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4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 1 of 56
by Julie Zhang and Hans Juvkam-Wold
November 2000
Lesson 4
U-tubing Concepts
Subsea Mudlift DrillingBasic Technology
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 2 of 56
• U-Tube Concept in Conventional Drilling • U-Tubing in DGD• Equilibrium After U-Tubing• Factors Affecting U-Tubing rate• Effect of Water Depth, Mud Weight, Mud
Viscosity, Pipe Size, Nozzle Size and Well Depth Below Mud Line
• Hookload during U-tubing• Special considerations• Examples of U-Tubing
Contents
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 3 of 56
U-tube in Conventional Drilling
• Drilling, under static conditions, with uniform mud weight throughout, the hydrostatic head in the drillstring and annulus are equal.
• The U-Tube is therefore balanced.
• However, there are exceptions. Such as: – At beginning of increasing
mud weight.– Cementing– Lots of cuttings in annulus– Gas in annulus
Riser
Annulus
Mudline
Drillstring
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 4 of 56
Subsea Mudlift Drilling
BOP
Thus, the well can be treated as a U-tube
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 5 of 56
When does free-fall (U-tubing) occur?
• During pipe connection
• Pipe trips in or out of the hole
• Wiper trip
• Logging/Gyro
NOTE: U-tubing will occur only when there is no drillstring valve (DSV) in the drillstring
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 6 of 56
U-tube in DGD
Sea water Hydraustatic
Sea
water
Thus, pressure inside drillstring and outside drillstring are not statically balanced
In DGD, the pressure at the inlet to the subsea mudlift pump is maintained ~ at seawater hydrostatic pressure
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 7 of 56
Static Pressure - DGD
Pressure
Dep
th
Annulus and Return Line
Drillstring pressure
Mud level
Balance
Seawater Hydraustatic
Pressure
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 8 of 56
U-tube in DGD
• When the rig pump is shut down, Free-Fall or U-Tubing occurs in DGD . . .
• because the pressure at the inlet to the subsea mudlift pump is maintained ~ at seawater hydrostatic pressure
Drillstring
Annulus
Mudline
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 9 of 56
Pressure
Mud level
Balance
Annulus
Mudline
Seawater
Hydraustaticbalance
Final Equilibrium Condition in DGD
The final mud level depends on two factors:
1. The relative densities of mud and sea water
2. Water depth
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 10 of 56
Pressure
Mud level
Final Equilibrium Condition in DGD
Below the mudline the pressures are balanced inside and outside the drillstring.Above the mudline the height of mud inside the drillstring is essentially balanced against a column of seawater.
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 11 of 56
Balance
Base Case Assumptions for Simulator Runs(made with Dr. Choe’s DGD Simulator)
Water Depth 10,000 ft (Range 4,000 - 10,000)
Mud Weight 15.5 lb/gal (Range 11-18)
Mud Viscosity 111/65 (Range 2/1 - 400/300)
Drillpipe ID 4.762 in (Range 2 - 6)
Nozzle Sizes 16/32 in (Range 12/32 -
24/32)
Well Depth 20,000 ft (12,000 - 20,000)
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 12 of 56
Pressure
Mud level
Balance
0
100
200
300
400
500
600
0 5 10 15 20 25
Time (min)
Flo
w R
ate
(g
al/
min
)
Initial Circulation Rate - before rig pump shutdown
Decreasing Flow Rate - as the fluid level drops - and the driving head decreases
Flow changes from turbulent to laminar
Approaching Static Fluid Level
Dynamic effects as the fluid rapidly slows down
U-tubing Rate in DGD
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 13 of 56
Factors Affecting U-Tube Rate in DGD
• Water Depth
• Mud Weight
• Mud Viscosity
• Drillpipe ID
• Nozzle Sizes
• Well Depth Below Mud Line
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 14 of 56
Effect of Water Depth on Free-Fall Rate
Water depth is one of the driving forces
behind the u-tube.
As water depth increases:
• The u-tubing rate increases
• The final fluid level in the drillpipe is
lower
• The time to reach equilibrium increases
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 15 of 56
U-Tubing Rate vs. Time for
Various Water Depths
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 16 of 56
Effect of Mud Weight on U-Tubing Rate
Mud Weight is the other driving force
behind the u-tube.
As mud weight increases:
• The u-tubing rate increases
• The final fluid level in the drillpipe is lower
• The time to reach equilibrium increases
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 17 of 56
Mud Level in Drillpipe vs. Time
for Various Mud Densities
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 18 of 56
Effect of Mud Viscosity on U-Tubing Rate
Mud Viscosity is one of the factors that
influences the u-tubing rate.
As mud viscosity increases:
• The u-tubing rate decreases
• The time to reach equilibrium increases
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 19 of 56
Flow Rate vs. Time for Various Viscosity
0
100
200
300
400
500
600
0 10 20 30 40 50 60
Time (min)
Flo
w R
ate
(g
al/
min
)
2/1
111/65
300/200
400/300
U-Tubing Rate vs. Time for Various Mud Viscosities
600/300 RPM
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 20 of 56
Effect of Drillpipe ID on U-Tubing Rate
Drillpipe ID is another factor that controls the u-
tubing rate.
As the drillpipe ID increases:
• The u-tubing rate increases (less friction)
• We might expect that the time to reach
equilibrium would increase (more mud to
drain), but this is not necessarily so - see
graphs
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 21 of 56
U-Tubing Rate vs. Time for
Various Drillpipe IDs
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 22 of 56
Effect of Nozzle Sizes on U-Tubing Rate
Nozzle Size is another factor that controls the u-
tubing rate.
As the nozzle size increases (3 nozzles
assumed):
• The u-tubing rate increases
• The time to reach equilibrium decreases
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 23 of 56
Flow Rate vs. Time for Various Nozzle Sizes
0
100
200
300
400
500
600
0 5 10 15 20 25 30
Time (min)
Flo
w R
ate
(gal
/min
)
12/32 in
14/32 in
16/32 in
18/32 in
20/32 in
22/32 in
24/32 in
U-Tubing Rate vs. Time for Various Bit Nozzle Sizes
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 24 of 56
Effect of Well Depth BML on U-Tubing Rate
Well Depth BML is yet another factor that affects
the u-tubing rate.
As the depth below mudline increases:
• The u-tubing rate decreases (more friction)
• The time to reach equilibrium increases
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 25 of 56
Flow Rate vs. Time for Various Wellbore Depths
0
100
200
300
400
500
600
0 5 10 15 20 25
Time (min)
Flo
w R
ate
(gal
/min
)
12000 ft
13000 ft
14000 ft
15000 ft
16000 ft
17000 ft
18000 ft
19000 ft
20000 ft
U-Tubing Rate vs. Time for Various Wellbore Depths
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 26 of 56
Pit Gain during U-tubing
• During U-tubing, quantities of fluid will return to surface and may appear to be pit gain (kick!).
• For instants, in 10,000 ft water depth, with 18.5 ppg mud, 6.5 “ DP. Surface pit gain is expected to be 180 bbls at the end of U-tubing process.
• In 1500 ft water depth, with 14 ppg mud, 5” DP, 11 bbls pit gain are expected.
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 27 of 56
Hookload during U-tubing
• During U-tubing, Hookload will decrease mainly due to loss of mud inside the drillstring.
• For the 10,000 ft water depth case, a 120,000 lbf reduction in hookload will be expected at the end of U-tubing.
• While for the 1,500 ft water depth case, a 2,800 lbf reduction in hookload is expected.
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 28 of 56
Hookload during U-tubing Cont.
• In a deep-water case, hookload can be used to predict fluid level by simply subtracting the weight of mud u-tubed from the hookload.
• In a shallow water case, this method can not be used to predict fluid level.
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 29 of 56
Hookload vs. Mud Level at 1500 ft water depth
0
100
200
300
400
215,000 217,500 220,000 222,500 225,000
Hookload, lbf
Mu
d l
evel
, ft Actual Hookload
Estimated Hookload only consider mud level drops
Estimated Hookload considering only the
mud level drop
Hookload vs. Mud Level at 1,500 ft Water Depth
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 30 of 56
Hookload vs. Mud Level at 10,000 ft water depth
0
1000
2000
3000
4000
5000
6000
600,000 650,000 700,000 750,000 800,000 850,000
Hookload lbf
Mu
d l
evel
, ft
Estimated Hookload only consider mud level
Actual Hookload
Hookload vs. Mud Level at 10,000 ft Water Depth
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 31 of 56
Special Considerations
• DSV should be installed as often as possible. However, U-tubing still could occur in cases, such as when the DSV does not work properly.
• After U-tubing, the Drillstring must be filled up with mud before full circulation can be resumed.
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 32 of 56
Special Considerations - Cont.
• Restriction pipe can be added in the drillstring to allows circulation at lower rates with full drillstring.
As length of restriction pipe increases, frictional
pressure increases. U-tubing rate decreases.
As ID of restriction pipe decreases, frictional pressure
increases. U-tubing rate decreases.
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 33 of 56
U-Tubing while...
• Making a 5-minute Connection
• Making a 30-minute Connection
• Shutting Down Before Making a Trip
• Re-Starting After Making a Trip
Some Examples
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 34 of 56
• Circulating Prior to Making the Connection
• Shut Down Surface Pump
• U-Tubing in Progress
• Connection is Complete
• Restart Circulation
The 5-Minute Connection
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 35 of 56
Circulating Prior to Making a Connection
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 36 of 56
Circulating Prior to Making a Connection
Elapsed Time = 0 sec
Standpipe Pressure = 5,517 psi
The 5-min Connection
Circulating at 700 gpm-both pumps
Fluid Level in DP = 0 ft
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 37 of 56
Shut Down Surface Pump
Elapsed Time = 30 sec
SP Pressure = 0 psi
The 5-min Connection
MLP Circulating at 392 gpm
Fluid Level in DP = 237 ft
MLP P_out = 8,237 psi
Pit Gain = 4.10 bbls
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 38 of 56
U-Tubing in Progress (1 min)
Elapsed Time = 60 sec
Pit Gain = 8.83 bbls
The 5-min Connection
MLP Circulating at 378 gpm
Fluid Level in DP = 505 ft
MLP P_out = 8,227 psi
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 39 of 56
U-Tubing in Progress (1 1/2 min)
Elapsed Time = 90 sec
Pit Gain = 12.9 bbls
The 5-min Connection
MLP Circulating at 365 gpm
Fluid Level in DP = 755 ft
MLP P_out = 8,218 psi
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 40 of 56
U-Tubing in Progress (2 min)
Elapsed Time = 120 sec
Pit Gain = 17.3 bbls
The 5-min Connection
MLP Circulating at 351 gpm
Fluid Level in DP = 1,005 ft
MLP P_out = 8,208 psi
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 41 of 56
Connection Complete - Start Rig Pump
500 gpm (elapsed time = 5 min)Elapsed Time = 5 min
Pit Gain = 40.0 bbls
The 5-min Connection
Pig Pump at 500 gpm MLP at 268 gpm
Fluid Level in DP = 2,295 ft
MLP P_out = 8,157 psi
SP Pressure = 0 psi
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 42 of 56
Drill Pipe is ~Full - Rig Pump @ 500 gpm
(elapsed time = 16 min)Elapsed Time = 16 min
Pit Gain = 0 bbls
The 5-min Connection
Pig Pump at 500 gpm MLP at 404 gpm
Fluid Level in DP = 2 ft
MLP P_out = 8,245 psi
SP Pressure = 0 psi
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 43 of 56
Resume Drilling - Rig Pump @ 500 gpm
(elapsed time = 17 min)Elapsed Time = 17 min
Pit Gain = 0 bbls
The 5-min Connection
Pig Pump at 500 gpm MLP at 500 gpm
Fluid Level in DP = 2 ft
MLP P_out = 8,321 psi
SP Pressure = 1,540 psi
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 44 of 56
Effect of Rig Pump Rate
0
500
1,000
1,500
2,000
2,500
0 2 4 6 8 10 12 14 16 18
Elapsed Time, min
Flu
id L
ev
el i
n D
rill
Pip
e,
ft
The 5-Minute Connection
700 gpm
500 gpm
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 45 of 56
• Circulating Prior to Making the Connection
• Shut Down Surface Pump
• U-Tubing in Progress
• U-Tubing is Complete
• Restart Circulation
Shutdown Prior to Making Trip
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 46 of 56
U-Tubing in Progress (10 min)
Elapsed Time = 10 min
Pit Gain = 64 bbls
Prepare for Trip
MLP Circulating at 159 gpm
Fluid Level in DP = 3,699 ft
MLP P_out = 8,104 psi
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 47 of 56
U-Tubing Complete (25 min)
Elapsed Time = 25 min
Pit Gain = 73 bbls
Ready for Trip
MLP Circulating at 0 gpm
Fluid Level in DP = 4,292 ft
MLP P_out = 8,060 psi
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 48 of 56
Pressure
Mud level
Balance
0
100
200
300
400
500
600
0 5 10 15 20 25
Time (min)
Flo
w R
ate
(g
al/
min
)
Initial Circulation Rate - before rig pump shutdown
Decreasing Flow Rate - as the fluid level drops - and the driving head decreases
Flow changes from turbulent to laminar
Approaching Static Fluid Level
Dynamic effects as the fluid rapidly slows down
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 49 of 56
Effect of Rig Pump Rate
0
1,000
2,000
3,000
4,000
5,000
0 10 20 30 40 50
Elapsed Time, min
Flu
id L
ev
el in
Dri
ll P
ipe
,
ft
U-Tubing Associated with Trip
700 gpm
500 gpm
Stop Rig Pump at 0 min. Re-Start at 30 min
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 50 of 56
Pit Gain During Connection
0
20
40
60
80
0 10 20 30 40 50
Elapsed Time, min
Pit
Ga
in,
bb
l
Pit Gain Associated with Trip
700 gpm
500 gpm
Stop Rig Pump at 0 min. Start at 30 min
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 51 of 56
Mudlift Pump Rate
0
200
400
600
800
0 10 20 30 40 50
Elapsed Time, min
ML
P R
ate
, g
pm
MLP Rate Associated with Trip
Rig Pump @ 700 gpm
@ 500 gpm
Stop Rig Pump at 0 min. Start at 30 min
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 52 of 56
Comparison
Rig PumpDown Time
(min)
2255
3030
Rig Pump Rate
(gal/min)
700500700500
700500
Max. Fluid Level Drop
(ft)
1,0051,0052,2952,295
4,2924,292
Total Time(min)
4.58.3
10.016.3
3846
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 53 of 56
Effect of Rig Pump Down Time
Rig Pump Rate at 500 gpm0
1,000
2,000
3,000
4,000
5,000
0 10 20 30 40 50
Elapsed Time, min
Flu
id L
evel
in D
rill
Pip
e, f
t
Fluid Level in DP vs. Time
2 min5 min
Connection
30 min Connection
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 54 of 56
• U-tubing depth is reduced
• Pit gain is reduced
• Time to reach equilibrium is reduced
• Time to fill drill pipe is reduced
What about Shallower Water Connections?
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 55 of 56
MUDLIFT
U-Tubing in DGD
~SEAWATER HYDROSTATIC
PRESSURE
BOP
STATIC FLUID LEVEL
FLOATER
10,000’
20,000’
4. U-tubing Concepts Confidential to SMD JIPConfidential to SMD JIP Slide 56 of 56
by Julie Zhang and Hans Juvkam-Wold
November 2000
The End
4. U-tubing Concepts
Subsea Mudlift DrillingBasic Technology