New Separator Internals Cut Revamping Costs
Authors: Swanborn Rambout A. , Koene Frits , de Graauw Jan 
Published: December 2007

This article describes the debottlenecking operation of the production separators on the Shell Leman AK platform in the southern North Sea. The original, compactly built separators became overloaded through decreasing wellpressures. Gradually, they were forming a serious restriction in the production flow sheet. Two alternatives were considered to overcome this problem:extending the platform and installing new separators orretrofitting the old separator vessels with state-of-the-art internals.

The second alternative was chosen because of considerably lower costs (totaling only 10% of first alternative) and considerably shorter downtime. This separator upgrade was a joint project between the operator of the field and their central research and engineering facilities and Burgess Manning, Technical U. of Delft, and CDS Engineering BV.

Background

The debottlenecking operation formed part of a larger project in which gas production of an aging gas field was boosted through, among others, the installation of new compressors. Fig. 1 depicts the simplified process flowsheet of the Leman AK platform. The function of the platform is to remove liquids from the produced gas and to secure transport of the gas onshore through additional compression. The produced well fluids first pass through the slug catcher, which removes the bulk of the liquid. Downstream of the slugcatcher two parallel separators remove the finer liquids, so the gas is" fit" to enter the centrifugal compressors.

Recently, well pressures were at a level of 690 kPa, with typical flow rates of 5.66 109 m3/d per vessel. It was doubtful whether the second stage separators functioned effectively under such conditions, although the original design stated that the separators could handle 16.99 109 m3/d per vessel. After the planned installation of the compressors, we expected flow rates of 4.96 109m3/d per vessel at pressures of 290 kPa. The vendor of the original separators re-rated maximum flow rates per vessel at these pressures at 2.03 109 m3/d, and it became clear that the separators would not sufficiently guard the compressors against incoming free liquids. Therefore, we considered it necessary to improve the performance of this separation stage before installing the new compressors.

Original Separator Design

The original separators (Fig. 2) consisted of 152-cm-diameter, 363-cm-longhorizontal vessels with 11 horizontally mounted, 20.3-cm cyclone tubes in the top half of the vessel. The length of these cyclones was 250 cm. Although this arrangement functioned well under original design conditions, we observed a clear deterioration of performance with increasing flow rates. In the end, nomore liquid was separated at all (Fig. 3).

Options To Improve Separation Performance

The two main alternatives to improve the performance of the second separation stage were (1) to replace the current separators with bigger one sable to handle the increased flow rates or (2) to remove the internals from the current separators and install high-throughput retrofit internals.

The generally accepted advantage of new vessels (Option 1) was that we were certain that performance would improve. The disadvantage of this option is that a platform extension would be required because of lack of space on the platform.

The primary advantages of the retrofit option are the much lower costs and much shorter downtime. The disadvantages were that previous retrofit operations had seldom been carried out under these conditions and that a range of novel engineering techniques had to be applied to guarantee success. The feasibility and consequences of each of these alternatives will be detailed.

New Separators

If new separators were to be designed for this operation, current design and engineering procedures prescribe the following. Vertical vessels would be 200cm in diameter and have a length between tangents of 290 cm. Weight per vessel would be 11,500 kg. The cost of a new separator per vessel would be $200,000.Because the existing horizontal separators are difficult to remove, a considerable amount of platform reconstruction/extension work would be necessary. Estimated total costs were $1.25 million. Completion time (from project start in 1993) was projected for mid-1995.

Debottlenecking Existing Separators

The success of this alternative depends on fulfilling the following requirements.Selection of a gas/liquid separation internal that can be fitted inside the existing vessels and that can process the current flow rates.Proper flow distribution to and from the internal, taking into account that the remaining area inside the vessels will be restricted.A geometry that will allow the internal to be brought in through themanway/outlet nozzle and be mounted inside the vessels within a short period of time (3 days) after removal of the current internals.

Selection of a Suitable Internal. Under the conditions specified, the operating principle of the separation internal will rely on inertial forces. That is, centrifugal forces are used and separation is achieved because of the difference in density between the gas phase and the contaminants. The current internals consisted of 11 20.3-cm cyclone tubes that function along this principle. Under original design conditions, entrance velocities of the cyclones were 25 to 30 m/s and G forces in the cyclones were 300 to 500 G.

Normally with inertial forces as the principal force of separation, particles as small as 5 mm can be separated, which is in line with the purpose of the separators. The use of different separation principles (e.g., diffusion)only leads to larger separators, which was unwanted in this case.

Inertial Separator Internals. Two main types of inertial separator internals exist: vane-type and cyclone-type separators.

Vane-Type Separators. In vane-type separators, the gas flows through a bundle of parallel, curved blades. The droplets in the gas are flung onto these blades (Fig. 4A) and subsequently drained. Various designs are on the market, but all have one property in common; that is, these internals require a relatively large area and also are sensitive to poor distribution and subsequent local overloading in restricted surroundings.

For this particular application, 3.5 m2 of intake area of a sophisticated up flow vane-type internal would be required. The available area in the vessel between the tangents is only 5.5 m2. Fig 4B shows a possible mounting configuration. Because of the almost impossible task of achieving an even distribution across the vane area, we rejected this option.

Cyclone-Type Internals. In cyclones, the gas/particulate-phase mixture is brought into a spinning motion.

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