Two-phase separation/pumping to make subsea debut on Marimba

Solution for medium/short deepwater tie-backs

Massimo Benetti
Marco Villa
ENI-Agip E&P Division

At the top of the VASPS unit are the fluid pressure housing and the helix. After a number of turns, gas is opened to the gas annulus and flows into the separator. The liquid then flows through the liquid discharge tubing by means of an electric submersible pump. [20,191 bytes]
Schematic of the VASPS subsea hardware. [15,476 bytes]
Development of small and marginal fields represents one of the most challenging issues in offshore technology. In mature offshore areas, characterized by an extensive pipeline and flowline network, new projects mainly involve exploitation of marginal reservoirs, which in many cases are tied back to existing facilities, usually located 10-20 km away.

Because of the limited reserves in place, efforts are focused on minimizing development costs and increasing the amount of reserves recoverable. In these cases, the application of a subsea separation system like VASPS would benefit the overall development. VASPS imposes a reduction on the wellhead flowing pressure, thereby increasing the production rate and extending the field's life.

VASPS is an innovative two-phase (gas-liquid) separation and pumping system conceived initially to be installed in a dummy well, 30-70 meters deep, with the separator head standing above the mud line. Since the system is derived from standard casings and wellhead components, installation and maintenance is executed using established drilling and completion procedures.

The unit consists of three concentric casings. The outer casing (pressure housing) has various nominal joints of 30-in. diameter with an overall height of 30-70 meters. The intermediate casing (helix separator joint) is a 16-20-in. pipe with plates welded on its outside diameter, forming a spiral that is in contact with the internal surface of the pressure housing, forming a spiral conduit. Finally, the inner, liquid discharge tubing is an 8-10-in. screwed tubing string within the bore diameter of the helix joint providing the liquid outlet path and forming (with the inside of the helix pipe) the inner gas annulus.

At the top of the pressure housing, the multiphase stream from the wells enters into the spiral. Inside the unit, the fluid is directed to the bottom of the separator guided by the spiral conduit, which enhances the separation by introducing centrifugal forces.

Gas separated from the liquid flows through the holes on the helix pipe wall, passes into the gas annulus and flows up into the gas expansion chamber. The gas leaves the unit from an outlet and naturally expands in a dedicated pipeline up to the surface treatment facilities, where it typically enters the second or third stage separation.

Degassed liquid flows down the helix into the liquid sump where the final degassing takes place. The liquid then enters the liquid discharge tubing, where an electrical submersible pump is installed to boost the liquid phase before it flows out of the VASPS into a dedicated flowline. VASPS' internals are designed to provide a high degree of separation of gas from liquid with a very limited amount of entrained gas in the liquid phase, thereby allowing use of single phase pumping and instrumentation systems.

Compared with a multiphase pump, VASPS has a higher pumping efficiency as it acts on the liquid phase, although two flowlines are required to deliver the gas and liquid phases separately. The technology looks most promising for medium and short distance subsea developments or in deepwater fields.

Development program

VASPS has been developed in four phases since 1990. In phase II, a 12,000 b/d prototype, 20-in. in diameter and 14 meters high, was designed, built, installed, and tested in ENI's multiphase test loop in Agip's Trecate onshore field production facilities in Italy. This loop operates with light crude from several wells with a maximum flow rate of 15,000 b/d and an average associated GOR of 170 cf/bbl at working conditions of 60 bars, 55 degrees C. Up to 5,000 b/d of water can be injected into the wellstream.

The prime aim of the tests on the VASPS separator was to verify its efficiency and operability in a live field. Various tests in over 200 different conditions were performed. Separation efficiency was proven to be very high - over 96% with flowrates greater than 12,000 cu meters/hr. For the commercialized version, a demister will be introduced which should increase liquid separation efficiency to over 99%.

During the third, pre-subsea phase, the VASPS separation was validated with high viscosity hydrocarbons at Petrobras' Atalaia Field test loop in Brazil. This facility, in use since 1994 for testing multiphase pumps/meters, works with oil and water in closed loops and with the gas in an open loop, with flow rate capacities of up to 100 cu meters/hr of oil, 60 cu meters/hr of water, and 6,000 cu meters/hr of rich gas supplied from a local field. As anticipated, separation performance decreased as oil viscosity increased.

For the Atalaia trials, a centrifugal pump was integrated into the system with tests performed in steady and transient conditions to evaluate the pump's effect on the system's operability. In the event, the pumping system was tested for more than two months in start-up, transient, and steady conditions. These tests confirmed the system's high operability, and also the suitability of the software used to simulate and design the VASPS control system.

Subsea demonstration

Results from these earlier phases persuaded the project's participants - ENI-Agip, Mobil North Sea, and Petrobras - to progress a final fourth phase aimed at installing and operating a full-scale subsea version of VASPS in a live subsea field. Following evaluation, they selected Petrobras' well 7-MA-1D-RJS in the Campos Basin's Marimba Field, at a water depth of 435 meters.

This well is an existing satellite equipped with a wet christmas tree, producing gas-lifted oil to the P-8 semisubmersible platform. The latter receives production from 15 satellite wells in total, and is also equipped with gas lift facilities. P-8's processing capacity is 50,000 b/d of oil and 1.2 MMcm/d of gas.

Produced oil is treated in a two-stage separation system with two production trains. Gas separated on the automatic separator passes through a boosting compression system so that it can be sent, with the gas from the first-stage production separator, to the main compression system. This raises the gas pressure to the required level either for export or for use in artificial lift.

The existing well will be disconnected from the flowline to be connected to the VASPS, located in 395 meters of water and 550 meters from the christmas tree. Between the VASPS and the platform, a new 1,050 meters long, 4-in. diameter gas-bearing flowline will be installed, while the existing 6-in. line will be used to transport oil.

The main objective of this final phase is to demonstrate that VASPS is a viable subsea separation and pumping system, and in particular to:

Prove the integrity and reliability of the subsea hardware
Verify the system's controllability in a subsea environment
Demonstrate the concept's cost-effectiveness and its advantages in enhancing production.

For this purpose, the VASPS unit must be kept operating for 10 months without major failures. The plan currently is to install the unit on Marimba. Prototype subsea hardware will include:

Temporary guide base (TGB): A 30-in, 65-meter long conductor will be installed through the TGB. This conductor will be assembled from standard 12-meter lengths, each 30-in. OD.
Flowbase: Designed to provide isolation of flowlines when retrieving the VASPS unit, without the need to disconnect the lines. The flowbase will be latched to the 30-in. casing.
VASPS head assembly: This upper section of the unit contains the separator head gas expansion chamber, choke valve, liquid and gas outlet connections, fluid inlet connection, process monitoring equipment, ROV interface panel, electrical power penetrating device, and test lines. The assembly can be removed from the VASPS unit.
VASPS separator: Comprises the pressure housing with its internal components - helix, liquid discharge tubing (LDT), ESP assembly, and still wells for level control.
Pressure housing: Contains the helix unit, liquid discharge pipe, and ESP. At the top, a profile will be included for landing and connecting to the conductor and also to interface with the expansion chamber.
Helix separator joint: Formed by a central pipe with a spiral helix plate welded around the outside diameter.
LDT: Carries the ESP and forms the conduit whereby the pumped oil flows from the VASPS unit to the flowline.
Still wells: Inside the separator is one still well suspended by the top plug to provide a conduit for measurement of the VASPS level.
ESP: Will be suspended by the LDT from the pressure cap in the separator head that carries the electrical connector. The pressure cap can be electrically disconnected and removed complete from the VASPS unit, along with the LDT and ESP. The ESP will provide a flow rate up to 12,000 b/d, 80 bar at maximum flowrate, and featuring a 1,375 V, 60 Hz motor controlled by a variable speed drive at the surface.

Reference

Case study of the VASPS concept for the exploitation of subsea fields, presented at the Profitable Small Field Development conference organized by IIR in London, October 1998.