Mooring System Design for a Circular Hull Shape FPSO Floater With Spar Like Responses
Authors: Beck John W. , Vandenworm Nicolaas 
Published: October 2011

A reliable mooring system is a critical aspect where survivability is considered. Total loss of a structure can occur during mooring system failures. With the advancements of the next generation circular hull shape FPSOs; Satellite Service Platform SSP320 Plus has the ability to utilize a spread mooring design that can be arranged to achieve the most cost effective and reliable design compared to a traditional ship shape FPSO. An additional factor with these increasingly large floaters that have a rectangular hull design is the gyration distance from the tipping point, which generates a significant larger moment than on a circular hull form. Thus, on a ship shape vessel, the mooring spread has limited structural real estate to sea fasten its winches and chain devices.

SSP320 Plus platform's new circular hull design provides better stability with the capability to accommodate a dry tree riser system with minimal yaw excitation whereas traditional ship-shape FPSOs motion varies considerably from head to beam seas.

Torpedo, suction pile and suction embedded plate anchor (SEPLA) have been considered for the permanent mooring system that will be installed in regions off the coast of Brazil. Torpedo pile anchor tends to be favored for these locations however the three anchors mentioned provide different advantages and cost values.

The study also focused on a pure technical evaluation of the influence to "peak shave" high loading forces when the sea state reaches an extreme event by introducing thruster assist positioning, which can be favorable for an omni-directional hull shape. This results in installation of fewer mooring lines and introduces a higher safety contingency thus giving the mooring system a more cost effective design and the ability to survive extreme events with minimal offsets.

Model tests were conducted in LabOceano, Brazil, with MARIN (Maritime Research Institute) and Oceanica Offshore. Tests were executed for three different loading conditions, ballast, fully loaded and 50% loaded cargo. These tests confirm that the circular shaped hull FPSO response with a larger natural period and hydrodynamic responses similar to a SPAR platform. To further design and analyze a mooring system to meet the design standards, a parametric model was created and analyzed within AQWA. AQWA provided the ability to carry out a frequency domain as well as time domain dynamic analysis of several different mooring line configurations considering a variety of global environmental conditions.

Mooring design results from this extensive study confirm that the mooring materials are well within acceptable market parameters and the mooring systems can be adapted/flexible to allow a variety of services and applications for deep and ultra deep waters.

Introduction

Exploration for oil and gas has begun to move into ultra-deep water, thus presenting a challenge to oil companies as a result of the absence of pipeline infrastructures. In these deepwater locations, the use of floating production, storage and offloading (FPSO) offers a favorable solution. Traditional FPSO's are known to be ship-shaped or box shaped which creates a disadvantage when permanently moored in one location for very harsh environments. These traditional FPSO's require a costly turret system to weathervane or align themselves with the predominate seas to minimize the motions and vessels stresses.

With the development of the next generation circular hull shape FPSO's; Satellite Service Platform SSP320 Plus has the ability for oil production to store cargo in ultra-deep water with similar motions of a SPAR platform.