Gravity Base Structure (GBS) design

EXMAR investigated to use its C-FLNG in a mild offshore environment with limited water depth. The C-FLNG process plant allows for very limited vessel motion and EXMAR invited MULTI.engineering to design a Gravity Base Structure (GBS) to dock the C-FLNG at the intended offshore location.

The GBS was also to be designed as a submersible heavy lift barge suitable for transit conditions. The GBS sits on a prepared seabed suitable to take the distributed loads involved with the offshore environment and combined loads of the GBS and C-FLNG.

Challenges

The challenges involved with designing a GBS for an object with a large waterplane area lie mainly in the large varying forces on the combined GBS and C-FLNG due to waves and tide.

Generally, a GBS will have a small waterline area minimizing wave loads. Also, the current and wind loads are to be considered, where especially the current loads for a large bottom supported object are to be carefully determined.

A further complication was the presence of a floating and moored LNG tanker to store the produced gas. The nearby presence of a large ship influences the wave loads to a large extend, due to shielding and reflection effects.

Result

MULTI.engineering designed the GBS as semi-submersible heavy lift barge, with intact and damage stability characteristics associated with the operations (transport and ballast down to the seabed). The decommissioning after the lifetime was also considered to ensure that the GBS could be re-floated. The GBS was minimized in main dimension to reduce loads and costs.

Added value

3D diffraction multi-body analyses were carried out by MULTI.engineering to determine the effect of the LNG tanker presence and to determine the optimum distance and location to the C-FLNG.

On the decks of the stability columns and bow, the foundation for temporary mooring provisions were designed, used to manoeuvre the C-FLNG over the GBS for mating.

The scope was executed in just 2 months