ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 26 OF 230 REVISION 0 4.
4.1.
CASING DESIGN
INTRODUCTION
For detailed casing design criteria and guidelines, refer to the ?Casing Design Manual?. The selection of casing grades and weights is an engineering task affected by many factors, including local geology, formation pressures, hole depth, formation temperature, logistics and various mechanical factors.
The engineer must keep in mind during the design process the major logistics problems in controlling the handling of the various mixtures of grades and weights by rig personnel without risk of installing the wrong grade and weight of casing in a particular hole section. Experience has shown that the use of two to three different grades or two to three different weights is the maximum that can be handled by most rigs and rig crews.
After selecting a casing for a particular hole section, the designer should consider upgrading the casing in cases where:
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Extreme wear is expected from drilling equipment used to drill the next hole section or from wear caused by wireline equipment. Buckling in deep and hot wells.
Once the factors are considered, casing cost should be considered.
If the number of different grades and weights are necessary, it follows that cost is not always a major criterion.
Most major operating companies have differing policies and guidelines for the design of casing for exploration and development wells, e.g.:
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For exploration, the current practice is to upgrade the selected casing, irrespective of any cost factor.
For development wells, the practice is also to upgrade the selected casing, irrespective of any cost factor.
For development wells, the practice is to use the highest measured bottomhole flowing pressures and well head shut-in pressures as the limiting factors for internal pressures expected in the wellbore. These pressures will obviously place controls only on the design of production casing or the production liner, and intermediate casing.
The practice in design of surface casing is to base it on the maximum mud weights used to drill adjacent development wells.
Downgrading of a casing is only carried out after several wells are drilled in a given area and sufficient pressure data are obtained.
ARPO ENI S.p.A. Agip Division 4.2. 4.2.1.
Casing Profiles
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 27 OF 230 REVISION 0 PROFILES AND DRILLING SCENARIOS
The following are the various casing configurations which can be used on onshore and offshore wells. Onshore
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Drive/structural/conductor casing Surface casing
Intermediate casings Production casing
Intermediate casing and drilling liners Intermediate casing and production liner Drilling liner and tie-back string.
Offshore - Surface Wellhead As in onshore above.
Offshore - Surface Wellhead & Mudline Suspension
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Drive/structural/conductor casing Surface casing and landing string
Intermediate casings and landing strings Production casing
Intermediate casings and drilling liners Drilling liner and tie-back string.
Offshore - Subsea Wellhead
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Drive/structural/conductor casing Surface casing
Intermediate casings Production casing
Intermediate casing and drilling liners Intermediate casing and production liner Drilling liner and tie-back string.
Refer to the following sections for descriptions of the casings listed above.
ARPO ENI S.p.A. Agip Division 4.3.
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 28 OF 230 REVISION 0 CASING SPECIFICATION AND CLASSIFICATION
There is a great range of casings available from suppliers from plain carbon steel for everyday mild service through exotic duplex steels for extremely sour service conditions. The casings available can be classified under two specifications, API and non-API. Casing specifications, including API and its history, are described and discussed in the ?Casing Design Manual?. Sections 4.3.1 and 4.3.2 below give an overview of some important casing issues.
Non-API casing manufacturers have produced products to satisfy a demand in the industry for casing to meet with extreme conditions which the API specifications do not meet. The area of use for this casing are also discussed in section 4.3.1 below and the products available described in section 4.3.2.
4.3.1.
Casing Specification
It is essential that design engineers are aware of any changes made to the API
specifications. All involved with casing design must have immediate access to the latest copy of API Bulletin 5C2 which lists the performance properties of casing, tubing and drillpipe. Although these are also published in many contractors' handbooks and tables, which are convenient for field use, care must be taken to ensure that they are current. Operational departments should also have a library of the other relevant API publications, and design engineers should make themselves familiar with these documents and their contents.
It should not be interpreted from the above that only API tubulars and connections may be used in the field as some particular engineering problems are overcome by specialist
solutions which are not yet addressed by API specifications. In fact, it would be impossible to drill many extremely deep wells without recourse to the use of pipe manufactured outwith API specifications (non-API).
Similarly, many of the ?Premium? couplings that are used in high pressure high GOR conditions are also non-API.
When using non-API pipe, the designer must check the methods by which the strengths have been calculated. Usually it will be found that the manufacturer will have used the published API formulae (Bulletin 5C3), backed up by tests to prove the performance of his product conforms to, or exceeds, these specifications. However. in some cases, the
manufacturers have claimed their performance is considerably better than that calculated by the using API formulae. When this occurs the manufacturers claims must be critically examined by the designer or his technical advisors, and the performance corrected if necessary.
It is also important to understand that to increase competition. the API tolerances have been set fairly wide. However, the API does provide for the purchaser to specify more rigorous chemical, physical and testing requirements on orders, and may also request place independent inspectors to quality control the product in the plant.
ARPO ENI S.p.A. Agip Division 4.3.2.
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 29 OF 230 REVISION 0 Classification Of API Casing Casing is usually classified by:
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Outside diameter Nominal unit weight Grade of the steel Type of connection Length by range
Manufacturing process.
Reference should always be made to current API specification 5C2 for casing lists and performances.
4.4. 4.4.1.
MECHANICAL PROPERTIES OF STEEL General
Failure of a material or of a structural part may occur by fracture (e.g. the shattering of glass), yield, wear, corrosion, and other causes. These failures are failures of the material. Buckling may cause failure of the part without any failure of the material.
As load is applied, deformation takes place before any final fracture occurs. With all solid materials, some deformation may be sustained without permanent deformation, i.e. the material behaves elastically.
Beyond the elastic limit, the elastic deformation is accompanied by varying amounts of plastic, or permanent, deformation, If a material sustains large amounts of plastic
deformation before final fracture. It is classed as ductile material, and if fracture occurs with little or no plastic deformation. The material is classed as brittle.
4.4.2.
Stress-Strain Diagram
Tests of material performance may be conducted in many different ways, such as by
torsion, compression and shear, but the tension test is the most common and is qualitatively characteristics of all the other types of tests.
The action of a material under the gradually increasing extension of the tension test is
usually represented by plotting apparent stress (the total load divided by the original cross- sectional area of the test piece) as ordinates against the apparent strain (elongation
between two gauge points marked on the test piece divided by the original gauge length) as abscissae.
A typical curve for steel is shown in figure 4.a.
From this, it is seen that the elastic deformation is approximately a straight line as called for by Hooke's law, and the slope of this line, or the ratio of stress to strain within the elastic range, is the modulus of elasticity E, sometimes called Young's modulus. Beyond the elastic limit, permanent, or plastic strain occurs.
If the stress is released in the region between the elastic limit and the yield strength (see above) the material will contract along a line generally nearly straight and parallel to the original elastic line, leaving a permanent set.
ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 30 OF 230 REVISION 0
Figure 4.A- Stress - Strain Diagram
In steels, a curious phenomenon occurs after the end of the elastic limit, known as yielding. This gives rise to a dip in the general curve followed by a period of deformation at
approximately constant load. The maximum stress reached in this region is called the upper yield point and the lower part of the yielding region the lower yield point. In the harder and stronger steels, and under certain conditions of temperature, the yielding phenomenon is less prominent and is correspondingly harder to measure. In materials that do not exhibit a marked yield point, it is customary to define a yield strength. This is arbitrarily defined as the stress at which the material has a specified permanent set (the value of 0.2% is widely accepted in the industry).
For steels used in the manufacturing of tubular goods the API specifies the yield strength as the tensile strength required to produce a total elongation of 0.5% and 0.6% of the gauge length.
Similar arbitrary rules are followed with regard to the elastic limit in commercial practice. Instead of determining the stress up to which there is no permanent set, as required by definition, it is customary to designate the end of the straight portion of the curve (by definition the proportional limit) as the elastic limit. Careful practice qualifies this by designating it the ?proportional elastic limit?.