Abstract This paper presents Fitness-For-Service (FFS) studies from projects in which large as-laid gaps existed between flanges of pipeline and tie-in spool. The reasons for the large as-laid gaps are explained. The first case study is for a spool tie-in with water depth of about 140m. Incongruity of duration between operation turnaround and spool fabrication implied the need to perform the metrology in pipeline’s operating while tie-in in shutdown. A gap of 300mm was estimated and was considered in the finite element analysis (FEA) of the tie-in. The gap included margins for fabrication and metrology misalignments and pipeline contraction due to shutdown of the existing pipeline. An element removal and addition technique was used to simulate the initial gap and tie-in operation. The second case study is for a spool tie-in with water depth of about 50m. A gap of about 2.6m between the flanges of the pipeline and tie-in spool was found. Air bags were used to facilitate the tie-in. An automation tool was used to rebuild the surveyed spool shape after tie-in and the locked-in stress level was checked. Then all the functional loads were added to check the spool’s integrity. For both case studies, the spools’ integrities satisfied all the design criteria. This paper provides an insight into the analysis of spool tie-ins and considerable engineering reference significance. The work uses advanced automation tools to validate the design and installation methods. Lessons learned are also presented for future spool tie-in projects.
Dai et al. (2025) studied this question.