OTC 8196 Prevention of Severe Slugging in the Dunbar 16“ Multiphase Pic)eline

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 Alwyn 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 thar 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 csrried 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 ElIon (Subsea wells) and the oil and gas from Dunbsr 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 riserinduced severe shrggi ng 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 particukw of the severe slugging phenomenon. 2, to present the operating strategy and the contiol 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 barg is given in Fig. 5. This map was based on the results of dynamic simulations performed with the OLGA pipeline software. The different pipeline flow regimes that can be encountered in the operations of the Dunbar pipeline me: stable flow, hydrodynamic slug flow at high flowrates and riser-induced severe slugging flow at low flowrates. The Dunbar Pipeline flow regime map shows that hydrodynamic slugging is predicted well above the maximum design capacity of the pipeline and is therefore not a concern for the receiving process facilities, On the other hand it shows that the Dunbar 16” Multiphase Pipeline is subject to riser-induced severe shrgging over a wide range of production rates, in particular when the ElIon gas is unavailable. The severe slugging phenomenon occurs in multiphase pipelines at low flow and low GOR production when the gas rate is insufficient to continuously lift liquid up the riser. It is not the object of this document to go into details regarding the severe