The Experience from Field Operation ofja Subsea Multiphase Booster

ABSTRACT The subsea multiphase production - based on the transportation over long distance of the untreated oil-well fluids (oil, water and gas) - is expected to be one of the most efficient tool for economic exploitation of deep offshore and marginal fields. A long term testing campaign on a multiphase screw pump was successfully completed in 1990 at the AGIP Trecate onshore oil field and the results confirmed the industrial viability for such a kind of equipment for surface application. Then, a subsea version of an improved multiphase twin screw pump has been integrated into a Subsea Multiphase Boosting Unit and installed on the Prezioso Field, offshore Sicily, in Summer 1994. Long term testing under real operating conditions wereinitiated affer a successful start-up of the Unit. To Authors' knowledge, this is the first world-wide subsea installation of an electrically driven multiphase pump operating with live oil. The paper presents first a description of the marine twin screw pump concept adopted for the subsea applicationincluding the main features of the complete boosting unit and the adopted solutions to allow itto operate under different conditions. Then, the project implementation activities kom the onshore integration through the installation, commissioning and start-up operations are described. Moreover, the results of the initial functional tests are discussed with particular reference to the screw pumphydraulic performance as well as to the behavior of the pump pressure compensation and seal/lube oil systems. Transient and steady state conditions experienced by the system are finally characterised and the early evidences of its long term performance are discussed. INTRODUCTION The industrial qualification of any developed multiphase boosting equipment requires the field operational expedience to prove the suitability of the proposed systems in handling the untreated well fluids under real operating conditions. The underwater operation poses additional requirements to be fulfilled in order to achieve the expected performance and reliability levels. To this end, the development of the marine concept of a multiphase twin screw pump has been undertaken and a long term subsea testing campaign was initiated. The testing campaign has been designed to cover the most probable and realistic operating conditions expected for an industrial application of such a kind of multiphase boosting equipment. The characterisation of the hydraulic and mechanical performance of both the pump and its associated auxiliaries constitutes the mainobjective of the tests together with the assessment of the overall system reliability. This campaign is believed to provide a significant contribution to a better understanding of theperformance of such systems in the different operating conditions as well as of the issues related to the installation, start up and maintenance. MARINISED SCREW PUMP The pumping system includes a subsea screw pump in vertical configuration driven by a water filled electric motor and equipped with a simplified lube oil system; all the system is totally pressure balanced at the process pump inlet conditions by means of special pressure transfer vessels, (Fig. 1). The developed concept of the subsea screw pump resulted in a system configuration that achieves the features summarised here below.