In the oil and gas industry, Floating, Production, Storage and Offloading (FPSO) is built in ship shaped either it made from conversion or new build. The problems are the motion and vessel stress. However, the industry mitigated this problem by developing turrets and swivels, which allowed the ship-shaped vessels to weathervane. Though swivels and turrets allow ship-shaped vessels to weathervane, they are costly, have long lead times and are typically available from only few specialized designers and fabricators. Swivels and turrets also have associated maintenance requirements and potential downtime (from leaking seals, for example).
The ship-shaped FPSOs are subjected to significant bending loads due to hogging and sagging and, as a result, are subject to fatigue damage. The fatigue problem is exasperated when using hulls built after 1985 where high tensile strength steel was used extensively to reduce weight. The requirement of stiffening steel, resultant increase the cost of construction.
Figure 1: FPSO
To overcome short comings associated with using traditional ship-shaped vessels for FPSOs, the industry is now developing fit-for-purpose FPSOs. The new FPSOs are being designed to have similar motion characteristics from all directions and to eliminate yaw excitation. This eliminates the need for a costly turret and swivels, minimizes the bending loads and fatigue and increases the storage capacity per plated area. Round-shaped FPSOs also have the advantage of being more easily approachable by service and installation vessels with minimum collision risk.
Figure 2: New Round FPSO
One way to minimize FPSO fabrication costs is to reduce the plated area (i.e., reduce steel tonnage) for a given storage capacity. In general, for any type of simple body, the shorter the longest distance between two points is, the smaller the surface area per volume.
Figure 3: Area To Volume Ratio
A simple example is illustrated in Figure 3 where a rectangle having typical length, width and height ratios for a ship-shaped FPSO is compared with a cylinder of typical ratios for a round-shaped FPSO. Both bodies have the same volume, but as can be seen in the figure, the surface area is about 50% larger for the rectangle. This illustrates one of the major advantages of a round-shape FPSO; it can have less plated area for a given storage volume, which minimizes the steel tonnage and associated costs with the plated shell structure of the hull. These savings are even further amplified when one considers that new-built FPSOs require a double hull.
Another concern with converted tankers is crack propagation and fatigue in structural connections. Mitigating these problems can require significant structural stiffening and frequent inspections. Extreme bending loads and stresses are significantly higher in a long slender body, such as a traditional ship-shaped FPSO, as compared to a more compact body such as the round-shaped FPSOs. Figure 4 is helpful in illustrating this phenomenon.
Figure 4: Forces, Bending Moment And Stresses In A Rectangle And
Cylinder Subjected To Wave Loading
Consider the two bodies shown in Figure 4 to be under the influence of a typical long period wave. The figure shows the wave when the trough passes the midpoint of the vessels. As the figure indicates, the center of gravity (i.e. the midpoint) is supported by very little buoyancy for the ship-shape with the buoyancy being concentrated at the bow and the stern of the vessel. This means that the buoyancy forces at the bow and the stern (the arrows pointing upwards) will be significantly larger for a ship shape than for a round shape, where the buoyancy is more evenly distributed along the vessel due to the more compact shape. Due to the lower buoyancy concentration at the bow and the stern and the shorter moment arm (midpointto-bow/stern distance), the bending moment arm is smaller than a comparable traditional ship-shaped FPSO. As a result the reduction in bending moments comes from both the reduction in buoyancy force differential between the center and bow/stern and the reduction in moment arm for a round-shaped vessel.
