Traditionally the S-lay installation loading condition using a stinger is not considered in the structural design of a Pipeline In-line TEE Structure(ILT) during FEED, and is usually only considered during the detailed design. However in a recent FEED project exercise, the S-Lay installation loading condition was found to govern the structural design of the ILT.
Large pipelay forces and moments due to curvature are expected during installation, which required the ILT to be analysed in order to meet the structural integrity requirement during S-Lay installation. Such a design issue requires the attention of the subsea structure designer early in the design process particularly for a large diameter pipeline, in which a high installation loading is expected.
However there are several design challenges in such a design practice. The approach to estimate the pipelay loadings has to give adequate conservatism, considering the range of potential installation vessel configurations. In particular, a high bending moment at the sagbend is expected due to the large size of the pipe, which complicates the design of the ILT structure design and design optimization.
These challenging design issues were extensively explored in a recent project. Advanced numerical modelling was used to assist the design, analysis and optimization. A suite of numerical analysis packages were employed including FS2000, Orcaflex and ABAQUS. The pipe resistance envelope during pipelay was developed using DNV limit state criteria. This pipe resistance envelope was used to derive an upper limit to sag bend bending moments for the local ILT analysis. This paper presents details of the methodology, discussions and conclusions for the engineering challenge described above.