Fpso Model Tests And Analytical Correlation
Authors: Huang E.W. , Beynet P.A. , Chen Jing Hui , Li Jian Xun , Lu Zi Guang 
Published: October 2008

This paper presents model test and analytical correlation results of a single point moored Floating Production, Storage and Offloading tanker based system (FPSO). Both Permanent and Disconnectable turret mooring system are considered. Analytical calculation of green water effects, vessel motions, turret and mooring line loads are compared with model test results. Frequency and time domain analysis techniques of calculating line tensions are demonstrated and results are compared to the model tests measured time histories.

The analysis procedure discussed in this paper is aimed toward estimating I)motions at bow relative to the water surface, 2)mooring line tensions and 3)turret design loads for a turret moored FPSO with a reasonable degree of accuracy. it is demonstrated that mooring line tensions can be calculated accurately with a finite element time domain computer simulation of the line dynamics provided the input fairlead motions are correct. Thus the method of computing time histories of mooring line tensions based on free-floating vessel motions is applicable for other spread moored floating systems in the same range of water depth.

CONCLUSIONS AND RECOMMENDATIONS

The model test results confirm the feasibility of both types of turret moored systems and validate the analytical tools and procedures. Results derived from the time domain line dynamic analysis agree well with the model tests. The dynamics of slack line are over estimated by using the frequency domain approach. Two methods of API mooring line fatigue calculation are compared.

This paper demonstrates that a frequency domain quasistatic approach provides an efficient tool for initial sizing of mooring components. The maximum global mooring line and turret loads may be estimated by combining the frequency domain line dynamic results with the quasistatic approach. A time domain line dynamic approach can confirm and/or predict the maximum line tension with a high level of confidence, provided the vessel motion at the line top can be prescribed accurately.

INTRODUCTION

A turret moored 140,000 DWT tanker system is being designed for 1,000 feet of water in the South China Sea. The turret moored tanker is an attractive concept for both production facilities and offshore storage. Design of the weather vaning turret mooring system and riser system connected to the seafloor are two key system components for FPSO development. In the design of a turret mooring system the prediction of the extreme motion responses of the moored vessel is of the utmost importance. A moored tanker exposed to severe environmental conditions experiences motion responses in two distinct frequency ranges:Wave frequency responses around the wave period range (12-15 seconds).Low frequency responses around the tanker natural period range (100 seconds);

The wave frequency response is generally well predictable, whereas estimates on the low frequency responses are more difficult and may vary by an order of magnitude depending on analytical tools and design procedures. Often a well executed model test program in parallel with analytical approach is imperative to ensure a reliable design of the turret mooring system.