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by Julie Zhang and Hans Juvkam-Wold

Subsea Mudlift Drilling. Basic Technology. by Julie Zhang and Hans Juvkam-Wold. November 2000. Lesson 4. U-tubing Concepts. Contents. U-Tube Concept in Conventional Drilling U-Tubing in DGD Equilibrium After U-Tubing Factors Affecting U-Tubing rate

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by Julie Zhang and Hans Juvkam-Wold

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  1. Subsea Mudlift Drilling Basic Technology by Julie Zhang and Hans Juvkam-Wold November 2000 Lesson 4 U-tubing Concepts Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  4. BOP Subsea Mudlift Drilling Thus, the well can be treated as a U-tube Confidential to SMD JIP

  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 Confidential to SMD JIP

  6. Sea water Sea water Hydraustatic 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 Confidential to SMD JIP

  7. Static Pressure - DGD Annulus and Return Line Drillstring pressure Balance Mud level Depth Seawater Hydraustatic Pressure Pressure Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  11. 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) Balance Confidential to SMD JIP

  12. Initial Circulation Rate - before rig pump shutdown Dynamic effects as the fluid rapidly slows down Decreasing Flow Rate - as the fluid level drops - and the driving head decreases Flow changes from turbulent to laminar Approaching Static Fluid Level U-tubing Rate in DGD Balance Mud level Pressure Confidential to SMD JIP

  13. Factors Affecting U-Tube Rate in DGD • Water Depth • Mud Weight • Mud Viscosity • Drillpipe ID • Nozzle Sizes • Well Depth Below Mud Line Confidential to SMD JIP

  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 Confidential to SMD JIP

  15. U-Tubing Rate vs. Time for Various Water Depths Confidential to SMD JIP

  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 Confidential to SMD JIP

  17. Mud Level in Drillpipe vs. Time for Various Mud Densities Confidential to SMD JIP

  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 Confidential to SMD JIP

  19. U-Tubing Rate vs. Time for Various Mud Viscosities 600/300 RPM Confidential to SMD JIP

  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 Confidential to SMD JIP

  21. U-Tubing Rate vs. Time for Various Drillpipe IDs Confidential to SMD JIP

  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 Confidential to SMD JIP

  23. U-Tubing Rate vs. Time for Various Bit Nozzle Sizes Confidential to SMD JIP

  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 Confidential to SMD JIP

  25. U-Tubing Rate vs. Time for Various Wellbore Depths Confidential to SMD JIP

  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. Confidential to SMD JIP

  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. Confidential to SMD JIP

  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. Confidential to SMD JIP

  29. Hookload vs. Mud Level at 1,500 ft Water Depth Estimated Hookload considering only the mud level drop Confidential to SMD JIP

  30. Hookload vs. Mud Level at 10,000 ft Water Depth Confidential to SMD JIP

  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. Confidential to SMD JIP

  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. Confidential to SMD JIP

  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 Confidential to SMD JIP

  34. The 5-Minute Connection • Circulating Prior to Making the Connection • Shut Down Surface Pump • U-Tubing in Progress • Connection is Complete • Restart Circulation Confidential to SMD JIP

  35. Circulating Prior to Making a Connection Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  42. Drill Pipe is ~Full - Rig Pump @ 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 Confidential to SMD JIP

  43. Resume Drilling - Rig Pump @ 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 Confidential to SMD JIP

  44. The 5-Minute Connection 700 gpm 500 gpm Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  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 Confidential to SMD JIP

  48. Initial Circulation Rate - before rig pump shutdown Dynamic effects as the fluid rapidly slows down Decreasing Flow Rate - as the fluid level drops - and the driving head decreases Flow changes from turbulent to laminar Approaching Static Fluid Level Balance Mud level Pressure Confidential to SMD JIP

  49. U-Tubing Associated with Trip Stop Rig Pump at 0 min. Re-Start at 30 min 700 gpm 500 gpm Confidential to SMD JIP

  50. Pit Gain Associated with Trip Stop Rig Pump at 0 min. Start at 30 min 500 gpm 700 gpm Confidential to SMD JIP

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