After a subsea mudslide near the mouth of the Mississippi River Delta, several sections of production or export pipelines were severed. The pipelines could not be retrieved to surface because they were buried deeply in the mud. Coiled tubing was successfully used to clean and abandon these pipelines in-situ under 200-feet of water. The coiled tubing was conveyed through a flexible riser from a Dynamic Positioning mono-hull vessel. The lengths of the four sections of pipeline cleaned were 2400-feet, 1200-feet and 1800-feet with diameters of 8–10 inches.
Coiled tubing has been used for years for workover purposes on oil and gas wells. A limited number of coiled tubing applications involve cleaning out pipelines from production platforms. In all these applications coiled tubing is conveyed through a solid metal riser with known friction factors.
During hurricane Georges in September 1998, several operators in the Gulf of Mexico had their pipelines in the South and Main Pass areas severed by mudslides. After repairs were completed, four pipelines were scheduled for abandonment. Because divers were unable to locate one end of each of these pipelines, these could not be flushed using conventional means. Normally, offshore operators abandon pipelines either by removal for disposal onshore or by abandonment in place. This requires approval from the Minerals Management Service. Both abandonment alternatives require that the pipelines be thoroughly cleaned to ensure that pollution does dot occur.
Divers use jetting equipment to expose the pipelines on the sea floor. The exposed ends can then be retrieved to surface and flushed to a facility. In the present case, the sections were buried more than 20 -feet in the mud and could not be retrieved.
An initial attempt to remove the hydrocarbons was made by applying suction to the exposed end of the pipeline. This was not successful as a mud plug was formed at the buried end. The only remaining option was to enter the pipeline, in-situ under the sea, with coiled tubing. Due to the fact that the pipelines were located in a mudslide and the water was too deep for a jack-up boat, a Dynamically Positioned (DP) vessel was used. Coiled Tubing was placed on the deck of the vessel and the work was performed through the moon pool.
The four sections to clean and abandon were three 8-inch sections, two 1200-feet and one 2400-feet, and a 1800-feet long 10-inch pipeline section. These pipelines were located in South Pass and Main Pass sections of Gulf of Mexico, southeast of New Orleans. The 8-inch pipelines were utilized to transport produced fluids between platforms. These pipeline sections contained approximately 10% hydrocarbons. The 10-inch pipeline contained 100 % hydrocarbons
Usually well intervention operations offshore are performed from either a platform or a jackup boat. For the reasons mentioned above and the need for diver support, it was then decided to use a dynamically positioned monohull vessel.