The first step in considering the required surge volume for transient increases can be made by considering the equilibrium profile.
25
Figure 7.6 Amethyst pipeline holdup profile
Figure 7.6 shows the equilibrium holdup for the Amethyst pipeline in the Southern North Sea.
The line is 30” diameter and connects two unmanned production platforms with the Easington gas processing terminal 30 miles away. The working volume of the slug catcher is 3800 bbls, with a maximum liquids rate of 6000 The liquid loading is typically 6.5
of condensate and 0.54 of water and methanol. It can be seen that for normal gas flowrates in the region of mmscfd the equilibrium liquid holdup is well in excess of the slugcatcher capacity, hence pigging at equilibrium conditions could overfill the process facilities. Higher gas rates than those available would be required to give equilibrium holdups below the surge volume. In fact this pipeline was designed to be pigged daily, and hence the liquid expected to be received was the order of 1760 bbls at 250 This design allows for a missed pig, ie two days between sphere launches, without causing liquid handling problems.
The holdup profile shows a rapid rise in the liquid content at gas rates below 200 mmscfd, hence operating in this region could cause problems even without pigging, as relatively small gas rate perturbations can cause large quantities of liquid to be swept from the pipeline. For example a 10% increase in the gas rate from 200 to 220 mmscfd could remove 4750 bbls of liquid and, possibly, swamp the slug catcher.
Shortly after the start-up of the Amethyst pipeline system the pig launcher failed and it was decided to investigate the consequences of continuing production, and allowing the pipeline liq-uids to build up to equilibrium values. A decision was taken to limit the minimum gas to 250 mmscfd, and hence to avoid possible uncontrolled sweep-out due to inherent
Section 7. Transient Flow BP Manual
fluctuations. The pipeline was operated without pigging over the winter, where it took several months to obtain an equilibrium holdup, which was estimated to be in the region of 18000 bbls from a mass balance. This is within 17% of the value predicted by the old segregated flow mechanistic model in MULTIFLO, but is 213% higher than the value predicted by the Eaton cor-relation. Some of the simple transient analysis outlined below was used to investigate how to resume pigging operations.
A simple approach to sizing a vessel to handle the liquid produced by a gas rate increase would be to consider the change in the equilibrium holdup and ignore the effect of the liquid pump-out rate. For example if the gas rate were increased from 250 mmscfd to 350 mmscfd the equilibri-um liquid removed would be 15373 8018 = 7355 bbls. Hence the gas rate could be increased in two steps from 250 mmscfd to 300 mmscfd which would remove 3800 bbls and then 300 mmscfd to 350 mmscfd which would remove 3555 bbls. A 7355 bbl surge volume would be required if the increase were made in one step without taking account of the liquid pump-out rate. The maximum gas available is 350 mmscfd which gives a pipeline inventory of 8018 bbls, and hence it is still necessary to ‘walk in’ the first pig.
We will use the Amethyst case to illustrate a simple way of determining the effect of the out rate on the required surge volume. Consider the case of an increase in the gas rate from 250 to 350 mmscfd.
At 250 mmscfd the equilibrium holdup is 15373 bbls At 350 mmscfd the equilibrium holdup is 8018 bbls Hence the difference in holdup is 15373 8018 7355 bbls.
Next calculate the initial and final liquid production rates from:
Liquid rate liquid loading x gas
Initial liquid 250 mmscfd x 7.04 1760 Final liquid = 350 mmscfd x 7.04 = 2464
Calculate the duration of the transition time for the transient, which is the length of time over which the high occurs. If it is assumed that all the liquid in the line accelerates to the equilibrium liquid velocity corresponding to the final gas rate, then the transition time is the same as the residence time at the final rate, ie:
Transition time (final holdup final flowrate) = (8018 2464) 3.25 days The transition is the sum of the final and the increase due to the rate change and is given by:
Transition = Final + (holdup change /transition time)
2464 + = 4727
It is seen that based on this method the surge can easily be handled by using the 6000 pump-out capacity of the Easington terminal. Figure 7.7 shows the predicted liquid outflow profile.
Figure 7.7. Simplified liquid outflow profile during ramp-up
Liquid
This approach can be extended to investigate the trade-off between the required slug catcher surge volume and the pump-out rate using the relation below:
Surge volume transition time x (flowrate in out)
= Tt x
If the pump-out rate is fixed at the final equilibrium value the required surge volume is:
3.25 ( 4727 2464 ) = 7355 bbls ie. the change in equilibrium holdup.
If the pump-out rate is 4727 then this method shows that no surge volume is required.
The solution to the equation is the linear relationship shown in Figure 7.8. It can be seen that for a surge volume of 3800 bbls a minimum pump-out rate of 3550 is required.
The relationship shown in Figure 7.8 can be subject to large inaccuracies at the extremes as the tends to zero and at the final transition flowrate, the reason being as follows; at zero pump-out rate the surge volume is equal to the transition multiplied by the transi-tion time, whereas in practice liquid continues to flow into the vessel at the final equilibrium flowrate, hence the required surge volume becomes infinite as the pump-out rate goes to zero.
When the pump-out rate is equal to the transition the above method indicates that no surge volume is required. However in practice the during the transient is not usually constant, and typically peaks at the start. Hence the solution for a surge volume of zero is a
rate equal to the peak during the transient.
Section 7. Transient Flow Multiphase Design Manual
Surge volume (BBLS)
16000
8000
6000
0 I I I I I I I I
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Liquid pump-out rate
Figure 7.8 Surge volume as a function of processing rate
The same simple method can also be applied to the estimation of the outlet profile dur-ing a decrease, as follows: Consider a reduction in the gas of the Amethyst pipeline from 350 mmscfd to 250 mmscfd.
First calculate the residence time at the final flowrate, this is also assumed to be the transition time:
Transition time (final holdup final flowrate) (15373 1760) 8.73 days
then:
Transition Final (Holdup change Transition time)
= 1760 (7355 8.73) = 918
Hence the hand calculation method predicts an initial fiowrate of 2464 falling to 918 over a 8.73 day transition period after which the rate increases to 1760 This is illustrated in Figure 7.9.
Example 7.1 illustrates a simulation of the shutdown and restart of Pompano wells and Example 7.2 gives a comparison with the simple hand calculation method.
Liquid outflow
rate (BBUD)
2500
2000
1500-
1000-500
-4 -2 0 2 4 6 8 10 12
Time (Days)
Figure 7.9 Simplified liquid outflow profile during ramp-down
Section 7. Transient Flow Design
THIS PAGE IS INTENTIONALLY
LEFT BLANK
7.3.2 Use of transient computer codes for other boundary condition changes
There are other changes in the boundary conditions of a pipeline apart from changes, for example outlet pressure changes, which are often required to be simulated by transient analysis. With general transient simulation programs such as PLAC there is a wide variety of boundary conditions that can be changed. For example PLAC uses the FILL component to specify a boundary and the BREAK component to specify a pressure boundary. When running PLAC the user can specify signal variables which are used to vary the conditions at the inlet or outlet and to control the action of valves. One of the simplest ways of changing a boundary condition is to select time as the signal variable, and to the define the flowrates at the fill as a function of time. In this way rate changes can be simply entered as a table of values.
assumes a linear change of the variable with time. There are 25 signal variables available in PLAC at present, allowing a wide range of boundary condition changes such as pressure, temperature, flowrate. heat transfer and flow area. It is beyond the scope of this chapter to con-sider all combinations, and hence reader is directed to Example 7.3 as an illustration of a boundary condition problem.
Thermal transients such as pipeline cool down and warm up are important to the estimation of the amounts of inhibitor that is required to prevent hydrate formation during periods of zero or low flow. Here WELLTEMP can sometimes be used to screen cases for detailed analysis by more sophisticated transient two-phase flow codes.
Pipeline pack and draw is also often to investigate the survival time of a given system and the ability to meet the gas sales contract.
Section 7. Transient Flow BP Design Manual
THIS PAGE IS INTENTIONALLY
LEFT BLANK
Section 7. Transient Flow