Abstract

The CO2-SCC susceptibility of tensile wires of flexible pipes was investigated. Wire specimens were tensioned in four-point bending jigs and exposed to different simulated annulus environments at high CO2 partial pressure. Unloaded specimens were exposed to the same test environments and corrosion rates were calculated from their mass loss. Artificial sea water solutions at different temperatures were used as electrolyte. Some specimens were pre-corroded in aerated artificial sea water before exposure to the CO2 environments. The corroded surface of the specimens after removal of corrosion products was analyzed using scanning electron microscopy. Cracks associated with localized attacks were observed in the tensioned specimens exposed to some of the simulated conditions. Temperature and pre-corrosion in aerated sea water influenced cracking susceptibility significantly.

Introduction

Unbonded flexible pipes used for transporting process fluids in offshore oil and gas production systems have a complex structure, with alternate polymer and metallic layers. Tensile armors are metallic layers constructed by the helical wrapping of high strength carbon steel wires, and they are responsible for the integrity of the pipe. These armors provide axial strength and torsion resistance to the pipe so that it can sustain its own weight and resist to stresses associated to environmental conditions and vessel motion.1,2 The tensile wires are confined in the annulus of the flexible pipes, which is limited by the inner polymer layer (internal pressure sheath) and the external polymer layer (outer sheath). The annulus is originally dry, but during operation it can be flooded with sea water due to damages to the outer sheath or filled with condensed water following the permeation of H2O molecules from the bore.2,3 When carbon dioxide permeates from the bore through inner polymer layers and arrives at the annulus, it dissolves into water and forms carbonic acid, generating an environment which is corrosive to the steel wires.4,5 Corrosion proceeds according to Equation 1 but in the confined annulus environment, because of the low ratio of free volume to steel surface area (V/S), the accumulation of corrosion products is fast, and supersaturation is quickly achieved.6 In the absence of oxygen, iron carbonate (siderite, FeCO3) precipitates according to Equation 2 and forms a film on the steel surface.7-11 Depending on the conditions in which this film is formed, it will give a varying degree of corrosion protection.8,11,12

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