1996. Durability of concrete structures in the North Sea.

The 1970s saw the birth of a new kind of structure: the concrete gravity platform. The technical and economic advantages of this novel concept were rapidly understood by the oil industry. Numerous concrete offshore platforms were erected in the North Sea to allow the production of petroleum and gas from the seabed.

The concrete offshore platforms built and installed in the North Sea during the last 20 years have been a success story. To date, 24 such platforms are in operation, the first of them being 20 years old, i.e. having completed their design life.

These concrete structures exhibit remarkably sound and reliable behaviour, considering the very hostile marine environment they must withstand. Exposure to huge waves, strong currents, violent gusts, and the aggressive action of seawater has never impaired their functional performance.

In more technical terms, concrete offshore structures offer a high level of durability, “durability” being defined as the property of remaining serviceable during the design life, without or with minimal maintenance and repair expenses.

However, the general perception that concrete offshore structures offer a high level of durability has to be quantified. In order to address this issue, the Concrete Sea Structures Commission created an ad hoc Working Group on the durability of concrete offshore structures, with the task of producing a State-of-the-Art Report on the behaviour of North Sea concrete platforms. This Group is listed on page 2 of this report.

Their work was not easy, since the process of quantifying durability is complex and must primarily rest on a close examination of existing platforms, allowing an assessment of their soundness. One of the difficulties arises from the fact that people do not pay attention to healthy situations, just as they disregard trains or airplanes that arrive on schedule. As a matter of fact, only damage to structures due to insufficient durability can be analysed and assessed. Instead of looking at the positive performance of structures, we are led to describe a small number of untypical cases where damage has occurred. One should be fully aware of this negative approach, which tends to overestimate the relative importance of malfunctions in a few parts of a few platforms.

A study of the durability of concrete structures in the North Sea is therefore essentially a review of cases where a lack of durability occurred and required remedial actions. The reader of this document will notice that the number of such cases is quite small, although extensive investigations were performed.

In order to list these cases, the Working Group launched an inquiry with the Norwegian Petroleum Directorate, the oil companies, the certifying authorities, the contractors, and the consultants involved in the design, construction, and maintenance of these North Sea concrete structures. The document that resulted from these consultations may therefore be considered as an exhaustive study of the behaviour of concrete platforms over the last 20 years.