The retrofit of cathodic protection facilities to two offshore pipelines is described. The reasons for the premature failure of the initial system, and for the selection of the retrofit method are discussed. The design of the retrofit facilities, comprising sleds bearing 1.8 tonnes of aluminium alloy, is described, and results of pre and post-installation potential surveys compared. A number of conclusions for future retrofit operations are drawn.
In early 1981, cathodic protection potential measurements were mode by a remote controlled vehicle (RCV) on six offshore pipelines that had been installed in 1969 – 1970. These pipelines were the first to be installed in Bass Strait (refer Figure 1) and were laid directly an the seabed. The results showed that portions of two lines - a 500 mm oil pipeline between Kingfish B and Halibut platforms and a 600 mm oil pipeline between Halibut platform and shore - were no longer fully protected. Potentials measured platform and shore - were no longer fully protected. Potentials measured on the lines were as positive as -700 mV, to a silver/silver chloride reference electrode (full protection levels are considered to have been achieved if the potential is more negative than -800 mV).
Both pipelines had originally been coated with 2.5 mm of a hot applied glass fibre reinforced coal tar enamel, and a concrete weight coating varying in thickness between 25 and 75 mm. Each line had been fitted with sacrificial zinc anode bracelets, at spacings of up to 1300 m, weighing up to 320 kg and designed to last for 20 years. The RCV was used to closely examine these anodes, and it was found that the anodes had been completely consumed after only 11 years.
The premature depletion of the anodes is thought to have been due to a combination of the following reasons:
Pipeline operating temperature: of the pipelines with anodes that were inspected, only those pipelines in hot oil service had suffered complete anode consumption. Anode depletion had occurred on onlythose portions of the lines that had carried oil at temperatures estimated to be above 60C.
Ineffective pipeline/platform electrical isolation: both pipelines had faulty insulated flanges.
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