The initial casing design, is not an exact reflection of the service loads, as it assumes that the casing string is not fixed at either end. It must be ensured that the incremental stresses resulting from changes in pressure, temperature, and applied point loads once the casing is cement do not cause it to fail.
Also instability, i.e. occurrence of buckling, should be checked for.
With respect to stress in the axial direction, the as-cemented axial stress, upon which all subsequent changes are superimposed, is that due to the static load immediately after the cement slurry has been displaced to its final position. The effect of cement gellation on buoyancy is not well known. The design factor, takes these unknowns into account.
For the uniaxial analysis, calculation of the resulting incremental axial force will enable the total actual axial force to be compared against the pipe axial capacity. Also the reduced axial force, Fa*, can then be compared to the critical buckling force. The collapse and burst loads can be compared to the uniaxial collapse and burst capacities as documented in API Bull. 5C3. For the more advanced analysis the equivalent stress (sVME) should be compared to the yield strength (sy).
The following sections describe the pressure, temperature, and point loads that may result in incremental stresses for which a design check is required.
1. Pressure loads
Actual axial forces
- The actual axial force within both uncemented and cemented sections of the casing should be determined for the burst and collapse load cases as established in the initial pressure load design.
- The actual axial force resulting from anticipated changes to the as-cemented internal and external fluid densities should be calculated e.g. increased internal fluid density for deeper drilling, reduced external fluid density due to mud deterioration with time.
- The actual axial force resulting from anticipated changes to the as-cemented internal and external surface pressures must be calculated e.g. increased external surface pressure due to a live annulus, increased internal surface pressure due to a pressure test against a retrievable packer.
Collapse and burst loads
1.If in the initial design the poor cement bond scenario was used in the collapse design, the possibility of a live annulus has already been taken into account.
If however, the good cement bond scenario was adopted, but possible annulus pressures are to be checked for, a check should be made of the maximum allowable annulus pressure. Depending upon this pressure, a judgement must be made between design of the casing and control i.e. bleed-off of any such pressures. Also possible leak off at the casing shoe will limit the pressure development in the annulus under consideration.
Possible burst of the outer casing and collapse of the inner casing should be addressed under such circumstances.
2.Some well servicing operations e.g. stimulation treatments result in a considerable increase in the bottomhole pressure. Any communication path behind the pipe will allow possible pressurisation to extend outside the zone that is directly affected. This may result in a collapse load being applied to any casing section which is not itself internally pressured, e.g. casing above the packer or bridge plug.
It is therefore advisable that the design of the production casing, to be set across the reservoir subject to stimulation operations, is checked for ability to withstand these pressures.
Reduced axial forces
The value of reduced axial force, Fa*, in the uncemented section resulting from changes in pressure should be checked against the critical buckling force.
2. Temperature loads
Actual axial forces
1.The axial force within both cemented and uncemented sections of the casing, resulting from linear thermal expansion caused by a change in temperature of the casing material, should be determined. Specially the forces generated by injecting cold fluids can be considerable.
The effect of wellhead thermal growth for offshore platform wells should be taken into account in such calculations.
2.The axial force resulting from thermal expansion of fluids contained in sealed annuli should be determined. Whether the casing should be designed to withstand these loads will depend on the ability to bleed off these pressures. Such pressures will not return once bled off. This in contrast to pressures caused by a live annulus.
Collapse and burst loads
Burst and collapse loads resulting from thermal expansion of fluids contained in sealed annuli should be determined. Whether the casing should be designed to withstand these loads will depend on the ability to bleed off these pressures. Such pressures will not return once bled off. This in contrast to pressures caused by a live annulus.
Possible burst of the outer casing and collapse of the inner casing should be addressed under such circumstances.
Reduced axial forces
The value of the reduced axial force in the uncemented section resulting from temperature changes as mentioned in Section 3.2.4.3.1 should be checked against the critical buckling force.
3. Point loads
Production packer
The most common example of a point load is that due to a production packer set in the production casing and to which a load is applied by landing the tubing in compression/tension. The resulting actual axial force, both above and below the production packer, should be checked.
Retrievable packer
A pressure test with a retrievable packer does not only introduce pressure loads onto the casing but also a change in the axial stress. The resulting axial stress, both above and below the retrievable packer should be checked.
Conductor casing
One particular form of a point load is the surface loading of the conductor casing of any well. The applied load in this instance is the weight of the inner casing strings, the wellhead and BOP or Xmas tree, and the completion tubulars.
Reduced axial forces
The value of the reduced axial force in the uncemented section resulting from point loads as mentioned previously should be checked against the critical buckling force.