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effect on the mooring line tensions in the line dynamic mooring analysis.

If the turret position is moved from a bow position to a position one-third the

vessel length back from the bow, the intact minimum mooring line tension safety

factor can improve by 15 to 18%.

In addition to considering the optimum position of the turret with respect to

mooring line tension, it is equally important to evaluate the impact of the turret

incursion into the vessel hull, and cost the impact. The following items can have a

major design and cost impact on the vessel development:

 Loss of cargo tank volume

 Loss of longitudinal strength

 Introduction of additional stiffening steel to dissipate turret loads

 Diameter of the turret structure derived architecturally from the space

requirements for the risers, the moorings and the turret equipment.

 Size of the turret bearing required to efficiently transmit mooring loads between

the turret shaft and the FPSO hull.

When considering the total impact of the turret on the hull, the bow turret

has proven more cost effective in both benign and harsh environments. The bow

turret can be configured in two ways:

1. Integral bow turret (built within tanker bow) (fig. 6.4)

2. Cantilevered bow turret (fig. 6.5)

A fully weathervaning vessel has opex advantages over a controlled

heading/limited rotation vessel, but the inherent requirements of a swivel joint for

each flow path imposes practical limitations on the number of flow paths that can

be provided for a fully weathervaning vessel.

Whilst all turret systems are disconnectable, the term is only used for turrets

having the facility for quick connection and quick disconnection (QCDC). Most of

the turret systems that have been designed for fairly benign weather and shallow

water are disconnected when a typhoon is expected.

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