Abstract
Due to significant levels of transversely imbalanced riser, topsides and structural loads, a conventional monohull FPSO with a large number of deep-water risers hung on one side is not optimal in terms of load carrying capacity, ballasting operation and cargo loading/offloading operation, among other deficiencies. An asymmetric hull form, as applied to the Mero 3 project for the Santos Basin offshore Rio de Janeiro, has proven to be advantageous. In this paper, the asymmetric hull form of the Mero 3 FPSO is presented. The stability, global strength, vessel motion, wave loads, ballast and cargo operations as well as wet tow performance of the FPSO are discussed.
The global performance of the Mero3 FPSO has been examined both numerically and through model tests. Extensive stability verification and loading analysis have been performed using the NAPA software. Comprehensive diffraction analysis and wave basin model tests have been conducted to quantify vessel motion, slamming and green water characteristics. A modified dam-break solution is proposed to determine green water height and speed profiles on the FPSO deck for structural and equipment protection design (Xia et al., 2022). A novel frequency-domain approach is developed to provide both extreme and fatigue design loads for the bilge keel (Xia et al., 2023). The wet tow performance is studied through towing tank model tests and SafeTrans simulations.
Compared to an initially proposed symmetric hull form design, the asymmetric hull form results in more even transverse cargo distribution. It is shown to reduce ballast operation, reduce structural strengthening requirement and improve cargo loading/offloading operability, in addition to improving storage and ballasting capacities. The hull form exhibits excellent motion performance in the Mero3 field that is susceptible to swells. In the 100-year return design conditions, the maximum roll angle is below 6.5 degrees. This makes normal process operation in the 100-year return storms possible. The favorable motion performance also brings benefits to the fatigue lives of the steel catenary risers (SCRs) and the mooring system. Wave slamming on the hull becomes a minor issue with the ballast draft of more than 14m. Design for green water impact is also manageable with improved green water profile quantification developed during the project. Based on the results of the towing tank model tests, no fish-tailing stability problem is expected during the wet tow. Conventional two or three tug tow from yard to field is feasible.
For the first time, the offshore industry is seeing the successful application of an asymmetric monohull for a large FPSO, to be installed in the deep-water Santos Basin offshore Rio de Janeiro. The asymmetric hull form offers optimal global performance in many respects for applications similar to the Mero3 FPSO.
