Pipework options

Much research has been done on the performance of various pipework materials when used on saltwater systems. Copper-nickel alloys have been specified for seawater use for over 50 years and are the materials of first choice for seawater pipework and condenser service for many of the world?s navies and merchant ships. They have a remarkable combination of good resistance to both corrosion and biofouling in seawater and, because they can be easily welded and fabricated, they are an obvious choice for pipe systems and heat exchangers. They are also used in desalination, power plants and offshore fire water systems, and for the sheathed protection of oil and gas platform legs and risers. A protective film that forms on the alloy?s surface protects it from both corrosion and biofouling. However, while this passive film is responsible for the alloy?s resistance to corrosion, pitting, crevice attack, stress corrosion cracking and resistance to velocity effects, excessive turbulence can damage the film.

While there is an abundance of 90-10 copper-nickel and aluminium-brass pipework used in the marine industry, the use of 70-30 is confined to submarines where its high strength is advantageous.

The low cost option is to use mild steel or, mild steel with a liner. DERA has looked into the possibility of using coatings of suitably inexpensive metal, which are hard/strong enough to withstand erosion and are anodic (or at least not cathodic) to the steel substrate. However, studies have shown that the installed costs of lined pipework can result in only a minor costs advantage over using 90-10 copper-nickel tubing.

Steel protection

Mild steel pipework can be protected by using an impressed electrical current system such as Cathelco?s. The system is based on the electrolytic principle and is therefore extremely reliable and safe in operation and is particularly effective in eliminating biofouling. The system features a control panel that can be fitted in the engine room, or any other convenient space, which supplies an electric current to anodes positioned in the ship?s seachests or strainers.

It provides an impressed electrical current to the anodes and is designed with digital displays, which enable crewmembers to instantly monitor the status of the system. Apart from periodic checks, the equipment is completely automatic.

The anodes are made of copper and aluminium and are mounted in a sea chest. In operation, the copper anode releases ions, which are carried throughout the engine cooling system by the flow of seawater. This creates an environment where barnacles and mussels cannot settle or multiply, eliminating risk of blockages or constrictions in the pipework. At the same time, the aluminium anode produces ions that coat the internal surfaces of pipes to produce a layer that suppresses corrosion.

The system can also be used on cupro-nickel pipework systems where soft iron anodes are substituted for aluminium, which reduces the effects of impinging corrosion by depositing an oxide film on the interior surfaces of pipes.

While attempts to build seawater systems from standard grade stainless steels have been unsuccessful, several thousand tonnes of superaustenitic stainless steel are now in service on offshore platforms and there are now a number of high performance austenitic and duplex alloys available. However, these alloys exception protection, but at a price.

The lighter option

One option that is guaranteed not to corrode is the use of epoxy pipework. Ameron manufactures a glass-fibre reinforced epoxy pipe system that it calls Bondstrand. The company claims that it has a number of advantages that include:

l Fast and easy installation

l Light weight

l An installed cost that equals traditional steel piping

l Little, or no maintenance

l Long service life.

While the capital cost of the pipe may be high, the adhesive joining system means there is no need for a skilled welder and, as the pipes are light, it alleviates the need for lifter gear. Ameron says that the Bondstrand pipework can also be installed much quicker than steel or alloy systems.

The company has developed a number of alternative systems that offer temperature resistance, high-pressure resistance and chemical resistance.