Analysis of Complex Wellhead Load Events for Conductor and Surface Casing Strings

Abstract

Abstract The analysis of mechanical and thermal-induced wellhead growth and resultant loads can be critical for tubular stress design and failure analysis of offshore platform wells. Standard simulation tools are typically used for this analysis. However, some common complex wellhead loading events are not explicitly accounted for in currently available software. Analysis of wellhead growth and loads from first principles is used to investigate these complex load events. Particular attention is given to conductor and surface casings and some non-intuitive results are discussed. A case study is presented to illustrate a range of field applications. The analysis and related methodology apply to complex load events including: tension from conductor tension units (CTU's) or top-tension riser (TTR) systems, order of CTU overpull and landing of subsequent strings, lift-off of strings without lock-down, lift-off at mud-line hanger (MLH) systems and loss of pre-existing casing strings due to structural failure. It is commonly recognized that standard single-string casing design which ignores wellhead movement is conservative for most casing strings. Similarly, it is understood that this single-string analysis is non-conservative for the outermost string. However, the extent and nature of non-conservatism as well as identification of all strings for which the standard analysis is truly non-conservative is not always understood. Also, the effect of sequencing CTU overpull and landing subsequent strings is also not always recognized. The analysis methodology and case studies discussed in this work provide an explicit and intuitive approach to multistring wellhead movement and loads which can be implemented in spreadsheet calculations to model each step in the loading sequence on the wellhead. The nature of wellhead loads on the outermost conductor or surface string compared to inner strings is explained. The effect of CTU loads and the sequence of application are also explained. Certain types of wellhead load events, which are not accommodated using available commercial software tools, are modeled and analyzed. Even when numerically based software simulation tools do accommodate the load conditions considered, a closed-form analytical model based on first principles is still useful for understanding the nature of loads and also for first-order verification of numerical results. 1. Introduction The analysis and modeling of wellhead growth and the resultant casing load distribution has been extensively reviewed and discussed in the industry literature. Stahl and Baur (1983) analyzed the effects of composite loads on offshore platform conductor design with specific emphasis on buckling and stability of the conductor or riser. Kocian et. al. (1990) applied simple linear elasticity to the analysis of conductors and surface casing loaded in compression by the tensile loads of inner tubing strings with a focus on land well applications. The need to go beyond single-string service life analysis and to model the interdependent interactions among all the tubular strings in a multistring system joined at the wellhead has been addressed with advanced numerical models. Adams (1991) and Adams and MacEachan (1994) presented a comprehensive numerical model which iteratively solved for the coupled axial and radial response of the wellbore system subject to combined axial loading, thermal casing expansion and annular pressure buildup. This solution method incorporated standard analytical equations from single-string theory into a matrix stiffness solution such as used for finite-element analysis of frame structures.