Active feedback control as a solution to severe slugging

Severe slugging in multiphase pipelines can cause serious and troublesome operational problems for downstream receiving production facilities. Recent results demonstrating the feasibility and the potential of applying dynamic feedback control to unstable multiphase flow like severe slugging and casing heading have been published (Refs. 4,9,1,5 and 2). This paper summarizes our findings on terrain-induced slug flow (Ref. 2). Results from field tests and as well as results from dynamic multiphase flow simulations are presented. The simulations were performed with the pipeline code OLGA2000. The controllers applied to all of these cases aim to stabilize the flow conditions by applying feedback control rather than coping with the slug flow in the downstream processing unit. The results from simulations with feedback control show in all cases stable process conditions both at the pipeline inlet and outlet, whereas without control severe slug flow is experienced. Pipeline profile plots of liquid volume fraction through a typical slug flow cycle are compared against corresponding plots with feedback control applied. The comparison is used to justify internal stability of the pipeline. Feedback control enables in many cases a reduced pipeline inlet pressure, which again means increased production rate. The paper summarizes the experience gained with active feedback control applied to severe slugging. Focus will be on extracting similarities and differences between the cases. The 1 Author to whom correspondence should be addressed: Present adress: Scandpower AS, P.O.Box 3, N-2027 Kjeller, Norway. Fax: +47 64 84 45 00, E-mail: Kjetil.Havre@scandpower.com main contribution is to demonstrate that dynamic feedback control can be a solution to the severe slugging problem. Introduction Multiphase pipelines connecting remote wellhead platforms and subsea wells are already common in offshore oil production, and even more of them will be laid in the years to come. In addition, the proven feasibility of using long-distance tie back pipelines to connect subsea processing units directly to on-shore processing plants makes it likely that these will appear also in the future. Such developments are turning the spotlight on one of the biggest challenges for control and operation of offshore processing facilities and subsea separation units: controlling the feed disturbance to the separation process. That is, smoothening or avoiding flow variations at the outlet of the multiphase pipelines connecting wells and remote installations to the processing unit. Common forms of flow variations are slug flow in multiphase pipelines and casing heading in gas lifted oil wells. In both cases the liquid flows intermittently along the pipe in a concentrated mass, called a slug. The unstable behaviour of slug flow and casing heading has a negative impact on the operation of offshore production facilities. Severe slugging can even cause platform trips and plant shutdown. More frequently, the large and rapid flow variation causes unwanted flaring and limits the operating capacity in the separation and the compression units. This reduction is due to the need for larger operating margins for both separation (to meet the product specifications) and compression (to ensure safe operation with minimum flaring). Backing off the plant's optimal operating in this way reduces its throughput. A lot of effort and money has been spent trying to avoid the operational problems with severe slugging and reduce the effects of the slugs. Roughly speaking, there are three main categories of principles for avoiding or reducing the effects of slugs: 1. Design changes 2. Operational changes and procedures 3. Control methods A. Feed forward control B. Slug choking C. Active feedback control An example of a typical slug handling technique involving design changes is to install slug catchers (on-shore) or increase SPE 71540 Active Feedback Control as the Solution to Severe Slugging Kjetil Havre, ABB Corporate Research, and Morten Dalsmo, SPE, ABB 2 K. Havre, M. Dalsmo SPE 71540 the size of the first stage separator(s) to provide the necessary buffer capacity. A different compact process design change is reported in Ref. 10, where the authors introduce an additional small pressurized closed vessel upstream the first stage separator in order to cope with slug flow. An example of operational change is to increase the flow-line pressure so that operation of the pipeline/well is outside the slug flow regime (Refs. 13,6). For older wells with reduced lifting capacity this is not viable option. For gas lifted wells an option would be to increase the gas injection rate (see Ref. 1). For already existing installations where problems with slug flow are present and for compact separation units, these design and operational changes may not be appropriate. Control methods for slug handling are characterized by the use of process and/or pipeline information to adjust available degrees of freedom (pipeline chokes, pressure and levels) to reduce or eliminate the effect of slugs in the downstream separation and compression units. The idea of feed-forward control is to detect the build-up of slugs and, accordingly, prepare the separators to receive them, e.g. via feed-forward control to the separator level and pressure control loops. The aim of slug choking is to avoid overloading the process facilities with liquid or gas. These methods make use of a topside pipeline choke by reducing it’s opening in the presence of a slug, and thereby protecting the downstream equipment. The slug choking may utilize measurements in the separation unit and/or the output from a slug detection device/algorithm. For a more complete assessment of current technology for slug handling refer to Ref. 9. However, in this assessment, active control methods are not properly addressed. Recently, results demonstrating the feasibility and the potential of applying dynamic feedback control to unstable multiphase flow like severe slugging and casing heading have been published (Refs. 4,9,1,5 and 2). Like slug choking, active feedback control makes use of a topside choke. However, with dynamic feedback control, the approach is to solve the slug problem by stabilizing the multiphase flow. Despite the promising results first reported in 1990 (Ref. 4) the use of active slug control on multiphase flow has been limited. To our knowledge only two installations in operation has stabilizing controllers installed. These are the Dunbar-Alwyn pipeline (Refs. 5,9) and the Hod-Valhall pipeline (Ref. 2). One reason for this might be that control engineering and fluid flow dynamics usually are separated technical fields, i.e. the control engineers have limited knowledge about multiphase flow and the experts in fluid flow dynamics have limited insights into what can be achieved with feedback control. Indeed, when presenting the results on the Hod–Valhall pipeline (Ref. 2), we had a hard time in convincing several of the fluid flow dynamics engineers that one can avoid the slug formation in severe slugging by active control. Hence, one objective of this paper is to provide insights and understanding into how feedback control can be used to avoid severe slugging, and thereby contribute to bridging the gap between control and petroleum engineering. Previous work Elimination of terrain and riser-induced slug flow by choking was first suggested by Schmidt et. al. (Ref. 13). Taitel (Ref. 6) state that stable flow can be achieved by using a choke to control the pipeline backpressure. He state that an unstable system can still operate around equilibrium steady state provided a feedback control system is used to stabilize the system. Furthermore, he refers to Schmidt (1980, Ref. 14) who found experimentally that it is possible to stabilize the flow by choking at the top of the riser upstream the separator. Taitel used stability analysis to define a theoretical control law. The control law relates the backpressure to the propagation of the slug tail into the riser. In Ref. 6 Taitel claims [sic] “It is interesting to observe that, to a good approximation, little movement of the choking valve is needed for such a control system. This makes it possible to set the valve in a precalculated constant value’’. In the experiments reported in the paper, no feedback control system is used. Instead, the choke is fixed in a pre-calculated position. Note that the derived stability condition is related to quasi-equilibrium flow conditions with bubble flow in the riser and no or limited propagation of the slug tail into the riser. From control theory it is well known that feedback control is needed to operate in an unstable operating point, otherwise disturbances will push the operation out of the desired operating point. Our conclusion is that the quasi-equilibrium flow conditions comprises a stable operating point with an unnecessary high riser base pressure which must be higher than the corresponding pressure which can be achieved by applying stabilizing feedback control. Furthermore, we believe that the riser base pressure at quasi equilibrium flow conditions is equal to, or larger than, the peak riser base pressure with slug flow. Typical flow maps showing the slug flow region's dependency on the pressure, justifies these statements by fact that the slug flow region shrinks with increasing pressure and that the bubble flow region lies above slug flow region (see Fig. 1 and Fig. 2). In Ref. 4, experiments on suppression of terrain-induced slugging by means of a remotely controlled control valve, installed in the riser top, are presented. Manual valve closure about 80% was necessary to remove the terrain-induced slugging, with a pressure difference about 7bar over the valve. In automatic mode the valve was controlled by PI algorithm with the pressure over the riser as the input signal. Terraininduced slugging was successfully alleviated with PI control algorithm ope