API Casing Strengths – Audit
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API Casing strengths Worksheet, release 10, issued 30 August 2019. AUDIT version.
This worksheet exists in two versions. They are identical apart from the way they are formatted. The Work version hides intermediate calculations and allows the user to see the results just below the inputs. This is useful for quick “what-if” games, changing various inputs to see what works best. The audit version displays all intermediate calculations.
ALL the figures published in API 5C2 for burst, tension, collapse and biaxial have been checked against this worksheet. Only three are different by more than rounding errors. These are noted at the bottom of the worksheet.
Description
This worksheet takes a set of inputs typical for tubular design. It uses the formulae defined in the referenced documents to calculate the tubing design parameters and will account for the effects of temperature, internal pressure and tension. All tension and burst results have been verified OK against API 5C2 Table 1. All biaxial collapse resistances have been verified against API 5C2 Table 4.
Mathcad uses metric units internally so if imperial units are entered, they are converted to metric. Intermediate results are metric. Both units will be given as outputs.
There is no tutorial version of this worksheet but all of the individual elements have tutorials with other worksheets, such as Collapse. There is an Audit version however which allows the user to verify all of the procedures used.
User input
| 4.5″ 9.5 ppf4.5″ 10.5 ppf4.5″ 11.6 ppf4.5″ 13.5 ppf4.5″ 15.1 ppf5″ 11.5 ppf5″ 13 ppf5″ 15 ppf5″ 18 ppf5″ 21.4 ppf5″ 23.2 ppf5″ 24.1 ppf5.5″ 14 ppf5.5″ 15.5 ppf5.5″ 17 ppf5.5″ 20 ppf5.5″ 23 ppf5.5″ 26.8 ppf5.5″ 29.7 ppf5.5″ 32.6 ppf5.5″ 35.3 ppf5.5″ 38 ppf5.5″ 40.5 ppf5.5″ 43.1 ppf6.625″ 20 ppf6.625″ 24 ppf6.625″ 28 ppf6.625″ 32 ppf7″ 17 ppf7″ 20 ppf7″ 23 ppf7″ 26 ppf7″ 29 ppf7″ 32 ppf7″ 35 ppf7″ 38 ppf7″ 42.7 ppf7″ 46.4 ppf7″ 50.1 ppf7″ 53.6 ppf7″ 57.1 ppf7.625″ 24 ppf7.625″ 26.4 ppf7.625″ 29.7 ppf7.625″ 33.7 ppf7.625″ 39 ppf7.625″ 42.8 ppf7.625″ 45.3 ppf7.625″ 47.1 ppf7.625″ 51.2 ppf7.625″ 55.3 ppf7.75″ 46.1 ppf8.625″ 24 ppf8.625″ 28 ppf8.625″ 32 ppf8.625″ 36 ppf8.625″ 40 ppf8.625″ 44 ppf8.625″ 49 ppf9.625″ 32 ppf9.625″ 36 ppf9.625″ 40 ppf9.625″ 43.5 ppf9.625″ 47 ppf9.625″ 53.5 ppf9.625″ 58.4 ppf9.625″ 59.4 ppf9.625″ 64.9 ppf9.625″ 70.3 ppf9.625″ 75.6 ppf10.75″ 32.75 ppf10.75″ 40.5 ppf10.75″ 45.5 ppf10.75″ 51 ppf10.75″ 55.5 ppf10.75″ 60.7 ppf10.75″ 65.7 ppf10.75″ 73.2 ppf10.75″ 79.2 ppf10.75″ 85.3 ppf11.75″ 42 ppf11.75″ 47 ppf11.75″ 54 ppf11.75″ 60 ppf11.75″ 65 ppf11.75″ 71 ppf13.375″ 48 ppf13.375″ 54.5 ppf13.375″ 61 ppf13.375″ 68 ppf13.375″ 72 ppf16″ 65 ppf16″ 75 ppf16″ 84 ppf16″ 109 ppf18.625″ 87.5 ppf20″ 94 ppf20″ 106.5 ppf20″ 133 ppf |
| Short RoundLong RoundButtress NormalButtress Special ClearanceOther |
Select a casing
Select a connection type.
For the OD and connection selected, additional weight per
If no connection data exists in the worksheet, you can enter a figure here for the additional weight per connection. If no value is entered in this field, then the worksheet will instead use the API nominal weight per foot to calculate downhole stresses.
| lbkg |
Manually enter the additional weight per connection. Wtconn =
| 42 |
| ftm |
Enter average joint length. L =
| 50000 |
| kgflbfNkN |
Enter axial force at the depth of interest (tensile is +ve). Fa =
| 75 |
| FCK |
Enter the temperature at the depth of interest. Temp =
| 1000 |
| psiPakPaMPa |
Enter the internal pressure at the depth of interest. pi =
| 80 |
Enter the number part of the steel grade eg for L80, enter 80 (units are kpsi). Gr =
| 1 |
Enter design factor for collapse. DFc =
| 1.15 |
Enter design factor for burst. DFb =
| 1.31 |
See the recommended DF in the results below.
Enter design factor for tension. DFt =
Click here when any values are modified to update the result.
Calculations
Calculate the pipe cross sectional area.
Calculate the API Minimum Yield Strength
From API 5CT, OD tolerance for pipe smaller than 4.5″ is +/- 0.031″ (0.019 mm). For 4.5″ or larger, OD tolerance is +1.00%, -0.5%.
From API 5CT, tolerance on wall thickness is +0 -12.5%. The nominal wall thickness is therefore also the maximum wall thickness. Calculate minimum wall thickness.
Under the API 5CT manufacturing tolerances, the largest ID will occur when OD is largest and wall thickness is thinnest.
The smallest ID will occur when OD is smallest and wall thickness is greatest.
Drift diameter depends on the pipe OD and is defined in Table 54 of API 5CT. It is related to the nominal inside diameter.
Convert temperature to centigrade
Calculate the ID for the smallest possible cross sectional area (minimum steel used in manufacture).
Calculate the possible cross sectional areas for the different OD’s and wall thicknesses.
The recommended design factor in tension accounts for dimensional tolerances and limits stress to 90% of API MY stress.
Calculate the possible API Minimum Yield Strengths (uncorrected for temperature)
Calculate the possible API Minimum Yield Strengths (corrected for temperature and with design factor)
Calculate the API Minimum Internal Yield strength from the API 5C3 formula of the pipe body. Note the constant 0.875, which shows that API assume that the wall will be the thinnest it can be under the wall thickness dimensional tolerances.
and withDF and TCF;
The formula for Yield Collapse should be used when the D/t ratio is less than Dtyp;
NOTE – formula 36 in ISO/TR10400 is incorrect and uses Minimum Yield Strength fymn when it should use Minimum Yield Stress σymn. The equation here is from API 5C3.
The formula for Plastic Collapse should be used when Dt is above DtYp but less than;
The formula for Transition Collapse should be used when Dt is above DtPT but less than;
The formula for Yield Collapse should be used when Dt is less than Dtyp.
The formula for Plastic Collapse should be used when Dt is above DtYp but equal to or less than Dtp.
The formula for Transition Collapse should be used when Dt is above Dtp but equal or less than DtT.
The formula for Elastic Collapse should be used when Dt is above DtT.
Calculate the axial stress σa arising in the pipe as a result of the tensile or compressive load.
To calculate the reduced collapse resistance for a pipe under axial tension, the yield stress is modified by the following formula;
The revised empirical constants and collapse resistances are now recalculated as before;
The ranges for Dt to use to select the correct formula are also revised with the modified yield stress.
The formula for Yield Collapse should be used when the D/t ratio is less than Dtyp;
The formula for Plastic Collapse should be used when Dt is above DtYp but less than;
The formula for Transition Collapse should be used when Dt is above DtPT but less than;
Calculate for each of the four formula; only one should return a result.
Apply the Temperature Correction Factor and Design Factor;
Adjust the final figure with Biaxial, TCF and DF for internal pressure of or .
Adjust for internal pressure.
Weight per unit length of pipe;
Weight per unit length, including connections;
Percentage of axial stress related to Minimum Yield Stress
Results
Maximum possible pipe OD within API tolerances
Minimum possible pipe OD within API tolerances
API nominal wall thickness
Ratio of OD to nominal wall thickness, D/t ratio
Minimum possible wall thickness within API tolerances
API nominal ID
API Drift Dia
ID for minimum amount of steel used within tolerances
Nominal cross sectional area
Minimum possible cross sectional area
Recommended Design Factor for tension
Temperature Correction Factor
API Minimum Yield Strength, nominal, ambient
API Minimum Yield Strength, with TCF and DF
Pipe body API Minimum Burst Strength, ambient
Pipe body API Minimum Burst Strength, with TCF and DF
Pipe body axial stress at nominal CSA
API collapse resistance, ambient, no axial force
API collapse resistance with TCF, DF, Biaxial & internal pressure
Plain end pipe weight, nominal dimensions
Pipe weight average with connections, nominal dimensions
CHECK CONNECTION STRENGTHS vs PIPE STRENGTHS AND USE THE LEAST!!
Worksheet references
API Specification 5CT, 5th Edition, April 1 1995 “Specification for Casing and Tubing (U.S. Customary Units)”.
API Bulletin 5C3, 6th Edition, October 1 1994 “Bulletin on Formulas and Calculations for Casing, Tubing, Drill Pipe, and Line Pipe Properties”, also ISO/TR10400:2007 which is expected to replace API 5C3 during 2008.
API Bulletin 5C2, 21st Edition, October 1999 “Bulletin on Performance Properties of Casing, Tubing and Drill Pipe”.
Formulae for Temperature Correction Factor taken from the Exxon Casing Design Manual.
SG of steel from http://www.simetric.co.uk/si_metals.htm taken as 7.85.
Version 1 of this worksheet released on 8 January 2008.
Release 8 released after testing against API 5C2 21st Edition for burst, tension, collapse and biaxial. The only significant differences were found in Table 4 (biaxial) as follows, with all other results the same within normal rounding;
9.625″ 40ppf Grade 80 at -10,000 psi axial load, this worksheet showed 3167 psi vs API 3107
7.625″ 45.3ppf Grade 95 at -10,000 psi axial load, this worksheet showed 14,241 psi vs API 14,330
9.625″ 43.5ppf Grade 110 at 25000 psi axial load, this worksheet showed 4205 psi vs API 4130
Release 8a fixed a small problem in the casing list, where the 8.625″ 44ppf casing appeared at the end of the list rather than in the correct order.
Release 9 of this worksheet added a recommended Design Factor in tension for the selected pipe. This accounts for dimensional tolerances to compare nominal cross sectional area with the minimum possible cross sectional area and limits the uniaxial tensile stress to 90% of API Minimum Yield Stress.
Release 10 made a correction to the factor to calculate the minimum wall thickness, -12.5% of nominal. The factor 0.85 had been used rather than 0.875.
