Sizing and Selection Criteria for Subsea Mutiphase Pumps
Bozorgmehrian, Manoochehr 1976-
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The main application of subsea multiphase pumps is in mature assets; when the reservoir pressure is not high enough to boost the oil flow, or in the green fields or tie-backs where the host facilities are far from the well head. In all subsea boosting projects, the pumps are expected to work non-stop for a long period of time (3-7 years) without any intervention. To satisfy this requirement alone, a highly reliable integrated system including electric motors, sealing parts, couplings, bearings and connections is necessary. There are no published standards for subsea pump selection and sometimes there is a big gap between proven experience and what is claimed by the manufacturers. An independent research study that covers all parameters and priorities can be helpful. In the present research, all available operator or manufacturer’s reports and published papers were reviewed and a criterion was developed based on these steps: 1- Since the evaluation of the various lifting and boosting methods is the prerequisite of pump selection and sizing, the conventional subsea separation system was compared to a multi-phase pumping one. 2- All applicable types of subsea pumps including Helico-Axial (HAP), Twin Screw Pump (TSP) and Electrical Submersible Pump (ESP) were compared, and their pros and cons were discussed. The auxiliary equipment of each one such as conditioning tank and bypass line were also explained. The characteristic curves and applicable operating ranges of each pump were analyzed. 3- Sometimes different suppliers offer a wide range of products, which need to be verified for specific working conditions; therefore all parameters in pump type selection such as Gas Volume Fraction (GVF), water cut, differential pressure, head, flow rate, fluid viscosity, RPM, sand content, water depth, Opex, Capex, reliability and asset life were discussed. 4- Material selection as a critical part of the pump selection was studied thoroughly. In this regard, all applicable types of corrosion and cracking including CO2 corrosion, Pitting and Crevice corrosion, Galvanic corrosion, Microbial Induced Corrosion (MIC), Hydrogen Induced Cracking and Sulfide Stress Cracking were studied to determine which part of the pump is susceptible to which types of corrosion. Material selection was conducted based on this survey and applicable standards such as Norsok M001 and NACE MR0175. Finally, applicable material grades were introduced. 5- Above mentioned parameters and the latest project records have been applied in designing a practical procedure which determines the priority of the parameters and the sequence of pump selection process. Applicable calculations and requirements of API 610, API 17A and DNV A-203 standards have also been incorporated. Measures were introduced to prevent some operational problems reported in historical records. 6- The procedure begins with the study of flow regime to calculate the differential pressure of the pump. Then it leads to calculating the shaft power. Other steps are dedicated to the specific pump type, whether it is Helico-Axial or twin screw. The calculations are more dependent on the vendor data for TSP; however, the formulas that can lead to proper sizing such as axial velocity, efficiency, slip and power were provided. Balancing, sealing and coupling standards that are applicable to both types were also explained at the end of the study.