The Shadow Effect on the Dynamics of a Shuttle Tanker Connected in Tandem With a FPSO

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. The problem of dynamic instability that arises in several ship mooring systems, like SPM and SMS subjected to the environmental forces, may also be present in the tandem system. Although the tandem mooring is a common procedure in the offshore oil industry, there are few publications related to the theme. Among these, there are none concerned with the environmental forces interference caused by FPSO on shuttle ship, here called as shadow effect. It is well known that the dynamic behaviour of a moored ship, in particular SPM system, is hardly affected by the environmental forces. Therefore, it is expected that shadow effect on the environmental forces acting on the shuttle ship will cause great influence in its dynamic behaviour, and consequently in the dynamics of whole FPSO-shuttle system. These phenomena could be observed in experiments with single point moored shuttle ships with and without the FPSO in upstream position. Therefore, the shadow effect should be considered in analysis of dynamic behaviour of two ships connected in tandem. Among the commercial simulators that analyse tandem systems there are none that consider shadow effect, making their analysis different from the real world. This paper presents an empirical model of the current shadow effect. The model was implemented in a numerical simulator, named DYNASIM. The comparison between numerical results and experimental one showed that the proposed model is effective.