The objective of the casing design is to define a set of casing strings (casing scheme), capable of withstanding a variety of external and internal pressures, thermal loads and loads related to the self-weight of the casing. These casing strings are subjected to time-dependent corrosion, wear and possibly fatigue, which downrate their resistance to these loads during their service life.
Objectives of casing design
- protect from sloughing shales or moving salt formations;
- isolate the reservoirsfrom unwanted fluids
- protect fresh-water horizons;
- provide a means to handle kicks;
- conduit for produced fluid;
- conduit for drilling, logging and completion tools;
- provide a smooth conduit for future casing and tubing strings;
- support wellhead equipment and subsequent casing strings;
- provide a means of anchoring the blowout preventers and Xmas tree.
Casing strings are subjected to corrosion, wear and fatigue, which down rate their resistance to these loads during their service life. The interaction between the casing strings - which may lead to annular pressure build-up or wellhead movement also merits attention.
Casing design information should be available at the wellsite, to ensure that the operating envelope remains within the design criteria.
Casing design steps
- collection of all the relevant data by a multi-disciplinary team
- selection of the casing scheme which is most cost-effective over the entire life cycle of the well (Casing/tubing represents ±15% of the drilling expenditure).
- definition of the various load cases to which each casing string is likely to be subjected.
- evaluation of the casing string to withstand the applied loads.
The interaction between the casing strings - which may lead e.g. to annular pressure build-up or wellhead movement - also merits attention.
Sequence of design criteria considerations
- Burst
- Collapse
- Tension
- Couplings
- Multi-axial corrections
- Cost
For pressure less than 8000 psi, biaxial corrections should always be applied. If pressure is greater than 8000-10000 psi, tri-axial analysis should be performed.
Sequence for graphical design
- Maximise the load
- Minimise the backup
- Calculate the resultant
- Select the design factor
- Calculate the “design line”
Casing catalogue specifications
Nominal diameter
For steel casing, drill pipe and tubing it is the OD of the tube.
For line pipe and fiberglass casing it is the ID of the tube
Grade:
Example: F25, H40, J or K50, L75, L or N 80, C90, C95, C100, C110, P105, P110, Q125, V150
The first letter doesn’t mean anything specific except:
C: Corrosion (less strong more expensive but corrosion resistant)
C 100, C110 can also be used for H2S applications
N80 40 lbs/ft 5750 psi is 10 % cheaper than C75 40 lbs/ft 5390 psi
L: H2S application
- HRC < 22
- Same hardness cross-wall: No stress concentration. The difference between HRC inside and outside the pipe wall is less than 3.
J55 may be welded on the side
K55 seamless
Average Yield Stress = Minimum Yield Stress + 10000
Collapse
Based on D/t ratio, various collapse modes can be considered:
- Yield collapse: loading the inside of the pipe to reach yield stress. No deformation
- Plastic collapse: empirical
- Transition collapse: made up by the API
- Elastic collapse: theoretical calculation
Burst
2 equations available:
- Barlow: thin wall cylinder
- Lamé: thick wall cylinder. To be used when D/t correspond to Yield collapse. this will give approx. 5% more strength.
Design factor:
Should account for wear and should not be the same for a vertical well followed by 2 days drilling vs a deviated well with 30 days drilling. Do not use Spiral hard face stabs accross casings.