by Julie Zhang and Hans Juvkam-Wold

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


• 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


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


Subsea Mudlift Drilling

BOP

Thus, the well can be treated as a U-tube


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


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


Static Pressure - DGD

Pressure

Dep

th

Annulus and Return Line

Drillstring pressure

Mud level

Balance

Seawater Hydraustatic

Pressure


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


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


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.


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)


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


Factors Affecting U-Tube Rate in DGD

• Water Depth

• Mud Weight

• Mud Viscosity

• Drillpipe ID

• Nozzle Sizes

• Well Depth Below Mud Line


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


U-Tubing Rate vs. Time for

Various Water Depths


Effect of Mud Weight on U-Tubing Rate

Mud Weight is the other driving force

behind the u-tube.

As mud weight increases:


• The final fluid level in the drillpipe is lower



Mud Level in Drillpipe vs. Time

for Various Mud Densities


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



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


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


U-Tubing Rate vs. Time for

Various Drillpipe IDs


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 time to reach equilibrium decreases


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


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)



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


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.


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.


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.


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


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


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.


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.


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


• Circulating Prior to Making the Connection

• Shut Down Surface Pump

• U-Tubing in Progress

• Connection is Complete

• Restart Circulation

The 5-Minute Connection


Circulating Prior to Making a Connection


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


Shut Down Surface Pump


SP Pressure = 0 psi


MLP Circulating at 392 gpm


MLP P_out = 8,237 psi

Pit Gain = 4.10 bbls


U-Tubing in Progress (1 min)








U-Tubing in Progress (1 1/2 min)













Fluid Level in DP = 1,005 ft



Connection Complete - Start Rig Pump

500 gpm (elapsed time = 5 min)Elapsed Time = 5 min



Pig Pump at 500 gpm MLP at 268 gpm



SP Pressure = 0 psi


Drill Pipe is ~Full - Rig Pump @ 500 gpm

(elapsed time = 16 min)Elapsed Time = 16 min

Pit Gain = 0 bbls





SP Pressure = 0 psi


Resume Drilling - Rig Pump @ 500 gpm

(elapsed time = 17 min)Elapsed Time = 17 min

Pit Gain = 0 bbls





SP Pressure = 1,540 psi


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


• 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



Elapsed Time = 10 min

Pit Gain = 64 bbls

Prepare for Trip





U-Tubing Complete (25 min)

Elapsed Time = 25 min

Pit Gain = 73 bbls

Ready for Trip





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


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


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


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


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


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


• 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?


MUDLIFT

U-Tubing in DGD

~SEAWATER HYDROSTATIC

PRESSURE

BOP

STATIC FLUID LEVEL

FLOATER

10,000’

20,000’


by Julie Zhang and Hans Juvkam-Wold

November 2000

The End

4. U-tubing Concepts

Subsea Mudlift DrillingBasic Technology

by Julie Zhang and Hans Juvkam-Wold

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Transcript of by Julie Zhang and Hans Juvkam-Wold