Offshore/subsea systems

Profit Increase With New Subsea Boosting Products

Emphasis on identifying more-efficient subsea boosting solutions has led to a number of initiatives in the industry.

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Emphasis on identifying more-efficient subsea boosting solutions has led to a number of initiatives in the industry. A new multiphase-pump technology has been developed that will expand the operation envelope for subsea boosting and create better opportunities for more-effective offshore-field development. A parallel, and equally important, advance has been the development of a new heavy-duty 6-MW subsea motor.

Introduction

In 2011, a decision was made to abandon twin-screw technology because of the low sand-handling resistance and limited differential-pressure generation with multiphase fluids that these pumps had demonstrated during in-house testing. Thus, a development project was initiated.

Multiphase-Pump-Development Project

For this development project, the main target was to increase pump performance in the following ways:

  • From 4,000 to 6,000 rev/min
  • From 3 to 6 MW
  • From 500 to 1,000 actual m3/h
  • From 50- to 150-bar differential pressure at 70% gas-volume fraction (GVF)

Reaching these targets would require design of a new motor to operate at higher speeds at twice the power.

Multiphase-Pump Technology

It was evident from recent experience that the pump design had to use the rotodynamic principle (dynamic energy transfer to the liquid) and not the ­positive-displacement principle. Before this multiphase-pump development, a hybrid multiphase pump with both multiphase impellers and radial impellers had shown very good test results when operating with multiphase fluids.

The multiphase impeller design has commonly been a helicoaxial design, which has been the standard until now. However, on the basis of previous experience and tests, a different technology was chosen. This was a mixed-flow impeller in which the flow channel was partly axial and partly radial. This design uses centrifugal action to provide more pressure generation from each impeller. This principle can be termed a helico-mixed-flow design.

Multiphase-Pump-Prediction Model

From promising initial pilot pump tests, the authors were able to prepare a prediction model for the multiphase pump. The prediction model is, in itself, a challenging piece of work because the fluid conditions will change considerably through the pump. It is necessary to use a stepwise approach and calculate the fluid parameters for each impeller stage.

The model has been used in preparation of Fig. 1, which shows a conservative operation envelope for 70% GVF with the new technology using full power (6 MW at 6,000 rev/min) with a 10-stage multiphase pump compared with the old technology.

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Fig. 1—Boosting potential; comparison of new and old technology.

Multiphase-Pump Design

The next step was to design, build, and test the full-sized prototype pump. This was designed to generate 100-bar differential pressure at 70% GVF. This is an eight-stage pump that will be used for qualification testing.

The speed increase to 6,000 rev/min is a significant technology leap when it comes to rotodynamic stability and vibration. For this pump and motor, generous shaft and bearing sizes were selected in order to secure a good stability margin. To be selectable for both high and low differential pressures, the pump had to be multistage, with several impellers assembled in series, where the number of stages would be adjusted to fit the desired differential pressure. The modularized design was defined to allow any number of stages. Further, the pump had to be selectable for a large range of fluid capacities. Therefore, a range of different impellers and diffusors was defined. They all fit inside the same geometric envelope and on the same shaft diameter.

Another important aspect to consider was the compression and consequent volume reduction of the gas because of the increasing pressure through the pump. This required impellers of smaller and smaller capacity through the pump, depending on the suction pressure and the actual GVF. This could easily be accommodated by arranging the required impellers. The range covers actual fluid capacities up to 1500 m3/h.

The pump design had to be tolerant of dry running, at least for shorter times. The two pump bearings are arranged to operate in the same barrier fluid that is protecting the motor, and two mechanical seals are used to hold back the barrier fluid from the process side of the pump. In order to handle impeller thrust forces, the back-to-back impeller arrangement was chosen. This means that a number of impellers are placed in the opposite direction to that of the first impellers on the suction side. In this way, the impellers will counteract the unbalanced pressure forces acting on the impeller shrouds.

Motor Technology and Design

The induction motor was chosen as the preferred technology over the permanent-magnet motor, on the basis of special subsea conditions. This choice was backed by a number of other factors: longer operational experience subsea, easy startup, and favorable mechanical robustness. To increase the operating temperature, and thus reduce the winding cross-sectional area and the size of the cooler, it was decided to develop a new cable with insulation with a copolymer of tetrafluoroethylene and hexafluoropropylene. This removed the temperature limitations and resulted in a reduction in both motor and cooler size.

A subsea-pump motor is liquid-filled. This will cause a significant viscous friction loss, especially in the rotor/stator gap, where the peripheral velocity will be very high when increasing speed. It is therefore very important to keep the rotor diameter as small as possible and instead increase the rotor length. For this new motor, the authors found an optimal configuration with an iron-length-to-rotor-diameter ratio. The ­motor-test results were excellent, showing an 84% efficiency at 5 to 6 MW and at 6,000 rev/min. The shaft-vibration level shows extremely stable levels for the whole speed range.

Condition Monitoring

The best way of monitoring pump rotors is by the use of proximity probes located close to the bearings. This technique is used for topside pumps. The subject company has worked on making new pressure-resistant probes that are qualified for 15,000 psi. The probes are now installed in the new pump and motor. Each probe contains three proximity sensors: two radial and one axial. The probes allow continuous and accurate rotor measurements. This enables wear analysis that identifies when the pump should be pulled for maintenance. The probes can also help the operator to avoid running in rough operating conditions.

Qualification Testing

A key factor in qualifying this technology is full-scale testing. No new technology will be deployed subsea until it is tested to a minimum Technology Readiness Level of 4 or 5. A new test facility has been built that includes a multiphase test loop, a new electric driveline, a new control system, and new measuring, analysis, and data-storage capabilities.

Subsea-Boosting Case Example

A case study assumes a medium oil field located at a 1500-m water depth with the reservoir 200 m below the mudline and with a 30-km stepout from the host facility. The field has four production wells and two water-injection wells. The water cut is high, increasing up to 60% after a few years and ending at 90 to 95%.

It is assumed that the cost of the field is USD 950 million including gas lift at riser base and USD 1.030 billion for the boosting-pump case, regardless of pump size and power. It is further assumed that a topside host is available at a 30-km distance; no cost is added for this other than necessary topside installations. The oil price is assumed to be USD 39/bbl, and the cost of topside water treatment is assumed to be USD 1/bbl.

A comparison was made to see the difference between no subsea boosting (use of gas lift at riser base) and subsea multiphase pumps at 2, 4, and 6 MW, respectively. The case study yielded three key learnings:

  • Effect on increased oil recovery: The positive effect of subsea boosting is very high when introducing just 2-MW boosting. The total recovery increased by 22%. Adding more power will help increase the total recovery, but with reduced effect. The effect of boosting is best in the first 10 years.
  • Effect on net present value (NPV): When considering NPV, subsea boosting has a significant positive effect. It increases from USD 118 million to USD 544 million when using 6-MW boosting power. More oil is produced earlier with more power.
  • Effect on field profitable production time: The increase in oil-production rate with subsea boosting pumps will shorten the production time of the field. The production is stopped when the cash flow turns negative. The case with gas lift and 2-MW boosting requires 22 years before production is unprofitable. Use of 4-MW boosting requires 17 years; use of 6-MW boosting power requires only 15 years of production.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 27747, “Profit Increase With New Subsea Boosting Products,” by Åge Hofstad and Hans Christian Nilsen, Aker Solutions, prepared for the 2017 Offshore Technology Conference, Houston, 1–4 May. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.