Unbonded Flexible pipes have inner armour of stainless steel made from a helix structure of an S-shaped continuous strip. Typically the selected stainless steel grade has been AISI 316L (UNS S31603). The recent development of lean duplex materials has provided attractive alternatives due to the increased strength, good corrosion resistance and lower cost. The carcass in an unbonded flexible pipe is exposed to the conveyed fluid or gas but protected from marine seawater surroundings. This paper describes acceptance criteria for autoclave testing in CO2 / H2S environments. Observations are made on initiation of pitting and selective corrosion on LDX 2101 (UNS S32101). Results from both short and long term tests are evaluated in order to validate an acceptable duration for autoclave testing.
Unbonded flexible pipes are typically used offshore for risers connecting floating production units with wellheads on the seabed and for flowlines resting on the seabed. The flexible pipes transport oil, gas, water and other fluids used in oil and gas production. Flexible pipes are often used instead of rigid pipes due to the ease of installation and superior fatigue properties. The innermost layer, the carcass, is made from a stainless steel strip typical 316L (UNS S31603). Stronger alternatives such as 22% chrome duplex (UNS S32205) and 25% chrome duplex (UNS S32750) has been used for deep water applications in addition to fields with highly corrosive production fluids. The resent developments in lean duplex stainless steel grades have provided an interesting alternative material with better mechanical properties, similar corrosion resistance at a very competitive price compared to austenitic 316L grade. Structure of a flexible pipe The most important layers in an unbonded flexible pipe are the Carcass, the inner lining, the pressure armouring, tensile armouring and outer sheath. A typical flexible pipe is illustrated in Figure 1. Additional layers providing the flexible pipe with insulation or internal anti friction tapes may also be integrated, however that is regarded outside the scope of this paper. The design and qualification of flexible pipes are given in API 17J1. A short introduction of the primary layers is given below: The carcass is folded from a stainless steel strip. The most important purpose of the carcass is to carry the inner lining in an accidental state where the pipe is empty and the outer sheath is damaged. In such a case the external water pressure acts on the inner lining, meaning that the carcass is the only structural load carrying layer. Typically, the material for the carcass has been 316L. The inner liner is the pressure barrier for the contained media. The inner lining is extruded on the carcass. The material is selected according to the maximum operating temperature and the water content in the contained media. The material is typically polyethylene, polyamide (Nylon) or polyvinylidene fluoride (PVdF). The pressure armouring is composed of cold worked carbon-manganese steel wires with an interlocking profile. This layer supports the inner liner when the pipe is pressurised.
