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Disconnectable moorings.

A mooring philosophy to avoid extreme loads.

Permanently mooring ship shaped floating production units on station, from commissioning through to demobilization, was a key step in the development of floating production. The first permanently moored FPU was Shell Castellan, which was installed in 1977. Although this was followed by many other permanently moored systems, the deployment of BHP Petroleum’s Jabiru Venture in the Timor Sea, offshore Australia, introduced a new mooring philosophy.

Installed in 1986, the Jabiru Venture was the first FPSO in the global fleet to operate in an area subject to cyclones. To avoid the extreme loads resulting from these cyclones, a disconnectable mooring system was designed to moor the vessel up to a certain threshold storm condition, above which the FPSO would disconnect and sail to protected waters.

Since the installation of the jabiru Venture, the industry has deployed a further 24 disconnectable mooring systems which allow ship-shaped units to leave their station to avoid extreme events, including tropical storms, sheet ice, or impact from icebergs.

Disconnectable mooring systems can be categorized as external turrets, internal turrets, and tower soft yokes. The number of each, including their geographical location, is shown below - this is current at the end of 2021, and another disconnectable tower soft yoke is currently under development for the Gulf of Mexico.

The predominant category of disconnectable mooring is the internal turret, with 13 examples deployed to date in a large range of water depths and environmental conditions - I’ve summarized the main characteristics of these systems below.

Note 1: Riser Count incudes risers and umbilicals, including future provisions.

Note 2: 10,000 year non-hurricane storm survivability check performed.

The table excludes systems designed to stay on station in all conditions, but which nevertheless incorporate a mechanism to allow disconnection of the production unit from the mooring system - these will be the subject of another blog.

For more information on the design or history of disconnectable mooring systems, please get in touch or take a look at my paper ‘A Historical Review of Disconnectable Moorings for Ship Shaped Floating Production Units, Newport A., Proceedings of the Thirty-first (2021) International Ocean and Polar Engineering Conference, Rhodes, Greece, June 20 – 25, 2021, ISBN 978-1-880653-82-1; ISSN 1098-6189’.


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