Towards an H2S Free Environment in Flexible Pipe Annulus Thanks to a New Anti-H2S Polymer Layer

Abstract

Due to H2S pressure build-up in annulus, the use of sour service steel grades is required in some cases. For ultra deep water fields where the flexible pipe top tension is critical, high weight of the structural steel layers combined with lower mechanical properties (about 950MPa) of the sour grade materials is a disadvantage. The generic flexible pipe design avoids any direct contact between the bore environment (production fluids, injection fluids, etc) and the steel layers of the flexible pipe. However, diffusion of active components such as water, carbon dioxide, hydrogen sulfide, etc, through the thermoplastic sheaths could lead to corrosion exposure of metallic layers between the thermoplastic sheaths. In order to reduce the weight of the flexible pipe structure to reduce riser top tension, high strength grade steel layers (>1200MPa) can be used with reduced thickness, considering that the steel layers will not be exposed to H2S coming from the bore fluid. This is now achievable even when H2S is present in the bore by use of a new reactive polymer layer, which will ensure a H2S free environment in the flexible pipe annulus. The new “Anti-H2S” layer will chemically react with the H2S, leading to a complete absence of H2S exposure to steel layers during the service life of the pipe. In addition, the “Anti-H2S” layer makes it possible to propose flexible pipes for applications with high H2S content for which no acceptable solution would otherwise be found. This paper will present the theoretical background supporting the new Anti-H2S layer concept, along with the qualification works carried out both at laboratory scale and on full flexible pipe structures. It will also focus on the design/calculation methodology applicable to this new sheath. Introduction of the “Anti-H2S” layer results in that: - Flexible pipes can now be proposed for applications with high H2S contents; - Sweet service high strength steel grades can be used in flexible pipe annulus even with the presence of H2S into the bore of the pipe; - The weight of the flexible pipe is reduced.