The improvement in our understanding of the fatigue lives of offshore welded tabular structures has led to a change in our inspection philosophy For the older platforms we are able to predict very accurately where and when trouble can be expected The newer structures have been designed to ensure that all nodal welds have fatigue lives in excess of three times that of the required working life of the platform, so there are no particular areas of concern. Any propagating defects in the new platforms will be due to fabrication faults which evaded detection during construction Because the locations of these faults cannot be predicted, it is not possible to direct inspection programmes at any specific areas of high risk Therefore a rapid method is required for checking primary members in the newer and older platforms for propagating defects ROVs can check for gross damage, but it is desirable to detect defects as early as possible The philosophy behind flooded member detection 1s that because all jacket members are seal welded in air at one atmosphere during construction, water within a member must indicate a through-thickness defect The challenge has been to find an accurate, rapid and cheap method of checking jacket members for flooding and it has proved remarkably difficult.
A number of techniques have been used for flooded member detection All have their advantages and disadvantages, most of which I do not intend to discuss in this paper because I wish to take a positive approach to some new developments.
The technique currently favoured by the industry uses ultrasonics. The principle of operation (Fig 1) is that if a member contains water, ultrasound will be transmitted through the water, reflect at the water/air or water/steel interface on the opposite side, and return to give a signal on an A-scan screen which is unmistakable Unfortunately, badly corroded internal or external surfaces, or internal debris, will scatter the ultrasound (Fig 2), giving a ‘non-flooded’ signal which may be false To detect horizontal members that are less than half full, the probe has to be held at the 6 o'clock position This is where debris and loose corrosion products will collect, so the problem of ultrasound attenuation is particularly severe The quality of the reflected signal will also(Fig.1 and Fig. 2 are available in full paper) depend on how close to the 6 o'clock position the probe is placed, on the angle of the member to the horizontal, and on any ripple on the water surface inside the member
In some recent offshore trials conducted by BP, the ‘hit rate’ was indeed found to be variable Tests on samples on the surface and on the wet bell staging under water gave a high detection rate approaching l00%, and it was discovered that the signal quality was considerably improved by cleaning the steel surface where the probe was applied to bright shining metal (BSM) However, when tried on two known flooded members, cleaned locally to BSM, the detection rate reduced markedly.
