An Empirical Model of Current Shadow Effect

Abstract

The ship based Floating Production Storage and Offloading system (FPSO) has been largely used in the recent offshore oil exploration. In most of the cases the oil stored in FPSO is offloaded to a shuttle ship that is connected by a hawser in tandem configuration. Although the tandem mooring is a common procedure in the offshore oil industry, there are few publications related to the theme. Among these, the papers concerned with the current flow interference due to the presence of FPSO on upstream of shuttle ship, here called current shadow effect, are even more uncommon. Neither among the commercial simulators that analyze tandem systems there is none that considers the shadow effect of environmental conditions, providing an analysis different from the real world. In previous studies, the authors presented some experimental results showing that the dynamic behaviors of shuttle ships subject to current were strongly influenced by the presence of a FPSO in an upstream position. This phenomenon was expected since the FPSO disturbs the downstream current field acting on the shuttle ship. Observing this fact, a tentative empirical model based on the disturbed current field was developed. The disturbed current field was obtained through a 3D CFD program and the hydrodynamic forces acting on the shuttle ship were computed using the local cross flow method. At that time, a particular experimental case was analyzed and the proposed model showed to be effective. However, during the model evaluation, it was realized that the proposed model wasn’t able to reproduce some experimental cases. This paper presents the last developments on the shadow effect model and a more comprehensive comparison between experimental and numerical results.