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winds, and currents are significant issues to consider in design and
operation.
Dynamic/impact-pressure actions arising from green water,
sloshing, and slamming are also issues to be resolved both in
design and for operation, particularly in harsh weather areas.
Careful consideration of an adequate station-keeping
system and adequate design considerations of systems, such as the
riser system, are necessary in order to avoid difficulties due to
vessel motions. The riser system used for ship-shaped offshore
units is usually flexible (rather than rigid). There are several
methods of mooring the ship-shaped structures, including turret
moorings, articulated towers, and soft yoke systems, which permit
the unit to weathervane, that is, rotate according to the direction of
external forces. Thrusters can assist the mooring system to reduce
forces and motions.
In relatively benign environmental areas, FPSO systems
may be spread-moored; also, rigid risers may be acceptable.
However, in harsh environmental areas – for example, with
revolving tropical storms such as typhoons in the South China Sea
and tropical cyclones offshore of Northwestern Australia – careful
consideration is 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
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