Ted Allan, Harvey Lee, Barry Robinson and Colin Reilly report on a grouting equipment system developed to meet a difficult offshore task.
The latest Arkutun Dagi Gravity Base Platform was installed in 35m of water in the Sakhalin field, Russia, in mid 2012. One of the main issues, critical to the success of the installation and subsequent platform stability, was the requirement for multi-compartment under base grouting, using a special 'flexible' grout formula, which had to be completed within a tight 72 hour period - the expected maximum 'weather window' providing suitable conditions for the grouting vessels and crew to operate safely.
An estimated 6100cum of grout (comprising cement, sodium silicate and sea water) had to be pumped into the 17 underbase compartments, via a manifold control panel to an extensive piping system, in a continuous operation over the 72 hour period.
ULO Systems was contracted to develop a grouting equipment system that could perform the above task.
This system needed the capability to store 2260 Tonnes of cement, and 360 Tonnes of liquid Sodium Silicate, house and operate all required equipment, and deliver the specified mix and quantities on a continuous basis to complete the installation successfully within the specified timeframe.
In order to provide enough storage for the 2260 tonnes of cement, ULO had three new 100 tonne capacity horizontal silos fabricated, and these were used together with a series of 40 tonne,50 tonne and 100 tonne existing units as well as vessel inboard P tanks to achieve the required infield storage capacity.
All of the required equipment (except for some of the silos) had to be housed on a DSV including 100 per cent redundancy on all operating equipment.
ULO set about developing a mixing and pumping system, based around conventional technology, which would not only have the required capacity to perform the work, but would also, be in modules which were transportable by conventional road transport and sea freight, and, fit into the confined deck space available on board the DSV. These space limitations presented an additional challenge to the ULO team, particularly because 100 per cent contingency equipment capability had to be available for immediate operation if required.
Although grout mixing/pumping equipment of sufficient capacity was available in the market, they were physically too big to fit into the available space on the vessel, so ULO had to design and build new modular units which would, when assembled, work in unison with the RJM (Re circulating Jet Mixer) to ensure continuous mixing and grout delivery, and still fit inside the available deck space footprint.
The functionality of the system was dependent upon the RJM having a mixing tank of the correct size so that the vortex action in the mixer tank was maintained, which was critical to the efficient mixing of the seawater, sodium silicate and cement in sufficient quantities to meet the demand and on a continuous basis, whilst coupled with the high capacity delivery pumps
The set up was also fitted with a sophisticated quality control system which incorporated non nuclear densitometers, and a series of flow meters, which were linked back to the Quality Control room via a computer linked data logging system.
After a detailed technical evaluation and design effort, ULO built and tested the system successfully at its facility in Sharjah, United Arab Emirates, using an identical grout mix to that specified for the offshore installation and by simulating the conditions expected at the offshore location as closely as possible.
Equipment function testing and a comprehensive laboratory testing program using the cubes made from the mixes, confirmed that the equipment was capable of exceeding project requirements in terms of performance and quality, and that the grout would develop the necessary strength after one week, to meet the 10 year storm condition.
The final equipment configuration was capable of mixing and pumping up to 200cum per hour.
During the testing, it was established that, to prevent dilution of the mix, the grout had to be pumped at a rate of 7-15cum per hour, until approximately 10 per cent of each sub compartment cavity was filled, then the pumping rate was steadily increased to 160cum per hour. The grout mix also needed a high viscosity to allow it to be pumped from the work vessel, up 25m to the top of the GBS, then down 50m into the platform shafts to the distribution manifold and then into the under base cavities.
After mobilising all equipment and sodium silicate to Singapore, a four day load out and equipment testing programme was undertaken, including the delivery at the dockside of the 2260 tonnes of cement into the ULO silos and inboard tanks on several vessels.
In the field, the grout delivery system was set up such that there were two manually controlled manifolds in the GBS shaft, each fed by a 4-inch grout hose from the pumps on the vessel. One manifold was used to fill large compartments and the second was used to fill two separate, smaller compartments simultaneously, meaning that three compartments were being filled with grout ant any point in time.
The offshore campaign went very smoothly and the under base grouting operation was successfully completed in 71 hours, using 15 ULO specialist personnel to manage and control the grouting operations.
Following the success of the project, which was carried out on schedule and without any downtime or injuries, ULO is now equipped to provide similar services in the future having done all of the research and development and built the equipment, to handle such difficult, high volume specifications.
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Ted Allan is Regional Manager, Asia ULOSEA Pte Ltd; Harvey Lee is CEO ULO Systems; Barry Robinson is Equipment Manager, ULO Systems; Colin Reilly is Operations Manager, ULO Systems. ULO Systems LLC is based in Sharjah, UAE. www.ulosystems.com