Repair of a damaged ship hull is time and cost consuming in shipping industry. A damage is something that often sets the ship operation out of business and every part that has an interest, are looking for ways to minimize the stay in a repair yard. Double plates are often used to repair damages on a ship hull, like buckling, corrosion, wastage and cracks of plates. Current standards rate repair with doublers as a temporary approach to fix the structure, which means that within a certain time the ship must once again seek a shipyard to fulfil the repair with a more, both time and cost consuming repair. The permanent method today is by replacing the the damaged section with a new one. Many shipyards though consider a doubler to be sufficient as a permanent repair, but lack of documentation and experimental material data result in temporary repairs. This thesis contributes to broaden the extent of experimental material by a thorough finite element analysis, with use of the computerized program Abaqus, of various kinds of thickness ratios, imperfections and load conditions. All have been done with regard of the current standards and guidelines. The results of the analysis consists of both eigenvalue/buckling loads and ultimate capacity. The effect of the doubler, contributes to make the structure to be almost as strong as an intact plate and all systems consider damage on a stiffened panel. The result complies the thoughts of the shipyards, that a doubler can safely be used as a permanent repair.In addition, it has been tried to develop a semi-analytical tool that could simplify and decrease in great extent the rate of the calculation time. A semi-analytical method creates possibilities to calculate the eigenvalue/buckling loads for a conservative estimation in the design of the structure much faster then in a finite element program. A procedure of this kind already exists for intact single plate with or without stiffeners and are implemented in a programme named PULS. The model created for doubler in this thesis will be able to conduct calculations for rather stiff systems. We have selected some limitations in the way of boundary conditions and chosen displacement field. The semi-analytical model has been implemented into a Matlab script.