Developments in Testing and Manufacture of Thick-Walled Pipe

Over the past 20 years there has been a considerable development of the capability to design and manufacture thick walled pipe to manufacture pipelines to operate in ultra-deep water. Design guidance is available in DNV OS F101 [1] in which the safety from pressure collapse failure during pipeline installation is determined by the use of a safety factor. The safety factor has been calibrated using the Load and Resistance Factor Design (LRFD) method in comparison with collapse pressure test results available at the time of preparation of DNV guidance. Because of the huge financial implications of loss of a very long pipeline during installation in ultra-deep water it has been the practice further to base the design of such a pipeline on specific pipe joint collapse tests in conjunction with the DNV information. Pressure testing full-scale pipe joints is an expensive undertaking that requires a suitable pressure chamber. Only a few chambers capable of applying pressures corresponding to very deep water are available in the world and transport of the pipes from the pipe mill to a suitable test facility may be very inconvenient and certainly expensive. This paper describes an alternative approach which could provide data that would enable the preparation of a safe approach specific to the pipeline diameter and design water depth. The approach could enable optimisation of the pipe design, particularly the pipe wall thickness. The proposed method is based on replacing costly full scale pipe tests by corresponding tests on ring specimens cut and machined from manufactured pipe joints. The proposal to use ring testing as the basis for design has been included successfully in the design of pipe for a recent ultra-deep water project [2]. The paper describes equipment used to subject the rings to external pressure and reports on tests carried out to validate the correspondence between the ring collapse pressure and that for the pipe joint from which the ring was obtained. Based on results from such tests it is concluded in this paper that ring pressure collapse testing is indeed a valid method to use as the basis for the design pipes in the next stage of ultra-deep water, i.e. increasing the capability to install pipeline in water depths down to 3500m from the current maximum level of 2500m.