Torsional Loading of Subsea Structures

Abstract

This paper demonstrates that the torsional-effect of horizontal loads on a subsea structure can represent a governing load-case for the foundation. It is intended to alert potential designers to this aspect as there is currently not much specific guidance in the public domain. Codes of practice for foundation design of offshore platforms are routinely used for the design of subsea structures, and the design is often performed by engineers whose technical-speciality is not necessarily geotechnics. The design can be performed strictly according to such a code but the underlying intention of the code can be overlooked; specifically, that that stability for all credible load-cases be considered in order to achieve a design that is fit-for-purpose. The importance of torque loading is initially demonstrated through examples of simple shallow foundation limit-equilibrium, and expanded with discussion of more realistic cases. It is shown that the sliding capacity could quite conceivably be about half that which could be determined using a traditional code approach, albeit inappropriately. The influence of torque on piled foundations is also discussed, with particular reference to well-head protection frames that are supported on the well’s grouted conductor. Relationships between axial and torsional capacity are explained and it is shown that latter is limited by the number of high-torque connectors. It is also shown that it may be inappropriate to directly apply conventional t-z design methods to determine the distribution of torque.