Multistring analysis of wellhead movement

This paper presents simple models to describe casing loading and wellhead movement during both the installation (mechanical) phase as well as the production (thermal) phase. Input parameters include hook load during installation and temperature change during production. The theory is simple and the formulations are suitable for hand-calculations and spreadsheet. The presented models are applicable to both conventional and HP/HT (high-pressure/high-temperature) wells. The term multistring analysis refers to the involved casing strings which are connected at the wellhead and therefore, the applied mechanical or thermal disturbances will cause wellhead movement that depends upon the interaction of the different tubulars.A field case is presented and analyzed using presented theory. This is an offshore well where the wellhead suddenly dropped several feet due to a collapsed surface casing. The incident took place during wellhead cooling following a routine shutdown. The expected wellhead contraction due to cooling did not occur. After an extended period with no wellhead movement, the wellhead suddenly released. A corroded segment of the surface casing collapsed and the wellhead dropped.