Sub-sea flexible pipelines corrosion mechanism in OIL and Gas well

#Subsea #flexible #pipelines corrosion mechanism in OIL and Gas well
High levels of carbon dioxide in production fluids corrode steel armor wires in flexible pipes connecting the subsea wells to floating production platforms. A better understanding of the corrosion mechanism, or a switch to composite pipes, could improve safety and reliability.
When flexible pipes were first introduced 30 years ago, it was thought that the interior of the pipes remained dry and noncorrosive.
However after a decade of operation operators realized that internal corrosion has led to corroded wires even in externally undamaged flexible pipes.
The standard steel-reinforced flexible pipe starts with a corrugated liner made by interlocking S-shaped pieces of duplex stainless steel or high-strength steel alloyed with nickel or molybdenum (Carcass).
To make this structure gas- and hydrocarbon-tight, a thermoplastic polymer liner, typically polyamide, Poly vinylidine fluoride (PVDF), or high-density polyethylene (HDPE), is extruded over the steel carcass (Inner liner).
Then, layers of high strength carbon steel wire are wound helically or circumferentially to reinforce the pipe to handle stress from pressure inside and outside the pipe, as well as tensile stresses from subsurface currents.
A slippery polymer tape, about a millimeter thick, separates each layer of wound armor wire and allows the metal layers to move freely on top
of each other and thereby eliminate wear.
After a pipe has been wound with enough steel reinforcement layers needed for a particular service, it is covered with insulation layers and a tough thermoplastic polymer, such as HDPE.
The most common damage is to rupture external sheath, often through accidents during installation or a ship collision.
This may allow seawater to flood the interior metallic layers of a pipe, causing severe and rapid corrosion of steel armor wires.
Several investigations proofed that the corrosion resulting from such collision is caused by seawater interacting with carbon dioxide that originally seeped into the pipe from the production fluid, not from oxygen gas introduced following an accident.
Corrosion in undamaged flexible pipes, begins at the polymer liner surrounding the steel carcass of a flexible pipe.
Since this liner is in contact with fluid from the well bore, carbon dioxide and water vapor from the production fluid seeps through to reach the annulus.
The annulus is the space between the inner and outer polymer sheaths.
Inside the annulus, water vapor condenses along the layers of steel armor wires.
Then Carbon dioxide dissolves in the condensed water produces carbonic acid, H2CO3, which reacts further with iron in the steel to generate iron(II) and carbonate ions.
These dissolved ions quickly reacts to form a precipitate of iron carbonate.
Carbon dioxide corrosion can generate pits in the surface of the wire.
With repeated stress, cracks might form from these pits and cause the steel wires to snap.
The detailed mechanisms of carbon dioxide-induced corrosion of steel armor wires yet to be investigated and hence more research is necessary to well understand the complete mechanism with all factors.
References:
Understanding corrosion of flexible pipes at subsea oil and gas wells www.cambridge....
Bhge.com
MDPI.com
Simeros.com
4subsea.com
SUBSEA PIPELINE INTEGRITY AND RISK MANAGEMENT book
NOV.com

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