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i am associated with a design/engineering/construction company in india; the company recently received a requirment for verifying and completing the process design for an early production system in middle-east; our client is the equipment-package supplier who won the bid based on internal estimation and the inputs from the client at the time of bidding.
the original schematic given by the client at the time of bidding is rudimentary and part of it is reproduced below:(the scheme continues to a line heater, electrostatic dehydrator, storage and pumping)
The well-head flowing pressure and temperature is given as 200 barg and 82 deg C; production flow is 3000 bopd with max of 50% water-cut. The scope includes supply of desander (sand trap) to remove production sand from 10000 ppm (w/v) to 10 ppm; the original schematic indicated removal of gas in a three phase separator downstream of the sand-trap (the design GOR is 12000 scf/bbl, leading to ~35 mmscfd gas flow).
The separator was quoted as 150# vessel and is shown with a conventional pressure control valve on the vapour side.
When i received the schematic and the orignal quote of equipment size and rating, i raised three questions:
1) once well-engineers decide on a fixed choke and production flow quantum,, the down-stream pressure is decided on the choke size, GOR, oil/water properties ; down-stream equipment cannot have a back-pressure control as shown, instead well-engineers need to give the arrival pressure and temperature after the choke.
2) both sand-trap and separator need to be designed and operated at a higher pressure-rating; in order to meet my client's low pressure quote, i suggested a HCV at the inlet to kill the excess pressure and link its operation to the inlet pressure of the three-phase separator.
3) the original quote had a sand-trap quoted by a US-based equipment supplier having two numbers of 18"/8' vertical sand-traps. I told them the size is quite small for the flow of gas as well as the amount of sand indicated.
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while i was trying the convince the client to include an HCV ( i was also unhappy to see a control valve in a sand-laden production stream) without which there is no meaningful pressure control in the loop, the end-client got a wave from my observation of undersized de-sander.
Based on an existing operation unit eleswhere, they suggested removal of vapours from the desander. the vapours are removed under forward-pressure control instead of conventional back-pressure control; The liquids are moved out through overflow trough/siphon to avoid the level control. Effectively, it means separation of vapours in the desander and the liquids (oil+water) will flow to the three-phase separator. Though the idea has a merit of removal of vapours in the first equipment itself, it failed to answer how the liquids are regulated.
End-client well-engineers also understood the pressure-drop issue and said they would go for adjustible choke which will result in a lower pressure at the inlet.
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my questions to the forum:
1) Since the client has said they have an operational unit where they have a unit with D/s pressure control on the vapour side, i am trying the visualize the schematic; i believe the three-phase separator wier has to be in level with the overflow trough of the desander. Any thoughts from others?
2) i still dont think a simple adjustible choke is solution to the original issue of varying the input pressure to the unit. Though logically a fixed choke followed by a HCV is theoretically equivalent to a variable choke, the functions of the choke and the HCV cannot be combined into a single adjustible choke, A choke is decided to maintain a production flow quantum. once the flow is established the HCV kills some pressure reducing the inlet pressure to the early production system. But an adjustible choke may not maintain the flow and pressure simultaneously. So which is the better option?
3) my final design is a vertical sand separator (three vessels in parallel, each of 30" dia, 12' tall) with gas outlets combined and provided with a down-stream pressure control to supply gas at ~ 6 barg to the fuel system. The vessels are rated to meet the maximum flow line pressure (900#). A feed regulating HCV is provided on the liquids outlet before the separator inlet. The separator is provided with conventional pressure and level controls. the vessel is rated 150#
I shall include the proposed schematic in the next post; meanwhile, i would request other forum members to review my post, and my thought-process in the development of the new schematic
regards
neelakantan
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original schematic.jpeg 61.8KB
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