DESIGN OF A FLOATING PRODUCTION

FPSOdes3

This is an example of designing FPSO.

Design of a Floating Production, Storage, and Offloading (FPSO) System and Oil Offtake System For Offshore West Africa

Project Definition

Floating Production, Storage, and Offloading (FPSO) systems comprise a new branch in offshore technology. In keeping with the innovative spirit of the offshore industry, this design of an FPSO will implement this moderately unused expertise to utilize the oil fields of the West African coast. A shallow  water depth of 27 m (88 ft), storm generated swells, low daily production output, and various regulatory bodies govern the overall design.

General Arrangement

Two separate designs have been considered throughout the project. The first option is a conventional ship shape, while the other is a more creative square shape. Both facilities include processing modules scaled from existing vessels. The ship shape, weight of 211,000 metric tons and draft of 16 m (52.5 ft), has longitudinally arranged oil storage tanks and ballast tanks along the side, and under each separate tank. The square shape, weight of 207,000 metric tons and draft of 9.25 m (30.35 ft), has radially arranged oil tanks and ballast tanks. This configuration lends to the increased stability over the other design. The storage capacity criterion is based on the total daily production output. The intended shuttling tanker, a 650,000 BBL Aframax, will be used to move the product from West Africa to the United States. In order to be economical, only full loads will be shuttled. Based on an output of 20,000 BBL/day, the total lift cycle is approximately thirty days. This will guarantee that the full storage capabilities of the tanker will be utilized.

Systems

Mooring

The purpose of the mooring system is to keep the vessel on station at the site. The mooring system includes mooring and anchoring. There are several types of mooring available for use on a FPSO. For this design, a catenary spread-mooring system will be analyzed using the MIMOSA software package. After optimization, the 12-line mooring system consists of line lengths equal to 250 m (820.21 ft) and factors of safety ranging from 2.5 to 3 for an intact system, and 1.4 for a damaged system.

Offloading

The tandem-stern offloading approach was selected based on the safety, cost, and reliability factors. A floating hose, carried by a workboat, connects the two vessels and provides a means to transfer huge amounts of product in a relatively short amount of time. As a result of being located directly behind the FPSO, the tandem configuration also helps to eliminate the exposure of environmental forces on the shuttling tanker.

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Analysis

Environmental Loads

An excel spreadsheet is used to analyze the environmental loads. It calculates forces induced by the wind and current. These forces are dependent on the wind speed, current speed, and the bow and beam areas. For the traditional FPSO design, the environmental loading results show that currents in the Ukpokiti field site are relatively strong in the beam seas. This is expected due to the major swells that approach the Nigeria delta. The bow seas show the smallest environmental forces, and so the FPSO will be moored in the direction of the bow. The bow sea forces for the traditional FPSO is 47.8 kips (212.6 kN), and the bow sea forces for the square FPSO is 552.4 kips (2457.2 kN). The loading for the square shape is nearly equal for both bow and beam seas.

Hydrodynamics

To limit the effects of the natural motions of the ship and square shape designs, the natural heave, roll and pitch periods of the structure were considered. The natural period and the wave exciting level are important parameters for estimating the amplitude of motion of the floating vessel. Due to the large water-plane area of FPSO, the natural periods of heave is in the range of wave periods. This is the reason why the FPSO motion characteristic is poor relative to other floating structure (OTRC2002). The period of maximum wave height from the Met ocean data provided by ConocoPhillips gives a period of maximum wave height ranging from 13.3s to 13.8s. There are produced from swells. The heave period of the ship shape FPSO, 7.97 seconds, is close to the maximum environmental periods, but is still allowable.

Stability

StabCAD is an analysis tool that checks for data consistency, determines heeling and righting arms and the allowable KG to meet the criteria set by ABS MODU regulations (ABS 1997). Hydrodynamic, intact stability and damage stability analysis were performed using StabCAD. Intact stability shows that for the 100% capacity and the 30% capacity cases the area ratio of 1.4 is satisfied. Damage stability shows that when one side ballast tank is damaged regulations are satisfied for both cases. There is 14 degrees between the first intercept and the second intercept, regulations require 7 degrees. Also at some 13.5 degrees, which is before the downflooding angle the righting arm is twice that of the heeling arm at the same angle is a damage requirement.

Cost

In every creative venture, cost is a major factor in the design process. For the FPSO at Ukpokiti, general estimations are made to determine the budget for the design. The cost breakdown was done for both the ship-shape and the square-shape options, using the 8-line and 12-line mooring systems. The total cost for the ship-shape option is 373 million dollars and 360 million dollars for the 8-line and the 12-line system, respectively. The total cost for the square-shape option is 448 million and 443 million for the 8-line and the 12-line systems, respectively.

 PDF document:  Designing of an FPSO and oil offtake system

See also: GUIDELINES FOR MEASUREMENT OF LIQUEFIED GASES

 

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