Corrosion-Resistant Unbonded Flexible Pipe for Acidic Annulus Service
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Solution Overview
Problem
Unbonded flexible pipes used for subsea transportation of CO2 and H2S-containing fluids face premature degradation due to corrosion and acidification in the annulus, leading to loss of anti-bird cage protection and reduced lifetime, as conventional carbon steel armor materials fail to maintain a stable pH environment when exposed to high CO2 and H2S levels.
Innovation Solution
The use of corrosion-resistant materials for tensile and pressure armor layers, combined with an anti-bird cage layer featuring elongate elements of steel, titanium, and advanced polymers like PVDF and PEEK, wound with specific pitches to enhance durability and resist acidic environments, while maintaining structural integrity and preventing radial buckling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional carbon steel armor materials are used in the annulus, then the pipe structure is simple and cost-effective, but the armor layers corrode when exposed to high CO2 and H2S levels, leading to loss of anti-bird cage protection and reduced lifetime
Solution Approach 1:
The patent changes the material composition parameter of the armor layers from conventional carbon steel to corrosion-resistant materials such as stainless steel, nickel alloys, or composite materials. This parameter change maintains the structural function while eliminating the corrosion issue caused by CO2 and H2S exposure in the annulus, thereby extending the pipe's service lifetime.
Solution Approach 2:
The patent employs composite material structures for the armor layers, combining different materials with complementary properties. For example, using stainless steel or nickel alloys that provide both mechanical strength and corrosion resistance, or creating multi-layer composite armors where outer layers protect against corrosion while inner layers provide structural support. This resolves the contradiction between reliability and corrosion susceptibility.
2Reliability
If corrosion-resistant materials are used for armor layers, then durability in acidic environments is improved, but the pipe complexity and manufacturing cost increase
Solution Approach 1:
The patent applies local quality by using corrosion-resistant materials specifically in the annulus region where CO2 and H2S accumulation creates acidic conditions, while other parts of the pipe can use conventional materials. The anti-bird cage layer and armor layers in direct contact with the annulus environment are made from corrosion-resistant materials, whereas other structural components may use standard materials, thus balancing durability with complexity.
3Duration of action of stationary object
If the anti-bird cage layer is made from conventional materials, then manufacturing is simple, but the layer degrades prematurely due to acidification in the annulus, losing protection against radial buckling
Solution Approach 1:
The patent changes the material composition of the anti-bird cage layer from conventional materials to corrosion-resistant materials that can withstand acidic environments. This parameter change ensures the layer maintains its mechanical properties and protective function against radial buckling throughout the intended service life, even when pH in the annulus drops due to CO2 and H2S accumulation.
4Strength
If armor layers are made from carbon steel, then mechanical strength is adequate and cost is low, but corrosion resistance in high CO2 and H2S environments is insufficient
Solution Approach 1:
The patent uses composite material structures for armor layers that combine mechanical strength with corrosion resistance. Examples include stainless steel alloys that provide both strength and corrosion resistance, or multi-layer composite armors where outer layers resist corrosion while inner layers provide structural support. This resolves the contradiction between maintaining adequate mechanical strength and achieving sufficient corrosion resistance in high CO2 and H2S environments.
Data Source
AI summary
A subsea installation including an unbonded flexile pipe for subsea transportation of a H2S and/or CO2 containing fluid. The unbonded flexible pipe includes from inside and out, a pressure sheath defining a bore for transportation of the fluid, a tensile armor and a liquid impervious outer sheath, wherein the tensile armor is of corrosion resistant material(s) and the tensile armor includes at least two cross wound layers of elongate armor elements, which are wound with a long pith and wherein the pipe further includes an anti-bird cage layer including at least one elongate element wound with a short pitch onto at least one of the tensile armor layers, and wherein the at least one elongate element includes or consist of steel, titanium and/or fibers of carbon, basalt, polyethylene, PVDF (polyvinylidene fluoride or polyvinylidene difluoride) PEEK (polyether ether ketone) PVC (polyvinyl chloride), LCP (liquid crystalline polymer) or any combinations thereof.