More than 2000 years after Archimedes formulated his famous law, there is today still some disagreement as to the physical understanding and actual acceptance and application of Archimedes' Principle. This paper is prepared in the support of the full validity of Archimedes' Principle, always calculating buoyancy as the weight of displaced fluid, without exceptions.

There are numerous examples of confusion and disagreement in literature and industry in the understanding and implementation of axial force theory for pipe exposed to external and internal pressures. In 1962, Lubinski et al (1) described how tubing exposed to internal or external pressure behaved different to what was expected from calculations. The gap was defined as a fictitious force. In the 50-year period since then, the terms ‘true force’ and ‘effective force’ have emerged from the many discussions and research activities performed to understand pipe behavior. True and effective forces represent certain physical conditions and parameters that are normally easily described and represented in diagrams; however, there is still an ongoing discussion as to what is the mechanical model and explanation, especially for effective forces. The gap between true and effective forces has been subject to many discussions. Several attempts have been made to bridge this gap. Today it is often referred to C.P. Sparks (3, 4) who presented a very simple relation between the true and the effective tension and the influence of internal and external pressure. A classic discussion theme is; will a cylindrical pipe connected to bottom see buoyancy when there is no bottom flat exposed to pressure? It is generally agreed that this pipe see no true axial force from the surrounding water, however, the Sparks equation implies that the pipe see an effective tension equal to the the weight of displaced fluid above a given level. The last contribution to the discussion is Samuel/Kumar's paper “Effective Force and True Force: What are they?” presented at SPE/IADC Drilling Conference, 2012 (2). The objectives of the present paper are; to offer an understanding of the mechanics of effective forces, to demonstrate the physical effect of effective forces from laboratory tests, to underline the relevance and importance of applying effective forces in design and dimensioning of drill pipe and risers, to give examples of true and effective forces in a few classical well situations and finally to present a method for understanding and assessment of effective compression in a Macondo look alike case.