The dynamics of electrical-submersible-pump populations and the implication for dual-ESP systems

This paper contains analytical expressions illustrating the growth, renewal, and decline of single electrical-submersible-pump (ESP) populations and their transition to dual-ESP systems. Critical assumptions are justified with published data, and the commercial implications of the conclusions are presented. Discussion starts with possible misinterpretations of instantaneous run time (IRT), which is a commonly adopted measure of pump reliability. Statistical renewal theory is then used to quantify the sensitivity of workover-delay time to rig availability and to determine the expected change in failure rates when replacing an existing pump population with more reliable units. The transition is shown to be relatively insensitive, even to large improvements in reliability. Dual ESPs are considered as a means of improving the run life. When analyzed as a standby redundant system, they are predicted to exhibit a gamma distribution. Under certain conditions, the replacement of an existing pump population with dual ESPs is predicted to result in a faster reduction in the workover rate and confer additional commercial benefits compared with a single ESP with the same mean time to failure (MTTF). The first dual-ESP system for the Milne Point field, Alaska, was successfully run in a Kuparuk well in April 2000. It is concluded that dual-ESP systems deserve greater consideration. The material is of interest to vendors designing ESP systems and those managing fields that use ESPs, particularly those involved in incentive-based contracts.