Subsea Testing Of A Multiphase Pumping Unit Prototype

Abstract

Marginal fields, and in particular those in deep wafers, call for innovative technologies to enable economical exploitation. The subsea development is gaining acceptance as one of the most promising options and appears the only one able to offer a viable rate of return when moving into deep waters. Most of the anticipated development applications call for either a high level of complexity in case of multiwell systems or a reduced equipment set up for limited recoverables. Subsea multiphase boosting of untreated well streams is also proposed as an alternative to natural drive. The ENI Group has been devoting great attention to multiphase production and boosting for several years. Different concepts of multiphase boosting equipment have been developed and manufactured. In particular, the application of a twin screw pump has been thoroughly investigated through laboratory and onshore field endurance testing. The Offshore Industry then required long term testing to prove the suitability of a subsea prototype boosting unit to perform in real operating conditions. To this end underwater testing has been undertaken at Agip Prezioso oil field off Sicily through the design and fabrication of a subsea boosting unit housing a vertical twin screw pump, as described in the paper. First, the twin screw pump as developed and manufactured by Nuovo Pignone for underwater application is described with special focus on the pressure balancing, lubrication and seal oil systems. The overall subsea trial system configuration is then provided together with an outline of both the support facilities set up and the test conditions allowed by the site characteristics. Moreover, the key features of the subsea prototype boosting unit are presented and the basic criteria for its design and manufacturing are discussed with particular reference to the requirements for the multiphase pump monitoring and ease of retrieval for maintenance purposes. Finally, the paper outlines the underwater tests implementation program together with the operating philosophy of the system. INTRODUCTION Several alternatives are proposed by the offshore industry for the safe and economical exploitation of marginal fields. For many applications, multiphase systems as opposed to conventional technologies can represent the answer to the problems related to such developments by moving to the sea bottom the overall production plant in a configuration whose economics are much less affected by environmental parameters and are almost exclusively dependent on the magnitude and productivity of the reservoir. Multiphase boosting in particular appears to be a viable solution, [1]. Additional requirements are also posed by deep water application. In such demanding conditions all aspects of the system may become decisive for both the technical feasibility and the economics of the field development. Deep water design therefore requires an integrated approach to installation, operation and maintenance. Since 1985 Agip, Snamprogetti and Nuovo Pignone have been active in the field of multiphase production and transportation.