Rigid spools are commonly used at riser base of hybrid riser systems such as STTRs (Single Top Tensioned Risers) or BHORs (Bundle Hybrid Offset Risers) for the deep water oil fields. The rigid spools design is critical and governed by numerous operating constraints such as dynamic loadings (due to motion of the vertical riser, Vortex Induced Vibration, slugging…), soil properties and soil/spools/structures interaction. Recent surveys on existing brown fields have highlighted unexpected issues on existing rigid spools. The constraints of the brown fields are numerous when the replacement of a rigid spool is envisaged: existing structures developed initially for a rigid solution at riser base giving load limitations, congested field layout, installation constraints due to existing subsea structures and pipelines, and downgraded soil conditions (trenches due to actual rigid spools). To solve these several issues, an innovative flexible spool solution has been developed, combining unbounded flexible pipe structures in a "free hanging" configuration to accommodate the vertical riser motions with mattresses to release the soil downgraded conditions. Based on the experience acquired on these brown field projects and lessons learnt from hybrid risers’ technology, a specific solution has also been developed for new projects (green fields). A steep wave solution is introduced via a reverse wave configuration with distributed buoyancy over the flexible spools length. Such proposal allows a direct connection between the riser base assembly and the flowline termination, solving the embedment topic and easing accommodation of the hybrid riser motions. This paper deals with the recent issues with the rigid spools design and describes the solutions developed with flexible spool configurations to provide a more reliable riser base spools system for both brown fields and green fields.
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