Strengthening Effect From Steel Pipe Cladding in Pure Bending

Clad steel pipelines consist of a typically 3 mm thick internal corrosion resistant alloy with metallurgical bonding to the outer carbon steel pipe. The method of clad pipe manufacture gives intrinsic rise to disbonded areas (defects) in between the two materials, and there is concern that such disbondment defects may trigger local buckling of the cladding when the pipe is subject to severe bending, e.g. when installed by the reeling method. This paper addresses the possibilities for local buckling of the cladding material with the steel pipe in pure bending. Disbonded areas are studied numerically, and the critical defect size regarding local buckling of the cladding is established and compared with the allowable defect size as defined in the manufacture specification. It is shown that allowable disbondment defects of 500 mm2 are only 1/20 of the necessary area for local buckling during installation by the reeling method. In traditional pipeline design, the possible strengthening effect from the cladding on a steel pipe is not taken into account. In this paper, the strengthening effect from the cladding with the pipe in pure bending is studied. For the pipeline analysed it is shown that the deformation controlled local buckling resistance of the clad steel pipe is at least equivalent to a full thickness carbon steel cross-section. Hence, for a clad steel pipeline installed by the reeling method, the required wall thickness can be calculated by assuming the total wall thickness to be solid carbon steel.