Use of copper-nickel alloy materials for offshore sea water piping
Authors: Lim L.H. 
Published: December 2007

Copper-nickel has been used for many years in the ship industry. Use of this material in the petrochemical industry has been generally limited to small diameter tubular heat exchangers.

Use of large (i.e. 24 in. diameter) bore copper-nickel piping material in corrosive service is a relatively new piping material in corrosive service is a relatively new development in the petroleum industry.

This paper discusses advantages, disadvantages and economics for this type of application, leading to Occidental's decision to use this material on the Claymore A platform. Comparison is made of copper-nickel material use versus conventional anti-corrosion techniques traditionally used in the petroleum industry. Also design and construction problems met are discussed.

WHY USE COPPER-NICKEL ALLOY MATERIAL FOR CLAYMORE PLATFORM SEAWATER PIPING?

A study review was made and conclusion reached to use this material for the following main reasons:Platform safety - other alternative material may render the fire and safety system inoperative in time due to scale blockage of critical valves and equipmentEconomically attractive if based on a long platform production life, say 15 years. Prevention of one platform production life, say 15 years. Prevention of one platform shutdown of several days will pay for higher platform shutdown of several days will pay for higher cost of this material over bare carbon steel piping.Other alternative material may damage injection wells by carrying corrosion products into the wells.

These advantages are felt to outweigh the known problems associated with this material: problems associated with this material:Limited number of suppliers capable of manufacturing large size pipe and fittingsNeeds detail design engineering attentionNeeds special handling and packaging careNeeds specialist type welders

Detail of advantages

Platform safety. About 60 per cent. of the seawater piping system on the Claymore platform will be used for piping system on the Claymore platform will be used for the fire and foam system. Under normal conditions this system is partially filled with air saturated stagnant sea water. Judicious selection of piping material which will not corrode is therefore essential. Alternative piping material selection may accumulate corrosion products or scale in the system, The corrosion products will block critical valves and equipment and may render them inoperative when needed during a fire. In addition, platform insurance cost coverage may be reduced by using a platform insurance cost coverage may be reduced by using a corrosion free piping system in fire fighting services.

Economically attractive. Use of copper-nickel alloy material is found to be the most economic attractive alternative in the long term, compared with other choices below.–Use of internally epoxy lined carbon steel piping was considered. Our estimates show, however, that lined piping will have to be replaced at least once during a platform life of about 15 years. This estimate assumes a good quality epoxy coating application.–Use of bare carbon steel piping in sea water service, we feel is out of the question. Advantage of lower initial capital investment will be negated later with frequent platform shutdowns and pipe replacement

CORE

Protection of water injection wells. Alternative pipe material may produce corrosion products. These pipe material may produce corrosion products. These products may be carried through the water treating products may be carried through the water treating facilities and damage the injection wells.

Effect on pipe design capacities. Internal pipe corrosion will seriously affect pipe smoothness and this will result in a poor friction factor for fluid flow. Piping susceptible to corrosion will be, in time, undersized for the original designed throughput rates.

The favourable friction factor of copper-nickel and its anti-fouling characteristics allow the reduction in pipe size design in many cases.

Lighter material. Copper-nickel material is about 50 per cent. lighter than comparable carbon steel. In addition, corrosion allowance does nor need to be added to the pipe wall thickness design. Less weight is an obvious advantage for offshore installations.

Ease of installation offshore. Prefabricated spool pieces are light and could be made to fit on the spot pieces are light and could be made to fit on the spot offshore by cutting and rewelding if needed. Refitting epoxy lined pipe, of course, needs relining. Prefabricated epoxy piping for ease of installation requires for this reason a substantial higher number of flanged spool pieces compared with copper-nickel spools. pieces compared with copper-nickel spools. Detail of disadvantagesLimited number of suppliers and availability of large size pipe and fittings in copper-nickel material.Needs design adaptations.