One corrosion control method used on pipelines is cathodic protection (CP). This is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded “sacrificial metal” to act as the anode. The sacrificial metal then corrodes instead of the protected metal.
“An offshore production field is a very complex system,” Flanery said. “Ideally, all the different components and their separate corrosion protection needs should be carefully planned at the design stage.” For example, oil and gas flows from the reservoir, through the subsea tree and, typically, to a manifold or pipeline end termination (PLET) via a jumper pipe. Fluids pass along the pipelines to a production platform for processing before being sent to a tanker or onshore facility for further processing. (A jumper is a short flexible or rigid length of pipe that is used to connect a flowline to other components.)
“You cannot just look at a pipeline in isolation,” Flanery added. “It is always part of a much larger system.” It is important to ensure there are no design gaps between the corrosion protection systems of two adjacent assets, such as a flowline and a manifold.
Both Flanery and Grapiglia commented that segments of a production system are often designed in isolation, with little or no consideration being given to how they might be integrated. Different contracts cover supply and installation with operation often covered by yet another contract.
The consequence of this disjointed and ad hoc design and installation is that after just a few years of operation, ‘new’ fields are showing signs of early failure of some of the CP systems protecting the equipment. “A lot of companies think that the protection system is fairly straight forward and they can save money by undertaking basic text book designs,” Grapiglia said. “It is better to invest a little extra at the start and avoid massive costs later to replace the whole protection system.”
One area that has been neglected in the past is the interface between onshore and offshore pipelines. Traditionally the offshore pipelines are designed to one standard and the onshore to a different one. The “shore crossing” has been a ‘no man’s land’. Research is currently under way in Australia to assess the best way to deal with this section, as well as develop best practices which can be used for future projects.