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materials selection option must also take into account the C02 and H2S

levels in the produced water.

While different oil companies may have different guidelines for material

selections, the following is the general basis of these guidelines, with some

variations in the corrosion rate limits and corrosion allowances:
* 1. For non-corrosive fluids, carbon steel with 1.5 mm corrosion

allowance is used. The corrosion allowance is specified to account for

offset conditions.

* 2. For fluids resulting in a corrosion rate of less than 0.13 mm/yr,

carbon steel with 3-6 mm corrosion allowance is selected. This is the

case if the corrosion is controlled using corrosion inhibitors.

* 3. In cases when the corrosion rate is higher than 0.13 mm/yr,

corrosion resistance alloy (CRA) is specified. While the general

corrosion for CRAs is generally negligible, compatibility with the

environment regarding stress corrosion cracking and localised forms of

corrosion requires special attention.

Materials Applications

In the past, plain carbon steel was the material of choice for seawater,

firewater, process piping and equipment. Corrosion control was mainly

addressed by painting, galvanising, or concrete lining. Because of excessive

corrosion and often-needed replacements, stainless steels AIS1 303, 304, 316

and 321 were used in piping of chemicals and hydraulic oil, and even in

seawater systems. External corrosion attack soon appeared, and only 316

proved to be resistant to the offshore atmosphere. The Cu-Ni alloys were also

used for their resistance to seawater corrosion. However, the Cu alloys are

found to be sensitive to seawater velocities because they can suffer erosion-

corrosion at high velocities (V > 3m/s, 10 ft/s) and some pitting corrosion

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