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corrosion allowance is required, 316 stainless steel is used and also 22 Cr

duplex stainless steel is often used when higher strength is required. The 25

Cr duplex stainless steel or the superaustenitic materials are sometimes

specified if an even higher strength is required.

* Often, the piping in made of 25 Cr or superaustenite and valves are

manufactured using 22 Cr because of its better availability. The material

selected for equipment such as vessels and pumps must be compatible with

the attached piping. For low-pressure vessels, internally coated carbon steel

and anodes are used. For high pressures, carbon steel with a corrosion-

resistant alloy cladding is the preferred option. Some vessel manufacturers

prefer applying internal Inconel alloy 625 weld overlay of 0.125-0.2” (3-5

mm) thickness.

Material for HPHT Applications

High pressure, high temperature (HPHT) offshore fields are

characterised by pressures in the order of 15,000 psi and temperature in the

order of 180°C (350°F). In addition, the produced fluids contain C0 2 of about

4% and H 2S of about 50 ppm. These conditions present several challenges that

include the need to use higher strength materials, larger thickness and heavier

components. The use of high strength and thick materials makes it necessary
to address issues such as stress corrosion cracking, welding, brittle fracture,

heat treatment requirements, handling, etc. The high temperature adds an
additional challenge because the material’s allowable strength is reduced with

temperature. At 180°C (350°F), the strength is reduced by 5% for steel and up to
20% for cold worked alloys such as duplex stainless steel. At high temperature,

design issues associated with buckling due to thermal expansions also become
important. The combination of corrosive fluids and high temperature increases the

corrosion rate of carbon steel and reduces the effectiveness of inhibitors.

Therefore, expensive corrosion-resistant alloys are used.

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