Abstract There are many different levels at which the performance and response of a steel structure can be studied. This paper reviews various schemes and outlines the results of recent work in the North Sea. The scope of the work includes a theoretical dynamic analysis of the structure, the design, installation and performance of a vibration monitoring system and the computer analysis of measured vibration spectra for the purpose of evaluating the technique as a means of monitoring the structural integrity of the installation. The system described is an offshore platform inspection tool capable of utilizing the divers time more effectively by directing them to areas of structural concern. Vibration spectral energy distributions for a N. Sea platform are presented as well as a more convenient form of presentation designed to aid the handling and interpretation of the data. The Need for Monitoring For some time, it has been clear that it is very desirable to develop systems of remote monitoring to enhance diver inspection techniques: when the statistics are studied, it is evident that such schemes are really essential. There are 52 fixed steel jacket structures in the British sector alone in the North Sea, the great majority of which are in water depths greater than 50m. Within the certification requirements there is a demand for re-certification every fifth year and a programme of annual surveys. In practice this is often replaced by a fairly steady and systematic series of annual surveys by divers. There are three main causes of failure, corrosion, fatigue and accidents, and it is very difficult to select areas that are most likely to have suffered. Finding that 10% of the structural members are in good condition is only superficially reassuring. The real point is to see if instrumentation can concentrate diver and other inspection activity on areas of greatest vulnerability. In the British sector of the North Sea, there are about 600 nodes on steel jackets that are below the 50 metre depth. This depth is generally recognized as the limit of air diving. With an average length of 23m of weld per node, this suggests about 14km of weld that requires inspection. The estimated cost of saturation diving, manned and unmanned vehicles used in the northern North Sea is about £50 million in 1977 and about 40% of this is concerned with inspection of members: the rest is concerned with items such as scour, anodes, cleaning of marine growth, etc. (statistics on maintenance requirements are taken from Ref. 1).