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.
The possibility of fretting damage in pressure armor wires of flexible pipes has been investigated. A novel experimental facility which is capable of simulating nub/valley contact conditions with dynamic slip, representative of actual pipe loading, has been developed. The test setup is equipped with a state of the art data acquisition system and a controller with transducers to measure and control the normal load, slip amplitude and friction force at the contact in addition to the hoop stress in the wire. Tests were performed with selected loading and the fretted regions were examined using an optical microscope. Results show that the magnitude of contact loading and the slip amplitude have a distinct effect on surface damage. Surface cracks originated from fretting scar were observed at very high contact loads in mixed slip sliding while surface damage predominantly due to wear was observed under gross slip. The position of surface cracks and the wear profile have been related to the contact pressure distribution. The evolution of friction force and surface damage under different slip and normal pressure conditions are presented. The effect of a general grease lubricant on friction behavior is also discussed.
Fasteners manufactured with Inconel 718 alloy are being widely used in offshore and subsea applications due to the material’s high strength, when compared to other nickel alloys, and its inherent corrosion resistance. However, concerns have been raised over its utilization in applications where cathodic protection or impressed current systems are in place. These concerns relate to the susceptibility to hydrogen embrittlement that Inconel 718 alloy may present depending on its processing, microstructure, hardness and actuating stresses. Over the last few years, much has been discussed on the suitability of the alloy for subsea applications. The development of special thermal cycles for the ageing of the alloy has been necessary to provide a consistent material with a maximum hardness of 35 HRc, and a microstructure free of detrimental phases without jeopardizing the overall mechanical properties of the alloy. Wellstream has developed a test programme focused on the assessment of Inconel 718 behavior when subjected to cathodic protection systems. Through this programme, it was possible to demonstrate the suitability of Inconel 718 alloy in subsea applications when the resulting microstructure and hardness are properly controlled, and bolt loading is within normal working limits.
A pressure armour layer is an essential feature of un-bonded flexible pipes. The layer is made of an inter-locked helically wound metal wire of profiled section, whose primary use is to provide the circumferential strength of the pipe to resist internal pressure. The general design philosophy of the layer is defined in API 17J in terms of the stress “utilisation” factor that specifies the maximum allowable average hoop stress in the layer, which is conventionally produced by the elastic stress analysis. During pressure armour layer manufacturing (a cold forming process), the armour wire is however subjected to a sequence of cyclic bending and twisting deformations which take it beyond its material elastic limit. This paper presents FE structure models for investigating the detailed local and residual stress variation during the forming process, and the subsequent stress relaxation as a result of the factory acceptance test (FAT). A study case is presented for illustrating the typical stress and strain behaviour after FAT pressurization. The paper also introduces X-ray diffraction technology as a method for residual stress measurement on full scale samples.
The new offshore areas being explored in Brazil presents higher concentration of CO2 compared with most existing offshore fields. The presence of these more aggressive environments has led to the development of new technologies. Due to the construction characteristics of flexible pipes, any increase in CO2 concentration in the conveyed fluid will, in turn, increase the CO2 concentration in the pipe annulus, subjecting the metallic armor layers to a more aggressive environment. Evaluation of the CO2 effects of corrosion fatigue behavior in tensile armor wires is therefore of vital importance. A comprehensive corrosion fatigue experiment for tensile armor wires in environments up to 10 bar of CO2, has been established and the experimental results have shown a fatigue life reduction in the tensile amour wires due to higher levels of CO2.
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