Abstract

Drag reducer additives (DRA) to increase pipeline capacities have been used extensively on the Oseberg Field in the North Sea since 1991. This paper describes different tests that have been performed to find the best and most cost-effective drag reducing chemical. The tests have been carried out in the 75 mile 28 in. crude oil pipeline from Oseberg Field Center to Sture on the Norwegian coast. and in the 9.5 mile 14 in. multiphase pipeline carrying full wellstream fluid from Oseberg C to Oseberg Field Center. Both conventional gel-type DRA, and a new generation type additive have been used. The new generation DRA is an emulsified powder-product with a polymer content of about 20-25% as compared to conventional gel-type product with a polymer content of about 5-8%. This new type turns out to be about four times more efficient than conventional gel-type DRA, and gives a cost-saving of about 25% compared to conventional type products.

This paper also describes a method to measure drag efficiency as a function of linefill, and shows the relationship between injection rate of DRA and wax-deposits in a pipeline.

Use of drag reducing chemical has increased capacity of the Oseberg Transportation System (OTS) by 25% from 620,000 bopd to 770,000 bopd. While use of conventional DRA had no effect on capacity of the multiphase pipeline (MTS), test results using the new generation DRA have shown a reduced pressure loss in the line of about 50%, corresponding to about 10% capacity increase.

Introduction

Polymeric drag reducer additive to increase flow rate in a crude oil pipeline was first commercially used in the Trans Alaska Pipeline in 1979. Today, several oil-field operators in the North Sea use DRA to increase capacity.

Drag reducer additives are long-chained polymers with a very high molecular weight. As drag reduction is a near-wall phenomenon, it is necessary to inject the chemicals downstream pipeline pumps to avoid the high shear degradation that occurs within a pump.

Norsk Hydro has used drag reducers for transportation of crude oil from the Oseberg Field to the oil terminal at Sture on the Norwegian coast since 1991.

The Oseberg Field development includes a Field Center consisting of a production-quarters platform. Oseberg A, and a drilling/wellhead platform, Oseberg B. Two subsea wells are tied back to Oseberg B.

Oseberg C, a production-drilling-quarters platform, was installed in the northern part of the field in 1991. A subsea well on the Gamma North satellite field is connected to the C platform. A multi-phase pipeline (MTS) transports unprocessed well fluid to the Oseberg Field Center as Oseberg C has limited separation capacity.

Stabilized crude oil (specific gravity of 0.85) is transported by Oseberg Transportation System (OTS) in a 28 in. pipeline to Sture. The design capacity of the line is 620,000 bopd. Other pipelines connected to OTS through Oseberg Field Center include a 16 in. pipeline from Oseberg C and the Veslefrikk field (operated by Den Norske Stats Oljeselskap, Statoil), a 12 in. pipeline from Brage field (operated by Norsk Hydro) and a 16 in. pipeline from Frigg/Froy fields (operated by Elf Aquitaine).

Field Tests of DRA

Several tests have been carried out on Oseberg Field Center to find the best and most cost-efficient drag reducer. The first test was carried out in September 1991.

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