Project Execution and Operational Experience With the First Permanent FSO System in the Gulf of Mexico
Authors: Bastiani Adolfo , Karve Shashank V. 
Published: October 2008

This paper presents details of the project execution and operational experience for a Floating Storage and Offloading system installed in the Gulf of Mexico. In addition to being the first permanently-moored FSO installed in this area, the system was designed with several unique features driven by the following Client requirements:15-year on-station life.No loading downtime.No offloading downtime.

This paper describes the design and construction solutions. Operational data from the first six months are also presented; these data are an indication of how well the design and operating objectives were met.

Introduction

The Client is developing the Cantarell field offshore the Yucatan Peninsula in the southern Gulf of Mexico (illustrated in Figure 1), and specified an FSO system for the field development. The Client specification requested both the supply and operation of the FSO for a period of 10 years, with the possibility of a five-year extension. MODEC was awarded the contract on 01 June 1997, and commissioning at site was stipulated to be 21 June 1998; thus, this was a "fast track" project.

A significant feature of the FSO system was the requirement that there be no interruption in loading, either for weather or mechanical problems. The area is subject to hurricane conditions, which hampers offloading of crude to tankers. In order to accommodate both weather downtime and shuttle tanker scheduling, the FSO was required to store several days' production. Further, the maximum offloading rate was to be much higher than the maximum anticipated loading rate.

Significant features of the Client's specification included:Continuous on-station life of 15 years.Continuous loading of crude oil 365 days per year through dual flexible risers.Permanent Single Point Mooring capable of station keeping in 100-year survival conditions in a design water depth of 246 feet.

Offloading Tanker Size Ranges:Side-by-side: 55,000 to 250,000 Deadweight Tons (DWT)Tandem:55,000 to 350,000 DWT

Characteristics of the system include:Normal loading rate: 350,000 BOPD of stabilized crude.Maximum loading rate: 850,000 BOPD.Minimum storage capacity: 1,750,000 barrels.Offloading to two locations - a tandem or a side-by-side moored shuttle tanker, either individually or simultaneously.Loading metering.Offloading metering with prover.Offloading rates:Single Export TankerSide-by-side: 80,000 bbl./hr.Maximum wave height for offloading: 4 metersTandem:55,000 bbl./hr.Maximum wave height for offloading: 5 metersSimultaneous Offloading to two tankersSide-by-side: 80,000 bbl./hr.Tandem: 40,000 bbl./hr.

The crude oil product is stabilized to about 7 psi Reid Vapor Pressure, with an API gravity of approximately 220.

Vessel Size Considerations

Continuous on-station duty necessitates "continuous inspection", which requires emptying and internal inspection of each cargo tank at five (5) year intervals. The vessel, in addition to providing the specified minimum storage of 1,750,000 barrels, must have additional storage capacity toaccommodate this inspection, which dictates a minimum vessel capacity of at least 2,000,000 barrels.