Severe wax deposits had built up in the Valhall subsea crude oil pipeline over a period of years. The equipment and procedures used to completely remove these solids from the line are described, with particular reference to the unique risks and difficulties associated with offshore pipelines.
The Valhall oil field, located in the Norwegian Sector of the North Sea, has been developed with three steel jacket platforms standing in 220 feet of water. The crude oil and gas produced are sent via separate 20 inch pipelines to the Ekofisk complex 23 miles away, where Valhall production is mingled with oil and gas from other North Sea oil fields. The gas is then pipelined to Germany, and the oil is piped to England.
Production began at Valhall in October 1982, but no pigs were sent through the oil pipeline to sweep out accumulations of water or solids for the first few years. During this period, wax components in the oil were .depositing in the oil pipeline, as the high paraffin content oil was cooled to below its cloud point of about 80°F in the line. In addition, the unstable chalk formation from which the oil was produced occasionally flowed out of the wells with the oil, and some of this chalk was pumped into the pipeline. A rising pressure drop across the pipeline confirmed that deposits were building up in the line. Calculations done in mid-1985 indicated that approximately 7,500 barrels of solids were deposited in the pipeline.
In 1984 the Norwegian governmental authorities required that an internal inspection of the Valhall pipelines (consisting of a geometry survey followed by a metal loss survey) be done. The metal loss survey involves sending an "intelligent" pig through the line to measure the pipe wall thickness and record the dimensions and location of any internal or external anomalies. In order to run this device a very clean pipeline is essential, which meant that the deposits in the oil pipeline would have to be completely removed.
Solids removal from a liquids pipeline using a pig always involves a certain element of risk, because if the solids pile up during removal they and the pig may form a plug which can stop the flow. In an extreme case, a very solid plug can prove impossible to pump out, and a portion of the pipeline may have to be cut out and replaced before flow can resume.
Pipelines are the lifeline of most offshore oil installations, since production must stop if the pipelines are not operational. Moreover, subsea pipeline repairs are expensive, may require a long production shutdown, and are technically challenging. For these reasons, any pigging operation in an offshore pipeline is engineered to reduce the risk of line blockage to an absolute minimum. Due to the inherent risk of a line blockage with a solids plug and/or stuck pig, many operators are hesitant to clean subsea pipelines more than that necessary to hold pressure losses to a reasonable level.
