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The aim of this paper is to present the modernization of an old cathodic protection system installed on the legs of an offshore platform and to discuss the results of the research performed in order to investigate the anticorrosive effectiveness of this solution.

The modernization of the cathodic protection system consisted of connecting to the platform legs a sacrificial anode system placed at the sea bottom. The effectiveness of the modernized system was assessed on the basis of the measurements of the potential profiles of the platform legs, i.e. the distribution of their potential over the entire length.

After connecting sacrificial anode systems, the potential of the legs kept changing towards more negative values. The extent of cathodic polarization increased, and already after a week the potential of the legs fell within the range of full cathodic protection almost along their entire length. The polarization was higher at the lower parts and decreased close to the water surface, which was a result of the distance of a given fragment from the anodic system. The results of the measurements confirmed the correctness of the design assumptions and the effectiveness of the performed modernization.

The results obtained show that the modernization of the cathodic protection system on the legs of an offshore platform consisting of connecting sacrificial anode systems placed at the sea bottom can ensure the achievement of full cathodic protection potential for long legs (these were 80 m long in this study).

A 2000 km pipe line for oil and gas services 18 oil fields and gas plants from Red Earth and Grand Prairie to Edson and Edmonton in Alberta, Canada. A total of 28 rectifiers provide soil side corrosion control with an operating output level of about 420 amps.

3. Unsolved problems and future developments The previous sections may indicate that cathodic protection, at least if installed to the best standard practice, will solve all immersed corrosion problems offshore. This would be incorrect. However, the history of those structures in the North Sea with complete records of effective cathodic protection indicates that the existing techniques do largely result in safe structures for long periods.

Equipment and typical results from cathodic protection surveying systems. In the simplest form, cathodic protection surveying of fixed offshore platforms is achieved by the so called ‘dipping’ technique, dipping a reference electrode into the sea and measuring a steel/sea potential with respect to it via an indicating voltmeter and a metallic connection to the topside steelwork. This procedure is allowed in NACE RP‐01–76 REF 36 but the standard does address the importance of placing the electrode close to platform members, distant from anodes and into areas of greatest shielding. The conventionally undertaken dip survey, particularly in geographic regions with substantial sea currents which cany the electrode away from structure members, is nothing better than a general indication of the overall level of protection. The probability is of errors indicating better levels than actually exist, due to the IR related voltage drops in the sea between the electrode location and the platform member.

The application of cathodic protection to the prevention of corrosion in steel structures immersed in sea water is a well established method. Monitoring such systems helps to ensure that the desired level of protection has been achieved and maintained for the prevailing conditions. This level is determined by the steel/ electrolyte (the sea water) potential. However, as it is impractical to measure this value directly a standard reference electrode, immersed in the electrolyte, is used and a value measured between this electrode and the structure. The measured value depends on the type of electrode employed.

Last month Raychem Ltd gave details at a press conference at the Swindon production and development centre of its Anodeflex cathodic protection system for pipelines and also refinery and process chemicals tanks bottoms. A demonstration was also given of the new Rayseal electrically shrinking sleeve for the repairs of low pressure leaks in gas distribution networks (see report in our ‘Materials’ section). As pioneers in heat shrinkable plastic sleeving, this latter development is a ‘natural’ for Raychem.

Since this paper was first presented at the International Symposium of Corrosion and Protection Offshore in Paris, May 1979, Wilson Walton International report that their system for remote monitoring of offshore structures and pipelines is the only one of its type in use in the North Sea area. They add that six major oil companies have now ordered this type for North Sea jackets.

Acorn steps up colour powder coating production. Acorn Exal, Milton Keynes, is planning to double its colour powder coating output. This, according to the company, is because of the way demand is going. It says that the market for colour powder coatings, which are used primarily in architecture, is growing rapidly.

An extensive investigation simulating the behaviour of coal tar protective systems on steel pipelines under concrete weight coatings has been carried out. In particular the study has been made of the behaviour of synthetic primer/high temperature coal tar enamel with glass reinforcement under the stresses caused by expansion of the steel pipe within its concrete weight coating. The possibility of shearing of the sacrificial anodes has been given special consideration. At high oil temperatures there tends to be an initial creep of the pipe which gradually ceases within 1/2 weeks, but the total movement amounts merely to a few millimeters. This has been found to apply up to temperatures in excess of 100°C. In order to establish this conclusion, laboratory studies were carried out by three different techniques and the effects evaluated. At elevated temperatures some coal tar oils pass from the enamel to the concrete, resulting in an increase in softening point of the coal tar enamel of up to 20°C. The resistance to creep of the enamel is thus greatly increased while the integrity of the corrosion protective coating is maintained. The conclusion that the coal tar enamel system provides a fully reliable protection at the highest submarine oil temperatures is borne out in experience. A North Sea line has been operating satisfactorily at 113°C, for the last four years.