The problem of inspection of concrete structures has been studied, particularly in relation to the detection of corrosion of steel embedded within, or attached to, the structures. The paper describes the possible corrosion processes involved and inspection techniques for offshore use. At present only visual surveillance is being considered for the statutory surveys to be carried out in the North Sea, no suitable alternatives being readily available. Development of techniques based on concrete sampling, electrolytic inspection methods, and other non-destructive tests, are discussed in the paper, as .well as possible problems in their use.
The value of installing permanent corrosion detection devices at the construction stage, is also considered since any in-service corrosion, particularly for example, in the prestressed cables, might ultimately affect the safety of the structure. It may be simpler to monitor rather than undertake underwater inspection of even selected critical regions of the large surface areas involved.
In U.K. waters in the North Sea, the British Government regulations (1) require the certificate of fitness of fixed offshore structures to be renewed every five years. This will be done following a major survey of the structure and its installations. In addition, an annual survey will also be carried out, on a smaller scale to the major survey, to ascertain that the general condition of the structure is still satisfactory. Det Norske Veritas(2)and Bureau Veritas(3)have published brief guidelines for these surveys.
These requirements apply to concrete as well as steel platforms. Whilst considerable effort has gone into the development of techniques for the inspection of steel platforms, relatively little attention has so far been paid to concrete gravity 'Platforms. For example, the F.I.P. recommendation (4 give no definite guidelines for inspection procedures.
There are two major problems associated with the inspection of gravity platforms.
The vast surface area (typically 40 acres) of concrete means that only a small proportion of the structure can be surveyed.
Although the total amount of steel in a concrete platform is similar to that in an equivalent steel jacket, it is buried within the high quality concrete as reinforcement or prestressing.
Thus, when carrying out visual surveys with divers or underwater television cameras, the first sign of damage will be when corrosion has reached a stage where spalling or cracking has occurred. There is considerable evidence to suggest that the corrosion problems associated with steel in concrete, particularly when submerged, are not expected to be significant, but it is nevertheless important to develop techniques which will give an earlier warning of any potential damage.
In 1974, the U.K. Government commissioned two 'state-of- the-art reports on the performance and inspection respectively of concrete structures in the marine environment. The first - "Concrete in the Oceans"(8)- recommended nine research projects on materials and structural performance, of total value £325,000; seven of these are now underway, jointly sponsored by government and industry.
