Abstract

The occurrence of the severe slugging phenomenon in multiphase pipelines can cause serious and troublesome operational problems within the receiving process facilities.

This paper describes: the pipeline operability studies on the Dunbar to Always multiphase pipeline regarding riser-induced severe slugging; the control scheme selected to prevent this phenomenon; and the subsequent operating experience. The pipeline operability studies involving dynamic simulations concluded that riser-induced severe slugging could occur over a wide range of production rates, in particular when the ElIon gas is unavailable.

The challenge for TOTAL was to develop an operating strategy and a control scheme in order to eliminate severe slugging and operate the pipeline without significant process upset.

An automatic control scheme was selected which offered a cost effective solution and ensured the safety of the existing process installations. Its originality was that it was based on a Riser Base Pressure Control.

Since the initial start-up in December 1994, this selected scheme has proven to be successful and has enabled the pipeline operations in the riser-induced severe slugging region to be carried out without any difficulty.

Introduction

The Dunbar field is situated in the UK sector of the northern North Sea, approximately 22 km to the South of Alwyn North field which has been operated since the end of 1987 (Fig. 1 and Fig. 2).

The recent development of the Dunbar field and its satellite ElIon comprises a production platform linked to the Alwyn North platform by a 16' multiphase pipeline (Fig. 3). The gas from El- Ion (Subsea wells) and the oil and gas from Dunbar are co-mingled and the overall well-fluid production is routed without separation to the Alwyn North processing and treatment facilities.

The Dunbar 16' Multiphase Pipeline has been designed to export the Dunbar and Ellen production to Alwyn North (NAB platform) at a maximum flow of 49000 BOPD and 7.9 MMSCMD. Over a wide range of production rates, in particular when the ElIon gas is unavailable, the pipeline operation is in an unstable region where riser- induced severe occurs.

The purpose of this document is :

  1. . to give a short description of the flow regimes predicted in the DUNBAR 16' Multiphase Pipeline and in particular of the severe slugging phenomenon.

  2. to present the operating strategy and the control system developed by TOTAL in order to prevent severe slugging and to operate the pipeline without occurrence of significant process upsets.

  3. and to give a feed-back of the pipeline operation at low flow rates

1. Flow regimes and severe slugging

The pipeline profile of the Dunbar 16' pipeline is represented in Fig. 4. The flow regime map for this pipeline operated at a NAB riser-base pressure of 74 is given in Fig. 5 This map was based on the results of dynamic simulations performed with the OLGA pipeline software.

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