Inlet diverter improves Split Flow separator efficiency on floating facilities

May 1, 2003
Prosernat developed the Split Flow sepa-rator concept 10 years ago for gas/oil/ water separation on floating facilities.

C. Rigaill
C. Streicher

Prosernat, IFP
Group Technologies

M.N. Lingelem
Norsk Hydro

J. Falcimaigne

Prosernat developed the Split Flow sepa-rator concept 10 years ago for gas/oil/ water separation on floating facilities. Norsk Hydro validated the first unit and implemented it on its Troll Olje semi-submersible platform in 1995. Today, more than 20 Split Flow separators are operating on various floating installations in the North Sea.

Main features of the Split Flow separator are:

  • Central inlet of the gas/liquid mixture with central outlets for the separated water and oil phases and gas outlets are at both upper ends of the separator. Level instruments are also positioned in the central part of the vessel. The center of the separator is where liquid level fluctuations are minimal, hence the positioning of the liquid outlets and control instruments allows for optimum liquid quality and overall operation control
  • An inlet diverter enables a homogeneous gas and liquid distribution on the top of the partition tray. The latter, situated just below the inlet diverter, provides a primary liquid/liquid separation through a cascade effect. The tray's design allows sustained liquid distribution despite the motions (rolling or pitching) induced by the floating production system.

The bottom section of the separator had anti-wave dampeners installed. Depending on the liquid/liquid separation efficiency, these can be of double perforated plate type or structured packing to facilitate water droplet coalescence.

The first Split Flow unit was implemented on the Norsk Hydro Troll B platform after extensive laboratory testing at Heriot Watt University in Scotland (at reduced 1/7 scale). The Troll B Split Flow unit comprises one test separator, three production separators, and one downstream electrostatic coalescer. The unit has been designed to treat 30,000 cu m/d of oil with high emulsion, along with a high water/oil ratio (handling up to 24,000 cu m/d of water). In 1995 it was the largest such package installed on a floating facility.

Ever since, operating results have been consistently excellent, typically.

Furthermore, the unit's production capacity has been increased by 50% over the design capacity, without any loss in performance.

Snail's progress

A recent improvement to the concept has been the introduction of a new inlet diverter, known as the "Snail," conceived by Norsk Hydro. This comprises a low velocity cyclonic channel that enables a smooth reduction of the momentum of fluids entering the separator, thereby improving separation efficiency and also reducing the risk of foaming.

The Snail concept was initially developed as part of an initiative to design a compact and reliable subsea separator, and was tested successfully on the Troll pilot subsea processing station. Norsk Hydro then decided to apply this concept in a Split Flow separator for its Fram Vest subsea development.

In the development of subsea processing stations, significant importance is attached to simplicity and durability of solutions. Interven-tions inside subsea separators are highly undesirable as are retrieval operations. Efficiency of subsea separators is also important to minimize size – more so than for topsides applications. The separator inlet section is an important contributor to the overall separator perfor- mance. A poor inlet section will require a larger separator and, possibly, a need for further internals in the separator.

Design goals for an inlet device include:

  • Good performance for a wide variety of flow-rates and combinations of phase flow rates
  • Low shear so as not to reduce droplet sizes
  • Resistance against erosion
  • Robustness with respect to slug loads and vibrations
  • Simplicity and low fabrication cost
  • Simple installation.

During initial design of the Troll Pilot subsea processing system, the ideal inlet device was envisioned as a long pipe with moderate velocities and with the potential to vent off gas along the pipe. As a long pipe would not easily fit into a compact subsea processing station, however, alternative configurations were investigated, including the Snail. The initial concept comprised a spiral configuration with an open top.

As the initial design was somewhat too "curved" for efficient fabrication, simpler geometries were introduced, while retaining the basic configuration. The device's name came from its resemblance to a snail's shell.

The Snail's properties with respect to slug loads, vibrations, and overall functionality were tested on a large-scale model on an ice-covered fjord in Norway using firewater pumps for liquid supply.

All experiments were successful, with the ultimate test employing the actual oil-water-gas combination from the Troll field. An experiment using a model of scale 3.5:1 showed that separation was almost an order of magnitude better than for the standard momentum breaker (around 100 ppm oil in water as compared to 1,000 ppm for the standard system). However, when using the Snail as inlet device, only modest additional improvements resulted through use of plate package internals, so these were not installed in the subsea Troll Pilot separator.

Fram Vest

The new inlet device's spiral configuration appeared to offer significant potential for many other applications, including floating platforms, hence the decision to include a Snail in a Split Flow separator for the Fram Vest project. In this case, the separator was to be mounted in front of the existing separation train of the Troll C platform to de-bottleneck the existing production system prior to the Fram Vest tie-in.

As the anticipated fluid velocities were significantly different from those of the pilot tests, the extrapolations of the results to the actual Fram Vest conditions were validated through computational flow dynamics (CFD) simulations. Subsequent design of the Fram Vest Snail device was then based on the existing test results, scaled to the specific operating conditions, and verified by simulations in two-phase flow.

Cut-away of Prosernat's Split Flow separator:
Click here to enlarge image

The CFD code Fluent (version 5) was used to study flow behavior. This code can calculate the flow of several stratified incompressible fluids. The design studies employed a liquid (oil) and a natural gas, with properties (density, viscosity, and surface tension) corresponding to the design service conditions.

In addition to the Snail's internals, two external domains were included in the model, beyond the gas and liquid outlets, to apply realistic boundary conditions and to provide representative flows in the outlet sections. Above the gas outlet, a fluid domain was modeled between the top cover of the Snail and the wall of the pressure vessel. Below the liquid outlet, a fluid domain was also modeled between the outlet and the internal plate located under the device.

At the device's inlet, true liquid and gas velocities for the actual phase section fractions were used to ensure conservation of the fluid's energy, as well as its volumetric and mass flow rates.

As the slug characteristics were given in term of superficial velocities, the true fluid velocities were calculated with the hydrodynamic model used in IFP's multiphase transient analysis software Tacite.

Other boundary conditions were:

  • The mean separator pressure was applied at the boundary of the modeled domain at the top of the device, with a small vertical gradient given by the gas specific gravity
  • The lower outlet of the device was modeled with a 100-mm deep liquid layer flowing on the internal plate system, covered by a gas domain. The separator pressure was applied on the boundary of the gas domain with the same gradient as in the upper part. Several schemes were assessed previously, but this one provided the best results and insured that the outlet flow could be a mixture of liquid and gas.

An analysis was performed with the Snail sizes and inlet flow conditions previously tested. This provided a reference case to validate the model and simulation results and a basis to estimate (by comparison) whether the new design would be acceptable. Subsequently, one simulation was performed for the actual Fram Vest design operating conditions, with two further simulations corresponding to two cases of slug conditions.

The CFD simulations proved first that the initial Snail dimensions were well adjusted to the flow conditions considered previously, but that liquid flow was more perturbed with the new inlet flow conditions. Although most of the liquid stayed confined in the device, partial overflows appeared above the internal spiral wall of the device for the two slug cases. Some liquid droplets became detached and were carried over through the gas outlet.

In light of these findings, the height of the Snail's vertical drum was modified to avoid overflows of liquid over the spiral internal wall and carry-over of liquid droplets by gas. In addition, a horizontal stiffener placed at top of the spiral wall, on the interior side, was modified to contribute to prevent liquid overflows.

The simulations showed that the new inlet diverter concept developed initially for the subsea Troll Pilot could be modified for Fram Vest. CFD simulations enabled validation of the extrapolation rules that were subsequently employed to optimize design of the Snail for this new project.

Implementation of the Snail in a Split Flow separator allows simplification of the internals in the bottom section of the separator, where oil and water are separated. This will help minimize maintenance operations. It is now thought that the concept could be extended to many different types of separation applications, including the revamp of existing under-performing separators.