Deep-water wells for oil and gas extraction make structural components, such as casing and tubing , work in extremely harsh environmental conditions that accelerate component degradation and increase failure probability. Therefore, it is important to properly design casing strings under these operative circumstances. To do this, a Resistance-Based Probabilistic Design (RBPD) approach is here complemented with the quantification of the probabilistic safety margin that the design load does not exceed the casing strength. Wear is taken as the main degradation mechanism during drilling and the accidental event of a kick load is considered to affect the performance of the casing string. By so doing, we consider all uncertainties affecting the casing burst strength and evaluate the string safety by the Order Statistic (OS) method.