Debottlenecking of The Machar Slug Catcher
Authors: Kvamsdal Dag , Rhodes Elizabeth 
Published: October 2011

The BP Machar slug catcher is a combined three phase separator and slug catcher located at the ETAP platform in the British sector of the North Sea. With an original design throughput of some 70,000 gross barrels of fluids a day, the slug catcher was barely able to cope with half that quantity maintaining an acceptable outlet quality. The plant had posed a major bottlenecking problem and was adversely affecting oil export from the subsea development.

The oil production from the Machar slug catcher is limited by the amount of water following the oil. The amount of water in the oil exceeds the water handling capacity of the downstream separation system. BP initiated a broad program in 2006 to look at all possible options for debottlenecking the separator vessel. The upfront work included a scanning of the vessel using neutron back scattering. The scanning of the vessel identified a thick dispersion layer in the vessel. The thickness of this dispersion layer was varying considerably for the different scans. The varying thickness of the dispersion layer in the vessel is linked to the varying flow rates into the vessel due to the slugging nature of the inflow. The liquid slugs will result in large variations of residence time for the fluid inside the vessel during the slug cycles. The option chosen based on the initial investigation by BP were to upgrade the separator internals.

The debottle necking project identified that the vessel would handle stable flow conditions, however the large changes in the inflow of the liquid makes unseparated liquid spill over to the oil side of the vessel. To achieve acceptable separation performance the slugging inflow had to be addressed. The main philosophy is containing the dispersion layer upstream the weir during the liquid slug and then allow the dispersion to break during the gas surge following the liquid slug. The retrofit of the vessel included new inlet section, perforated plates covering the full cross section of the vessel, an increased weir height and new level settings.

The retrofit of the internals allowed the vessel to be operated with higher oil volume rates with drastically improved separation quality. The amount of dry oil produced from the Machar subsea field increased from 14 200 bbl/d before the modification to 15 800 bbl/d after the modification. The increase in oil production has paid back the cost of the retrofit in a matter of days. The amount of water carried over to the second stage separator has been reduced from 15 vol% before the modification to 3 vol% after the modification and is no longer the limiting factor for production.

One of the lessons learned from the trouble shooting and retrofit of this vessel is that even though compact slug catching is an attractive technology to many processes it may create problems for the phase separation. Compact slug catching is here understood as vessels using regulation systems with rapid response to be able to handle slugs. The rapid response of the liquid outlet valves assures that the levels in the vessel can be controlled with a reduced liquid hold up volume. However a compact vessel with rapid response will not efficiently smooth out the liquid flow. The flow variations for the liquid can only be dampened using a holding volume and a slow response to the level changes and accept level changes in the vessel. If a rapid response system is used the separation equipment on the liquid side of the vessel will have to handle the variation of liquid flow.

For the BP Machar slug catcher the liquid volume is sufficiently large to stabilize the flow variation however the separator vessel were not designed to handle the large flow variations with respect to the separation performance prior to the modifications of the vessel.