Crude-Oil Foaming Problems at the Sullom Voe Terminal

Abstract

Summary Early in the program of commissioning the Ninianstabilization trains at Sullom Voe terminal, problems of severefoaming in the first- and second-stage separators occurred. This resulted in massive carry-over of crude into the gaslines. Injection of a conventional anti-foam compound intothe crude alleviated the problem somewhat. At highergas/oil ratios (GOR's), however, conventional anti-foamagents did not control the foaming, adequately. BP Research Centre at Sunbury investigated the foaming characteristics of the crude and developed, with others, a novel foam inhibitor that effectively prevented foamgeneration in the separators. Also, gamma ray monitoring equipment with alarmsand emergency trips was installed on the separators toprotect the downstream gas-compression trains. protect the downstream gas-compression trains. Brent crude had similar, but less severe, foamingcharacteristics. These problems were solved in the same wayas those with the Ninian crude. Introduction The Sullom Voe terminal in Shetland processes andexports oil and gas from offshore fields in the EastShetland basin, about 100 miles 1161 km) northeast of theisland. The fields are divided into two groups-the Ninian Pipeline Group and the Brent Pipeline Group. Unstabilized crude from each group flows through a 36-in.191 -cm] -diameter pipeline to the terminal. The Ninian pipeline crude comes from two fields, whilethe Brent pipeline crude is from five fields. Theproperties of the crudes from each field are different, properties of the crudes from each field are different, particularly the foaming characteristics. particularly the foaming characteristics. Parallel stabilization trains-two for the Ninian crudeand three for the Brent crude-are located at theterminal. The two crudes have different gas and watercontents and different pressures. Their processing facilitiesand operating conditions are similar but not identical. One Ninian stabilization train was commissioned toprove the plant and to provide gas for power and steam prove the plant and to provide gas for power and steam generation. Very early in the commissioning operation, however, severe foaming occurred in the second-stage(low-pressure [LP]) gas/oil separator, which operated at 0.7 bar gauge [10 lbf/sq in.], when the temperature ofthe initial dead-crude feed was raised to 914 deg.F[490 deg.C]. As a result, large quantities of crudewere carried into the flare relief system. BP Sunbury Research Centre confirmed the severefoaming tendency and recommended a suitable anti-foamagent. Adjusting the operating conditions had little effecton the foam level, but injecting the anti-foam agent andinstalling adequate foam detection and trip systemsbrought it under control. At a later stage in the commissioning process, a verystable foam persisted in the first-stage (high-pressure[HP]) gas/oil separator, which operated at a pressure of1.7 bar gauge [24.7 lbf/sq in.], despite the injection ofconventional anti-foam agent. Further laboratory and planttrials resulted in the development of an improvedantifoam agent that destabilized this foam. As gas rates wereincreased to facilitate the commissioning of downstreamequipment, a degree of unpredictability in separatorperformance became evident owing to the varying quality of performance became evident owing to the varying quality of crude. Adequate techniques to control foam level finallywere developed. They called for appropriate rates ofantifoam addition supported by trip systems. This techniqueprevented significant carry-over of crude into downstream prevented significant carry-over of crude into downstream compressor suction drums. Later, when the Brent stabilization trains werecommissioned, a similar. less severe foaming tendency wasobserved. The phased commissioning program allowedinstallation and assessment of the performance of parallelplates (Dixon plates) in the separators as a method of plates (Dixon plates) in the separators as a method of breaking down foam. Description of the Crude Stabilization Processes Crude from the Ninian and Brent pipelines enters theprocess plant where it is dehydrated and stabilized. The process plant where it is dehydrated and stabilized. The Ninian and Brent pipelines operate at pressures of 26 and 9bar gauge 1377 and 130 lbf/sq in. 1, respectively. The crudestreams are processed separately in that each pipeline feedsa number of parallel dehydration/stabilization trains-two for the Ninian and three for the Brent. Each train isdesigned to produce 330,000 B/D 152 466 m 3 /d] of stabilized crude. One Ninian and two Brent trains are currently operating, and the remaining Brent and Ninian trains willbe commissioned shortly. Figs. 1 and 2 are simplified block diagrams of theNinian and the Brent dehydration/stabilization processes. Ninian Stabilization. Cold Ninian unstabilized crude at26 bar gauge [377 lbf/sq in.) is heated in three stages: first. by exchange of heat with hot stabilized crude;second, with steam condensate; and third, with medium-pressure steam. pressure steam.JPT P. 2211