Midline bulkheads are often used as a restraining mechanism in High Pressure / High Temperature (HP / HT) pipe-in-pipe (PIP) systems. Their primary function is to share the loads between the inner and outer pipes and / or to minimize the extent of damage in an operational or installation incident. They can also be used during reeled pipelay to facilitate the reel-to-reel or trip-to-trip weld tie-in. Design of a reeled bulkhead, which is categorized as a “pipeline component” under DNV-OS-F101, requires the careful adaptation of a Pressure Vessel Code (PVC), a subsea pipeline code and compliance with the additional requirements of reeling, welding and fabrication. Modern pipeline codes, such as DNV-OS-F101, are LRFD based codes in which different limit states (failure modes) have been formulated and calibrated based on a given probability of failure. The PVCs, however, do not specifically address the pipeline design and careful consideration in selecting load factors, load combinations and the analysis method are necessary.
This paper summarizes a procedure that has been adopted in design of reeled midline bulkheads. The methodology can also be used in design of reeled end bulkheads. Firstly, the initial geometric dimensions of the reelable bulkhead are defined and its reelability is confirmed. Then, appropriate load combinations are identified from ASME BPVC Section VIII – Division 2. Finally, a series of Finite Element Analyses (FEA) are performed to show the fitness-for-service of the bulkhead. The importance of selecting an appropriate and justifiable “code break” is highlighted here.