Structural Integrity Monitoring Using Digital Processing Of Vibration Siqnals

Abstract

INTRODUCTION THE NEED FOR STRUCTURAL MONITORING As the search for offshore gas and oil is pursued into deeper and deeper water the environment in which the necessary structures must exist becomes increasingly hostile. Because of these more arduous conditions there is a higher probability of structural failure and because or the increased capital cost of each structure and- the difficulty of repair, there is a much higher pen8.lty for catastrophic failure. In parallel with the increased inspection requirement there is a major increase in the difficulties of subsea diving. Not only does the cost rise steeply with depth, but the available weather windows diminish, the productivity per diver drops and the danger to human life, though al).precautions are taken, must inevitably rise. Although diving expertise and equipment are continually improving, many are convinced that it will be impossible to inspect platforms in deeper water, such as the northern North Sea, with anything approaching the same thoroughness and detail that is now afforded these platforms in the southern half of that sea. In addition to these factors, the large capital cost of each structure increases the cost level at which undersea repaire are profitable, and, at undersea welding technology improves, it cannot be long before major undersea repairs are under taken. Quite obviously such repairs will be greatly assisted and cheapened if structural damage can be detected at an early stage before the primary failure can include secondary damage. The preceding arguments lead inevitably to the conclusion that increased and more frequent comprehensive inspection is required and, moreover, that either diving inspection should be made easier and quicker or else an alternative method developed. This paper describes just such an alternative method for overall assessment of platform structural integrity with the possibility that, in the future, it may be upgraded to provide more detailed information about the condition of individual members and joints. The new monitoring technique is based on the fact that, whereas a structure has a series of natural frequencies or resonances which are continually excited by the motion of the sea and wind, these natural frequencies are appreciably changed if a load carrying member, such as a brace or pile, breaks or loosens' materially. Using vibration analysis technique it is possible to measure the appropriate natural frequencies of such a structure from relatively few measurements taken at selected positions above the water level so that monitoring may proceed whatever the weather. The amount and distribution of the change in the natural frequencies due to a member failure varies depending on the position of the member within the structure and on the topology and degree of redundancy.