Overview
Five condensate wells are to produce into a subsea manifold, through a subsea tieback and up a riser to a platform. The oil and gas are then to be separated, with the oil pumped to shore and the gas compressed to shore. The expected production rate is 14,000 STBD and the system will be designed to accommodate between 8,000 STBD (turndown case) and 16,000 STBD should the wells produce more than expected. The engineer is asked to perform the following tasks:
1) Develop a compositional model of the hydrocarbon phases 2) Size the subsea tieback line and riser
3) Screen the for severe slugging at riser base 4) Determine the pipeline insulation requirement 5) Size a slug catcher
Exercise 1:
Develop the compositional PVT model based on the following data:Pure Hydrocarbon Components
Name Boiling Point
(°F) Molecular
Weight Specific
Gravity Moles
C7+ 214 115 0.683 12
Aqueous Component
Component Volume ratio
(%bbl/bbl)
Water 10
Method:
1) Use the <setup/compositional...> menu to enter the pure components given at the end of the case study. Select the pure hydrocarbon components from the component database. Multiple selection is possible by holding down the control key. When all pure hydrocarbon components have been selected, press the "Add>>" button.
2) Select the "Petroleum Fractions" tab and characterise the petroleum fraction "C7+" by entering the petroleum fraction name, the BP, MW, and SG in row 1. Highlight the row by pressing on the row “1” button and then press the "Add to composition>>" button.
3) Return to the "Component Selection" tab and enter the number of moles for C7+.
4) Generate the hydrocarbon phase envelope by pressing the "Phase Envelope" button.
Schlumberger
Exercise 2:
Size Subsea Tieback
Determine the required ID for the subsea tieback such that the separator pressure for the maximum expected rate is no less than 400 psia. The riser must be the same ID as the tieback. In addition, ensure that the errosional velocity is not exceeded. First, build the physical model as shown below with the following data:
Manifold
Outlet pressure 1500 psia
Temperature 176 ºF
Subsea tieback
Rate of undulations 0'/1000' (not hilly)
Horizontal Distance 6 miles
Elevational difference 0' (horizontal)
Available ID's 9,10,11 "
Heat Transfer:
Ambient temperature 38 º F
Pipe thermal conductivity 35 Btu/hr/ft/°F
Insulation thermal conductivity 0.15 Btu/hr/ft/°F Insulation thicknesses available 0.75" + 0.25"
increments
Horizontal Distance 0' (vertical pipe) Elevational difference 1600'
Insulation thermal conductivity 0.15 Btu/hr/ft/°F Insulation thicknesses available 0.75" + 0.25"
increments
Ambient fluid water
Ambient fluid velocity 1.5 ft/sec
Method:
1) Perform a System Analysis with the minimum, maximum and expected flow rates as the x-axis variable, and the available ID’s for the flowline and riser as Change in Step sensitivity
variables.
2) Determine the minimum flowline ID that satisfies the separator pressure requirement for the maximum flow rate.
3) Change the y-axis to display Errosional Velocity Ratio and check to ensure that the selected flowline ID does not exceed an errosional velocity ratio of 1.0.
Schlumberger
Result Pipeline and Riser ID:
Max. errosional velocity ratio for selected ID Min. Separator pressure for
selected ID Max. separator pressure for
selected ID
Exercise 3: Check for Severe Slugging
Based on the ID selected above, determine the likelihood of severe slugging occurring at the riser base. Severe riser slugging is likely in a pipeline system followed by a riser under the following conditions:
1. The presence a long slightly downward inclined pipeline prior to the riser.
2. Fluid flowing in the "stratified" or "segregated" flow regime (as opposed to the usual "slug" or
"intermittent" flow regime).
3. A slug number (PI-SS) of lower than 1.0.
Method:
1) Configure the y-axis of the System Analysis plot to display the PI-SS number. This represents the maximum value of the PI-SS number along the flowline.
2) View the Summary Report (Reports -> Summary File), to determine the prevalent flow regime at the riser base for the different rates.
Result 8000 STBD 14000 STBD 16000 STBD
PI-SS number at riser base
Flow pattern at riser base
Exercise 4: Select tieback insulation thickness
Using the tieback/riser ID selected above, determine the thickness of insulation required for both the flowline and riser such that the temperature of the fluid does not come within 10ºF of the Hydrate curve for all possible flow rates.
Method:
1) Start with an insulation thickness of 0.75”. Ensure that “phase envelope” is checked in the Report Tool (located upstream of separator) and perform a pressure-temperature profile with Separator (outlet) pressure as the calculated variable and with flowrates as the sensitivity variables.
2) Use the Series menu on the resulting plot to change the x-axis to Temperature.
3) Observe the production path on the phase envelope and its proximity to the Hydrate curve.
4) If required, perform successive runs while increasing the thickness by 0.25” each time until sufficient.
Result
Req. Insulation thickness
Exercise 5: Size Slug Catcher
Determine the required size of the slug catcher based on the largest of following criteria multiplied by a safety factor of 1.2.
1. The requirement to handle the largest slugs envisaged (chosen to be statistically the 1/1000 population slug size).
2. The requirement to handle liquid swept in front of a pig.
Method:
1.) Ensure that “slugging values” and “sphere generated liquid volume” are selected in the report tool.
2.) Under Setup -> Define Output, select 3 cases to print
3.) Re-run pressure-temperature profile open output report. This report provides the full output of each sensitivity with Report Tool selections appended to the bottom of each sensitivity output.
For each sensitivity, scroll down to this section and read the reported “1/1000 slug volume”
and “Total Sphere Generated Liquid Volume So Far”.
Result 8000
Notes on SGLV Calculation:
When a sphere is introduced into the line, it will gather in front of itself a liquid slug made from "all the liquid that is flowing slower than the mean fluid flowrate in the pipeline at any given point". Thus the crucial value that determines Sphere Generated Liquid Volume (SGLV) is the Slip Ratio(SR), which is the average speed of the fluid divided by the speed of the liquid. If the liquid and gas move at the same speed, the slip ratio will be 1, i.e. there is 'no slip' between the phases. In this situation the sphere will not collect any liquid, so the SGLV will be zero. Normally the liquidflows slower than the gas, i.e.. the slip ratio is greater than 1, so "some" of the liquid in the pipeline will collect in front of the sphere to form the SGLV. The only way that "all" of the liquid in the pipeline will collect to form the SGLV, is if the liquid velocity is zero, i.e.. the slip ratio is infinite. This cannot happen in a steady-state reality, so the SGLV is always smaller than the total liquid holdup.