FPSO fleet correlations: examining storage vs deadweight

By Mark J. Kaiser, Louisiana State University 

FPSOs have several advantages over other production facilities, but the most important is their field storage capability which avoids expensive pipeline infrastructure and allows direct offloading and delivery to market.

FPSOs have become increasingly standardized and new technology has delivered better and more reliable projects with lower lifecycle costs. One of the most direct depictions of this standardization is the relationship between storage and deadweight.

Storage capacity and deadweight are two key features of FPSOs, and the goal here is to examine the relationship between these two FPSO engineering design factors.

Storage capacity

The hull stores the stabilized crude and condensate until ready for offload, and provides storage for ballast and fresh water, chemicals, and fuel used in operations.

Before 1995, storage capacity for all but four of the 100 or so installed FPSOs were one million barrels (MMbbl) or smaller. After 1995, >1 MMbbl storage was more common and a few 2 MMbbl units began to appear.

Today, about 80% of the FPSO fleet has crude oil capacity between 500,000 and 2 million barrels (Table 1), and there are eight FPSOs with storage capacity greater than 2 million barrels. The maximum storage capacity in the current fleet is 2.3 million barrels.

Crude storage capacity depends on peak production rate, parcel sizes, and offloading frequency. The hull must allow sufficient storage volume to accommodate the required parcel size for shipping and allow a reasonable offloading frequency.

FPSO oil storage is usually sized for at least two weeks of oil production, and 7 to 14 days at peak production is most common, but much variation exists as shown in Figure 1.

The sample in Figure 1 includes 74 newbuild and 128 converted units and there was no noticeable difference in storage volume between the two hull types.

Tanker rule-of-thumb

Trading tankers are almost all storage, and a frequently encountered rule of thumb uses the conversion factor 7.5 barrels per DWT to approximate storage capacity.

Recall that deadweight tonnage (DWT) is the displacement of a fully loaded ship at maximum draught, minus the lightship displacement (hull weight).

For example, a VLCC with 320,000 DWT is roughly equivalent to a storage capacity of 2.4 MMbbl (=320,000 tonne·7.5 bbl/DWT).

The origin of the conversion factor probably arises from the nominal (average) exported crude with a density of 36 API (specific gravity of 0.845).

FPSO rule-of-thumb

For FPSOs, a smaller conversion factor should apply between storage and deadweight because topsides weight for FPSOs is significantly greater than trading tankers and a greater portion of storage is utilized for equipment and non-crude storage.

Taking these two factors into consideration, we show using a back-of-the-envelope calculation that for FPSOs, a conversion factor 6.0 bbl/DWT applies.

Using a random sample of 50 FPSOs, the topside weight was collected and compared to deadweight. Over 80% of the sample yielded topside weight between 25,000 to 35,000 tonnes (t), representing 5 to 15% of the reported FPSO deadweight. Since FPSOs also require storage for fresh water, chemicals and fuels, and may use the hull for equipment, effective storage is reduced further, perhaps another 10% of DWT.

The combined result of these two factors results in a smaller conversion factor compared to tankers, calculated at roughly 6 bbl/DWT = 7.5 – 0.2(7.5) bbl/DWT, where the 20% value comes from the arguments described above.

Although this is just a simple back-of-the-envelope calculation, it agrees surprisingly well with fleet data described next.

Storage x deadweight 

Graphing crude storage capacity against deadweight for the FPSO active and available fleet yields a slope of 5.9 barrels per DWT.

One might expect a different conversion factor depending on whether the FPSO is converted or newbuild, but the empirical data shows only a small difference, 5.8 bbl/DWT for newbuilds and 6.0 bbl/DWT for conversions (Figure 2).

Regression models

Fixed-term models using deadweight and storage capacity provide a robust model fit:

         Storage new = 0.0041 DWT + 425, R² = 0.66

         Storage conv = 0.0055 DWT + 115, R² = 0.77.

Storage is described in Mbbl and DWT is expressed in tonnes. The poorer fit model for purpose-built hulls is attributed in part to the greater diversity of newbuild designs.

Models without an intercept are as follows:

          Storage new = 0.0058 DWT, R² = 0.44

          Storage conv = 0.006 DWT, R² = 0.76.

Circular FPSOs were not included in the sample, and if gas FPSOs are excluded, the model fits improve since the samples are more homogenous.

Deadweight correlates with storage capacity, but one should always be mindful that when comparing individual units, a smaller deadweight FPSO could have greater storage capacity than a heavier deadweight unit.

Storage vs dimensions

Hull size determines cargo capacity, and the greater the dimensions of the hull the greater the storage capacity.

Storage capacity is correlated to the hull’s length L, hull effective area L*W, and hull block volume L*W*D as follows:

       Storage = 7.63 L – 949, R² = 0.50

       Storage = 0.101 L*W – 244, R² = 0.62

       Storage = 0.0028 L*W*D + 145, R² = 0.63.

       Storage is described in Mbbl and all dimensions are measured in meters.

Like the previous deadweight relations examined in Part 3 of this FPSO series, model fits improve with each additional variable because each new variable is both relevant and (mostly) independent from the previous variables:

      Storage = 4.43 L, R² = 0.89

      Storage = 0.085 L*W, R² = 0.93

      Storage = 0.003 L*W*D, R² = 0.93.