Modify DCS GAs Swing Logic

Discuss about the Modify DCS GAs Swing Logic.

The Distributed Control System (DCS) in our gas processing plant is out-of-date and requires a programming modification to be updated for two changes in our gas injection train:

The project is estimated to cost $29,385 and was not budgeted for 2017; the need for this project was only recently identified.

The gas injection swing logic serves the purpose of maintaining an appropriate injection pressure in the system. This logic resides within the DCS; when the Total Gas To Injection (TGTI) pressure reaches 19 MPa the logic sends a signal to the Remote Transmitter Unit (RTU) at each injection well which opens the control valve. This allows more gas to be injected into the wells, and relieves the system pressure back to a normal operating level (around 17.5MPa).  There are two reasons why this swing logic is needed:

• Medium-duration handling of excess gas injection volumes. This is to allocate sustained excess gas volumes properly and requires smooth control.  This occurs as production rates fluctuate from the field from activities like pipeline pigging and bringing new wells onto production.

Note that long-duration excess gas injection will be accommodated through injection target changes; it is not intended to be handled with the swing logic. As well, gas shortages will not be included in this scope of work; this situation will also be accommodated through target changes. 

The existing swing logic is designed to take the well’s existing injection target and multiply it by a factor of 1.75, to increase the valve position and open it to allow more gas. The main problem with this design is that there is less gas being sent to injection on a normal daily basis because it is being preferentially sent to sales instead. This results in many injection wells having a normal operating target of 0 e3m3/d. When the swing logic engages, the DCS will not send signals to the field RTUs to open the control valves for any wells that have an existing target of 0 e3m3/d, because 0 e3m3/d X 1.75 = 0 e3m3/d. This renders those wells ineffective when TGTI pressure reaches 19 MPa and it is likely that the TGTI system pressure will reach the set point of the high pressure shut down switch (22 MPa). This situation has happened twice within the last year.

There are two problems that result from compressor shutdowns:

1. Lost Revenue - The gas train of the processing plant will shut down. Excess gas is sent to flare (burned). Oil wells that have a high gas-to-oil ratio (GOR) must be shut in until the gas train is back online. Both situations directly lose revenue; during the last two compressor shutdown events lost revenue amounted to ~$150,000 each time.
2. Increased Cost - Reaching these high pressures often can result in damage to the compressor, which will result in unscheduled downtime and increased maintenance costs.

It is also important to note that high pressure shutdowns should only be utilized as a last resort. It is proper engineering design to ensure you don’t reach those shutdown levels as a part of normal operating procedure, to reduce the likelihood of failure of these protection devices.

Background

The remaining wells to be part of the gas injection swing logic are these “B” Pool injectors:

• 0/16-15-109-08W6/00
• 0/09-15-109-08W6/00
• 0/12-10-109-08W6/00
• 2/10-10-109-08W6/00
• 0/12-03-109-08W6/00
• 0/10-03-109-08W6/00
• 3/13-02-109-08W6/00
• 0/03-03-109-08W6/00

These wells have a combined maximum injection capacity of approximately 2240 e3m3/d. This number is based on current orifice plate sizes and trends of actual injection rates from 2014-2017.

Typical daily gas volumes during normal operating conditions in the plant are as follows:

• TGTI (treated gas to injection): 800e3m3/d
• TCPL Sales gas: 900-1400e3m3/d

This project proposes to upgrade the swing logic for the remaining “B” Pool gas injectors to open their control valves during a high-pressure swing situation even if their operating target is set to 0 e3m3/d. The following proposed control philosophy is based on ease of implementation; all controllers already reside in SCADA.

Control Philosophy

The proposed change for the new swing logic is to consider any control valves that currently have a target of 0 e3m3/d, and impose a set point change even if this condition exists. This will result in a consistent maximum swing tolerance volume.

B-Pool Chase Gas

The following 8 control valves will continue to be included in the B-Pool chase gas swing:

NLB 09-15 / 50-0915-FIC-62

NLB 16-15 / 50-0915-FIC-61

12-10 splitter - 12-10 / 50-1210-FIC-60

2/10-10 well - 2/10-10 / 10-FIC-573

SLB 3/13-02 / 54-FIC-93            

SLB 03-03 / 50-0703-FIC-61                  

SLB 12-03 / 54-FIC-61               

SLB 10-03 / 50-0703-FIC-60

Operator accessible ON (trip) and OFF (reset) pressures will continue to control the status of the B-Pool chase gas swing.  If the plant TGTI pressure is greater than the ON pressure for Level 1 (to be set at 19 MPa), the swing will be enabled and the B-Pool chase controllers will continue to be issued a set point of: Existing Set Point X % Flow Increase.  The % percent flow increase will be similar to the existing logic, at 75%.

If the plant TGTI pressure continues to increase and exceeds the ON pressure for Level 2, which will be set to 19.8 MPa, all controllers will receive a valve output command of 100%.

Once the plant TGTI pressure falls below the OFF pressure, all swing will be switched off and the B-Pool chase controllers will receive their original set points prior to the swing.  A minimum gap of 100 kPa is recommended between the ON and OFF pressures to allow smooth control.

Notes

1. The swing levels will be “cascaded” by using higher ON and OFF pressure set point for each subsequent swing.
2. Controllers in manual prior to a gas swing will not be affected by the swing (during or after).
3. Naming conventions will be consistent with the existing gas swing:
4. Pressure set points will be called “ON PRESSURE” and “OFF PRESSURE”
5. “ENABLED” will mean the logic is ready / standby, “DISABLED” will mean the logic is bypassed.
6. Swing status will be displayed as either “ON” or “OFF”

1. Continue to operate with existing swing logic, assuming there will be atleast two compressor high pressure shutdowns per year. This is a total of $300,000 in lost revenue, plus an increase in maintenance cost.
2. 0/15-03-109-08W6/00 and 0/01-03-109-08W6/00 are also gas injectors in “B” Pool, but currently do not have SCADA at site as there is no power available. They can only be controlled manually by the field operators. They are not part of the swing logic as there is no remote signal that can be sent from the DCS with this current design, therefore their injection capacity cannot be considered as part of the design. If added to the swing logic, they would provide an additional 480e3m3/d of capacity to the gas injection swing logic. This number is based on the current orifice plate size and well capability. The scoping for adding SCADA to these sites is not a part of the design of this project, but must be considered if sales gas targets are expected to increase past 1400e3m3/d in the future.

This project will reduce lost revenues by ~$300,000/year for a total spend of $29,385. This work will pay out in approximately 36 days, which is well under the Capital expenditure hurdle rate of 6 months.

Required internal resources include the DCS Maintenance Rep, Plant Operations, and Engineering. External programming resources are also required to complete this project; the selected vendor is Autopro. 

1. Project Engineer will write an upgraded Control Narrative.

A Capital AFE will be raised for this project.  Estimated costs are as follows (+/- 10%):

Engineering Manager                    ________________________________

Plant Superintendent                    ________________________________

Field Superintendent                    ________________________________

Maintenance Superintendent        ________________________

Grammatical errors, which I found, which include verb use problems and punctuation error.

I did not understand this part regarding the specification of the target that can range from the source of injection to the sink for injection.

The alternative section of the report is very neatly said and all the portions of lost revenues are very persuasive to apply for the new proposed project.