ILT3 - Surface Compressive Loads and Dynamic Axial Loading

Casing Design Workshop: Surface Compressive Loads and

Dynamic Axial Loading

(Last Updated 16 December 2015)

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Casing Design Workshop

Surface Compressive Loads and Dynamic Axial

Loading

Axial Design Check (Alternate)

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Dep

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Tension (1000 lb)

7 " Production Casing - Axial Design

Axial Plug BumpLoad

Axial DesignFactor = 1.6

32# P-110

Axial Design OverPull

38# Q-125

38# P-110

Production Casing (Alternate)

Casing Design Summary

7" Production Casing

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 biaxial effectsCollapse: 1.125Burst: 1.2

Section Weight

Cum. Weight

Unbuoyed

Safety Factor **

Grade Connection Bottom LengthSection Number OD ID Weight

Stop here for production

casing project

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© PetroSkills, LLC., 2016. All rights reserved._____________________________________________________________________________________________

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Some Axial Load Comments

At Plug Bump• No increase in hook load at plug bump• Hook load may be less than when run

– If negligible borehole friction, and – Cement more dense than displacement fluid

• Hook load may actually decrease when plug bumps if casing is stuck somewhere down hole

• Pressurizing casing internally at plug bump– Increases axial tension– Causes free casing to stretch

– Does not increase its weight

• Cannot measure this load change

Severe Lost Circulation While Cementing

Severe lost circulation during displacement• Annular fluid level drops• Annular hydrostatic pressure decreases• External buoyancy force is lost

Plug not yet seated• What to do?

– Continue to displace cement until plug seats?– Shut off pumps and let cement equalize?

Plug is seated• Nothing we can do

How does this affect our axial design?

Check our examples

13-3/8 in. surface casing:• Un-buoyed casing weight: 175,650 lbf• Weight of displacement fluid: 147,809 lbf• Total possible suspended weight: 323,459 lbf• Plus 1000 psi plug bump: 448,446 lbf• Casing joint strength (surface): 547,000 lbf

9-5/8 in. intermediate casing:• Un-buoyed casing weight: 456,750 lbf• Weight of displacement fluid: 324,115 lbf• Total possible suspended weight: 780,865 lbf• Plus 1000 psi plug bump: 841,341 lbf• Casing joint strength (surface): 1,105,000 lbf

Surface Compressive Loads and Dynamic Axial Loading ═════════════════════════════════════════════════════════════════════════

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© PetroSkills, LLC., 2016. All rights reserved.

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Check our examples

7 in. production casing:• Un-buoyed casing weight: 442,102 lbf• Weight of displacement fluid: 271,391 lbf• Total possible suspended weight: 713,493 lbf• Plus 1200 psi plug bump: 749,535 lbf• Casing joint strength (surface): 797,000 lbf

7 in. production casing (alternate):• Un-buoyed casing weight: 503,416 lbf• Weight of displacement fluid: 260,210 lbf• Total possible suspended weight: 763,626 lbf• Plus 1200 psi plug bump: 798,627 lbf• Casing joint strength (surface): 1,025,000 lbf

How does surface casing support the other strings?

Compressive Loading (Optional)

View Optional Slides

Compressive Load

Our example: surface casing supports subsequent casing, tubing, wellhead, and BOP



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What Load to Use?

Buoyed at running?• Realistic but higher than post plug bump

Buoyed post plug bump?• Minimum but what if pipe is differentially or friction stuck and does

not contract after release of pressure?

Plug bump?• Bump pressure does not affect hanging load on casing head

(internal pressure load)

We will use 875 klb

Capacity of Casing Body

We have 54.5 lb/ft K-55 on top of the string and 68 lb/ft K-55 on bottom

We could add a few joints of 68 lb/ft to top

Pipe body yield strengths (tension & compression):

Connection Compressive Strengths

What are the compressive strengths of threaded casing?• Proprietary connections: manufacturer specifies as a % of pipe

body yield, e.g. 60%. 80%, 100% (not always published)• API threads: no standards, some manufacturers have test values,

some use 40%, some use higher values• Stop rings increase compressive strength of non-shouldered

connections. How much?


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Alternatives

Use proprietary connections• Standard practice in many applications• Requires crossovers in many cases – cementing head, etc.• Threaded casing head must have same thread (no crossover)

Assure good cement to surface between surface casing and cut off conductor (as in our example)

• Vast majority of shallow and medium depth land wells and shallow-water wells

• Use conductor (welded or compressive connections) with casing head instead of surface casing for support

Other options• casing head with base plate resting on top of cut off conductor or on

reinforced concrete cellar foundation, etc.

What about shock loads while running casing?Dynamic Loading (optional)

View Optional Slides

Dynamic Loading Overview

Dynamic loading is beyond a fundamental casing design course

In general, dynamic casing loads are axial loads caused by:• Impulse loads due to sudden impact• Momentum loads due to change in velocity• Often a combination of both

– All impulse loads are accompanied by a change in momentum– Most momentum loads do not involve impulse



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Dynamic Loads in Casing

Impulse Loads (while running):• Actuating spider while pipe is in motion (tensile)• Shoe hitting ledge, bridge, or bottom (compressive)• Depends on impact velocity and material

Momentum Loads (while running):• All of the above• Any time pipe velocity changes• Depends on mass, acceleration, and material

Impulse Loads

Also called shock loads or impact loads

Timoshenko and Goodier derived a simple formula :

For steel in USC units it is:

v E

1780

where

relative velocity at impact, ft/s

maximum change in axial stress, psi

v

v

Things to Understand About the Formula

Impact is between a rigid body and an elastic bar (or tube in our case)

Impact is uniform across the end

Transfer of energy is instantaneous

The formula is independent of the mass of either object, in other words, no change of momentum is considered

It is an absolute upper limit that cannot actually occur in real materials


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Impulse Stress Chart

We can use the formula to plot a chart independent of tube size, length, or mass

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Relative Impact Velocity, ft/s

Rigid/Elastic Impact

Example:

A casing string being run at 5 ft/s is suddenly stopped by someone prematurely actuating the spider

A tensile wave of 8900 psi propagates down the tube below the spider.

If the cross sectional area of the tube is 12 in2, the tensile axial load below the spider has increased by 106,800 lbf

This seems reasonable?

Questions

If the casing is N-80 grade will it fail or even yield?

What if it is one joint?

What if it is 250 joints?

Does it really make no difference as to the length and mass of the casing string?

Let us look at this formula in another scenario



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Suppose . . .

A rigid plate is fixed on one end of a suspended N-80 casing joint:

An archer fires a rigid tipped arrow through the tube striking the center of the rigid plate at the other end with an arrow velocity of 300 ft/s → 534,000 psi tensile impulse load

Or a rigid .22 cal bullet at 900 ft/s → 1.6 million psi tensile impulse load

Are these even remotely reasonable?

Another Example

The elevator opens accidentally and a single joint drops 40 ft but is caught in the spider actuated by an alert floor hand, the floor hand is congratulated

Free fall velocity 50 ft/s → 89,000 psi

Same thing happens with a 10,000 ft casing string. Ignoring friction, 50 ft/s and 89,000 psi again. Is the floor hand going to be congratulated if he catches the casing this time?

Impulse and Momentum in Casing

Impulse loads associated with hitting a ledge or prematurely actuating a spider are real.

Load magnitudes are not accurately predicted by that formula• rigid bodies do not exist • instantaneous load transfers do not occur • impacted structure always absorbs some of the shock

Most dynamic load failures are caused by a fairly rapid change in momentum (which may also constitute an impulse load) but they always depend on the mass and other characteristics of both structures in the impact.


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Momentum

Momentum is mass times its velocity

Change of momentum is a force – Newton’s second law

It is primarily this force we contend with in dynamic loading of casing

p mv p mu

p mu p mu f ma

Change of Momentum

In one dimension (the longitudinal axis of the casing) this leads to an equilibrium equation:

Solutions exist for simple versions of this equation, but ü always depends on the inertial response of other structural components (spider, rotary, substructure, the soil under the rig, or even the structure and buoyancy of the drill ship)

This goes beyond our fundamental design course

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d uA Bu Cu D

dx

Bottom Line !

This overview is as far as we go on dynamic loading.

If you want to use the simple impulse formula go ahead and use it. Many do.

The purpose here is to help you understand it.



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Comments on Casing Selection for Design

Costs

Availability and logistics

Simplicity of design

Minimum number of cross-over joints

Corrosion considerations

Wear considerations

More . . . ?

Chapter Summary

Applied design factors to burst and collapse loads

Selected a preliminary design based on burst and collapse

Calculated axial load of selection

Applied axial design factor and adjusted selection if necessary

Examples• Surface Casing• Intermediate casing• Production Casing

Next Chapter (7):

Review sources of strength ratings

Learn how to account for combined loads

Finalize our designs by adjusting for biaxial loading


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Documents

ILT3 - Surface Compressive Loads and Dynamic Axial Loading