ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 31 OF 230 REVISION 0 As extension continues beyond yielding, the material becomes stronger causing a rise of the curve, but at the same time the cross-sectional area of the specimen becomes less as it is drawn out. This loss of area weakens the specimen so that the curve reaches a maximum and then falls off until final fracture occurs.
The stress at the maximum point is called the tensile strength (TS) or the ultimate strength of the material and is its most often quoted property.
The mechanical and chemical properties of casing, tubing and drill pipe are laid down in API specifications 5CT and 5C2.
Depending on the type or grade, minimum requirements are laid down for the mechanical properties, and in the case of the yield point even maximum requirements (except for H 40). The denominations of the different grades are based on the minimum yield strength, e.g.:
Grade H 40 J 55 C 75 N 80 etc.
In the design of casing and tubing strings the minimum yield strength of the steel is taken as the basis of all strength calculations
As far as chemical properties are concerned, in API 5CT only the maximum phosphorus and sulphur contents are specified, the quality and the quantities of other alloying elements are left to the manufacturer.
API specification 5CT ?Restricted yield strength casing and tubing? however specifies, the complete chemical requirements for grades C 75, C 95 and L 80.
Min. Yield Strength
40,000psi 55,000psi 75,000psi 80,000psi
4.5.
NON-API CASING
Eni-Agip Division and Affiliates policy is to use API casings whenever possible. Some
manufacturers produce non-API casings for H2S and deep well service where API casings do not meet requirements. The most common non-API grades are shown in the Casing Design Manual (STAP-P-1-M-6110-4.3).
Reference to API and non-API materials should be made to suit the environment in which they are recommended to be employed.
ARPO ENI S.p.A. Agip Division 4.6.
CONNECTIONS
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 32 OF 230 REVISION 0 The selection of a casing connection is dependant upon whether the casing is exposed to wellbore fluids and pressures. API connections are normally used on all surface and
intermediate casing and drilling liners. Non-API or premium connections are generally used on production casing and production liners in producing wells.
API connections rely on thread compound to form the seal and are not recommended for sealing over long periods of time when exposed to well high pressures and corrosive fluids as the compound can be extruded exposing the threads to corrosive fluids which in turn reduces the strength of the connection. Sealing on premium connections are provided by at least one metal-to-metal seal which prevents this exposure of the threads to corrosive elements, hence, retains full strength.
The properties of both API and non-API connections are described below.
4.6.1.
API Connections
The types of API connections available are:
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Round thread short which is coupled. Round thread long which is coupled.
Buttress thread which is coupled, with both normal and special clearance. Extreme line thread which is integral with either normal or special clearance.
Round thread couplings, short or long, have less strength than the corresponding pipe body. This in turn requires heavier pipe to meet design requirements, than if the pipe and coupling had the same strength. Problems like ?pullouts? or ?jump-outs? can happen with round thread type coupling on 103/4\doglegs, directional drilled holes. etc.
Buttress threads have, according to API calculations, higher joint strength than the pipe body yield strength with a few exceptions. Buttress threads also stab and enter easier than round threads, therefore, should be used whenever possible, except for 20\where special connections could be beneficial due to having superior make-up characteristics.
API round threads and buttress threads have no metal to metal seals. As stated earlier, the seal in API thread is created by the thread compound which contains metal which fill the void space between the threads. When subjected to high pressure gas, temperature variations, and/or corrosive environment this sealing method may fail. Therefore, in such conditions, connections with metal-to-metal seals, should be utilised.
According to API standards the coupling shall be of the same grade as the pipe except grade H 40 and J 55 which may be furnished with grade J 55 or K 55 couplings. For connection dimensions refer to the current API specification.
ARPO ENI S.p.A. Agip Division 4.7.
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 33 OF 230 REVISION 0 APPROACH TO CASING DESIGN
Casing design is basically a stress analysis procedure which is fully described in the ?Casing Design Manual?.
As there is little point in designing for loads that are not encountered in the field, or in
having a casing that is disproportionally strong in relating to the underlying formations, there are clearly four major elements to casing design:
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Definition of the loading conditions likely to be encountered throughout the life of the well.
Specification of the mechanical strength of the pipe.
Estimation of the formation strength using rock and soil mechanics.
Estimation of the extent to which the pipe will deteriorate through time and quantification of the impact that this will have on its strength.
4.7.1.
Wellbore Forces
Various wellbore forces affect casing design. Besides the three basic conditions (burst, collapse and axial loads or tension), these include:
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Buckling.
Wellbore confining stress. Thermal and dynamic stress.
Changing internal pressure caused by production or stimulation. Changing external pressure caused by plastic formation creep. Subsidence effects and the effect of bending in crooked hole. Various types of wear caused by mechanical friction. H2S or squeeze/acid operations. Improper handling and make-up.
This list is by no means comprehensive because new research is still in progress. The steps in the design process are: 1)
Consider the loading for burst first, since burst will dictate the design for most of the string.
Next, the collapse load should be evaluated and the string sections upgraded if necessary.
Once the weights, grades and section lengths have been determined to satisfy the burst and collapse loading, the tension load can then be evaluated.
The pipe can be upgraded as necessary as the loads are found and the coupling type determined.
The final step is a check on biaxial reductions in burst strength and collapse
resistance caused by compression and tension loads, respectively. If these reductions show the strength of any part of the section to be less than the potential load, the section should again be upgraded.
2) 3) 4) 5)
ARPO ENI S.p.A. Agip Division 4.7.2.
Design Factor (DF)
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 34 OF 230 REVISION 0 The design process can only be completed if knowledge of all anticipated forces is
available. This however, is idealistic and never actually occurs. Some determinations are usually necessary and some degree of risk has to be accepted.
The risk is usually due to the assumed values and therefore the accuracy of the design factors used.
Design factors are necessary to cater for:
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Uncertainties in the determination of actual loads that the casing needs to
withstand and the existence of any stress concentrations, due to dynamic loads or particular well conditions.
Reliability of listed properties of the various steels used and the uncertainty in the determination of the spread between ultimate strength and yield strength. Probability of the casing needing to bear the maximum load provided in the calculations.
Uncertainties regarding collapse pressure formulas.
Possible damage to casing during transport and storage.
Damage to the steel from slips, wrenches or inner defects due to cracks, pitting, etc.
Rotational wear by the drill string while drilling.
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The DF will vary with the capability of the steel to resist damage from the handling and running equipment.
The value selected as the DF is a compromise between margin and cost. The use of excessively high design factors guarantees against failure, but provide excessive strength and, hence, cost.
The use of low design factors requires accurate knowledge about the loads to be imposed on the casing.
Casing is generally designed to withstand stress which, in practice, it seldom encounters due to the assumptions used in calculations, whereas, production tubing has to bear pressures and tensions which are known with considerable accuracy.
Also casing is installed and cemented in place whereas tubing is often pulled and re-used. As a consequence a of this and due to the fact that tubing has to combat corrosion effects from formation fluid, a higher DF is used for tubing than casing.
ARPO ENI S.p.A. Agip Division 4.7.3.
Design Factors
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 35 OF 230 REVISION 0 Casing Grade Design Factor ? H 40 1.05 J 55 1.05 K 55 1.05 C 75 1.10 Burst L 80 1.10 N 80 1.10 C 90 1.10 C 95 1.10 P 110 1.10 Q 125 1.20 Collapse All Grades 1.10 < C-95 1.70 Tension > C-95 1.80 Note The tensile DF must be considerably higher than the previous factors to avoid exceeding the elastic limit and, therefore invalidating the criteria on which burst and collapse resistance are calculated. 4.7.4.
Application of Design Factors
The minimum performance properties of tubing and casing from the ?API? bulletin are only
used to determine the chosen casing is within the DF.
The following DF?s must be used in casing design calculations:
Burst
For the chosen casing (diameter, grade, weight and thread) take the lowest value from API casing tables columns 13-19. This value divided by DF gives the internal pressure resistance of casing to be used for design calculation
Collapse Tension
Use only column 11 of API casing tables and divide by the DF to obtain the collapse resistance for design calculation. Use the lowest value from columns 20-27 of the API casing tables and divide by the DF to obtain the joint strength for design calculation.
Note:
It should be recognised that the Design Factor used in the context of casing string design is essentially different from the ‘Safety Factor’ used in many other engineering applications.
The term ?Safety Factor? as used in tubing design, implies that the actual physical properties and loading conditions are exactly known and that a specific margin is being allowed for safety. The loading conditions are not always precisely known in casing design, and therefore in the context of casing design the term ?Safety Factor? should be avoided.