Automated off-shore studless chain inspection system

This work deals with the design and in-house operational results of a novel Automated Ultrasonic Inspection System (ASCIS) for studless chain links with diameter ranging from 111 to 185 mm, conceived and developed by INTERLAB Ingeniería Electrónica (IIE) for VICINAY Cadenas, a major industry devoted to manufacture of chains for both the shipbuilding industry and offshore exploration. The system employs an arrangement of transmitting and receiving probes specifically designed to detect possible flaws in the weld area of each link, while providing the additional benefits of an automated inspection system. Introduction: Chains used in mooring systems for offshore applications are subject to harsh mechanical and environmental conditions, which call for tight observance of strict quality standards [2] during manufacturing to guarantee continued performance throughout their operational life. In studless chains, each link is made from a steel bar bent to join its ends through a Flash Butt Welding (FWB) process (electrical non addition welding system), which leads to a concentration of manufacturing flaws in the soldered area. The main target of the project described herein was the development of an automatic system capable of detecting all possible flaws in the soldered area of chain links with diameters ranging from 111 to 185 mm, including those inner flaws which can not be reliably detected by an standard manual inspection. Additionally to this main objective, the designed system had to be capable of in-line link inspection, without hindering production even when ultrasonic inspection of 100% of the production is required . The system to meet all these requirements has been designed around ULTRASEN, a fully modular ultrasound system developed by IEEE for automatic and semiautomatic ultrasound inspection applications. It totally inspects the two sides of the welding zone of each link activating both acoustic and visual warnings when a flaw is detected. Additionally, graphical display, storage and hardcopy of inspection results are available to the operator. In this paper, we present the design and in-house operational results (during two years of continuous operation) of the this inspection system, the first one in a series of eight units that will be installed in VICINAY for automatic ultrasonic inspection of a 100% of their production. Results: According to the standard, manual inspection is performed with a single angle-beam probe, which the operator scans along the link side checking if the amplitude of any echo reflected from the solder area is larger than a pre-set threshold. This mode of operation is only valid for the detection of those imperfections within a plane parallel to the weld surface and located in the vicinity of the link surface (by corner reflection, see figure 1 centre), or those presenting a component normal to the ultrasonic beam (see figure 1 left). In order to enhance flaw detection, inspection is carried out from both sides of the solder plane. However, due to the nature of the link manufacturing process, occasional imperfections in the weld area can be expected to be more or less parallel to the solder plane, but not always in a position to permit corner reflection, as shown in figure 1. Figure 1. Flaw detection by single angle-beam transducer inspection. Inner imperfections normal to the probe beam (left) or edge imperfections parallel to the solder plane (centre) are detected, but inner imperfections parallel to the solder plane (right) are lost In the automatic system, system, the problem discussed above is overcome by a four-transducer layout, where a pair of transducers inspects the upper side of the solder plane, and the other pair covers the lower side. In this configuration, all transducers operate in a conventional pulse-echo mode, waiting for echoes coming from imperfections within the weld area. Additionally, each pair of transducers operates in a transmit-receive mode, where one of them emits an ultrasonic pulse into the link, and the other receives possible echoes. This mode of operation allows for the detection of planar inner imperfections parallel to the solder plane, as shown in figure 2 for the upper pair of transducers (operation of the lower pair symmetrical). Figure 2. Proposed dual transducer inspection During inspection, the transmitter and the receiver transducer movements must be synchronised (when one goes up, the other must go down by the same amount, see figure 2), so that the sum of their distances to the solder plane remains constant, according to: α tan × = + D h h Rx Tx (1) Where hTx and hRx are the respective heights of the transmitter and the receiver transducers over the solder plane, D is the link diameter, and α is the transducer beam angle. The chain links adjacent to the one being inspected set a practical limit that equals the link diameter, D, to the maximum height that any probe can reach during inspection. The maximum theoretical height for either probe can be computed by making the other zero in equation (1), which yields: ° ≤ ⇒ × = 45 tan α α D hMAX (2) Since the standard [2] specifies angle-beam shear waves in the range from 45o to 70o, it is clear that 45o probes are the only option to avoid clearance problems. With these 45o angle-beam probes, the sum of the heights of the transmitter and the receiver transducers over the solder plane must remain equal to D (see eq. 1), which ranges from 111 mm to 185 mm. A specific double folding mechanism (see figure 3) has been designed for the transducer clamp, so that it self-adapts to each link diameter. Scanning the probes vertically along the chain side (thus scanning diameters of the weld plane), and rotating them around the vertical axis to scan the next diameter carry out the inspection. 0 to D vertical scan and ±90o angular scan, as shown in figure 3 achieves full weld plane inspection.