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Ultrasonic inspection system for offshore mooring chains

Using the large seven-axis immersion tank acquired for the National Structural Integrity Research Centre (NSIRC), TWI is developing ultrasonic inspection techniques using array probes for in-situ inspection of critical links in mooring chains used to keep offshore vessels such as FPSOs in place. NSIRC MSc students are developing the techniques under the supervision of TWI experts, making use of advanced concepts in inspection. The techniques will be implemented by robotic underwater inspection systems, which will also be developed within the NSIRC facility in subsequent phases. This programme supports the long-term structural integrity of high-value industrial assets.

Project background

Failure of mooring systems for offshore structures, primarily used by the oil and gas industry, represents a critical threat to the assets themselves, human life and the environment. Considerable effort is made to ensure that the integrity of the chains is kept to high standards, and a key part of this is a drive to increase the precision and reliability of inspection methods. TWI has recently been involved in the inspection of a critical chain link on an FPSO, developing a specific inspection tool for implementation by divers. This tool was able to detect critical fatigue cracking which had developed during service. The current programme takes this further by aiming to (1) size and map accurately fatigue cracks within the body of the chain, (2) replace the diver with robotic systems to meet evolving health and safety regulations and (3) address all links on a mooring chain system.

CAD model of the mooring chain link being inspected
CAD model of the mooring chain link being inspected

Work programme

The first step in achieving these goals is developing an ultrasonic technique which can accurately scan the full volume of the chain and be sensitive enough to detect the tip of the fatigue crack. This requires the implementation of a complex 3D scan path for the probe and use of array ultrasonic techniques to focus the sound beams within the chain. The second step is developing an underwater robotic system to deploy this method on chains while in service.

The chain is first modelled in three dimensions using a combination of CAD and precision metrology methods. It is then placed in a virtual model of the tank containing the part, probe and motion axes, as shown in the diagram.

The system then calculates a three-dimensional trajectory for the probe to enable inspection of the full volume of the chain, while the phased array ultrasonic instrumentation focuses the sound to detect signals from the cracks, as shown in the diagram.

For more information, please email contactus@twi.co.uk

Conducting an inspection in the seven-axis immersion tank
Conducting an inspection in the seven-axis immersion tank
Phased array ultrasonic data showing location and severity of defects
Phased array ultrasonic data showing location and severity of defects
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