacity
- The Scanmachine can lift “larger” debris by using a gripper mounted on the suction nozzle, while the ROV working in collaboration, can relocate the debris to a safe deposit area for later handling if required
- Clogging of the suction hose can be avoided by utilizing a backflush unit mounted on the ejector, reversing the water flow and blowing out the obstacle clogging the hose
- Strainer on the suction nozzle inlet can be mounted to avoided big obstacles to enter the suction hose
- Heavy cofferdams with unknown conditions have been relocated by the orange peel grab
- The area around the cofferdam was dredged with Scanmachine, and then the cofferdam was lifted in two parts to a dedicated wet storage area
- Boulders can be picked up and dropped using the Scanmachine dipper, or using the Grab for larger boulders
- Concrete mattresses have been attacked in several ways, dependant on their condition
- Sometimes the seabed underneath a concrete mattress is dredged in order to let the mattress sink below seabed level
- Using a big ripper tooth has proven to be a success; the ripper has hooked into the mattress and then pulled it away, or pulled the mattress into smaller pieces for easy removal. This operation requires a strong and stable subsea excavator.
- A dozer-mounted winch used in collaboration with a ROV to winch the mattresses away is also an option
- Dependant on condition, some older grout bags have been removed only by suction, while others have necessitated the breaking of the grout bag into smaller pieces prior to re-location by suction
- The challenge is often to predict the condition, but prepare for everything