Caisson inspection can be crucial for long term integrity, especially of firewater systems. Graham Marshall, subsea project manager, Sonomatic
The deterioration and failure of caissons, and associated supports, is a core industry concern for several reasons. Key is the impact such deterioration can have on production. Should the firewater pumps be adversely affected or out of action, the platform is required to shutdown.
Caissons can suffer degradation that can lead to catastrophic failure if left unchecked, with potentially severe health, safety, and environmental consequences. Given the number of aging structures in operation, there is a growing focus on caisson integrity.
Repairs are costly and operators have indicated that they require effective inspection programs to identify and address deterioration before the structural integrity of a caisson is compromised. This involves regular inspection to assess the condition of caissons so that maintenance decisions can be made.
Before an inspection, it is difficult to assess the condition of a caisson visually, as one of the main degradation mechanisms in caissons is internal. This makes external visual inspection a relatively ineffective method of determining the extent of deterioration. Internal inspection is often not possible for pump caissons without a shutdown to remove the pumps.
External ultrasonic inspection is an option that has many benefits, though it too is challenged by the position of the caisson, which spans subsea, splash zone and topside boundaries. As a result, the inspection equipment involved must be capable of both subsea and topside inspections. ROV (remotely operated vehicle) deployment subsea is now the industry’s preferred method, as diver deployment has inherent safety risks and is also costly due to the requirement for a dive support vessel.
Sonomatic has recently carried out several caisson inspections using its MAG-Rover, a tethered crawler system capable of topside and subsea deployment.
The standard MAG-Rover unit is a steerable crawler driven by two magnetic drive wheels, which allow the scanner to attach to any magnetic surface. At the front is the ultrasonic transducer, which gimbals to follow the contour of the surface being inspected.
It was originally designed for the inspection of monopiles using time of flight diffraction (TOFD). This involved mounting the tool on the internal surface of the monopole whilst topside, and driving it down the monopole to the subsea environment. A straight line weld inspection probe ensured the collection of consistent data for analysis. Adapting this process to carry out caisson inspections was relatively simple.
To use the MAG-Rover for caisson inspection, the tool was required to attach to the external surface of the caisson. The original design of the tool included capability for TOFD weld and pulse echo inspection.
This capability was unchanged, but additional functionality was added to enable corrosion mapping. This involved the adding a corrosion mapping third axis to fully quantify the thickness profile of the caisson. For caissons, the MAG-Rover is attached by an ROV and controlled remotely topside. It is fully deployed by ROV with no diver intervention. Once mounted on the caisson, it is steerable and can be positioned in the area of interest.
This enables the accurate mapping of wall loss, so that the inspection team can identify early stage degradation.
The MAG-Rover and corrosion mapping software provide of full scan data, which allows for comparative analysis on subsequent inspections. The data collected can also be used to carry out fitness-for-service and remaining life assessments, which consider the effects of deterioration and enable operational decisions.
Corrosion engineering assessments, to validate the type of corrosion present, can also be carried out along with statistical analysis of the inspection data to provide a basis for definition of future inspection requirements. This stage is particularly crucial for caisson inspections, where corrosion is found, to ensure repairs can be performed before the damage impacts integrity, thereby reducing the risk of a major incident.
The program of inspection outlined above demonstrates how new technologies can work alongside existing systems and assessment methods to tackle important industry issues. It also shows how remote ultrasonic technologies can be developed and used for difficult to access and hazardous inspection locations. Not only does the MAG-Rover capture more detailed and accurate data than external visual inspection, it also captures the information in a way that ensures informed decisions can be made for the long-term maintenance of caissons.
Graham Marshall is Sonomatic’s subsea project manager. He joined the organisation in 2009. His role is focused on projects utilising ROV deployed inspection systems using advanced ultrasonic technology. Marshall was previously a towed array sonar specialist in the Royal Navy and since joining Sonomatic has become TOFD level ll qualified.