Abstract

Nowadays, deep-waters are the new frontier of the latest huge oil discovers. The most common technology for oil exploration in deep waters is based on the FPSO concept. Therefore, a key element for a successful exploration of deepwater oil fields is the accurate prediction of the unit motions in order to minimize the stop production due to excessive motions and maximum accelerations to which the process plant equipment and supports are subjected.

The excessive motions of a FPSO can be caused not only due to rough weather but also due to mild weather at beam sea conditions. Beam sea conditions are faced by FPSOs in a spreading mooring configuration or even in single point mooring configuration but moored in regions where the swell sea is important, for instance, in Campos Basin offshore Brazil. In such cases, the accurate evaluation of roll motions is of prime importance. Moreover, in order to reach valuable results, a critical step is the proper evaluation of the different damping effects in particular viscous damping induced by the bilge keel, or those of risers.

The state of the art assumption is that the roll damping is composed of two parts: the linear one and the quadratic one. The former accounts for the potential wave radiation and partly to some frictional effects, while the latter accounts for drag forces, flow separation, and other non-linear effects. In practice, the linear part can usually be neglected and the only remaining damping is the quadratic part. These fact strengths the role played by the non-potential damping in a reliable evaluation of the roll motion amplitude.

In this paper, we will show the implementation of a practical procedure for the evaluation of the roll motions including the non-potential damping of a FPSO moored in a spreading configuration at Campos Basin and its validation through the comparison of the numerical results to the model tests.

Introduction

Compared to the other modes of motion, the evaluation of the roll motion appears to be much more complicated and the corresponding results more uncertain. The reasons for that are mainly related to the evaluation of the roll damping coefficient, which is usually dominated by the non-potential sources of damping. The usual procedure for calculation of the ship motions is the use of the so-called diffraction-radiation software based on the potential flow theory. The damping calculated by this kind of codes is linear and related to the wave making. As far as the linear theory is concerned the horizontal modes of motion (surge, sway, yaw) are usually out of resonance in the common sea conditions (periods of 5 to 25 seconds). In those cases the role of damping is not very important so that the potential flow theory evaluate correctly the (linear) amplitudes of these motions. The only modes of motion that may enter in the (linear) resonance are vertical modes (heave, pitch and roll) and for these motions the role of the damping is essential.

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