6. Motion Compensation

1

TAMU - PemexOffshore Drilling

Lesson 6 Motion Compensation

2

Motion Compensation

Reentry

Tensioners

Heave Compensators

Passive Motion Compensation

Active and Semiactive Systems

3

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)

4

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

5

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?

6

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...

7

Pneumatic Riser Tensioning System

8

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

9


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.

10


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

11

Air reservoirreduces pressurechanges

F = PA

To support larger load,increase the

pressure

An AIR spring ...

Passive System

12

13

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

14

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)

15

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

16

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

17

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

18

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

19

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

20

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-

21

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

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

23

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.

24

Rucker Heave Compensator

Dual Pistons- on the travelling block

Large air cylinders below deck

Flexible hoses

25

Figure 7-9 Vetco dual piston Heave Compensator

L.P. hydraulic fluid throttled for damping

Piston balancing may be problem

26

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

27

Figure 7-13. Active Heave Compensator

28

Semiactive Heave Compensator

29

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.

30

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.

31

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.

32

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.

33

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.

34

UnbalancedBumperSubs

35

Buoyant Riser Module

36

Cantilever Jackup

Rig

Float out to location

Then lower legs to

seafloor

Then jack up

37

Jackup Rig Side View

38

Jackup Rig

Deck Plan

39

End of Lesson 6

Documents

6. Motion Compensation