EXMAR | Gravity base structure design

General info

  • Client: Exmar
  • Period: 2019


| Sustainable and efficient value chains for customers worldwide

EXMAR is a multi-disciplinary maritime and offshore solutions provider, designing and delivering sustainable and efficient value chains for its customers worldwide for the production storage, supply and transportation of oil and gas

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 the EXMAR invited Maridea 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.

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.

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

Maridea 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.
On the decks of the stability columns and bow, the foundation for temporary mooring provisions were designed, used to maneuver the CFLNG over the GBS for mating.
The GBS has no power systems, and to keep the design as simple as possible, any valve operation is manual from accessible positions above the waterline. Power for pumps for ballasting purposes is to come from external power sources. During the life on site any power needed on the GBS (lifeboat operations, navigational lighting) will be supplied by the C-FLNG.

The structural design of the GBS was made in 3D already in the concept phase allowing for accurate determination of the material take-off’s to ensure accurate pricing by yards already in early phases.

Scope of services provided

| Design of the GBS
| Full 3D structural design of the GBS and interface with C-FLNG
| FEM analysis of the GBS with interface to C-FLNG and seabed forces
| Load calculations for the GBS and C-FLNG with and without presence of LNG tanker
| Determination of optimum position nearby storage tanker
| Determination of seabed forces and interaction with GBS
| Stability calculation for all stages from construction of the GBS to mating with C-FLNG, wet transport and installation on site.
| System diagrams for ballasting and tank vent systems, with routing of the lines to determine MTO’s
| Optimization permanent ballast and selection of ballast mixture (seawater and sand)
| The scope was executed in just 2 months.


The principal dimensions and particulars of the GBS are:

| Length overall: 153.000 m
| Breadth moulded: 37.920 m
| Depth at side to main deck: 11.475 m
| Maximum Operational draught: 19.100 m
| Bow height: 22.950 m


The following main machinery is required:

Ballast pumps

| Self priming vertical centrifugal pumps : 2 x 500 m3/h at 2 bar