Deepwater Drilling

Offshore Drilling Operations

Deepwater Drilling

Heimo Heinzle

2 I Offshore Drilling Operations – Deepwater Drilling

Deepwater Drilling


Deepwater Drilling

Deepwater Considerations

� Water Depth

� Differential Pressures

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

� Currents, Tidal effects, Waves & Swells, Wind

� Temperature Differences

� Hydrates

� Forces effecting Riser Design

� Fracture Gradients

� Station Keeping (DP or Mooring)

� Rig Selection


Deepwater Drilling

� Riser Design

� Hydrates

� Dual Gradient / Managed Pressure Drilling


Deepwater Drilling Riser Design

(1) Deployment/retrieval analysis -- to determine the environmental window for running/pulling risers safely. (2) Operability analysis -- to determine the operating envelopes that define the required minimum top tensions and the allowable vessel offsets for each mud density. (3) Storm hang-off analysis -- to determine the limiting seastates in which the riser can be hung-off without buckling the riser. (4) Drift-off/drive-off analysis -- to define the radius of the yellow and red watch circles for dynamic positioned (DP) vessels. (5) Weak point analysis -- to identify the weakest part of the riser and well system under extreme vessel offsets. (6) VIV fatigue analysis -- to predict the accumulated fatigue damage incurred by vortex-induced-vibration due to currents.

Riser Study



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.



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.



Storm hang-off analysisDrilling 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 hang-off. In a storm hang-off, the LMRP is hung off at the bottom of the riser string normally without the BOP.



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.



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 drift-off.



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.

VIV fatigue analysis



Measurements to avoid VIV


Deepwater Drilling Hydrates

� Gas molecules encapsulated by water molecules

� 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

� Most often encountered on restart of operations

Hydrates


� Hydrates� Gas molecules encapsulated

by water molecules

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




Temperature Profile in relation to Water Depth


Prevention is essential:

� Well control – prevent hydrocarbons entering the wellbore

� Thermodynamic inhibitors – standard approach• 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



� Hydrates can form:

• While drilling

• While displacing

• During cementing operations

• During well tests




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

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

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

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

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

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


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

� Blends of salt & glycols give greatest level of hydrate suppression

Chemical Molecular Weight Density (sg)Mono-Ethylene Glycol 62 9.26 (1.11)Propylene Glycol 76 8.60 (1.03)Di-Ethylene Glycol (EMI-201) 106 9.28 (1.11)Di-Propylene Glycol 134 8.53 (1.02)Tri-Ethylene Glycol 150 9.33 (1.12)

Glycols



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)

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

� Alternatively, run SBM when riser attached



Gas Hydrate Inhibition - Salts

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

• Similar To Freeze Point Depression

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

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

� Blends of salt & glycols enhance level of inhibition

� Review application of particular salts based on local regulations



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

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

� MI provides testing facilities in Houston and Stavanger



Gas Hydrate Inhibition

1000

10000

32 36 40 44 48 52 56 60 64 68 72 76 80 84

Temperature [°F]

Pre

ssur

e [p

si]

2000

4000

6000

8000

M-I Bas e Flu id

Seawater

DI-waterM-I Bas e Fluid w/ 5

vol% EMI571

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

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



Drilling Through Gas Hydrate Zones

� Salt / Glycol saturated mud

� Maximize flow rate

� Control drill, avoid excessive ROP

� Select highest mud weight possible

� Set casing as fast as possible



Hydrates can form on outside of BOP

Restricts disconnect operations:

� Cone diverters - diverts gas away from connection

� Glycol injection ports - allows for hydrate dissolution

� ROV - chip away hydrates (inefficient)



Gas Hydrate Remediation� Do everything possible to avoid hydrate formation

� Be very careful with spacer design and when running casing

� Remediation is a costly and time consuming process

� Options include

• Depressurization (highly dangerous)

• Chemical (coiled tubing)

• Heat (coiled tubing)

• Mechanical (drilling)



Dual Gradient / Managed Pressure Drilling

� Fracture Gradient / Pore Pressure

� Dual Gradient Drilling

� Riserless Mud Recovery

� Managed Pressure Drilling



Fracture Gradient & Pore Pressure in Deep Water





Fracture Gradient:



Fracture Gradient:



Fracture Gradient:



Pore Pressure:



If the pore fluid cannot escape fast enough relative to the rate of loading then:

� Porosity decrease is delayed / retarded

� Part of the load is now supported by the pore fluid

� Pore fluid pressure becomes abnormal (greater than hydrostatic)


DEPTH

Dual Gradient Drilling

DGD vs. Conventional Riser Drilling

SEAFLOOR

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE DGD

SEA WATER HYDROSTATIC

PRESSURE

PRESSURE

MUD HYDROSTATIC

PRESSURE Conventional

FRACTURE PRESSURE

DGD vs. Conventional Riser Drilling

SEAFLOOR

PORE PRESSURE

MUD HYDROSTATIC

PRESSURE DGD


PRESSURE

PRESSURE

MUD HYDROSTATIC


FRACTURE PRESSURE

� Single Gradient Wells

� Wellbore contains a single density fluid

� Single pressure gradient

� Dual Gradient Well

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





Conventional Deepwater Casing Design:

Can result in 7+ casing strings !

Where to place/land them within wellhead ?



12.4 ppg mud

13.5 ppg mud

12.4 ppg mud

13.5 ppg mud

12.4 ppg mud

13.5 ppg mud

Pressure, psi

Depth

ft

Seafloor @ 10,000’

Seawater HSPSeawater HSP

23,880 psi

@ 37,500’

2 different fluid gradients



Casing Requirement Conventional

SEAFLOOR

PORE PRESSURE


PRESSURE

PRESSURE

MUD HYDROSTATIC


FRACTURE PRESSURE

Casing Requirement Conventional

SEAFLOOR

PORE PRESSURE


PRESSURE

PRESSURE

MUD HYDROSTATIC


FRACTURE PRESSURE



Casing Requirement DGD

DEPTH

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE


PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC

PRESSURE DGD

Casing Requirement DGD

DEPTH

SEAFLOOR

FRACTURE PRESSURE

PORE PRESSURE


PRESSURE

PRESSURE

DEPTH

MUD HYDROSTATIC

PRESSURE DGD



20” @ 12,500’

16” @ 13,000’

13 3/8” @ 14,000’

11 3/4” @ 15,000’

9 5/8” @ 17,500’

7 5/8” @20,000’

5 1/2 “ @ TD

20” @ 12,500’

16” @ 13,000’

13 3/8” @ 14,000’

11 3/4” @ 15,000’

9 5/8” @ 17,500’

7 5/8” @20,000’

5 1/2 “ @ TD

0.5 ppg standoff

no influx

20” @ 12,500’

16” @ 14,000’

13 3/8” @ 17,000’

11 3/4” @ 22,800’

9 5/8” @ TD

1.0 ppg kick, 50 bbl influx

Conventional Dual Gradient



Wellhead and BOP

Return Line

Drillpipe

Rotating Diverter

BHADrill String Valve

Mud Return and Pump

Seawater-Driven MudLift Pump

Seawater Filled Marine RiserSeawater Power Line,

Control Umbilicals

Seawater Pumps(Existing Mud Pumps)

Wellhead and BOP

Return Line

Drillpipe

Rotating Diverter

BHADrill String Valve

Mud Return and Pump

Seawater-Driven MudLift Pump

Seawater Filled Marine RiserSeawater Power Line,

Control Umbilicals


Seawater Power Line,Control Umbilicals




Return OutletsReturn Outlets



Drillstring Valve (DSV)

MUDLIFT

SEAWATER HYDROSTATIC

PRESSURE

BOP

STATIC FLUID LEVEL

FLOATER

MUDLIFT

SEAWATER HYDROSTATIC

PRESSURE

BOP

STATIC FLUID LEVEL

FLOATER





Diaphragm Pump

Stroke Indicator Sensor (Fixed)

Hydraulic Fluid In/Out

Mud In/Out

Diaphragm

Stroke Indicator Magnet Assembly

Stroke Indicator Tube (Moving)

Connection to Diaphragm



Alternative Dual Gradient Systems:

� Nitrogen Injection at Wellhead or below

� Injection of Hollow Glass Spheres at seabed

� Riserless Mud Recovery System (RMR)



Hollow Glass Spheres



� Mud can be used instead of pump & dump

� No riser� Smaller rigs and storage

capacity� Dual hydrostatic pressure

Riserless Mud Recovery:


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 losingreturns 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.


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Deepwater Drilling