Veba Hanze F2A Steel GBS Installation Provision of Underbase Grouting Services

North Sea, 2000

The Hanze GBS is a gravity based structure of steel construction. It contains twelve central oil storage cells, with the cells to the North and South ends of the structure being used for ballast water.

A void was formed between the underside of the GBS structure and the seabed due to the unevenness of the seabed. FoundOcean devised the solution to fill the void with a sodium silicate/cement grout in order to ensure an even load distribution between the structure and the seabed.

The silicate grout used by FoundOcean is formulated from cement, sodium silicate and seawater. The resulting grout has a specific gravity of 1.27. This low strength grout was used as it has similar compressive strength properties to the soft clay that the GBS sits on.

Sodium Silicate, also known as "water glass" has the property of absorbing many times its volume of water to form a fluid gel. When mixed with cement, the gel will hold the cement particles in suspension until curing has occurred with the resulting product resembling stiff clay.

For applications, such as underbase grouting, where high strength grouts are not required Sodium silicate has the added advantage of vastly reducing the quantity of cement needed.  In the case of the Hanze GBS the cement requirement was reduced from 1.4 tonnes of cement per cubic metre of grout to less than 350 Kg per cubic metre - a saving in excess of 75%.

The structural integrity of the platform was dependent on FoundOcean’s expertise in enabling the correct placement and retention of good quality grout in the voids beneath the base.

Grouting Equipment

For this particular project FoundOcean used its RJM14 Recirculating Jet Mixer, which comprised a cement surge tank, two electric powered jet mixing tanks (one primary and one backup) and a holding tank for the mixed product. In addition to the mixing equipment FoundOcean provided two triplex grout pumps (one as backup), which, dependent on slurry densities, can pump at a rates up to 0.63 m3 per minute.

A metering skid was supplied to add the correct proportion of Sodium Silicate to the water used for mixing the grout.  The skid was equipped with primary and backup silicate holding tanks and volume feedback mixing valve to add the correct amount of Sodium Silicate in accordance with the water flow rate. The silicate flow is controlled by linking the inventor to the water input meter on the grout mixer and both volumes are recorded and displayed in Litres Per Minute.

The mixing spread was additionally equipped with a hose winch, laboratory and workshop facilities and remote grout density monitoring equipment.

Equipment Summary

• Recirculating Jet Grout Mixer
• Grout Pumps
• Sodium Silicate Metering Skid
• Umbilical Hose Winch
• Cement Silos - 650 tonnes capacity
• ROV Grout Density Monitoring Probes
• Workshop Container
• Laboratory Container

Grout Delivery

Grout was mixed at a rate of between 40 & 45 m³ per hour and was transferred from the mixing tank to a ten-barrel capacity holding tank from where it was drawn off by the pump and delivered to the GBS via a flexible hose. Grouting of the compartments was carried out in pairs, with two penetrations in the dividing cell wall to permit grout flow between the first and second compartment of each pair. One primary and one secondary grout line was provided for each pair of compartments, together with two 8-inch outlet ports located in the outer wall of the second compartment.

Testing of the grout in the mixer was carried out by means of an acoustic density gauge built into the mixing circuit which was periodically checked by a pressurised slurry density balance.  Grout pumping was continued until the theoretical volume required to fill the void had been pumped, and good quality grout flow from the outlets had been confirmed by the ROV.

This project demonstrated FoundOcean’s ability to project manage the offshore grouting and grout monitoring for a specific gravity based structure. The scope of work included design of the underbase grout mix, development of detailed procedures for filling the void between the GBS underside and the seabed and a detailed contingency strategy.