Offshore Stone Columns
Offshore stone columns have been installed for many years.
The critical aspect of this product was always quality control or a lack thereof.
The finished product can be visually inspected only by divers and full scale load tests on offshore stone columns are virtually unheard of.
It is therefore of the utmost importance to rely on a column installation method that leaves no room for defective columns.
A large step to improve the quality of the installed columns was made with the introduction of what would evolve into Betterground’s Pressure Chamber Injection System (PCIS) at the Port of Patras, Greece, in 2001.
Since then the number of offshore projects as well as their technical challenges have increased. Today millions of meters of offshore stone columns have been installed in over 38 m water depth, with column length of over 25 m and diameters that can both be constant over depth or follow a tapered design, with the largest column diameters installed directly below seabed where marine deposits typically exhibit the lowest strength.
Pressure Chamber Injection System (PCIS)
The Betterground PCIS is the latest evolution of marine bottom feed stone column installation equipment.
As shown in the illustration, gravel is transported via a 6-inch gravel hose. In a variant of this system, the gravel can alternatively be placed into a hopper by a bucket.
Key to the PCIS is the airtight gate on the top of the receiver tank that locks the gravel from atmospheric pressure and puts it under an over-pressure that ranges between 2 to 6 bars, depending on the operating depth of the rig.
Under the applied excess air-pressure in the interconnected chamber consisting of receiver tank, silo tube and tremie pipe, gravel is injected into the soil at the tip of the tremie pipe near the Vibroflot’s nose, hence the name “Pressure Chamber Injection System”.
Compared with a Single Large Tank System, the advantage of the PCIS is that below the gate there is always exactly one full batch of gravel filled in with each opening of the airtight gate. This not only allows for fewer open-close cycles of this gate, but also for an extremely accurate recording of gravel batches placed in the ground at defined depths. In an aptly named “Double Lock” variant of the same system, two airtight gates enclose the receiver tank on its top and bottom end.
The following picture shows one of the earliest setups for a project in Patras, Greece.
More recent setups are given here.