Design of Cement Sheaths in Offshore Oil Wells Based on Structural Reliability

The design of cement sheaths in offshore oil wells is usually performed following a deterministic approach, which doesn’t consider explicitly the variability and uncertainties in material properties and loads. Cement properties can vary, for instance, due to the intrinsic nature of the cementation process. Load uncertainties are attributed to the random character of the loading environment or to inadequate knowledge of the physical phenomena associated with them. The objective of this work is to present a reliability-based methodology for designing cement sheaths, which uses probability distribution functions to describe material properties and loads, obtained from several laboratory tests and from literature review, respectively. Reliability calculations were performed combining the FORM (First Order Reliability Method) and a dedicated structural analysis computer program developed at COPPE/UFRJ named as TENCIM-1D. The proposed methodology allows an easy determination of failure probabilities with low computational costs. The importance factors derived from FORM help to identify variables that deserve a better statistical description. These results will help the development of future research, setting priorities in new lab tests and field surveys. Additionally, this methodology can contribute to change the way cement sheaths are designed, considering explicitly the uncertainties in materials properties and loads.