OFFLOADING OF VLCCS FROM A CALM BUOY: A TWO-BODY APPROACH TO COMPUTE LINE TENSIONS
Laurent Foulboux, Offshore Structure Department, Elf Aquitaine Production, Pau, FRANCE
Stephane Le Guennec, Naval Hydrodynamics Laboratory, Ecole Centrale de Nantes,
Nantes, FRANCE
ABSTRACT
ELF CONGO has recently instrumented a CALM buoy on its DJENO site, in order to monitor the line tension in the hawser during offloading of VLCCtankers. Environmental conditions and position of the tanker are recorded as well and provide a good data base. The very shallow water and the benign but noncolinear environmental conditions considered are responsible for tanker resonant motions. The system has been modeled numerically for calibration purposes. The buoy and hawser have been modeled by a single nonlinear spring. The tanker and tug vessel connected to it have been modeled in different ways, including a two-body approach. Nonlinear time domain simulations have been run to compute the mean position and slow drift of the tanker under waves, wind and current. Results of the analysis are presented and compared with on site measurements for several offloading conditions. Eventhough, there are still some uncertainties, a two-body approach gives substantial improvements in the line tension and tanker motions computation.
DYNAMIC ANALYSIS OF FPSO WITH A TANDEM OFFLOADING SYSTEM
K. Nishimoto, Hernani L. Brinati and Carlos H. Fucatu, Department of Naval Architecture and Ocean Engineering, University of Sao Paulo, Sao Paulo, Brazil.
ABSTRACT
Turret-moored tankers have been conceived by the oil industry as a Floating Production Storage and Offloading system (FPSO). The oil stored in ship hull may be offloaded by a shuttle ship connected to the EPSOby a hawser, in a so called tandem system configuration. The problem of dynamic instability, that arises with other ship mooring systems, like SPM and SMS subjected to the environment forces, may also be present with the tandem system. The present paper is concerned with the dynamic analysis of the tandem system. It is an extension of the research that the authors had carried out with the application of a new mathematical model to deal with the SPM dynamics. This formulation is a manoeuvering model specially developed to represent low speed ship motion.
It is presented, initially, the mathematical model that represent the dynamics of the tandem system. Particular emphasisis given to the description of the slow ship dynamics for the two vessels. Simulation of the mathematical model is used to analyse the dynamical behaviour of the tandem system. The effects of hawser length, wind and current speeds and wave angle of incidence are investigated.
