Introduction to Casing Design Loads (Collapse, Burst and tension Load)
1 Collapse Load
The design load case for collapse is based on a partial evacuation of the casing string.
For partial evacuation during the drilling phase, the internal pressure profile is based on mud losses to pore pressure. Full evacuation after a blow-out is evaluated as a special case.
For the production casing with gas lift facilities, full evacuation above the packer is used as worst design load case. The external pressure profile for collapse is constructed in two sections; that for the cement column and that for the annulus fluid column.
Corrosion, wear and downrating because of tension is treated separately.
The uniaxial design factor used for collapse design is 1.0. The collapse capacity should be downrated according to the maximum temperature to which the casing will be exposed when the collapse load can occur.
2 Burst Load
For the drilling phase, the worst case internal pressure loading corresponds to full displacement of the casing to gas and the well closed in at surface. Since the HC-water contact in the structure is known, the chosen gradient should be assumed to originate from this depth. When the gas/oil ratio is very low, the relevant oil gradient may be used.
For burst loading during the production phase, a tubing failure with CITHP acting on top of the packer fluid is used as worst design load case. The external pressure profile is equal to the one used for collapse load.
When evaluating the burst capacity of a casing, a down rating because of wear, corrosion, temperature and applied compression is required before the design factor is introduced.
The uniaxial design factor for burst design is 1.1.
3 Tensile Load
3.1 Installation Loads
The dynamic and static installation loads are experienced during the installation of the casing and during cementation and pressure testing.These loads should include:
- Casing weight (in air) loads
- Pressure (buoyancy) loads
- Bending loads
- Dynamic drag loads
- Shock loads
- Point loads
- Static drag loads
The maximum expected axial load during the dynamic part of the installation phase is the greater of:
Either:
Casing weight (in air) load
- Pressure (buoyancy) load
- Bending load
- Drag load
Or:
Casing weight (in air) load
- Pressure (buoyancy) load
- Bending load
- Shock load
The maximum expected axial load during the static part of the installation phase is the greater of:
Either:
Casing weight (in air) load
- Pressure (buoyancy) load
- Bending load
- Point load due to pre-cementation pressure test (against a retrievable packer)
Or:
Casing weight (in air) load
- Pressure (buoyancy) load
- Bending load
- Point load due to post-cementation pressure test (against the sealed float shoe)
The uniaxial design factor for tension design is 1.3.
3.2 Service Loads
The calculations on service loads are done to ensure that the triaxial incremental stresses in the pipe body resulting from changes in pressure, temperature, and applied point loads relative to the "as-cemented" condition, do not cause the casing to fail. Also instability, i.e. occurrence of buckling is checked for.
The calculations on service loads can be divided into three main groups as follows:
3.2.1. Pressure loads
Triaxial stress analysis
- Tangential stress
- Radial stress
- Axial stress
a) Increase in internal fluid density
b) Partial evacuation
c) Reduction in external fluid density
- Von Mises Equivalent stress
- Use of the WELLCAT program
Buckling potential analysis
- Reduced axial force
- Buckling resistance
3.2.2. Temperature loads
Triaxial stress analysis
- Linear casing expansion
- Annulus fluid expansion
Buckling potential analysis
- Reduced axial force
- Buckling resistance
3.2.3. Point loads
Example: Loads applied to production packers when installing the completion
The following design factors are applicable:
- Uniaxial tension design factor: 1.3
- Uniaxial compression design factor: 1.0
- Triaxial design factor: 1.25