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.
A new resin designed for use as a pressure barrier in high temperature / high pressure flexible pipes is introduced. The resin finally selected is a modified PVdF homopolymer. Material characterisation has been conducted in full compliance with the applicable API standard requirements. This has confirmed an excellent blistering resistance, chemical ageing resistance, low creep and dimensional stability compatible with the final product long term performance requirements. With a plasticizer content limited to 3% in mass, this new product is ensuring an optimum mechanical behaviour, not only at high temperature but also when low temperature conditions are a specific project requirement. Particular attention has also been paid to end fittings of the finished product in order to guarantee their suitability for high temperature thermal cycling conditions. Based on a specifically designed testing facility, short length pipe samples have been submitted to such thermal cycling tests as required by the latest API specifications. Having passed satisfactorily all characterisation and qualification testing required, Technip-Coflexip and Atofina are now in a position to introduce this new material, called ?- Flex£, which will provide the oil and gas industry with an even higher performance flexible product whilst reducing delivery lead times. Introduction Unbonded flexible pipes have been in operations since the early seventies and are available from a number of manufacturers around the world. This technology has proved its worth in a number of offshore developments worldwide, whether used in ultra deep water applications such as in Brazil [1] or used for high temperature / high pressure conditions in the North Sea. Flexible pipes are composed of a succession of extruded thermoplastics and spiraled/armoured steel layers, each layer being designed to resist a specific loading. Figure 1 below displays the basic build-up for a typical dynamic riser structure. Figure 1: Standard flexible riser construction(Available in full paper) Extensive work took place in the industry a few years ago to qualify new end fitting constructions compatible with severe thermal cycling associated with regular shut down conditions. This new termination assembly, certified by Bureau Veritas, has proved since then its total reliability in such service conditions. The methodology used in this process was, in particular, relying on pre-deplasticization of the Coflon£ material prior to end fitting mounting in case of high severity applications. Although technically sound and fully effective, this approach introduced additional delays in the end fitting mounting process which could be as high as several weeks, depending on the PVdF grade used and the anticipated operating conditions. This also made offshore repairs more cumbersome in case remedial work had to be undertaken under the form of inserting an additional end termination. A new thermoplastic product has therefore been developed which aimed at suppressing the need for such predeplasticization, enabling therefore shorter delivery lead times and reducing risks during the manufacturing process.
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