Responses of a Tanker-Based FPSO to Hurricanes

Abstract

Abstract A series of model tests were completed in the Offshore Technology Research Center wave basin to investigate the responses of a tanker-based FPSO to hurricane environments. Results indicate that transverse responses, particularly roll, are significantly influenced by non-parallel environments and directional seas. Introduction While FPSO.s have successfully used in many other areas worldwide, none have been used to date in the Gulf of Mexico. With increasing interest in their use in the Gulf, model tests were conducted at the Offshore Technology Research Center wave basin to examine the behavior of FPSO.s in wave, wind, and current conditions typical of the passage of a severe hurricane. FPSO.s for the Gulf of Mexico will likely be passively moored through a turret system so that the tanker can weathervane or rotate in response to the changing wave, wind, and current directions in a hurricane. In many areas where FPSOs have been used, the wind, wave and current conditions are relatively parallel or collinear. In such an environment, the FPSO is generally subjected to head seas. The typical design practice for such FPSOs assumes that the design wave, wind, and currents are parallel. The waves are assumed to be unidirectional or long-crested. However waves, winds, and currents in a hurricane can be quite non-parallel [1], and subject the vessel to quartering or beam seas that can significantly influence the response of a ship-shaped vessel. For example, conditions that result in quartering or beam seas can cause larger roll amplitudes than that would result from head seas. The extreme responses of a moored FPSO can be sensitive to non-parallel waves, winds, and currents. The extreme or design responses of an FPSO is recognized to be sensitive to non-parallel waves, wind, and currents [1,2], but few studies have addressed this issue. A recent study investigated response based design criteria for FPSOs in the Gulf of Mexico [3]. A frequency domain analytical model was used to predict the responses of an FPSO from hindcast hurricane waves, winds, and currents that were generally non-parallel. The effect of directional waves was approximated in the model. Results showed larger than anticipated roll that was due in part to quartering to beam seas that can result from non-parallel waves, winds, and currents, and in part to directional seas. Larger than anticipated roll has also been observed on operating FPSOs[4]. The model tests described here were designed and conducted to examine the responses of an FPSO in nonparallel wave, wind, and current conditions and wave directionality (i.e., short-crested waves) typical in the passage of a severe Gulf of Mexico hurricane. Results from these tests can be used to assess the importance of realistic descriptions of hurricane wave, wind, and current conditions on the simplifying assumptions often used in design practice.