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The exploitation of subsea hydrocarbons fields as well as large subsea gas to market infrastructures often requires pipelines to route across sharp slope transitions. Site conditions are sometime so critical to represent a severe design concern and to require costly seabed preparation works, extremely challenging and environmentally impacting. This paper presents an innovative solution that allows avoiding any preparation work as well as a novel installation method for its effective and safe implementation. Specifically, the paper presents a design case study for a gas export pipeline crossing a severe escarpment at the intersection between the Continental Shelf and the Continental Slope (Shelf Break). The rigid pipeline installed by normal continuous pipelay across such sharp slope transition would be subject to excessive bending due to localized imposed curvature that requires mitigation. Two alternative design solutions have been developed and compared: one based on excavation (trenching) to shape the escarpment edge and one introducing vertical bends on the pipeline string. The design solution based on trenching results in large excavation volume with associated environmental impact in term seawater turbidity and damaged seabed surface (biocenosis). The analysis results also show that the solution is very sensitive to trenching profile accuracy and profile long term stability. Constructability is discussed in terms of availability of technology vs water depth, soil uncertainties, excavation volume, excavation tolerances. The alternative design based on bends allows avoiding any seabed preparation works and all the associated issues. The analysis results demonstrate much higher performances in terms of pipeline integrity and safety throughout the pipeline life. However, its implementation requires special installation solutions as bends are not installable with the normal continuous pipelay method. The paper presents a new installation method (Saipem patented) that allows realizing vertical cold bends safely and effectively on the pipeline profile with no interruption of the continuous pipelay process. The novel idea is to realize the bends underwater, when the pipe is landing or close to landing on the seabed, by means of a remotely operated Underwater Bending Machine purposedly designed. The paper describes the new Underwater Cold Bending technology, presenting the engineering work done to support the demonstration of its robustness and reliability. The paper also presents a case study for escarpment crossing design developed according to two alternative design concepts, the standard and the novel one. The two solutions are compared, listing pros and cons of each one, highlighting relevant design issues as well as construction criticalities and risks. |
