The Underwater Repair Of Concrete Offshore Structures

ABSTRACT Methods for repairing concrete offshore structures underwater have received much less attention than those for steel structures damaged by boat collisions or fatigue. This paper describes a major research programme which has been carried out to develop proven underwater techniques for repairing cracked and spalled areas on concrete offshore structures. Materials have been tested and where necessary new materials developed for priming surfaces, for crack sealing prior to injection, for crack injection, for patching and for bonding steel to concrete. Preliminary screening tests on a large number of proprietary materials showed that few form a bond to high strength concrete underwater. Materials developed during the project are described together with tests which demonstrate the superior performance of these materials. Bond strengths are obtained sufficient to fully restore the strength of damaged high strength concrete. These materials have been used to repair underwater full scale structural elements, typical of sections from the towers of offshore platforms, which have then been tested to failure in the laboratory. Earlier research has suggested that the bond strength may deteriorate with time and so the long term durability of the repair is an important feature of the test programme. The paper describes specimens which are being exposed in splash zone conditions for a number of years. Another aspect of the research programme described in the paper is a study of the availability, effectiveness and limitations of equipment that either prepares the underwater surface for repair or applies the repair material. INTRODUCTION Reinforced concrete has been used in the marine environment since the turn of the century mainly for coastal installations. However, it was not until the exploitation of hydrocarbon reserves began in the North Sea that large concrete structures of long term importance were designed for offshore sites. Experience with coastal and small offshore maritime structures (e.g. Tongue Sands Fort in the Thames Estuary) has shown that, while concrete is subject to gradual deterioration, its inherent durability in the marine environment is impressive. This, together with relatively low bulk cost encouraged the use of the reinforced concrete gravity structure as an alternative to the potentially corrosion and fatigue prone piled steel structure. To date 13 major concrete structures have been installed in the North Sea in water depths generally in the range 100-150 metres. Now that production from these offshore platforms is becoming significant their strategic and economic importance is focusing attention on their long term performance and potential sources of deterioration and damage. In turn this leads to the consideration of methods of repair for the envisaged damage and the need to have proven repair techniques available should they be required. This paper describes a research project on underwater concrete repairs being carried out by Wimpey Laboratories, partly funded by the UK Department of Energy.