This paper presents a case study on the design, fabrication, and installation of a pipeline In-Line Tee (ILT) assembly connecting a branching flexible pipeline to a main rigid carbon steel pipeline. The ILT assembly includes two sub-assemblies: a piping sub-assembly (valves, flanges, an elbow, and a barred tee) and a structural sub-assembly (shroud, clamps, and tubulars). The final tie-in is achieved via a conventional subsea flanged connection by divers.

Key design considerations are highlighted, such as design modularity, preventing the ILT from over-rotating during pipelay, and providing adequate support to the piping sub-assembly during operational life. The modular design proved beneficial when a valve in the piping sub-assembly was damaged, allowing for a change in installation methodology without significant delay to the pipelay campaign. The causes behind the aforementioned valve damage are also discussed, leading to recommendations for improving future designs to ensure safer lifting and handling, given the top-heavy nature of the assembly.

Cost and schedule efficiencies are discussed, noting that the use of standard fittings and equipment optimised material delivery lead times and simplified onshore fabrication. Integrating the assembly into the S-lay process significantly optimised offshore execution time by reducing diving time and the number of subsea lifts required. The pipelay was completed within the originally estimated schedule, demonstrating the constructability and ease of installation of this design.

The paper also emphasizes the importance of collaboration and interface management. Following the project’s overall contracting strategy, the ILT assembly was designed by the Company's Detailed Design (DD) engineering consultant, pre-fabricated onshore by the Engineering, Procurement, and Construction (EPC) contractor, and installed offshore by an Offshore Installation Contractor (OIC). Rigorous inter-company reviews ensured that the ILT assembly footprint was within the pipelay tensioner clearance limits, highlighting the critical role of close collaboration in the project's success.