There is an increasing trend of pipelines operating at temperature in excess of 100 °C, particularly in deepwater production flowlines. These pipelines are generally of small diameter and often also operating at relatively high pressure. The combination of high temperature with high pressure leads to design challenges with respect to lateral buckling mitigation requirements.
In line with this development, a noble lateral buckling design mitigation scheme involving combined vertical and lateral offsets incorporated in a conventional sleeper concept has been developed [Ref. 1]. This so called Zero-Radius-Bend (ZRB) method significantly improves the buckle initiation reliability and its effectiveness has been field proven on many recent projects. However, ZRB track record to date has been limited to relatively shallow water in the range of 50m-100m water depth using conventional S-lay pipeline installation method.
This paper first describes the ZRB buckle mitigation method, its unique characteristics and ensuing advantages over the conventional sleeper method, followed by specific design and analysis considerations for deepwater application using reel-lay or J-lay installation method to create the lateral offset. The feasibility of ZRB is demonstrated through advanced three-dimensional finite-element (ABAQUS) modelling of pipe response, from progressive pipe laying, to pre-commissioning to first start-up and through to eventual operating cyclic loads from thermal and pressure cycles. Changes in pipe material post-yielding properties due to reeling/unreeling cycles and their impacts on buckle response are also considered.
Deepwater production flowlines also tend to be relatively short and, coupled with high operating and low ambient temperature, can lead to global axial walking response driven by the thermal gradient during the transient restart condition, following a full or partial shutdown. Excessive axial walking can have serious impacts on flowline tie-in spool design and connector loads. Axial walking can also interact and change the buckle profile affecting its overall characteristics. The effects of walking, its impacts on buckle response and tie-in spool design are demonstrated and captured in a single FEA model to illustrate the overall flowline behaviours under cyclically high thermal and pressure loads.
Reference (1): Subsea Australasia Conference 2011, "Lateral Buckling Design And Field Verification", Lim & Lau, et. al.