Evaluation of alternative methods for de-docking of FPSO-type units in the process of decommissioning

The current context is characterized by the increasing number of decommissionings of offshore oil production units, for which the unanchoring operation constitutes the last and crucial phase of the process, before the effective ocean towing of the unit to its destination location. Unanchoring maneuvers, however, fundamentally depend on the operability of the unit's original mooring tensioning systems, which are often degraded and/or inoperative, which makes their replacement unfeasible and difficult to handle during the decommissioning phase. This scenario imposes a series of technical and operational challenges, demanding alternative and effective solutions to guarantee the safety and efficiency of the entire process.
Thus, aiming to contribute to the proposed theme, this article establishes a detailed analysis of alternative methods of undocking maritime production units, adopting their provisional installed systems. In this study, a conceptual approach is adopted that integrates the use of ROVs (Remotely Operated Vehicles) and AHTS (Anchor Handling Tug Supply) type vessels equipped with the necessary tools to disconnect and handle the anchor lines. The analyzes are carried out for an FPSO (Floating Production Storage and Offloading) representative of the spread mooring type, using engineering computer programs for dynamic simulation in the time domain, involving the proposed alternatives subjected to the environmental conditions of the FPSO location, such as currents, waves and winds. Additionally, the assessment considers different operational scenarios to ensure a comprehensive approach.
The results point to both technical and operational advantages and disadvantages for different approaches to decommissioning mooring lines. Aspects such as tensions in the anchor lines, in the working cables of the support vessels, as well as other parameters were evaluated, including the variation in the unit's offset when subjected to different environmental conditions during the unanchoring process. This assessment contributes significantly to the evolution of these practices in the offshore industry, providing efficient and safe solutions for platform decommissioning.