Offshore concrete structures

Abstract

In the offshore industry there are two possible materials for the construction of the hull of any structure: steel and concrete. Steel is being widely used in the shipbuilding industry for merchant ships, warships, etc. Materials such as aluminium, glass reinforced plastic (GRP) or timber are also used in small units with lengths lower than 100m and with less adverse conditions than in the offshore industry. Nevertheless, some ships/barges have been built of concrete in the past, but these have been rather isolated cases and have not changed the industry practice. During the First and Second World War concrete was used due to the scarcity of other materials, so the series of barges by Alfred A. Yee were a rare exception. Other floating structures were also made in concrete, but they are almost anecdotal. However, the behaviour of these concrete structures, especially in terms of maintenance, has been excellent. Therefore, the fact that concrete has not had an adequate reception so far in shipbuilding does not mean that it will not be the material best suited to the offshore industry in the future. The extra displacement and associated fuel costs in concrete ships have been found to be prohibitive in the past. However the loss of mobility in a concrete hull compared to a steel hull can be perfectly offset by the advantages offered by concrete, as the shipping and offshore industry now have very different priorities. One of the main differences in these priorities is the importance of maintenance and resistance to fatigue, precisely the areas where concrete performs the better. Ships can easily be dry-docked for maintenance and repair, while in the offshore platforms these works have to be done in situ, so maintenance and fatigue are crucial to them. Besides these aspects, concrete has other advantages, according to a number of findings in several studies. This supports the fact that in recent years concrete offshore units have been built proving that in certain cases the benefits of steel are inferior to those of concrete. Concrete gravity base platforms type Condeep have been building since the first unit became operational in 1976, together with a few floating platforms, that have geometry similar to those made of steel (barges, semi-submersibles and TLP type platforms). Offshore LNG terminals and offshore industrial plants utilise emerging concepts in concrete that are lasting for years. The life of these barges can be designed for up to 200 years, such as the floating Nkossa barge, so they can be a good alternative to the construction of these facilities on land, thus avoiding landfills on the coast that degrade the already punished coastline of industrialized countries. The challenge is precisely to optimize their capital costs to compete for an offshore installation against a shore facility. The environment will undoubtedly benefit from this great challenge that lies ahead in the XXI century.