ABS Approves Petrobras MPSO Design for GOM Environment

Class society ABS has provided its basic design approval for Petrobras' Mono-Column Floater, Production, Storage and Offloading Unit (MPSO) intended for ultra deepwater operation in the Gulf of Mexico (GOM). The approval is significant as it consolidates the MPSO design concept as one viable option for the next phase of the Cascade Chinook field in the GOM. The MPSO is a short cylindrical mono-column floater that is also being considered for sites offshore Brazil.

ABS provided an early conceptual stage review of the design and issued its "approval in principle" or AIP in 2005.

The MPSO concept is a non ship-shaped floating production storage and offloading facility (FPSO) that breaks with the tradition of converting existing tankers into FPSOs. The MPSO is a short cylindrical mono-column floater being considered for sites offshore Brazil as well as the Gulf of Mexico.

"A round FPSO design was unheard of ten years ago," commented Luiz Feijo, ABS Project Manager for the MPSO. "Today we are not surprised by the designs being put forward for classification review. If we are not able to review a design to prescriptive rules then we take a risk-based approach for determining criteria," he added.

For example, the MPSO has some characteristics of a Spar but a much shallower draft. The design is such that it minimizes heave and pitch making it more suitable for the application of steel catenary risers (SCRs). With water depths pushing the 10,000 feet mark in some field developments, industry has voiced some concern with the possibility of riser fatigue caused by the motions of the FPSO assigned to the field. Feijo says this new Petrobras hull design is intended to reduce heave thus lessening the fatigue on the SCRs.

The MPSO was designed to be permanently moored to the seabed, remaining on station for its operational life. This presents a major advantage over the traditional ship-shaped FPSO which would require a disconnectable turret due to the environmental characteristics of the Gulf of Mexico.

The MPSO was assessed by ABS for its hull strength by evaluating load components, conducting fatigue assessments and reviewing the Global Motions and Stability reports. ABS also conducted a Hazards Identification Study (HAZID) to identify possible safety issues associated with the offloading system. Feijo says the intent is for offloading to be carried out using dynamic positioning (DP) class 2 shuttle tankers. ABS provided its approval of the DP system on the shuttle tanker design as well.

Since the unit is slated for Gulf of Mexico operations, ABS has provided regulatory compliance assistance to Petrobras. "Our extensive experience in the Gulf of Mexico and our ability to work closely with the U.S. Coast Guard as well as the Minerals Management Service has been a valuable contribution to the project," said Feijo.

Petrobras and Japan Oil, Gas and Metals National Corporation (JOGMEC) have been jointly working toward the practical application of the MPSO concept together with partners National Maritime Research Institute (NMRI) and IHI Group in Japan. The unit calls for storage capacity of 800,000 barrels.

Founded in 1862, ABS is a leading international classification society devoted to promoting the security of life, property and the marine environment through the development and verification of standards for the design, construction and operational maintenance of marine-related facilities.