Understanding of buoyancy in drill pipe and risers

Abstract

This thesis highlights that there is a wide spread use of the term “buoyancy” in the petroleum industry which can lead to misunderstandings. It is also evident that terms like “effective tension”, “true wall tension” and “apparent weight” induces more misunderstandings if not understood correctly. It has been shown that there is different ways of interpreting buoyancy forces and how they act on a submerged object. Several experiments have been used as illustrations to show that fluid need nonvertical sides to create a lift force (buoyancy force) on an object immersed in fluid although the object is displacing fluid. Further on it has been shown that the effective tension concept used in marine riser calculations can be misinterpreted because of the different buoyancy understandings. When calculating the effective tension the influence of the horizontal pressure acting on the riser is accounted for. In other terms, the effective tension is a three dimensional stress calculation which gives the needed top tension force (in one dimension) to prevent buckling as an answer. This calculation can be interpreted as if there were a “buoyancy force” present along the entire length of the riser, which is a contradiction to what has been presented in this thesis. An experiment has been conducted to show that effective tension concept gives correct results of the internal three dimensional stress state and thus is not just a “fictitious stress” or “fictitious force” as mentioned in several papers, but a stress state which can lead to failures if not accounted for.