|NDT.net||July 2006, Vol. 11 No.7|
ChainTest - automated inspection of mooring chains
A typical floating structure such as a floating production, storage and off-shore loading vessel (FPSO) may have as many as 14 mooring chains, which might amount to nearly 10km of chain in total. Mooring chains are subject to cyclic loads and, therefore, fatigue. Fatigue may result in crack propagation, which can cause a chain to break well below the ultimate strength of the material. Multiple mooring failures would cause a platform to drift, possibly causing loss of control of well-heads and causing risers to rupture. The major surveys specified by Det Norske Veritas (DNV) in 'Certification of Offshore Mooring Chain', require the chains to be inspected by magnetic particle inspection, a non-destructive testing (NDT) method for detecting surface and near surface cracks, as well as visual inspection to detect corrosion, chain breaks and distortion. All these techniques require the chain to be drawn up onto the deck or taken ashore.
The ChainTest robotic system aims to increase testing repeatability and consistency and hence the probability of detection, by employing improved and automated NDT techniques. It is proposed to design a robotic vehicle capable of climbing down the chain and carrying both NDT and cleaning equipment. At each link, the robot will stop, perform cleaning, carry out NDT and send data back to the operator on the offshore structure for analysis and recording. This approach is expected to increase the reliability of chain inspection. Figure 1 illustrates the concept and Figure 2 illustrates two of the NDT techniques to be used. Ultrasonic Guided Wave testing sends a low frequency ultrasonic wave through the chain link and detects damage such as cracks, corrosion or erosion. ACFM is to be used to specifically to detect weld cracks.