The present paper describes the study and the repair of a steel water pipe concerning the dimensional stability by finite elements methods, selection of materials and the repair procedure. The water steel pipe transports water from a dam to an electrical power. The structure presented some areas with corrosion and other parts with lack of material "pitting". There were some solutions for the repair, but due to atmospheric conditions, local access and the advanced corrosion conditions, it was decided that the right solution would be with composite materials. Composite materials turn out to be the most attractive solution to the repair because it can easily be done, possibility to transport the raw materials near repair local and good mechanical properties of the composite laminates. The finite element method predicted the number of carbon fibre layers necessary to recover the initial mechanical resistance of the water steel pipe. Dry fibres and a thermosetting matrix were selected according to the atmospheric conditions, its mechanical properties and the joining of dissimilar materials. A repair procedure manual was done according to the results obtained.