Ultra-deep water gas pipelines will be playing a major role in tomorrow’s energy supply. Diminishing onshore and shallow water gas resources together with geo-political and energy security concerns will be leaving no alternative but to accelerate the development of ultra-deep water reserves. Thick wall pipeline requirement to resist the collapse pressure is one of the most challenging aspects of such fields in terms of pipe design. In addition to direct costs of installing tremendous amounts of extra steel, there would also be a substantial carbon footprint in the global atmosphere.
DNV-OS-F101 is the most popular design code addressing the failure modes of offshore pipelines including system collapse. The standard and its safety factors are based on reliability analyses. There have been some efforts to improve the characteristic capacity of pipes towards more economic but still safe structures. Among them, there is a recent work to improve the pipe fabrication factor (?_fab) of heat treated UOE line pipes.
This paper will discuss the consequences of system collapse of ultra-deep water gas pipelines at depressurized shutdown condition. DNV-OS-F101 requires ‘’Medium’’ or ‘’High’’ safety classes for such a failure regardless of its spatial or temporal extents. Zero hydrocarbon release policies of oil companies and regulators are presumably the most important factor implying such a requirement.
Hydrate formation phenomenon will occur in case of any puncture in the pipe wall where the (LTHP) water enters the pipeline at a temporary shut-down condition due to the external over-pressure. As a result of water ingress, hydrates will form and partially or even completely block the collapsed pipeline. This will result in preventing further release of gas to ocean, hence ‘’Low’’ safety class would be allowed in that particular load scenario which is likely to be the governing one in terms of wall thickness calculations.