Slide 1 of 56

1
Subsea Mudlift Drilling
Basic Technology
by Julie Zhang and Hans Juvkam-Wold
November 2000
Lesson 4
U-tubing Concepts
2
Contents

• 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

3
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

Drillstring
Riser
Mudline
Annulus
4
Subsea Mudlift Drilling
Thus, the well can be treated as a U-tube
5
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
6
U-tube in DGD
In DGD, the pressure at the inlet to the subsea
mudlift pump is maintained at seawater
hydrostatic pressure

• Thus, pressure inside drillstring and outside
  drillstring are not statically balanced

7
Static Pressure - DGD
Annulus and Return Line
Drillstring pressure
Balance
Mud level
Depth
Seawater Hydraustatic Pressure
Pressure
8
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
Mudline
Annulus
9
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

Seawater Hydraustatic
balance
Balance
Mudline
Mud level
Annulus
Pressure
10
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.

Mud level
Pressure
11
Base Case Assumptions for Simulator Runs(made
with Dr. Choes 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)

Balance
12
U-tubing Rate in DGD
Balance
Mud level
Pressure
13
Factors Affecting U-Tube Rate in DGD

• Water Depth
• Mud Weight
• Mud Viscosity
• Drillpipe ID
• Nozzle Sizes
• Well Depth Below Mud Line

14
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

15
U-Tubing Rate vs. Time for Various Water Depths
16
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

17
Mud Level in Drillpipe vs. Time for Various Mud
Densities
18
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

19
U-Tubing Rate vs. Time for Various Mud Viscosities
600/300 RPM
20
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

21
U-Tubing Rate vs. Time for Various Drillpipe IDs
22
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

23
U-Tubing Rate vs. Time for Various Bit Nozzle
Sizes
24
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

25
U-Tubing Rate vs. Time for Various Wellbore Depths
26
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.

27
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.

28
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.

29
Hookload vs. Mud Level at 1,500 ft Water Depth
Estimated Hookload considering only the mud level
drop
30
Hookload vs. Mud Level at 10,000 ft Water Depth
31
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.

32
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.

33
Some Examples

• 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

34
The 5-Minute Connection

• Circulating Prior to Making the Connection
• Shut Down Surface Pump
• U-Tubing in Progress
• Connection is Complete
• Restart Circulation

35
Circulating Prior to Making a Connection
36
Circulating Prior to Making a Connection
Elapsed Time 0 sec
Fluid Level in DP 0 ft
The 5-min Connection Circulating at 700
gpm-both pumps
Standpipe Pressure 5,517 psi
37
Shut Down Surface Pump
Elapsed Time 30 sec
Fluid Level in DP 237 ft
Pit Gain 4.10 bbls
The 5-min Connection MLP Circulating at 392
gpm
SP Pressure 0 psi
MLP P_out 8,237 psi
38
U-Tubing in Progress (1 min)
Elapsed Time 60 sec
Fluid Level in DP 505 ft
Pit Gain 8.83 bbls
The 5-min Connection MLP Circulating at 378
gpm
MLP P_out 8,227 psi
39
U-Tubing in Progress (1 1/2 min)
Elapsed Time 90 sec
Fluid Level in DP 755 ft
Pit Gain 12.9 bbls
The 5-min Connection MLP Circulating at 365
gpm
MLP P_out 8,218 psi
40
U-Tubing in Progress (2 min)
Elapsed Time 120 sec
Fluid Level in DP 1,005 ft
Pit Gain 17.3 bbls
The 5-min Connection MLP Circulating at 351
gpm
MLP P_out 8,208 psi
41
Connection Complete - Start Rig Pump 500 gpm
(elapsed time 5 min)
Elapsed Time 5 min
Fluid Level in DP 2,295 ft
Pit Gain 40.0 bbls
The 5-min Connection Pig Pump at 500 gpm MLP
at 268 gpm
SP Pressure 0 psi
MLP P_out 8,157 psi
42
Drill Pipe is Full - Rig Pump _at_ 500 gpm
(elapsed time 16 min)
Elapsed Time 16 min
Fluid Level in DP 2 ft
Pit Gain 0 bbls
The 5-min Connection Pig Pump at 500 gpm MLP
at 404 gpm
SP Pressure 0 psi
MLP P_out 8,245 psi
43
Resume Drilling - Rig Pump _at_ 500 gpm (elapsed
time 17 min)
Elapsed Time 17 min
Fluid Level in DP 2 ft
Pit Gain 0 bbls
The 5-min Connection Pig Pump at 500 gpm MLP
at 500 gpm
SP Pressure 1,540 psi
MLP P_out 8,321 psi
44
The 5-Minute Connection
700 gpm
500 gpm
45
Shutdown Prior to Making Trip

• Circulating Prior to Making the Connection
• Shut Down Surface Pump
• U-Tubing in Progress
• U-Tubing is Complete
• Restart Circulation

46
U-Tubing in Progress (10 min)
Elapsed Time 10 min
Fluid Level in DP 3,699 ft
Pit Gain 64 bbls
Prepare for Trip MLP Circulating at 159 gpm
MLP P_out 8,104 psi
47
U-Tubing Complete (25 min)
Elapsed Time 25 min
Fluid Level in DP 4,292 ft
Pit Gain 73 bbls
Ready for Trip MLP Circulating at 0 gpm
MLP P_out 8,060 psi
48
Balance
Mud level
Pressure
49
U-Tubing Associated with Trip
Stop Rig Pump at 0 min. Re-Start at 30 min
700 gpm
500 gpm
50
Pit Gain Associated with Trip
Stop Rig Pump at 0 min. Start at 30 min
500 gpm
700 gpm
51
MLP Rate Associated with Trip
Stop Rig Pump at 0 min. Start at 30 min
Rig Pump _at_ 700 gpm
_at_ 500 gpm
52
Comparison
Rig Pump Down Time (min) 2 2 5 5 30 30
Rig Pump Rate (gal/min) 700 500 700 500 700 500

Max. Fluid Level Drop (ft) 1,005 1,005 2,295 2,2
95 4,292 4,292
Total Time (min) 4.5 8.3 10.0 16.3 38 46
53
Fluid Level in DP vs. Time
Effect of Rig Pump Down Time
Rig Pump Rate at 500 gpm
0
2 min
1,000
5 min Connection
2,000
Fluid Level in Drill
Pipe, ft
3,000
30 min Connection
4,000
5,000
0
10
20
30
40
50
Elapsed Time, min
54
What about Shallower Water Connections?

• U-tubing depth is reduced
• Pit gain is reduced
• Time to reach equilibrium is reduced
• Time to fill drill pipe is reduced

55
U-Tubing in DGD
FLOATER
SEAWATER HYDROSTATIC PRESSURE
10,000
STATIC FLUID LEVEL
MUDLIFT
BOP
20,000
56
Subsea Mudlift Drilling
Basic Technology
by Julie Zhang and Hans Juvkam-Wold
November 2000
The End
4. U-tubing Concepts