Service loads

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The previous article presented the methods to quantify the installation and as-landed loads. The purpose of this article is to present methods to quantify the changes in stress in the cemented casing due to departure from the as-cemented conditions caused by changes in pressure and temperature.

The temperature related service loads and the estimation of the increase in annulus pressures is discussed in detail previously.

The resulting changes in stress should be added to the as-cemented stress to result in a new stress state.

The as-cemented stress state should be calculated using the casing weight and pressures resulting from fluid columns present at the end of the casing cementation. This is because the effects of cement gellation on buoyancy loads are not well known. The design factor, takes these unknowns into account.

In the following paragraphs the methods to quantify the changes in stresses will be presented for a limited number of service loads. However, it will become clear that the manual solution of these problems is cumbersome. The presented equations are meant to allow a first order approximation of the relevant stresses, but computing tools should assist where in depth analysis is required.

Changes in tangential stress: the tangential stress, st, is solely dependent on the internal and external pressures.  As a result, change in this stress, can only result from pressure changes.

Change in radial stress: the radial stress, sr, is solely dependent on the internal and external pressures.  As a result, change in this stress, can only result from pressure changes.

Change in axial stress:  The general equation will be developed for the change in axial stress in a partially cemented string due to a departure from the as-cemented conditions. A modified form of this general equation will then be presented for a variety of common drilling and production operations conditions.

Increased temperature can affect a cemented casing string in two ways. Firstly, stresses will increase due to steel linear expansion effects. Secondly, an increase in fluid pressure will occur in sealed casing annuli, resulting in stress changes as described in the previous sections.

The most common example of point loading in a cemented casing string is the application of a tubing compressive or tensile load to a Annular Safety Valve, suspension packer or production packer set in the casing. Analysis of the resulting additional stresses in the casing depends on the location of the point load.

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