Equations to Calculate Collapse Strength for High Collapse Casing

As wells are drilled deeper, the external pressures applied to well casing become greater. Conventional America Petroleum Institute (API) casing strength cannot meet the strength criteria of high pressure, high temperature, and high H₂S (HPHTHS) gas wells which are called “3-high” gas wells. When high collapse casing (HCC) is applied in oil fields, it has obviously improved collapse properties in excess of API ratings. HCC shows a very high resistance to tension load, internal pressure, and collapse, as well as being highly resistant to sulfide stress corrosion cracking (SSCC), and it also can be used for deep and sour gas and oil fields. For imperfections of the API 5C3 collapse formula, the joint API/ISO work group ISOTC67 SC5 WG2b have proposed the current API Bulletin 5C3, and a new collapse strength model with manufacturing imperfections, such as ovality, eccentricity, residual stress, etc., improves the casing strength calculation accuracy and increases the benefits for casing strength design, rather than just using API Bulletin 5C3. The study on the new ISO collapse model has found that it is inappropriate to predict the collapse strength of the high collapse casing. As a result, on the basis of my work group results, a new high collapse model for predicting the collapse strength of all HCC has been presented. Numerical and experimental comparisons show that the “new high collapse model” predicts higher accuracy than that of ISO, and this will make great improvements in the casing design of deep and ultradeep wells on the basis of HCC material safety, which was guaranteed.