This paper presents numerical analysis of the behavior of a towed FPSO using an in-house computer code Ship Maneuvering Laboratory (SML). The background of the study was an attempt to explain some unexpected observations during a towing situation, where the position of the towed vessel was on the upwind side of the towing vessel and oscillating in yaw. The effects of rudder, wind and current were investigated initially. Then the study focused on the behavior of the towed FPSO with different hydrodynamic coefficients in an environment of steady wind varying from 1.0 to 25 knots with the wind direction on the starboard bow of the tug. The tug was assumed to move in a straight line at a constant speed.
It was found that with the calculated MMG coefficients the FPSO had large oscillations of track and yaw when the wind speed was low (1 knot). When the wind speed was increased, the FPSO moved to the starboard side of the tug (up wind) and kept oscillating and yawing but with less amplitude. This was similar to the FPSO behavior observed in the incident. This phenomenon of oscillating and yawing of the FPSO was not seen when using the measured PMM hydrodynamic coefficients. One possible explanation for the FPSO behavior in the incident was related to the specific values of the FPSO’s hydrodynamic coefficients.
Some possible methods to reduce the FPSO yaw oscillation are also provided. Numerical results showed that by adding drag created by a drogue to the FPSO or using a longer towing bridle can significantly increase the directional stability of the FPSO. A sensitivity study of the FPSO hydrodynamic coefficients is also given and some conclusions are provided.