The trend over the last few years in Coiled Tubing (CT) usage has been towards a greater preponderance of large diameter, heavier wall section and higher yield strength tubing being employed. These larger, stronger section tubing strings require larger forces to be applied than those previously experienced in the industry, in order to reel and unreel the pipe from the working reel or spooler. These forces are transmitted to the riser on which the CT injector and guide arch are mounted. In turn, the toolstrings which are now more commonly used in well intervention operations are longer, requiring longer risers to accommodate them. This causes higher loads to be imposed upon the wellhead on which the riser is mounted. The wellhead, or Christmas Tree, is the primary well containment on which other well control equipment, such as Blow Out Preventers (BOP), are attached. Does this change in operating parameters now mean that the Christmas Tree is being overloaded to the point of a potential breach in its integrity?

The advent of longer risers, which are subjected to greater loads from more powerful high pull capacity injectors and more powerful reels, gives concern as to whether or not the types of riser commonly assembled in the field are stable or liable to suffer catastrophic buckling collapse. This is not readily apparent, as the types of risers being used are composed of diverse structural elements; multiple BOP’s, API flanged spools, Pin & Collar quick union riser sections, flow tees, et cetera.

This paper will present a summary of an engineering study that was undertaken to explore these questions. The study examined the effect on stability imposed by riser internal pressure, riser configuration, pipe loads and support tie-downs, as well as the loads that are being transmitted to the Christmas Tree. The study was undertaken using both existing engineering principles and Finite Element Analysis (FEA). The result of the study was the development a formulaic model, to be used within a computer simulation program, that would account for these varied inputs.

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