Experience with Flexible Pipe in Sour Service Environment : A Case Study (The Arabian Gulf)

Abstract

Abstract The suitability of a flexible pipe was evaluated on a trial basis for a lift gas line in a sour oil field in the State of Qatar, in the Arabian Gulf. Flexible pipes have been successfully used in the oil and gas industries for transportation of methanol, benzene and gas condensates in wet sweet environment at temperatures of Uplo 80°C. However, there is little or no information available as to its corrosion resistance in sour service wells containing 6% co2with 3% mole H2S and at moderate temperatures. The present experience with a flexible pipe in the gas field of Qatar has shown that under sour service conditions, the Iayered, composite material can suffer severe degradation leading to failure. A detailed inspection and failure analysis of the flexible pipe forms the basis of this paper, The failure demonstrates the significant effects of stress Ievel, environmental aggressiveness, and Iocalised hard zones in promoting Sulphide Stress Cracking (SSC). Permeability of the sour gas through the composite layer of the flexible pipe resulted in varying degree of sulphide attack and hydrogen embrittlement depending on the susceptibility of the multi Iayeredmaterial. Introduction The application of flexible pipe for pipelines and risers is increasing due 10its low installation costs. Flexible pipes have been used123 in oil and water service with considerable rneasure of success. These pipes have been found suitable for transportation of water and methanol intermittently, and gas condensate containing abut 40% 0bcflane in the Wet sweet gas at temperatures of up to 80°C. However, there are limited operating experience available in the oil and gas industry with flexible pipes in both wet gas and sour gas services. The structure of the flexible pipe is illustrated schematically in Figure (l), and the various layers present are :Polyethylene (HOPE) external sheathFabric tapeCrosswound first armourFlat spiral second armoutZeta spiral reinforcementIntermediate pressure Rilsan sheathIntcdoeked stainless steel type 316L carcass The pressure and tensile armour wires are encapsulated between an outer (polyethane sheath) and an inner (Rilsan sheath) polymer barrier. The armour wires should remain unaffected by both sea water and the gases (CH H2Sand C02) as long as the polymer barriers are intact. A typical performance characteristic of the flexible pipe is the permeability to gases. All polymers are to some extent permcable4 to gases, and the amount of permeation depends on the type of gas and the type of polymer. In flexible pipe, permeability may allow gases to pass through the primary fluid or gas-containing layer and build-up in the interplay region. The diffusion of corrosive gases into the annulus means that the armour wires are exposed to a corrosive environment.