Riser Dynamic Analysis Challenges for Petrobras Barracuda Caratinga Project
Authors: Zhang H. , Hamilton J. , Haverty K. , Mourelle M. 
Published: October 2008

This paper presents analysis procedures and methodologies developed for Petrobras Barracuda Caratinga riser systems. The magnitude of the scope, combined with the complexity of interface design and fast-track project schedule imposed significant challenges for riser analysis efforts. The objective is to effectively and efficiently handle the analysis for such large-scale deepwater riser systems. A robust design wave approach is developed to evaluate the riser design loads realistically but not unduly conservative leading to an otherwise more expensive riser system design. Procedures were established to condense over 10,000 load cases by an order of magnitude to a manageable proportion of less than 700. A post processing software is customized to efficiently filter multiple analysis results databases and automatically produce report ready level summaries.

Introduction

The Barracuda Caratinga field development in the Campos Basin, offshore Brazil, comprises two FPSO tanker conversions (Barracuda: P-43 and Caratinga: P-48) with spread mooring systems, with over 420 kilometre of risers and flowline. As stated above, the sheer magnitude of the scope, combined with the complexity of the interface design requirement and the aggressive project schedule imposed significant challenges to the dynamic analysis for the riser systems. Procedures and methodologies were needed to handle the riser analysis work efficiently and effectively, enabling the engineering, fabrication and, where necessary, qualification testing of long lead items to commence.

The first challenge was to develop a robust regular wave methodology to evaluate the riser design loads practically and realistically. Considering the magnitude of the analysis work involved for two vessels, positioned in deepwater, with 5 riser design types and 84 risers, irregular wave analysis was considered impractical leading to demanding computation and post processing. On the other hand FPSO motions are typically more severe than other types of floating facilities due to their heave, roll and pitch natural periods being in the same frequency range as the wave energy. Preliminary regular wave analyses using maximum wave characteristics (Hmax, THmax), as specified in the Metocean data documents, resulted in overly conservative dynamic loads in the risers. This can occur, typically, in the case of a large FPSO with risers attached near the bow of the vessel, when the design wave period, as determined from wave statistics, coincides with the pitch natural period. An additional consideration arises from the fact that numerous risers are attached over a considerable length along the port side of the vessels. In such cases, a single design wave may not be applicable for all risers. This has lead to the adoption of a response based methodology for selection of the design waves.

A procedure has been developed to synthesize regular wave characteristics (Heq, Teq), which realistically simulated the expected maximum vessel motions and riser loads in random seastates, at each riser attachment location. An "Equivalent Regular Design Wave" approach was developed and has been verified by comparison with more rigorous random sea analysis.

The second challenge was to rationalize the number of load cases and environmental conditions considered during the design effort.