This paper describes both the challenges and development of a novel solution involving 3.5-in. diameter coiled tubing (CT) for deepwater pipeline commissioning applications. The work scope required that the complete solution be capable of multiple deployments and recovery operations using a single string of 3.5-in. CT from a floating support vessel. The project began with a detailed analysis of the existing available equipment and tools to determine their suitability and limitations for this application. Factors in this analysis included the limited vessel space available for surface equipment, crane capacity, and the suitability of equipment for working outboard on a vessel. This led to the planning, designing, and sourcing of suitable CT equipment. Trials were performed onshore to optimize the rigup, stackup, vessel layout, and assemblies handling. The combination of pre-operation planning and trials led to confidence in the new tools, work methods, and risk assessments. Because the purpose of the deployment work was to complete the commissioning work on several different marine pipelines and risers, the equipment and work methods had to be easily transferred between vessels. This paper presents and discusses the range of technologies that were developed and successfully applied for the first time globally to complete the project. These include the first fully sealable subsea quick-disconnect for CT, the first pump-through modular clump weight, and the first real-time, high-cycle fatigue (HCF) monitoring system to aid in CT pipe management. The deployment and recovery operations involved a wide range of challenges and led to the development of specific tools and methods for using large-diameter CT equipment. In addition to discussing the design and development of the solution, this paper presents the results and lessons learned from successfully using the large-diameter CT downline solution for deepwater pipeline commissioning applications.