Operability of Cantilevered Float-over Installations

Float-over installations are now a common method of installing large platform topsides modules. The idea of using floatover techniques for small minimal facilities platforms with topsides weighing in range 400 to 1200 tonnes has been shown to offer significant advantages, especially when coupled with a substructure installed by a jack-up rig. The technique can eliminate expensive crane barges from offshore installation projects. Recently, float-over installations of minimal facilities have been conducted by the cantilevered method in Malaysia by ICON Engineering Pty Ltd (ICON), an example of which can be seen in Figure 1.

Operability of Cantilevered Float-over Installations - Figure 1

Figure 1

 

The operation involves the platform topsides being loaded and transported to site on a barge, skidded over the barge bow, then accurately positioned and lowered onto the jacket. A 3-Dimensional drawing of this arrangement can be seen in Figure 2.

Operability of Cantilevered Float-over Installations

Figure 2

 

The relationship between fender restoring forces, barge motions and substructure stiffness is crucial to accurately predict motions of the barge at mating points and to ensure acceptable loadings on the jacket. The Australian Maritime College (AMC) in conjunction with ICON has commenced a research project with the primary objective to determine the range of sea states, within which a cantilevered float-over installation can be conducted safely. This has involved the setup and conduct of a series of scale model experiments to investigate the barge motions and the maximum wave induced contact loads between the barge and the jacket, when the two are docked together in the cantilevered arrangement. The bulk of the investigation is being conducted by Suzanne Hayne, a final year BEng (Ocean Engineering) student under the supervision of Dr Jinzhu Xia and Gregor Macfarlane of AMC, and Dr Yuriy Drobyshevski of ICON.

The test program completed to date has included motion decay tests in still water and dynamic testing in regular seas of varying wave height and period. Response amplitude operators of the barge motions and of the surge load on the jacket and fenders have been determined. The model of a typical symmetrical barge has been fabricated at AMC with two springs attached forward on a perpendicular support with a sliding shoe attachment, as can be seen in Figure 3.

Operability of Cantilevered Float-over Installations - Figure 3

Figure 3

 

This allowed the barge not to be restricted in pitch and heave, while being restrained transversely at the docking points. The barge with the spring system was fixed to a stiff support to simulate the barge being docked to the jacket, which reduces surge and sway motions significantly. The effect of varying the stiffness of the jacket and the fenders was investigated by installing a series of springs having various stiffness constants. The mooring system of the barge was also modelled, each line having the correct longitudinal stiffness and pretension. The dynamic loads in each mooring line were measured during each test run. Analysis of the experimental data is currently underway.

The present project is one of those that utilise the model testing capabilities and teaching programs at AMC for the needs of Australian offshore companies. Acknowledgement is given to ICON for their kind permission to publish the above details.