Market demand pushes subsea developments into ultra-deep water. Pipelines installed in such environment require very thick walls to cope with the extreme hydrostatic pressure. However, the design criteria for local buckling under combined loading in one of the prevailing submarine-pipeline design standards, DNVGL-ST-F101 (2017), are stated to be valid only for pipelines with a diameter-to-wall-thickness ratio (D/t) between 15 and 45. The European Pipeline Research Group (EPRG) evaluated whether the existing design equations can be used for very thick-walled pipelines with D/t below 15 that are subject to external pressure and bending, without affecting the level of conservatism underlying the code framework. The limit-state formulation for load-controlled behaviour is found to be increasingly conservative for thicker pipe walls.
Intecsea reviewed the status of deep-water pipeline technology for the EPRG Design Committee as part of EPRG Project 178/2015. This study covers a gap analysis and was concluded in 2016. Areas that require further research were identified and subsequently included in EPRG's research roadmap. An important gap is the absence of a reliable and valid limit-state formulation for local buckling of thickwalled pipe when loaded by a combination of bending, axial force and pressure. The limit-state formulations for local buckling included in DNVGL-ST-F101, which is widely used for design of subsea pipelines, are stated not to be valid when D/t is less than 15.
Deep-water pipelines installed to date require D/t in the order of 20 for external pressure design. This is the case in maximum water depths in the range of 2000–2500 m. When moving to ultra-deep water, the required D/t can be even lower than 15. This asks for technological advancements, which should also be sought in a fundamental review and a potential reformulation of the existing design equations.
In addition to the lack of adequate design equations, manufacturability of line pipe with an extremely thick wall, meaning very low D/t, may be beyond the capabilities of the leading pipe mills. This depends on the selected manufacturing method; this paper considers seamless (SMLS) and longitudinally arc-welded (SAWL) pipe. The design of pipelines with D/t below 15 needs documented information on what can be produced, and according to which specification.