Optimizations in the Design and Operation of an Offshore Hydraulic Pumping System
Authors: Kpodo F.K. 
Published: December 2007

This paper presents an overview of the design and operation of an offshore hydraulic pumping system on Platform Eva when it was converted from a power-oil to a power-water system in 1983. Also described are some useful economic and operational data obtained from experience with hydraulic pumping systems over several years of operation.

Introduction

Platform Eva is located in state waters offshore Huntington Beach, CA. The platform, constructed in 1964, was operated until 1983 with a closed power-fluid (CPF) hydraulic power-oil system. On July 1, 1983, Eva was converted from a power-oil to a power-water system.

The new system exhibits several special features in the design and operation of an offshore hydraulic power-water system. Emphasis was placed on maximizing operational efficiency. cost reduction, safety, and the efficient use of deck space. Platform Eva was refitted with canned electrical submersible pumps, designed within the constraints of limited deck space, electrical switchgear rating, and power-fluid properties. An evaluation was performed leading to the selection of produced water as power fluid over fresh water and seawater. Guidelines for good power-water quality have been established that specify fluid solid content, oil content, and lubricity level. A new oilfield filter has been introduced and used successfully to achieve the required power-water quality and improved downhole pump life. In a joint effort by Unocal Oil and Gas Div. and pump manufacturers, the normal downhole pump life has been improved more than 100% with metallurgical and mechanical improvements in the standard pump system. Severe problems with tubing leaks occurred shortly after the switchover from oil to water. These problems were traced to small amounts of CO2 that break out at low pressures in the turbulent regime of the fluid stream and were eliminated. Failure of splash-zone coating caused by thermal stresses and improvements thereof are also described.

Despite optimizations that improved operational efficiency of the power-water system, a comparative analysis of the old and new systems shows several economical and operational advantages of using power oil over power water as a hydraulic power fluid. The power-oil system, however, is potentially a higher safety risk.

General Information

The conversion from a power-oil to a power-water system was facilitated with the installation of temporary vessels and piping, which allows simultaneous operation of dual power-oil and power-water systems. Groups of wells were switched over to the power-water system by displacing and flushing oil in the side strings until the oil system was completely phased out. There were 25 active wells involved in the switchover, with a gross production of 6.000 B/D [950 M/d]. The switchover was achieved within 24 hours without a platform shutdown.

The total cost of the system conversion and the new power-water facilities was $1.4 million. The new facilities include seven canned submersible pumps, a freewater knockout (FWKO), filter packages, a water lubrication system, in-and-out turbine meters for each well, and a new control panel. Fig. I shows Platform Eva with canned electric submersible pumps in place.

There were two basic reasons for the conversion from a poweroil to a power-water system. First, use of the 20-year-old offshore facility raised some serious safety and environmental concerns. Exposed to the marine atmosphere, the facility had undergone gradual corrosion, as well as wear and fatigue processes. If operated at 3,000-psig [20.7-MPa] pressure, the facility's piping or equipment could develop a leak, leading to a potential oil spill in the ocean. A power-oil leak could also be ignited into a fire on the platform, endangering personnel, equipment, and environment. Fig. 2 shows an example of such an accident.

Second, making accurate oil-production measurements of marginal wells is necessary because the royalty payment is based on the number of wells producing economically (the more active wells on production, the less royalty paid). In a typical power-oil system, some of the power oil (about 4 to 8%) is leaked from the engine end into the produced fluid, making marginal wells appear to be producing more oil than they actually are. This condition, which is the opposite of the power-water system, made determining the economic status of a number of high-cut wells difficult. In a cooperative effort with the State Lands Commission to measure oil rates on high-cut producers accurately, we elected to convert to power water. This conversion has made it possible to identify which wells are producing oil economically.

Existing Power-Oil System

The existing power-oil system was a CPF system, incorporating basic functional components, such as electrically driven triplex pumps, subsurface equipment, 100-bbl [16-m] power-oil surge tank, a vertical heater/treater for treating the makeup power oil, and a control station.

As mentioned earlier, Platform Eva had 25 active wells, with a gross production of 6,000 B/D [950 m/d] (60 % cut) at the time of the conversion. The power-oil requirement for this production was about 11,000 B/D [1750 m/d]. Power-oil loss or makeup was 500 B/D [80 M/d] or about 5% of the supply.

Six 125-hp [93-kW] triplex pumps were operated in parallel. These pumps were circulating 11,000 B/D [ 1750 M /d] of power oil at 3,000 psig [20.7 MPa] through a common header to the control manifolds to operate the individual downhole hydraulic pumps. The total horsepower requirement for the system was therefore 750 hp [560 kW].

Spent oil from the hydraulic pumps was returned to the 100-bbl [l6-M] surge tank for recirculation. The makeup for power-oil loss downhole is obtained by simply diverting some dry producers to the heater/treater that, in turn, supplies the surge tank. Equipment for power-fluid filtration and lubrication was not needed.

Now Power-Water System

The new power-water system was designed to interface with much of the older equipment in the power-oil system.

JPT

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