By L G Cuthbert and T J Poskitt, Queen Mary College
Over the past few years a considerable amount of work has been devoted to developing instruments for monitoring the performance of the large piles used for pinned offshore production platforms. The work has followed two distinct paths, the first aimed at the measurement of strains and accelerations during driving and the second at studying the performance of piles in service. Of special interest in the second case is the redistribution of load between the piles and the mud-mat over a period of several years following installation. The systems used to do this can also be employed to look at pile axial and bending forces during severe storm loading conditions.
Although in theory it is possible that systems used for driving studies can also be used for long-term measurements, in practice a number of difficulties prevent this, the most important of these being the signal transmission line. For high accuracy and reliability this requires a cable link. To deploy this link during driving and subsequently when the pile is grouted to the leg is not practical because of the problem of passing the cable through pile guides and access to sensors on the inside wall of the pile when the hammer is positioned. Connectors which would appear to provide a way round this difficulty cannot be guaranteed over a long period of time to give the accuracy necessary. For this reason the approach at QMC has been to treat driveability and in-service investigations separately.
Workers in the offshore industry have mixed feelings towards tests on instrumented piles and the field monitoring of actual structures. These are due to two major factors, the first being cost. This can vary from a few thousand pounds to finance a small-scale laboratory test or field test to E2-3 million for a large offshore structure. The second concerns the performance of systems that have been used in the past: it is common to read of reports where expensive systems have failed to function at all, often for some quite simple reason. Quite naturally this has lead to some scepticism among oil companies and government agencies who sponsor much of this work.
It is the authors’xperience that the performance of a system should be such that at least 80% of the sensors function as designed. Once it is recognised that this goal can be achieved it is to be hoped that offshore operators will be more willing to monitor foundations. There is after all general agreement that a better understanding of foundations is most desirable and that this is allied to the necessity of assessing their in-service performance.