Global Analysis of Shallow Water FPSOs
Authors: Duggal Arun S. , Liu Y.H. Allen , Heyl Caspar N. 
Published: October 2008

The paper focuses on the global analysis of FPSOs in shallow water, with an emphasis on vessel motions and offsets, mooring and riser design. The paper emphasizes the complex hydrodynamic interaction between the vessel and the environment, and the prediction of the vessel responses unique to shallow water. This done by using two examples of FPSOs moored in shallow water using an external turret mooring system and a tower yoke mooring system. Results are presented that illustrate the unique nature of both mooring systems and their sensitivity to variation in the environment and system damping. The design of compatible riser systems for both mooring systems is also presented and discussed.

Introduction

The Floating Production, Storage and Offloading (FPSO) system is the most mature of all floating production systems with over 100 installed and operating worldwide. It is also one of the most versatile floating production systems being installed in water depths from 20 meters to over 1800 meters, for a wide range of environmental conditions and can be easily extended to water depths even greater than 3000 meters. The majority of the FPSOs have been installed in water depths less than 200 meters of water. This is in contrast with other floating production systems that are focused on deepwater (greater than 500 meters). The market for shallow water FPSOs or Floating Storage and Offloading Systems (FSOs) is still very strong with a large number of facilities being designed and planned for water depths of 100 meters or less.

From a global analysis perspective, shallow water FPSO/FSO systems can be much more challenging to analyze and design than ultra-deepwater systems. This is due to the:Environmental loading on the system in shallow water,"Hardening" nonlinear stiffness of the mooring system that at extreme offsets can result in large variability in loads,Low level of associated damping in the system, andDesign of an appropriate riser system for fluid transfer. The first objective of this paper is to provide the reader with the basic information to understand the analysis and design methodologies for a shallow water FPSO system focusing on the four items discussed above. Due to the focus on ultra-deepwater systems, the complexity and challenges of shallow water hydrodynamic design are not always well understood, and software tools and global analysis methodologies are being developed for these ultra-deepwater systems without much verification or application to shallow water systems.

The second objective of the paper is to review two typical shallow water FPSO systems and the associated riser systems and relate their response to the discussion above. The examples are based on actual mooring system designs and help illustrate the challenges and solutions available for mooring FPSOs in shallow water. These examples are also important as they apply to future shallow water concepts, e.g. LNG offloading terminals.

Typical Shallow Water FPSO Mooring Systems

Shallow water FPSO systems have been developed using a number of vessel-mooring configurations depending on site specific and project requirements. Typical configurations include mooring to a CALM buoy using hawsers or hard yokes, spread-moored, internal or external turret catenary mooring systems, or mechanical mooring systems like tower yoke moorings.