ROV Simulation of LRUT Deployment
One of the problems with any underwater Engineering task is that testing and prototyping equipment in the water is costly and high risk. One way to de-risk this operation is to simulate any underwater task as much as possible. With this in mind, Fugro GRL were tasked with producing a simulation for the deployment of the TWI Long Range Ultrasonic Tool (LRUT) as part of the Sub-C-Test Project.
The LRUT was one of the subsea NDT tools being developed in the Sub-C-Test project under the management of TWI. CAD models of the tool were provided by TWI which allowed a graphical model of the tool to be produced by FGRL. As part of the project, joint and hydraulic actuator components were developed by FGRL to allow a functioning LRUT to be simulated in DeepWorks. This then allowed a simulation to be run where the LRUT is deployed by an ROV onto a large steel jacket structure typical of likely future operations.
ACFM manipulator trials
ACFM is an electromagnetic technique designed to detect and size surface breaking defects. It can work through coatings with a minimal amount of cleaning, making it ideal for subsea inspection. Offshore structures typically employ a series of tubular sections welded together in a complex geometry. Cracks typically appear at the toes of the welds that act as stress risers. It is vitally important to inspect these welds for fatigue cracks, but the complexity of the weld geometry plus the challenge of conducting the inspection remotely subsea makes this an unusually complex task. The inspection of welds is particularly important for structures that are prone to cyclic fatigue loading such as offshore structures, that can suffer unusually high loads during storms.
The ACFM manipulator development had to be separated from the LRUT manipulator development because the ACFM probe requires a scanning motion, while the LRUT transducers remain stationery once clamped around a pipe.
ZUT were charged in building an ACFM manipulator that would:-
- be placed on the proper position by a small Remote Operating Vehicle (ROV),
- have a total weight that than be carried by the ROV not exceeding 25 kg,
- be remotely controlled by the operator from the surface.
Once placed in the vicinity of a pipe junction, a clamp on the manipulator closes automatically and the operator switches on electromagnets that attract two elements of the clamp to themselves. At this stage the operator can control the position of a transducer head using a joystick or switches. What is more, maintainer can see the result of the movement through the camera, which is installed on the manipulator.
The ACFM manipulator trials were conducted in the diver tank at TWI North (Figure 1). TWI North has a series of samples consisting of braces connected to a central chord. Each weld contained fatigue-like defects of varying sizes and depths. The manipulator was deployed successfully by A diver (Figure 2).
LRUT manipulator trials
The LRUT manipulator is in the form of a bracelet, with a mechanism to clamp the rings of LRUT transducers down and around the pipe once they have been positioned by the ROV. The manipulator then needs only two degrees of freedom for orientating the clamp for either horizontal or vertical pipes.
Although NDT systems for ROV deployment are already available, they are for deployment from large ‘work-class’ ROVs, which require expensive logistical support. These ROVs are not operated by project participant Dacon. Instead Dacon employ smaller observation class ROVs. These can be operated from small boats and can access confined spaces, which work-class ROVs cannot. Moreover, the zones operation for observation class ROVs is far wider than for work-class ones, that are restricted to areas such as the North Sea and Gulf of Mexico.
The challenge of the LRUT manipulator design therefore was to decrease the size and weight to a degree that it would operate from Dacon’s observation class ROV. The main components of the LRUT manipulator are shown in Figure 3.
These were mounted onto the Dacon ROV before undergoing laboratory trials.
Underwater trials were conducted in a tank at Dacon (Figure 4).