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Seawater

Offshore Platforms and FPSO’s

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USE OF COPPER ALLOYS IN OIL AND GAS PROCESS FACILITIES

Sea water system

Sea water system pressures are low (typically less than 20 bar). Temperatures are generally ambient seawater conditions, which may vary considerably internationally, but can get up to more than 30°C in the Middle East and other tropical locations. Piping is typically 90-10 Cu-Ni up to a maximum temperature of 100°C.

Velocity limitations in relation to piping diameter and good fabrication and commissioning practices optimise service life.

Valves and pumps would typically be in nickel aluminium bronze (maximum temperature of exposure would generally be limited to 75°C).

Strainers and filters generally have bodies in nickel aluminium bronze or GRP and have a combination of these materials for filter components.

Cooling water system

This takes water from the sea water system (in the case of direct cooling) and pipes it to heat exchangers in the process systems. Typically these heat exchangers are cooling pressurised gas in the gas compression train. Because temperatures can be very high in these systems, the choice of material can be limited.

The heat exchangers themselves tend not to be made of Cu-Ni alloys because a large amount of oil and gas production contains H₂S ranging from trace levels to substantial concentrations which could lead to high corrosion rates and pitting.

Cooling systems have high demand on reliable water throughput. The seawater piping can utilise 90-10 Cu-Ni as long as due attention is given to maximum velocity limits.

Where an indirect cooling water system is used (containing treated 'deoxygenated' water) the Cu-Ni alloys can be used for the piping to the main seawater/treated water heat exchanger. Again, maximum velocity limits need to be taken into consideration during design and operation.

Fire water system

Fire mains are often constructed in 90-10 Cu-Ni. The sprinkler and deluge pipe work are not designed for continuous use and so 90-10 Cu-Ni is successfully used for all of these sections. All of the necessary fittings for fire systems are also readily available in compatible copper alloys. Although Cu-Ni's have maximum velocity considerations for continuous flow, higher velocities experienced in fire mains can be accommodated for the relatively short periods they are in operation.

EEMUA Specifications covering 90-10 Cu-Ni Piping for Offshore Applications are :-

Tubes Seamless and Welded Publication 144
Flanges Composite and Solid Publication 145
Fittings Publication 146

(EEMUA is the Engineering Equipment and Materials Users Association, UK)