In the detailed design phase, the casing designer determines the material grade and casing wall thickness for each section of the casing scheme selected, which will allow it to withstand all realistically expected loads throughout the life of the well.
Selection of relevant load cases
Before design calculations can be performed for a given casing section, the casing designer must decide which load cases can realistically be expected to occur.
Uniaxial design
The design loads for the load cases selected are determined and compared with the resistances to burst or collapse tabulated in API 5C2 on the basis of the formulae published in API 5C3, after these values have been corrected to take the influence of corrosion, wear and fatigue into account and divided by the relevant design factor. The casing design obtained in this way is then checked to see whether the casing selected can withstand the loads occurring during installation (in particular the axial forces due to the total weight of the casing string down to the depth considered, and the shock and torsional loads experienced during setting the casing).
Uniaxial design generally leads to a conservative choice of tubular grade and wall thickness.
Triaxial design
Triaxial analysis is used to optimise casing design. As with the uniaxial approach, the influence of corrosion, wear and fatigue should be taken into account before the triaxial design factor is applied. Designs that did not meet the uniaxial design rules may know be acceptable following a detailed triaxial stress analysis.
Uniaxial design consists in comparing a uniaxial load (such as a pressure, an axial force or a torque) with a uniaxial load-bearing capacity. Triaxial design methods compare the combined effect of radial, tangential and axial stresses in the casing wall with the material yield strength and represent a more realistic assessment of the ability of the casing to withstand a given load. The stresses can be analysed by using a combination of Hooke's law, the Lamé equations and the Von Mises yield criterion.
Further design considerations
Connections
It is important to ensure that the casing connections withstand the loads to which they are subjected. Recent developments have led to a wide diversity of connection types and sealing compounds for use with casing connections.
Design for special cases
The design steps outlined above are applicable to any casing string or liner. However, special design measures are needed to ensure adequate design in special cases such as high-temperature/high-pressure wells, squeezing salt wells, horizontal wells, etc.
Probabilistic approach to casing design
Probabilistic methods of risk analysis, permitting quantification of the risk of failure associated with a given casing design, might allow further rationalisation of the design.