Underbase Grouting

Underbase grouting is used to provide evenly-distributed support to gravity based offshore structures.

FoundOcean gained initial experience in underbase grouting on the Thistle SALM installation project in 1978. The scope of work included pumping 2,000 tonnes of hematite into the base of the structure.

Gravity structures range widely in size and weight, and are constructed from either reinforced concrete or steel. When the structure is first set down on the seabed, the uneven profile of the seabed results in uneven contact with the underside of the structure. Support occurs only at the high points, with voids where the seabed is lower. This uneven support can lead to unacceptable structural loads. In order to provide a firm, even support to the underside of a gravity structure, a cement grout is pumped into the void between the seabed and the structure.

The ideal cement grout for underbase grouting will simulate the seabed soil in strength, with a low compressive strength and even consistency. FoundOcean’s underbase grouts resemble a stiff clay when fully cured, with 28 day strengths as low as 1 MPa.

The underside of the structure is normally divided into compartments which are grouted individually. This ensures proper and complete filling of the voids beneath the structure. Each individual compartment is equipped with fill and vent lines. The fill lines may run to a surface manifold, or terminate at a subsea ROV grout connector. As well as acting as an outlet, the vent lines show when the compartment is completely filled.

GROUT FORMULATION

Successful underbase grouting is dependent on a low density, low strength grout formulation. FoundOcean’s ultra low strength grout comprises cement, water and sodium silicate.  The addition of sodium silicate enables the cement particles to remain evenly dispersed through the mix during the curing process. During the mixing process, a series of samples are taken and tested for bleed and compressive strength.

MIXING SYSTEM

The low strength grout mixing system is based around FoundOcean’s high-output Recirculating Jet Mixer. Cement is stored in bulk cement tanks and delivered pneumatically the mixer. Water is delivered directly to the high pressure mixing jet. A dosing system adds the correct amount of sodium silicate to the mixing jet in proportion to the water flow rate. During mixing, the slurry density is monitored by densitometers mounted on the mixer units. The mixed grout is then pumped to the structure at a rate of up to 60 cubic metres per hour, per mixer.