Implementation of Magneto-Rheological (MR) Elastomer for Mitigating Fishtailing Instability in FPSO-Tanker In-Tandem System
Authors: Zainuddin M. , Kim M. , BhatAramanadka S. 
Published: October 2023

Abstract
The objective of this study is to analyze the implementation of MR Elastomer (MRE) Tensioner system in FPSO-Shuttle Tanker in-tandem system to mitigate fishtailing instability. It is also to identify the opportunity for digital twin implementation for similar application.
Fishtailing instability in the FPSO-Shuttle Tanker in-tandem system, and the implementation of MRE Tensioner in this system are analyzed by performing mooring-vessel-coupled time-domain analysis utilizing Texas A&M in-house CHARM3D program. Two types of FPSO are considered, which are turret FPSO and spreadmoor FPSO. West Africa metocean condition is used in the simulation as this region contains significant number of FPSO. First, two-body frequency-domain WAMIT simulation is performed to obtain the required vessel hydrodynamic data. These data, together with mooring, riser and hawser data are inputted into time-domain analysis (CHARM3D). The MRE Tensioner is modelled in CHARM3D and it is based on the physical MR Elastomer and Elastomer Tensioner system. The MRE Tensioner is then used as hawser connection on FPSO side. A simulation of FPSO-Shuttle Tanker with fishtailing instability is performed and used as baseline (called Fairlead case). Hawser angle, which is the angle the hawser deflects due to out-of-phase sway and yaw motions between the FPSO and Shuttle Tanker, is calculated in the post-simulation to measure the severity of fishtailing instability. Two MRE Tensioner cases for each FPSO type are simulated to investigate the stiffness level required to suppress fishtailing instability. The first MRE Tensioner corresponds to the 0A energizing current, which corresponds to the lowest MRE Tensioner stiffness. The second MRE Tensioner corresponds to the current level to get the appropriate MRE Tensioner stiffness that results in significant fishtailing suppression.
The Fairlead case exhibits significant hawser angle dynamics indicating fishtailing instability in both turret FPSO-Shuttle Tanker case and spreadmoor FPSO-Shuttle Tanker case. However, spreadmoor FPSO-Shuttle Tanker case exhibits benign motion than turret FPSO-Shuttle Tanker case. In MRE Tensioner cases, no significant improvement is observed in MRE Tensioner case with 0A energizing current or with the lowest tensioner stiffness. Significant suppression is observed when 5A energizing current is implemented in turret FPSO-Shuttle Tanker case. Fishtailing dampening is also observed when 10A energizing current is implemented in spreadmoor FPSO-Shuttle Tanker case.
The novelty of MRE Tensioner is its ability to adjust its stiffness which enables it to be utilized as means to suppress fishtailing instability for a given metocean condition by utilizing it as hawser connection. This in turn provides an opportunity to improve hydrocarbon offloading uptime. Furthermore, having the ability to control hawser connection stiffness through electrical current signal provides an opportunity for digital twin implementation.