Comparison of Models to Predict the Annulus Conditions of Flexible Pipe

Abstract

Abstract Corrosion and corrosion fatigue of the steel reinforcement wires will take place if the annulus of a flexible pipe is flooded with seawater. It will also occur under non-flooded conditions if water permeates from the bore and condenses in the annulus. The combination of water or seawater with permeated CO2 and H2S gases results in a corrosive environment that may cause stress corrosion cracking, hydrogen induced cracking, pitting, general corrosion or, in dynamic risers, corrosion fatigue. The type of corrosion, its severity, and the reduction in fatigue life will depend on the actual composition of the annulus environment. Until now, uncertainties about the annulus conditions have caused difficulties in assessing the remaining service life of flexible pipes with flooded annuli or even the service life under normal operating conditions. In this context, a review has been carried out on permeation, corrosion and corrosion fatigue in flexible pipe. This paper will focus on the first step of a service lifeassessment procedure: the modelling of the annulus condition. Introduction The flexible pipes in the offshore industry transport water and/or oil and/or gas. Due to permeation, some of the components in the bore fluid will permeate through the pressure sheath of the flexible pipe and into the annular space between the pressure sheath and the outer sheath. Vent ports in the end fitting of each pipe will vent the permeated gas if the gauge pressure on the vent valves is reached. After some time the conditions in the annulus will reach equilibrium. The time until equilibrium conditions depends amongst others on the temperature profile across the pipe, the pressure differential across the pressure sheath and the gauge pressure on the vent valves. In operation, water from the bore will permeate through the polymer pressure sheath. Depending on the conditions in the annulus, it may be present as condensed water or as water vapour. After dissolution of permeated CO2 and H2S, the condensed water will form a corrosive solution for the steel wires in the annulus. Sometimes the annulus is flooded with seawater, for example if the outer sheath is damaged or if an end-fitting plug is missing or badly installed. It is obvious, thatseawater in combination with the permeating gases will also cause corrosion of steel armour wires. The severity of corrosion depends amongst others on the composition of the aqueous solution and hence on the composition of the annular gas mixture. Predicting the annulus conditions in the design phase of a project is therefore important. A Round Robin annulus prediction exercise has been performed as part of a joint industry project with the three manufacturers of unbonded flexible pipe and a number of users. This paper describes some results of the case studies. It should be emphasized that the results are equally applicable to static and dynamic flexible pipes.