Study of Flow Improvers for Transportation of Bombay High Crude Oil Through Submarine Pipelines

Abstract

Summary During pumping of waxy crude oil through longdistance pipelines, crystallization of wax can cause serious problems. If the pipeline is shut down, the wax may gel the cooling crude and block the pipeline. Studies were made of several flow improvers and waxy crudes, and a bench-scale laboratory test unit was developed for evaluating such crudes. The test rig accurately simulates cooling conditions in the pipeline and satisfactorily predicts ability to restart flow of a particular crude. predicts ability to restart flow of a particular crude. Introduction In March 1974 the Oil and Natural Gas Commission (ONGC) of India discovered the Bombay High offshore oil field 160 km west-northwest of Bombay (Fig. 1). The field's oil-bearing zone has an area of about 700 km. The average water depth is about 700 m, and the vertical depth of the main limestone pay zone (L III) is 1350 m. After a number of wells were drilled to delineate the field, production of crude oil was started on May 21, 1976, through a well-cum-production platform. Initial production was about 4,300 B/D, platform. Initial production was about 4,300 B/D, and this gradually was raised to 80,000 B/D. During the initial phase of production, oil was stored in the tanker Jawaharlal Nehru anchored in the field to a single-buoy mooring. Crude oil from this tanker was dispatched to coastal refineries by means of daughter tankers, but in view of the production potential of the field, a 30-in.-diameter, production potential of the field, a 30-in.-diameter, 203-km-long submarine pipeline was laid to connect the field to the onshore terminal at Uran. Pumping of the offshore crude though this submarine line started on June 13, 1978, from Platform F. From the Uran terminal, the crude oil is transported further through feeder pipelines to refineries and to a tanker loading terminal at Butcher Island near Bombay.Bombay High crude oil is waxy, and it has a pour point on the order of 30 degrees C (Table 1). Pipeline point on the order of 30 degrees C (Table 1). Pipeline transportation through the submarine section was expected to pose problems because the average minimum seabed temperature in this region ranges from 22 to 24 degrees C, with a lowest recorded temperature of 20 degrees C. Therefore, after an emergency shutdown of the pipeline, gelling of the crude oil could make restarting difficult, if not impossible, after only a few days of cooling. Various measures were considered to keep the crude oil in an acceptable state of fluidity, even at the lowest seabed temperature. These included (1) dilution with less waxy crude, (2) crude oil conditioning, and (3) treatment with flow improvers.The dilution method was not possible because a less waxy crude oil is not available near the Bombay High field. Crude oil conditioning by proper heat treatment can improve the fluidity of some crude oils, as demonstrated in Assam, East India, but this would require a huge treatment plant on one of the offshore platforms at the field. Running costs such as energy and manpower are very high, so this was clearly not a viable approach. The response of Bombay High crude oil to such a conditioning procedure also would require evaluation. procedure also would require evaluation. The oilfield operators (ONGC), therefore, decided to use an additive treatment, and screening work was started with a large number of commercially available additives to assess their effect on the pour point of Bombay High crude oil. Only a few point of Bombay High crude oil. Only a few additives showed any significant effect, and these were tested further by ONGC to determine yield stress and plastic viscosity improvement. From the results of plastic viscosity improvement. From the results of this study, one additive (A) finally was selected for continuous use in the Bombay High crude oil submarine pipeline. JPT P. 2539