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Permanent mooring of FLNG and FSU vessels in ultra-shallow water

hero_mobile_1200x2400_B0112 – Permanent mooring of FLNG and FSU vessels in ultra-shallow water – Marine XII_
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9 October 2024

PERMANENT MOORING
OF FLNG AND FSU VESSELS IN
ULTRA-SHALLOW WATER

Marine XII is a flagship FLNG delivery project offshore Congo, West Africa, operated by Italian energy giant Eni. The early production phase involves two vessels moored in combination in very shallow water (only 20m), the FLNG “Tango” and FSU “Excalibur”, using a chain catenary spread mooring. The two vessels are close-coupled using a novel arrangement of Nylon hawsers. The seabed conditions at site consist of a thin layer of very soft silt over rock, not normally considered suitable for drag embedment anchors.

STEVSHARK®REX anchors and STEVADJUSTER® in-line tensioners from Delmar Systems were used to help successfully moor FLNG and FSU vessels in very hard seabed conditions, in shallow water.

SPREAD MOORING SYSTEM

The spread mooring system is comprised of 17 legs of 107mm diameter studless R4 chain arranged in four clusters. Originally drilled and grouted piles were specified as the anchor points, however a need arose during installation to replace some of these with drag anchors. Delmar Systems supplied a total of thirteen 33mT STEVSHARK®REX anchors with additional 16mT internal ballast and a further 17mT external ballast to provide the required holding capacity in the very hard seabed at site.


To enable accurate line length adjustment and pre-tensioning of the mooring, an in-line tensioner was included in the ground chain section of each mooring leg. For this, Delmar Systems designed and supplied our patented Mk1b STEVADJUSTER® to suit the 107mm chain, complete with mud mat and LTM shackle connection for the passive side chain. Both the anchors and tensioners were designed, manufactured and tested in the Netherlands with Bureau Veritas certification, the anchors being built to a tight delivery deadline of only six weeks! Anchors were transported to Congo by large North Sea anchor handling tugs, which were then used for the installation work.

ANCHOR INSTALLATION

Seven of the supplied anchors were installed in place of drilled and grouted piles, using bollard pull by tandem tugs, to 391mT at the anchor. In the hard rock seabed, drag lengths were quite short (typically 3-11m).

HOOK-UP AND TENSIONING
OPERATIONS

A STEVADJUSTER® was installed on every chain leg, with the active chain pre-installed. The FSU and FLNG vessels were hooked up to the prelaid mooring legs and the STEVADJUSTER® units tensioned in turn to the target load/length, using only a single vessel. First gas was sent ashore on schedule before the end of 2023.

FEATURES 

Client: Eni/Exmar
Application: FLNG, FSU
Location: Offshore Congo, West Africa
Water depth: 20m
Period: 2023
Products: 13 x 33mT STEVSHARK®REX anchors with 16mT internal ballast and 17mT external ballast, associated rigging, and 18 x 107mm MK1b STEVADJUSTER® in-line tensioners with mud mat and shackle.

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