Flexible Pipe Riser Annulus Corrosion Protection
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Solution Overview
Problem
Corrosion of armor wires and other steel elements within the annulus of flexible pipe risers in offshore hydrocarbon production facilities due to hydrogen sulfide, carbon dioxide, and water vapor diffusion, leading to corrosion fatigue and potential failure.
Innovation Solution
Circulating a corrosion-inhibiting fluid or buffer fluid through the annulus to protect armor wires and steel elements, which can include surface passivating agents and gas-flushing to remove corrosive gases, using a closed loop system with buffer fluid tubes integrated within the armor wire layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If armor wire layers are used to provide tensile strength, then the flexible pipe can withstand dynamic cyclic loads, but the armor wires are subject to corrosion from hydrogen sulfide, carbon dioxide, and water vapor diffusion, leading to corrosion fatigue
Solution Approach 1:
A buffer fluid is introduced as an intermediary substance between the corrosive environment (hydrogen sulfide, carbon dioxide, water vapor) and the armor wires. The buffer fluid circulates through the annulus and contacts the armor wires, forming a protective barrier that prevents corrosive gases and moisture from directly attacking the steel surfaces, thereby eliminating corrosion without compromising the tensile strength function of the armor wires
Solution Approach 2:
The buffer fluid creates an inert or non-corrosive environment around the armor wires by displacing corrosive gases (hydrogen sulfide, carbon dioxide) and maintaining a chemically stable atmosphere in the annulus. This inert environment prevents oxidation and corrosion reactions while allowing the armor wires to maintain their mechanical strength properties
2Device complexity
If the annulus is left empty or filled with air, then the flexible pipe structure remains simple, but corrosive gases and water vapor accumulate within the annulus, leading to hydrogen induced cracking and sulfide stress cracking
Solution Approach 1:
The buffer fluid acts as a mediating substance that fills the annulus space, providing a protective atmosphere between the inner pressure sheath and outer armor layers. This fluid intermediary prevents corrosive substances from reaching the steel elements while maintaining a relatively simple overall pipe structure without requiring complex additional components
Solution Approach 2:
The circulating buffer fluid system provides self-service corrosion protection by continuously or intermittently flowing through the annulus, automatically displacing corrosive gases and maintaining a protective environment. The system uses the circulation of buffer fluid to continuously renew the protective atmosphere around steel elements, providing ongoing corrosion prevention without external intervention
3Reliability
If a buffer fluid circulation system is implemented, then corrosion of armor wires is prevented, but the device complexity increases due to additional tubes and fluid circulation requirements
Solution Approach 1:
The buffer fluid circulation system serves multiple functions simultaneously: it provides corrosion protection to armor wires, displaces corrosive gases (hydrogen sulfide, carbon dioxide), controls temperature in the annulus, and prevents accumulation of harmful substances. This multi-functionality justifies the added complexity by delivering comprehensive protection and operational benefits beyond just corrosion prevention
Solution Approach 2:
The buffer fluid tubes are nested within the existing flexible pipe structure, specifically positioned within the annulus between the pressure sheath and armor layers. This nested configuration integrates the corrosion protection system into the existing pipe architecture without requiring separate external systems, thereby minimizing the increase in overall device complexity while achieving reliable corrosion protection
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Reduces corrosion incidence and fatigue of armor wires by continuously or intermittently flowing a protective fluid through the annulus, effectively preventing hydrogen-induced cracking and sulfide stress cracking, thereby extending the lifespan of the flexible pipe risers.
Implementation Method 1
circulating a corrosion-inhibiting fluid or buffer fluid through the annulus to protect armor wires and steel elements
Implementation Method 2
which can include surface passivating agents
Data Source
AI summary
Disclosed are methods, systems and apparatus for circulating fluid within the annulus of a flexible pipe used in a riser in an offshore hydrocarbon production facility. Fluid, such corrosion inhibitors, can be introduced into the annulus by pumping fluid from a storage tank located on a platform or vessel into tubes within the annulus. Use of the system to flow the fluid through the annulus can prevent or reduce corrosion of the steel members within the annulus and increase the fatigue life of the riser.