P43/P48 Global Motion and Stability Analysis: A Compromise Combination to Define the FPSO Operational Behavior
Authors: Portella Ricardo B. , Kameyama Victor , Wibner Chris , Maloney John 
Published: October 2008

Designed to operate in Campos Basin Offshore Brazil, the FPSOs P43 and P48 are a considerable mark on the offshore industry in terms of the number of innovative engineering procedures developed for their design.

As the FPSOs are going to operate as permanent floating production units at the same location for 20 (twenty) years, a complete dedicated design was performed considering the specific site conditions. This was carried out not only to determine mooring system and structural design aspects but also to predict and govern operational behavior in motion and stability, the object of this paper.

Motion analysis of permanent offshore floating units is normally performed taking into account the specific site environmental conditions. On the other side, stability analysis is normally performed with worldwide limits criteria. The result of this is a predicted motion behavior based on the typical loading conditions and site environmental data that are not necessarily aligned with the stability limits of the unit defined by a minimum GM curve, eventually causing unexpected operational conditions.

The main idea on the Barracuda and Caratinga project was to combine the stability limits of the unit with the predicted motion analysis typical operation conditions, defining a normal operation region where the FPSO will operate to guarantee a global performance in accordance with the predicted analysis.

Based on the minimum GM curve derived from intact and stability analysis, new maximum and minimum GM curves were determined based on mass properties combinations where the motion response will be permanently inside the design parameters.

The intent of this paper is to present the work performed to define the maximum and a minimum operational GM curves for the FPSOs, including motion and stability analysis, hull girder and local structural limits and a considerable amount of real operational considerations, that results in a general overview of a very refined global response design developed for a special project.

Introduction

Reviewing the usual procedures adopted to perform the stability analysis and global motion analysis of FPSOs, one realizes that the stability analysis normally considers the Unit as a ship calculated for unrestricted service. On the other hand, the global motion analysis considers the Unit as a fixed structure under the action of the specific site environmental conditions.

It is easy to understand that the stability of a ship-shaped offshore unit shall be similar to the stability limit of a ship with the same hull forms, but motion analysis are normally performed for the expected operational load condition, not for the stability limit of the unit.

For offshore floating units that are not able to store oil on board, like a semi-submersible platform, the stability limits are quite close to the expected load conditions, as there are few possibilities to vary the load of the unit the mass properties do not change very much.

On an FPSO, the oil storage capacity weight is much larger than the vessel lightweight and consumables combined. The different possible combinations of the cargo tank loading can result in considerable different mass properties values for the same draft.