Abstract

Coiled tubing (CT) pipe has been successfully used in the past to clean and abandon in situ subsea pipelines in offshore areas where conventional industrial approaches (i.e., pigging, flushing or suction) were not feasible. The technique of using CT has been explored before and the subject has been covered in previous SPE papers1. The basis of the previous work relies on the CT pipe's strength to be pushed through long, horizontal lines and its ability to convey tools and fluids at once.

One of the main limitations, however, in using CT for pipeline interventions in offshore environments as opposed to the traditional methods is the amount of equipment required on board. In most cases, deck space, loading constraints and operational factors such us seabed, heave, or current direction, fixed have posed significant challenges. These challenges have been addressed in some specific cases for production platforms, jack-up boats, or dynamically positioned (DP) monohull vessels. However, traditional coiled tubing intervention will not always provide a feasible solution.

A particular operationally challenged subsea pipeline intervention exists in the Congo delta. The pipeline, which linked two production platforms, had been plugged with organic deposits and two pigs from an initial cleanup attempt in November 2004. Subsequent conventional trials failed to break through these obstructions, so CT was proposed as a feasible alternative. The platform's deck space and crane load capacity, 4-knot sea currents rich in alluviums and products from the Congo river, and 46-ft seabed that used either jack-up boats or DP vessels not an option. This paper discusses the unique solution developed to address the operational and technical challenges of this difficult environment.

Objective

The objective of this mission was to cleanout a plugged sealine linking the 2 platforms, Libwa 3 and Tshiala 5. CT was proposed as a solution to clean out the asphaltenes from the pipeline and because this is a nonconventional application, several considerations were taken into account during the planning-engineering phase and during the operation. Few operations of this kind have been performed 1 and the geographical restrictions on this field (as explained later in this paper) have made this operation unique. These considerations and the professionalism of the team resulted in a successful operation where the client recovered the pipeline and could eventually put back into production all the HP wells that had been stopped for several years. The oil production in this field increased to 1,200 bbl/d (2,075 bbl/d of fluid was passing through the sealine after operation) with a drop in pressure of approximately 300 psi.

Background

Since November 2004, the pipeline between Libwa 3 and Tshiala 5 has been plugged by two pigs. Above the pigs, an unknown quantity of organic deposits had accumulated as a result of changes in temperature and pressure when the crude was flowing through the line. Hence, they had to use another sealine, creating a pressure increase along with a production loss in the other wells. Several unsuccessful attempts were made to restore the flowing capacity, primarily because

  • the Tshiala 5 platform faces the Congo Delta, creating currents up to 4 knots, or 7.5 km/h, making vessel operations difficult

  • lifting the pipeline to surface was also impossible because of the shallow depth, the composition of waters that are rich in alluviums and products of the Congo river, and the current that prevented the appropriate vessels from working

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