Split Mooring System Solution for the Congo FLNG Terminal

Abstract The Congo LNG project, developed by EXMAR and ENI, consists of two floating vessels: an FLNG production unit and an FSU, moored side-by-side in nearshore shallow waters approximately 20m deep. The vessels are positioned parallel to each other and connected by 20 Polyamides (PA, or Nylon) ropes, arranged in 4 clusters in a crisscross pattern. The two-vessel system is then spread-moored to the seafloor using 4 clusters of catenary mooring chains, with two clusters per vessel. The configuration is referred to as ‘Split-Mooring’, offers key benefits, including a simplified spread mooring layout and the elimination of interference between the mooring lines compared to individual moorings for each vessel. The mooring line arrangement on the FSU allows adequate space for an LNG Carrier (LNGC) to berth alongside the FSU for side-by-side LNG offloading operations. Additionally, the low-stiffness interconnecting ropes, which remain under tension even in extreme sea conditions, allow greater motion flexibility between the two vessels while reducing the loads on mooring components and structure foundations. This paper outlines the design and analysis details of the mooring system for floating LNG production and storage units offshore Congo in west Africa utilizing Split-Mooring solution. The paper details both technical and schedule-related challenges encountered during various engineering stages, including: Concept selecting and determining the unit layout with Split-Mooring solution. Particularly optimizing the vessel separating distance to meet the requirements for global mooring performance, inter-vessel operations for LNG hose, personal transfer system, clearance requirements in extreme conditions.As an innovative mooring solution, design validation through basin test and numerical calibrations were performed. The unique basin responses and test characteristics in shallow waters for mechanics and fluid coupled floaters were identified. Tri-party independent evaluations were performed with converging results.Mooring component selection of PA rope (Nylon) for permanent application, design parameter selection, vendor product qualification and risk mitigation through rope testing, in-place monitoring, and planned component change out.Mooring fatigue life, identified as critical during the early design phase, has been the design focus and concept studies. Adding an extra line on the prevailing environmental direction is found to be the most practical way to increase the fatigue life. The spread mooring pretensions must remain within an optimal range to satisfy both strength and fatigue life requirements.Challenges encountered during rope and mooring hookup.Installation and Post-installation Analysis: Design analysis for different installation scenarios; managing anchor changes due to schedule constraints; and post-installation tension reconciliation and fatigue reassessment.