Deepwater Drilling

Offshore Drilling Operations Deepwater Drilling Heimo Heinzle

Deepwater Drilling

2 I Offshor Offshore e Drilling Drilling Operat Operations ions – Deepw Deepwater ater Drilli Drilling ng

Deepwater Drilling Deepwater Considerations 

Water Depth

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Differential Differential Pressures

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Distance to Shore (Logistics, Ocean rather than Sea Conditions)

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Currents, Tidal effects, Waves & Swells, Wind

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Temperature emperatur e Differences Differe nces

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Hydrates

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Forces effecting Riser Design

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Fracture Gradients

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Station Keeping (DP or Mooring)

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Rig Selection

3 I Offshor Offshore e Drillin Drilling g Operations Operations – Deepw Deepwater ater Drillin Drilling g

Deepwater Drilling

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Riser Design

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Hydrates

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Dual Gradient / Managed Pressure Drilling

4 I Offshor Offshore e Drillin Drilling g Operations Operations – Deepw Deepwater ater Drillin Drilling g

Deepwater Drilling

Riser Design

Riser Study (1) Deployment/retrieval Deployment/retrieval analysis -- to determin determine e the environ environmenta mentall window for running/pulling risers safely. (2) Operability analysis -- to determine determine the operati operating ng envelopes envelopes that that define the required minimum top tensions and the allowable vessel offsets for each mud density. (3) Storm hang-off analysis -- to determine determine the limiti limiting ng seastates seastates in which the riser can be hung-off without buckling the riser. (4) Drift-off/drive-off analysis -- to define define the radius radius of of the yello yellow w and red watch circles for dynamic positioned (DP) vessels. (5) Weak point analysis -- to identify identify the weakest weakest part of the riser riser and well system under extreme vessel offsets. (6) VIV fatigue analysis -- to predict predict the accumul accumulated ated fatigue fatigue damage damage incurred by vortex-induced-vibration vortex-induced-vibration due to currents. 5 I Offshor Offshore e Drillin Drilling g Operations Operations – Deepw Deepwater ater Drillin Drilling g

Deepwater Drilling Deployment/retrieval analysis The purpose of deployment/retrieval analysis is to determine the environmental window for running/pulling risers safely. The main concern is stress. When a riser joint is lowered through the diverter housing, it is often in contact with the top or bottom edges of the diverter housing. Large contact force and bending moment can be developed in this region. This is caused by vessel motion or currents. Stresses during initial deployment just below the water surface can be high when the BOP is in the wave zone. Thus, BOP should be rapidly deployed past the keel of the vessel where wave and current velocities will be high.

6 I Offshore Drilling Operations – Deepwater Drilling

Riser Design

Deepwater Drilling Operability analysis The purpose of operability analysis is to determine the operating envelopes (windows) that define the required top tensions and the allowable offsets for each mud density. This is normally done by plotting a set of operating envelopes which shows the required tension as a function of offset, mud weight, and environments. These envelopes tell how much top tension should be pulled to avoid the riser string from buckling. They also show the offset range that the vessel should stay within to avoid excessive flex joint angles. Each plot refers to one water depth, mud density and environmental condition. A complete set of results should have one of these plots for several key mud densities and a couple of environmental conditions. 7 I Offshore Drilling Operations – Deepwater Drilling

Riser Design

Deepwater Drilling Storm hang-off analysis Drilling riser may need to be disconnected when the environmental condition deteriorates. If the riser is kept connected in this situation, the telescopic joint (or tensioners) might stroke out or the riser might clash with the moonpool. If the riser is disconnected, it then faces another potential problem: axial compression. Vessel’s heave motion can induce dynamic axial compression to the riser, particularly at the top portion. A storm hang-off analysis is used to determine the limiting sea-states in which the riser can be hung-off without buckling the riser. Riser hang-off can be done at least two ways: hard and soft hang-offs. For the hard hang-off, the telescopic joint is locked. In this arrangement, the riser is coupled to the vertical motions of the vessel. For the soft hang-off, the riser is allowed to stroke on the telescopic joint. In this case, the tension fluctuation in the riser is reduced. As a result, its weather envelope is larger than that of a hard hangoff. In a storm hang-off, the LMRP is hung off at the bottom of the riser string normally without the BOP. 8 I Offshore Drilling Operations – Deepwater Drilling

Riser Design

Deepwater Drilling Drift-off/drive-off analysis The purpose of a drift-off/drive-off analysis is to define yellow and red watch circles for dynamic positioned (DP) vessels. Drift-off analysis examines riser conditions when the vessel loses power of its thrusters and starts to drift off location. Drive-off is another situation where the vessel’s GPS or DP control system malfunction and consequently cause the vessel to drive to a false target location. This analysis determines when the DP operator should push the disconnect button to activate its emergency disconnect sequence (EDS). Most DP rigs are fitted with an EDS which is typically a push button that initiates a sequence starting from closing the BOP, shearing the drill pipe to disconnecting the LMRP. It is in place to prevent catastrophic damage to the well and riser system. The EDS sequence normally takes about 30 to 60 seconds to complete.


Riser Design

Deepwater Drilling Weak point analysis The purpose of a weak point analysis is to identify the weakest part of the riser and well system under extreme vessel offsets. In other words, it is to consider the worst event where a drive-off/drift-off occurs and the LMRP is not disconnected from the wellhead. When the vessel’s offset increases to a point that the telescopic joint (or tensioners) strokes out, the riser tension will start to increase rapidly. Weak point analysis can identify the points that first reach yield. It also helps in determining the required conductor and wellhead bending moment capacities. Weak point analysis is not routinely performed for every drilling operation. However, some government authorities still require it to prove the well integrity is satisfactory. In that case, the analysis must demonstrate that identified weak points do not reside anywhere below the BOP. It should show that hydrocarbon is always securely contained in the well system even in the worst scenario of a drive-off or driftoff.


Riser Design

Deepwater Drilling VIV fatigue analysis Under high current loads, a riser string might experience vortex induced vibrations (VIV). VIV are motions induced on bodies facing an external flow by periodical irregularities on this flow. This alternating shedding pattern causes the riser to vibrate perpendicular to the current direction. The vibration induces a small amount of stress that is not a concern in terms of strength, but may accumulate fatigue damage.


Riser Design

Deepwater Drilling

Measurements to avoid VIV


Riser Design

Deepwater Drilling Hydrates 

Gas molecules encapsulated by water molecules

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Ice like crystals formed from water and light hydrocarbons, which when agglomerated can block the flow path 

Can form at temperatures up to 18°C when pressure i s > 170 bar

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Most often encountered on restart of operations


Hydrates

Deepwater Drilling 

Hydrates 

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Gas molecules encapsulated by water molecules

Required conditions     

Cold temperatures High pressure Water Hydrocarbons Time (but can form instantly)


Hydrates

Deepwater Drilling Temperature Profile in relation to Water Depth


Hydrates

Deepwater Drilling

Hydrates

Prevention is essential: 

Well control – prevent hydrocarbons entering the wellbore

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Thermodynamic inhibitors – standard approach • • • •

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Salts (inorganic and organic) Glycol (soluble) Methanol, Ethanol Combination of salt & glycol

Kinetic inhibitors – not field proven in drilling • Chemical additives added to slow rate of reaction


Deepwater Drilling 

Hydrates can form: • While drilling • While displacing • During cementing operations • During well tests


Hydrates

Deepwater Drilling

Hydrates

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Plugging of choke and kill lines preventing their use in well circulation

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Formation of a plug at or below the BOPs, that prevents monitoring well pressures below the BOPs

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Formation of a plug around the drillstring in the riser , BOP's or casing that will restrict drillstring movement

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Formation of a plug between the drillstring and the BOPs to prevents closure

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Formation of a plug in the ram cavity of a closed BOP preventing full opening

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On outside of BOP/Riser preventing hydraulic connector to disconnect from wellhead (or LMRP from BOP)


Deepwater Drilling

Hydrates

Glycols Chemical Mono-Ethylene Glycol Propylene Glycol Di-Ethylene Glycol (EMI-201) Di-Propylene Glycol Tri-Ethylene Glycol

Molecular Weight 62 76 106 134 150

Density (sg) 9.26 (1.11) 8.60 (1.03) 9.28 (1.11) 8.53 (1.02) 9.33 (1.12)

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The lowest molecular weight glycols provide the most gas hydrate inhibition • High molecular weight glycols for shale inhibition

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Blends of salt & glycols give greatest level of hydrate suppression


Deepwater Drilling

Hydrates

Current Practices 

Attempt to fully inhibit drilling fluid against hydrate formation • Maximize Sodium Chloride (NaCl) concentration based on MW limitations (fracture gradient) - Maximum typically 23 wt% NaCl

• Boost inhibition with glycol 

If full inhibition not possible (typically in water depths 4,000 ft)

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• Have contingency hydrate inhibitive fluids on location to spot in BOP’s & choke/kill lines 

Alternatively, run SBM when riser attached


Deepwater Drilling

Hydrates

Gas Hydrate Inhibition - Salts 

Near saturated Sodium Chloride (NaCl) brine will provide 28.5 to 33.5°F of hydrate temperature suppression •

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Similar To Freeze Point Depression

Potassium Chloride (KCl) less effective than NaCl due to lower solution activity

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Below 18 wt%, NaCl more effective than Calcium Chloride (CaCl2)

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Blends of salt & glycols enhance level of inhibition

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Review application of particular salts based on local regulations


Deepwater Drilling Gas Hydrate Testing / Modeling 

Computer based simulation models available (e.g. MI Swaco) -

Improved Algorithms 8 Salts 6 Glycols Can model blends of 3 salts with 3 glycols

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If high operator concerned about hydrate formation in mud, schedule tests for hydrate formation

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MI provides testing facilities in Houston and Stavanger


Hydrates

Deepwater Drilling

Hydrates

Gas Hydrate Inhibition 10000

8000

M-I Bas e Fluid w/5 vol% E.G.

6000

Seawater

] i s p [ e r u s s e r P

4000

M-I Bas e Fluid w/15 vo l% E.G.

M-I Base Fluid 2000

DI-water

M-I Bas e Fluid w / 5 vol% EMI571 1000 32

36

40

44

48

52

56

60

Te mpe rature [°F]


64

68

72

76

80

84

Deepwater Drilling Drilling Through Gas Hydrate Zones 

Salt / Glycol saturated mud

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Maximize flow rate

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Control drill, avoid excessive ROP

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Select highest mud weight possible

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Set casing as fast as possible


Hydrates

Deepwater Drilling

Hydrates

Hydrates can form on outside of BOP Restricts disconnect operations:

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Cone diverters

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Glycol injection ports -

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ROV


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diverts gas away from connection allows for hydrate dissolution

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chip away hydrates (inefficient)

Deepwater Drilling

Hydrates

Gas Hydrate Remediation 

Do everything possible to avoid hydrate formation

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Be very careful with spacer design and when running casing

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Remediation is a costly and time consuming process

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Options include • Depressurization (highly dangerous) • Chemical (coiled tubing) • Heat (coiled tubing) • Mechanical (drilling)



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Fracture Gradient / Pore Pressure

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Dual Gradient Drilling

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Riserless Mud Recovery

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Managed Pressure Drilling


Dual Gradient / Managed Pressure Drilling Fracture Gradient & Pore Pressure in Deep Water




Dual Gradient / Managed Pressure Drilling Fracture Gradient:






Dual Gradient / Managed Pressure Drilling Pore Pressure:


Dual Gradient / Managed Pressure Drilling If the pore fluid cannot escape fast enough relative to the rate of loading then: 

Porosity decrease is delayed / retarded

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Part of the load is now supported by the pore fluid

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Pore fluid pressure becomes abnormal (greater than hydrostatic)


Dual Gradient Drilling 

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DGD vs. Conventional Riser Drilling

Single Gradient Wells Wellbore contains a single density fluid Single pressure gradient

MUD HYDROSTATIC PRESSURE DGD SEAFLOOR

FRACTURE PRESSURE

Dual Gradient Well

Wellbore feels seawater gradient to the seafloor, and DEPTH mud gradient to bottom

MUD HYDROSTATIC PRESSURE Conventional

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SEA WATER HYDROSTATIC PRESSURE

PRESSURE


PORE PRESSURE




Conventional Deepwater Casing Design: Can result in 7+ casing strings ! Where to place/land them within wellhead ?


Dual Gradient Drilling 2 different fluid gradients

Pressure, psi D e p t h

Seawater HSP Seafloor @ 10,000’ 12.4 ppg mud 13.5 ppg mud

f t 23,880 psi @ 37,500’


Dual Gradient Drilling Casing Requirement Conventional MUD HYDROSTATIC PRESSURE Conventional

SEAFLOOR

FRACTURE PRESSURE

SEA WATER HYDROSTATIC PRESSURE

PRESSURE 39 I Offshore Drilling Operations – Deepwater Drilling

PORE PRESSURE

Dual Gradient Drilling Casing Requirement DGD

SEAFLOOR

MUD HYDROSTATIC PRESSURE DGD

FRACTURE PRESSURE

DEPTH

DEPT H SEA WATER HYDROSTATIC PRESSURE PRESSURE 40 I Offshore Drilling Operations – Deepwater Drilling

PORE PRESSURE

Dual Gradient Drilling Dual Gradient

Conventional

20” @ 12,500’

20” @ 12,500’

16” @ 13,000’ 13 3/8” @ 14,000’ 1.0 ppg kick, 50 bbl influx

16” @ 14,000’

0.5 ppg standoff

11 3/4” @ 15,000’

no influx

13 3/8” @ 17,000’

9 5/8” @ 17,500’ 7 5/8” @20,000’

11 3/4” @ 22,800’ 5 1/2 “ @ TD 41 I Offshore Drilling Operations – Deepwater Drilling

9 5/8” @ TD

Dual Gradient Drilling Seawater Pumps (Existing Mud Pumps)

Mud Return and Pump

Return Line Seawater Power Line, Control Umbilicals

Drillpipe Seawater Filled Marine Riser Rotating Diverter

Wellhead and BOP

BHA


Seawater-Driven MudLift Pump

Drill String Valve


Return Outlets 43 I Offshore Drilling Operations – Deepwater Drilling

Dual Gradient Drilling Drillstring Valve (DSV)

FLOATER

STATIC FLUID LEVEL

SEAWATER HYDROSTATIC PRESSURE

BOP

MUDLIFT




Dual Gradient Drilling Diaphragm Pump Stroke Indicator Magnet Assembly Stroke Indicator Tube (Moving)

Stroke Indicator Sensor (Fixed) Hydraulic Fluid In/Out

Connection to Diaphragm Diaphragm Mud In/Out 46 I Offshore Drilling Operations – Deepwater Drilling

Dual Gradient Drilling Alternative Dual Gradient Systems:  Nitrogen Injection at Wellhead or below 

Injection of Hollow Glass Spheres at seabed

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Riserless Mud Recovery System (RMR)


Dual Gradient Drilling Hollow Glass Spheres


Dual Gradient Drilling Riserless Mud Recovery:  

Mud can be used instead of pump & dump No riser

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Smaller rigs and storage capacity

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Dual hydrostatic pressure


Managed Pressure Drilling


Managed Pressure Drilling The idea is to keep the static and dynamic pressure the same. How to go from static balance to dynamic (circulating) balance without either losing returns or taking a kick. This can be done by gradually reducing pump speed while simultaneously closing a surface choke to increase surface annular pressure until the rig pumps are completely stopped and surface pressure on the annulus is such that the formation “sees” the exact same pressure it saw from ECD while circulating.


Deepwater Drilling

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