Variation of Current Drag on Flotel in Side-By-Side Configuration with FPSO
Authors: Yenduri Anurag , Bukka Sandeep Reddy , Magee Allan Ross , Liu Jing , Hussain Anis Altaf 
Published: November 2020

A flotel is often used to house personnel and equipment for on-site maintenance of ageing FPSOs. Although tankers are designed to be dry-docked, operating FPSOs (which may be reconverted tankers) must be maintained in the field. The advantage to industry of on-site maintenance and inspection rather than dry-docking of FPSOs is crucial to cost-effectiveness. For multibody systems like FPSO and flotel in a side-by-side configuration, one of the primary questions is the time for which the gangway can remain safely connected between the two bodies. Operators always seek for a maximum possible uptime to operate the gangway, so that FPSO maintenance works are not halted for long durations during the harsh environments. The main factors limiting the operation of a gangway are its extension and rotation due to the relative motions between FPSO and flotel. Therefore, it is firstly important to accurately predict the hydrodynamic interaction between the two bodies for estimating their motions. In the present work, the variation of current drag on the flotel due to the presence of a turret-moored FPSO is investigated.

The current drag variation on the flotel is studied by modelling the multibody system in OpenFOAM. Steady state simulations are performed using simpleFoam algorithm to compute the hydrodynamic coefficients of pontoons and columns. SimpleFoam is a steady-state solver for incompressible, turbulent flow, using the SIMPLE (Semi-Implicit Method for Pressure Linked Equations) algorithm. The meshes for FPSO, flotel and fluid domain are created in ICEM-CFD and imported into OpenFOAM. A mesh convergence study is conducted. The simulations are performed for various current velocities and directions in combination with different drafts of FSPO. Finally, a lookup table comprising of current drag coefficients for several cases is prepared for future reference in designing the flotel.

A non-collinear environment is the worst scenario for turret-moored FPSO. In most cases, the wind-driven current direction is within ±45° relative to the wave heading. The drag load experienced by the flotel on the columns and pontoons varies as a function of the current direction, the draft and the heading of FPSO relative to the flotel and the location of the flotel (upstream or downstream of FPSO). As FPSO changes yaw angle, the flotel experiences current loads which sometimes vary abruptly due to the shielding effects. The paper presents novel results and explains how the hydrodynamic interactions affect the important current loads experienced by the flotel under various scenarios. The choice of orientation of the flotel relative to FPSO (pontoons parallel or perpendicular to FPSO) is influenced by and must be decided by considering the effect on the current loads. The effects of varying shielding need to be considered by the dynamic positioning system in order to develop robust control methods to maximize uptime for a given installed thruster power.

Several studies have been performed to investigate the hydrodynamic interaction between two bodies like cylinders, square columns, rectangular structures, etc. However, very little work has been on the hydrodynamic interaction between FPSO and flotel (semi-submersible platform), especially when this multibody system is subjected to current forces. The present study aims to fill this gap and aid the industry in the preliminary design and configuration of the flotel.