FINAL PRODUCTION CASING DESIGN WITH COMBINED LOADS EXERCISE

Instructions

Use the example well data from this document or the Powerpoint notes handout to complete the following graphs.

Production Casing – Collapse Design

Production Casing – Burst Design

Production Casing – Axial Design

When complete, scan or photograph your work and upload it. Post questions to the discussion board.

Production Casing

Here is the preliminary intermediate casing design we made in the previous chapter.

Here we might check the design at the bottom of section 2 at 12,041 ft, but upon examining the axial load at that point we see that it is in compression. We will do a quick check at the bottom of Section 3 at 9606 ft using a spreadsheet to do the calculations. It shows a safety factor of 1.00 without tension, and that may be a problem with tension.

Casing Design Summary

7" Production Casing

Collapse BurstJoint

Strength4 7 6.184 29 P-110 LT&C 3925 3925 113825 442102 2.42 1.08 1.333 7 6.094 32 N-80 LT&C 9606 5681 181792 328277 1.00 1.00 1.642 7 6.094 32 P-110 LT&C 12041 2435 77920 146485 1.00 1.74 high1 7 6.004 35 P-110 LT&C 14000 1959 68565 68565 1.04 2.00 high

0 0 00 0 00 0 00 0 0

Totals: 14000 442102Collapse & burst do not

Minimum Design Factors, k D Mud Weight: 15.3 include biaxial effectsCollapse: 1.125Burst: 1.2Tension: 1.6/100,000lb buoyed

** Safety Factor: Strength / (k D * Load)

Section Number OD ID Weight Grade Connection Bottom Length

Section Weight

Cum. Weight

Safety Factors **

Figure 7-16 Production casing, axial load, running case

The new collapse pressure is 8421 psi and the collapse pressure in the absence of tension is 8600 psi. That is a relatively small difference, but let us see what the actual design factor is now.

D

8421k 0.979

1.125(0.052 )(15.3 )( 9606 )

This safety factor is slightly less than 1.00 As a practical matter this may not be worth changing the design, however, we must all be aware of the consequences of such a decision. In the world today if there were to be an accident involving this well and this string of casing the fact that would carry a lot of weight in a courtroom or other litigation procedure would be the fact that we did not design the string to meet our own minimum design factor. So practical matters aside, we will change the design to be sure it is within our minimum design requirements. The easiest way to deal with this one would be to possibly raise the bottom of section 3 up to 9500 ft, i.e. reduce its length by 106 ft. Since both Section 2 and Section 3 are 32 lb/ft pipe this will not change the tension curve at all but it will change the tension at the bottom of Section 3 which we can still read from the same curve. The tension at 9500 ft is about 45,000 lb. So let us calculate the effect of this change.

Then calculate the design factor at 9500 with a new collapse pressure of 8403 psi.

D

8403k 0.988

1.125(0.052 )(15.3 )( 9500 )

Obviously 100 ft was not enough. One of the problems is that when we raise the top of the section we lower the hydrostatic collapse load, but we also find the tension is greater at the shallower depth. We could continue to try another 100 ft or so, but that is a waste of time. We will resort to an iterative technique which we will do graphically.

Here is how the technique works.

orf (D) 1.00

f ( D ) 1.00 0

We assume that all the calculations we do are lumped into something we will call a function of the depth, D, and if we select the correct depth it will give us the design factor we want, 1.125 in this case. So if we select the correct value for the depth y = 0 and if our guess is not correct then y ≠ 0 , which we have already done. Let us see what we have done so far.

1 1

2 2

D 9606 f ( D ) 1 0.979 1.000 0.021

D 9500 f ( D ) 1 0.988 1.000 0.012

Now we can plot a quick graph to predict what value of depth should give us a zero value of y.

After plotting the two points we draw a line through them to the axis where y = 0 . We see at this point that the depth that would give us the correct design factor is 9360 ft (assuming subsequent points plot as a straight line). We will check the design at 9360.

-0.03

-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

9000 9050 9100 9150 9200 9250 9300 9350 9400 9450 9500 9550 9600 9650 9700

f(D

) -

1

Depth, D

Collapse/Depth Interpolation

D2 = 9500 f t

D1 = 9606 f t

D = 9360 f t

We then check our design factor with a collapse value of 8384 psi at 9440 ft.

Ds

8384k 1.001

1.125(0.052 )(15.3 )( 9360 )

Now we are within our required safety factor. And our final design for this production string is:

Alternate Production Casing

Our alternate design for a shallow tubing leak was dominated by burst pressures and collapse in the presence of tension is not likely a question as you can see by the high safety factors. Nevertheless we have assumed

7" Production Casing

Collapse* BurstJoint

Strength4 7 29 P-110 LT&C 3925 3925 113825 442102 2.20 1.08 1.323 7 32 N-80 LT&C 9360 5435 173920 328277 1.00* 1.00 1.582 7 32 P-110 LT&C 12041 2681 85792 154357 1.00 1.74 high1 7 35 P-110 LT&C 14000 1959 68565 68565 1.05 2.00 high

0 0 00 0 00 0 00 0 0

Totals: 14000 442102* includes biaxial effects

Minimum Design Factors, k D Mud Weight: 15.2Collapse: 1.125Burst: 1.2Tension: 1.6/100,000lb buoyed

** Safety Factor: Strength / (k D * Load)

Cum. Weight

Section Number OD ID Weight Grade

Safety Factors**

Connection Bottom LengthSection Weight

this is a critical well and we will definitely check that in addition to the effect that compression has on the burst resistance.

The collapse load looked like this:

The burst loads are:

7" Production Casing (Alternate)

Collapse BurstJoint

Strength3 7 6.094 32 P-110 Tenaris Blue 4764 4764 152448 503416 2.53 1.00 1.472 7 5.920 38 P-110 Tenaris Blue 10238 5474 208012 350968 1.65 1.00 2.271 7 5.920 38 Q-125 Tenaris Blue 14000 3762 142956 142956 1.34 1.02 7.49

0 0 00 0 00 0 00 0 00 0 0

Totals: 14000 503416Collapse & burst do not

Minimum Design Factors, k D Mud Weight: 15.3 include bi-axial effectsCollapse: 1.125Burst: 1.2Tension: 1.6/100,000lb buoyed

** Safety Factor: Strength/(k D * Load)

Section Number OD ID Weight

Section Weight

Cum. Weight

Unbuoyed

Safety Factor **

Grade Connection Bottom Length

0

2000

4000

6000

8000

10000

12000

14000

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Dep

th (

ft)

Pressure (1000 psi)

Production Casing Design Load

Collapse DesignLoad

Collapse Load Line

Production

38 # P-110

32 # P-110

38 # Q-125

And the axial loads look like this:

We have three axial load curves here, and the question is which one do we use? The truth is we do not know. In the absence of friction we might assume the running load is the load when we reach bottom. When

0

2000

4000

6000

8000

10000

12000

14000

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Dep

th (

ft)

Pressure (1000 psi)

Production Casing Design Load

Burst DesignLoad

Burst LoadNear SurfaceTubing Leak

38 # Q-125

38 # P-110

32 # P-110

we cement the casing the plug bump load is the maximum axial load, but after releasing the plug bump pressure then the axial load reverts to the post plug bump load which is less than the running load because of the buoyancy of the more dense cement. When do the maximum collapse and burst loads occur? In this case both occur in the production phase after cementing. That would suggest that the post plug bump load is the one to use, but what if the casing stretches at plug bump, but does not contract (due to friction) when the plug pressure is released? Almost no one really worries about this sort of thing, especially in basic casing design. Most would use the neutral buoyed axial load curve without added pressures (which is labeled “running” on out chart). But for our critical case here let us use the maximum tension load (plug bump) as the residual load for bi-axial collapse and the maximum compressive load for residual bi-axial burst.

Bi-axial Collapse Check

Our safety factors are quite high, so there is not likely a bi-axial collapse problem. On bottom the pipe is in compression so there is no bi-axial collapse reduction there. The next point of concern would be at the transition point from Q-125 to P-110 at 10238 ft. The tension is only 52,000 lbf and the collapse load is 8145 psi (I am actually calculating these loads, but values read from the load plots are what we would usually use and are accurate enough.) We will use the spread sheet to calculate the reduced collapse.

Next, we calculate our safety factor with the reduced collapse rating, 14858 psi, and the 1.125 design factor.

SFD

Strength 14858k 1.62 1 OK

k Load 1.125( 8145 )

Bi-axial Burst Check

Often we do not do a bi-axial burst check. This is because for most casing strings the maximum burst pressures occur in the upper portion of the string where the string is in tension. So burst strength is enhanced by the tension rather than reduced. But this 7 in. production string we designed is different because the maximum burst pressures occur in the lower part of the string, part of which is in compression. Here we will use the post plug bump curve where the bump pressure has been released and the casing is further buoyed by the density difference of the internal displacement fluid and external cement.

On bottom the axial load is -128,000 lbf (compression) and the burst load is 13,744 psi and the API burst rating of the casing is 16,880 psi. Again using the spread sheet to calculate the bi-axial reduction of the yield:

And we check the safety factor using those results and a design factor of 1.20.

not acceptableSFD

Strength 16030k 0.972 1

k Load 1.20(13744 )

Now what? There is no longer any API grade pipe that will meet this load condition. We could run some V-150 grade casing, and that is not unusual. For now let us just go ahead and substitute V-150 pipe for the lower section, and leave the discussion until later. With V-150 (same weight):

We recalculate the safety factor:

SFD

Strength 19460k 1.18 1 OK

k Load 1.20(13744 )

Next, we check the bi-axial collapse at the bottom of the 38 lb/ft P-110 section at 10,238 ft. It is in tension at that point.

Now recalculate the biaxial collapse load at the bottom of each section. At bottom:

clpsSF

D load

p 19740k 1.57 1 OK

k p 1.125(11138 )

The bi-axial effects changed our design considerably from the preliminary selection that was initially based on the burst design line. There are many things we might want to consider here. Would we want to use a shorter length of V-150 and use some Q- 125 and eliminate the P-110 in section 2? When we picked the V-150 for bottom I used 38 lb/ft so as to be the same wall thickness as the pipe above it because it can be problematic for packers to have to pass through a small ID and then set in a larger ID, and since these weights are going to require special packers we need to keep things simpler. Probably a better alternative might be a string of P-110, Q-125, and V-150 all in the 32 lb/ft weight. But it is left as above because it has illustrated the bi-axial process.