Multiphase Pumping - Operation & Control

Abstract This paper reviews field issues related to the planning, installation and operation of the helico-axial multiphase pumps. Interest for multiphase production, which leads to simpler and smaller in-field installations, is primarily dictated by the need for more a cost effective production system. Multiphase pumping is essentially a means of adding energy to the unprocessed effluent which enables the liquid/gas mixture to be transported over long distances without the need for prior separation. The Poseidon helico-axial pumps, under normal operating conditions, are largely unaffected by process fluctuations at pump inlet (changes in pressure, liquid or gas flow rate). They have demonstrated a stable behaviour (self-adaptive capability with regards to instantaneous changes). A multiphase pump set is designed to operate under changing/fluctuating process conditions. An important issue related to pump operability and flexibility has to do with the driver selection: fixed speed vs. variable speed In some cases a fixed speed drive provides sufficient operational flexibility. In other cases variable speed can be chosen. Pump operation & control strategies are presented and discussed. Recommendations and guidelines for the field system designer are given. Introduction When the production of a marginal field or a group of remote wells is considered with an existing central gathering system the traditional options for field development are:–natural flow,–artificial lift,–in-field separation with crude oil transfer pumps, gas to flare, or gas compression systems. With the recent field deployment of numerous multiphase pumps (MPP) new approaches to field development and production have been demonstrated. Typically helico-axial MPP have accumulated over 35,000 operating hours with an average availability of approximately 90%. Considering multiphase flow transient behaviour the operability of MPP is highlighted in the case of a declining offshore field with an increasing water cut. Different levels of pump control are defined depending on the operational requirements. Building on previous ideas a novel control system is introduced. Pump Specification The multiphase pump selection cannot be based solely on one defined operating point (main duty point) as it is commonly done for process pumps or compressors for refinery duty. Field experience shows that actual operating conditions change during the field life and may be different from predictions. Therefore multiphase pumps should be designed to be capable of facing different operating parameters by having a wide working envelope. Flow rates and pressure predictions are bound with uncertainty. The design of production facilities is generally based on extrapolations from the results of exploration well tests, delineation wells and reservoir characterisation. As a result, when the production wells are drilled and completed, the actual production data may be different from the prediction. P. 193