Experiences in Developing Monitoring Systems and Operating Integrity In the North Sea

Abstract

ABSTRACT Offshore structures vibrate under the action of the sea, wind and on board machinery. The response is most marked at the natural frequencies of these structures which are dependent on the geometry, stiffness and-mass .of the structure and are independent of the excitation. Measurements of the responses of three platforms in the North Sea have been made over a period of nine months. Each platform has produced a unique signature which has remained constant within prescribed limits. Failure of any structural member in the platform will produce a change in the signature and changes of greater than 15% have been measured when minor modifications were made to one of the test platforms. This vibration analysis technique for the monitoring of offshore structures as a primary inspection aid is both technically feasible and practically possible. INTRODUCTION At the present time there are approximately 80 offshore fixed steel structures installed in the coastal waters of the United Kingdom and it has been predicted that by 1980 there will be around 150 steel structures in water depths varying from 30m to 300m. Underwater inspection is a requirement for these platforms to ensure their structural integrity. The Offshore Installations (Construction and Survey) Regulations which came into operation in May 1974 called for a Certificate of Fitness to be obtained for each platform by August 1975. When the certificates are granted they remain valid for a period of 5 years, renewal being conditional on a satisfactory survey of the entire installation. Periodic surveys are required between each of the main surveys. The suggested format for a main survey of steel structures is given in the guidance to the UK Offshore Installation Regulations and contains the following: "Visual examination of the whole structure above and below water to assess its general conditions, to detect obvious damage" If this inspection routine has to be carried out by divers operating in water depths up to 300m and over possibly 150 platforms, then even allowing for a percentage inspection each year, there exists an immense problem if a guarantee as to the structural integrity of each platform must be given (1). It must be borne in mind here, that this inspection forms only a small part of the total underwater commitment required to fulfill these regulations. However, alternative techniques are available to aid this primary inspection process and one such method relies on the changes which occur in the natural frequencies of a structure when load-carrying members are damaged. The principle and application of this technique to offshore structures is not new indeed laboratory and computer studies were carried out two to three years ago and much useful data obtained (2, 3, 4). The technique has also been used to verify the absence of damage in a small US Coastguard Tower (5). However, the technique had yet to be proved operationally offshore.