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required for the station-keeping with relevant mooring system designs.

FPSO systems may be either new builds or conversions from trading

tankers. Challenges for their structural design are mostly related to assessment of

limit states including ultimate limit states, fatigue limit states, and accidental limit

states as well as serviceability limit states. The 100-year return period is usually

considered for design onsite strength assessment, but tow considerations are based

typically on 10-year return period environmental phenomena. For operation,

relevant programs of inspection and maintenance must also be established to keep

the structural integrity and reliability at an adequate level.

Useful discussions of the technical challenges and technology gaps and

needs related to the use of ship-shaped offshore units to develop the offshore oil

and gas in deep and ultradeep water are given, for example, by Henery and Inglis

(1995), Birk and Clauss (1999), Bensimon and Devlin (2001), Lever et al. (2001),
Maguire et al. (2001), Le Cotty and Selhorst (2003), and Hollister and Spokes

(2004).
Over the past 25 years, ship-shaped offshore units have proven to be

reasonably reliable, cost-effective solutions for the development of offshore fields
in deep waters worldwide. These include FPSOs or FSOs operating in harsh

environmental areas and also waters of more than 1,000m depth; see FSO/FPSO

performance records by Single Buoy Moorings, Inc. for examples.

It is hard to say with precision exactly when ship-shaped units made their

appearance on the offshore oil scene. Certainly, oil storage and shuttle tanker-

mooring facilities using converted trading tankers existed in the late 1960s. The

first such vessels were connected by hawsers to catenary anchor leg mooring

(CALM) systems.

These then evolved into the now more familiar systems employing single-

point mooring, where the FSO Ifrikia was permanently moored to a buoy via a

rigid arm (rather than a hawser) in the early 1970s, with a concomitant increase in

operational reliability and reduced downtime.
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