Pipe-In-Pipe (PIP) systems are increasingly used where the primary objective is to prevent wax deposition and/or hydrate formation and to achieve a low Overall Heat Transfer Coefficient (OHTC) value. PIP systems are also increasingly considered as an additional protective layer against loss of containment (e.g. in Arctic pipelines) or to withstand the interaction of a third party (e.g. trawl gear impact) in lieu of costly pipeline burial.
At the end of each PIP section, either at a midline connection or end point tie-in, bulkheads are used as a way of transition from a double wall (PIP) system to a single wall (hub in the subsea structure) system. End bulkheads are designed using detailed Finite Element Analysis (FEA) in accordance with more stringent Pressure Vessel Codes (PVC) and manufactured by forging followed by heat treatment and detailed machining to the required dimensions. As the design of end bulkheads does not fall under the Pipeline Codes, a distinction (so called “code break”) may be required on where the governing code changes from the PVC to a Pipeline Code.
This paper firstly discusses the application and validity of internationally known PVCs (ASME BPVC Section VIII Division 2 , BS EN 13445 , PD 5500 ) that can be used in the design of end bulkheads. This is then shown in practice by using an example of a typical end bulkhead, designed to various PVCs. Finally, results are compared and conclusions and recommendations are made.