6. motion compensation
TRANSCRIPT
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TAMU - PemexOffshore Drilling
Lesson 6 Motion Compensation
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Motion Compensation
Reentry
Tensioners
Heave Compensators
Passive Motion Compensation
Active and Semiactive Systems
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Re-entry
It is possible to re-enter a borehole without using guidelines!
1. Use land-based navigation equipment to get the vessel in the vicinity of the well
or better still: Use GPS (Global Positioning System)
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Re-entry, cont’d
2. Lower reentry string to a safe distance above the wellhead
3. Use position location equipment to complete the job:
(i) Television camera
(ii) Acoustic device - pinger or transponder
(iii) ROV - Remote Operated Vehicle
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Heave Compensation
How do you maintain a constant tension on the marine riser - when the vessel
heaves?
How do you maintain a constant weight on the bit - when the vessel heaves?
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Deadweight Riser Tensioning System
Dead Weight(~constant
tension)
Dead Weight
Slip Joint
Marine Riser
Early design - OK up to 100,000 lbf. Attached to lower half of telescoping joint. Adds weight & is bulky...
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Pneumatic Riser Tensioning System
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Pneumatic/Hydraulic Heave Compensation System
Pneumatic/Hydraulic Tensioners: Take up much less room than
dead weights
Facilitate changing the tension by changing the air pressure
Can be used for the marine riser,
the guidelines and the drill string
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Pneumatic/Hydraulic Heave Compensation System
Passive Systems are the most popular
Require essentially no energy input Use an “air spring” with a variable spring constant Can keep the tension within ~ 15%
or even less.
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Pneumatic/Hydraulic Heave Compensation System
Active Systems
Require external energy through each load cycle Provide a highly consistent force But -- Have a high initial cost Have a high operating cost
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Air reservoirreduces pressurechanges
F = PA
To support larger load,increase the
pressure
An AIR spring ...
Passive System
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Consider Change from P1 and V1 to P2 and V2
P1 and P2 are absolute pressures
n = 1 for isothermal expansion or contraction
n = 1.41 for adiabatic expansion or contraction
1
2
1
2
P
P
F
F
n
V
V
P
Pand
2
1
1
2
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Change in Volume
P1 and P2 are absolute pressures
Isothermal Volume Change: PV = const. (slow; constant temperature)
Adiabatic Volume Change: PV1.41 = const. (fast; no heat flow)
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V
V Relative Change in volume
37180
411
1
1
411
2
1
1
2 .V.
V
V
V
P
P..
Pre
ssu
re R
atio
, P
2 / P
1
ADIABATIC
1.4
1.0
0.7
-0.1 +0.2
ISOTHERMAL
25.8.
1V0
V
V
V
P
P
1
1
2
1
1
2
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Theoretical reservoir size vs. pressure
fluctuation.
Allowable Pressure Fluctuation, %Vo
l. o
f R
eser
voir
/ V
ol.
of
Cyl
ind
er
15 300
15
30
Fig. 4-7
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Example
Consider a 14-inch piston with a 10-foot stroke. We shall hook two units (cylinders) to the reservoir. Determine reservoir size for 15% force variation.
Reservoir Volume = R m A L
4)-7 Figure (from 5 R
cylinder of volume
reservoir of volume lTheoretica
R = 5
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Example cont’d
Reservoir Volume = R m A L
= 5 * 2 * 10 * 1.07 = 107 ft3
m = 2 = number of cylinders
L = 10 ft = piston stroke
A = /4 (14/12)2 = 1.07 ft2 = piston area
VRES = R m A L
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Equation (on p. 170)
Theoretical force variations as a function of piston position and heave can be determined by a modification of the previous pressure - volume equation:
2
LL
2
L- and
1001L A mV
V1001
F
FE
n
1
2
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Where:
E = theoretical percentage error in force applied to the load
= force applied to the load relative to the force with the piston centered
A = piston area, ft.2
L = full piston stroke, ft.
V = reservoir volume +
L = length of the stroke from the center of the piston, ft.
3
2ft,
mAL
1
2
F
F
1001
1001 1
2
n
LAmV
V
F
FE
2
LL
2
L-
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Heave Down
Per
cen
t C
han
ge
in L
oad
0
15
-15-20 20
15
-15
0
010
Heave Up, ft
Reservoir Volume = 5 * vol. swept by piston
Adiabatic
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22 Lower sheaves are attached to the cylinder (and vessel), upper sheaves are attached to the piston rod
20 ft
5 ft1,500 psig WP
TypicalTensioner
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Motion Compensator
Principles ofOperation
Purpose:
Keep bit on bottom
with low bit weight change
Air pressure can support entire weight of drillstring. e.g. 200,000 - 40,000 = 160,000lbf. Reduce air press.
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Rucker Heave Compensator
Dual Pistons- on the travelling block
Large air cylinders below deck
Flexible hoses
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Figure 7-9 Vetco dual piston Heave Compensator
L.P. hydraulic fluid throttled for damping
Piston balancing may be problem
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Figure 7-10
Split travelling block
Hydraulically operated pistons can be locked in any position with remotely operated valves.
Air operated units must be mechanically locked in position because of compressible fluid in cylinders.
Single Piston Rod
Western Gear Heave Compensator
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Figure 7-13. Active Heave Compensator
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Semiactive Heave Compensator
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Bumper Subs
Fit into the drill collar string, but do not have the ruggedness of drill collars. To obtain a reasonable operating life from bumper subs, action has to be taken on the following points of concern:
1. Always run the bumper subs at the “neutral point” in the string.
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Bumper Subs, cont’d
2. Bumper subs are designed to stroke, and if operated at a single position, they will wear at that position.
3. Running the subs in tension minimizes the area through which the torque will be transmitted, and will cause excessive wear and fatigue.
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Bumper Subs
4. Running the joint in compression increases torque reversals and fatigue in the tool. It also increases “bit chatter” and instantaneous peak torque, causing unnecessary wear to the tool and the string. This is the fault of the operation, not the sub.
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Bumper Subs
5. From items 3 and 4 above, it is evident that once the string has been run, the weight on the bit is fixed. This weight should not be changed until the string has been pulled and the number of drill collars below the bumper sub changed.
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Bumper Subs
6.Do not separate the bumper subs in a drillstring for heavy drilling operations.
7.Special care and maintenance are required to obtain a reasonable operating life.
*Remember: bumper subs fit into the same part of the drillstring where drill collars and stabilizers have been destroyed. This is a severe test of workmanship & maintenance.
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UnbalancedBumperSubs
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Buoyant Riser Module
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Cantilever Jackup
Rig
Float out to location
Then lower legs to
seafloor
Then jack up
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Jackup Rig Side View
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Jackup Rig
Deck Plan
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End of Lesson 6