Casing wear by drillstring results in a thinner portion of casingwall and a reduction on casing burst strength (the ability tohold internal pressure). How to estimate the reduced casingburst strength on such a “crescent-worn” casing has been animportant issue in oil and gas industry, as it is directly relatedto how to safely design casing strings. A common approach isto estimate the reduction of casing burst strength of such worncasing from API burst strength equation with a linearreduction by the remaining wall thickness or the wearpercentage, equivalent to a “uniform-worn” casing model,despite a question on whether such a linear reduction of casingburst strength is over-conservative and may result in a highercasing cost.This paper presents a further study on hoop stress anddeformation of such a “crescent-worn” casing and discussesthe reduction of casing burst strength. The hoop stress in thethinner portion of such a “crescent-worn” casing is foundclose to that from a “uniform-worn” casing, when l°Calbending in the thinner portion of “crescent-worn” casing isignored. The reduction of burst strength of casing worn bydrillstring may still be estimated from casing yield or ruptureburst strength with a linear reduction by wear percentage forsweet service well conditions, while more and non-linearreduction of burst strength of casing worn by drillstring maybe needed for sour service well conditions, to prevent an“earlier” casing burst on sulfide stress cracking.
展开▼
