A Depla during scale testing. Images from Centre for Offshore Foundation Systems (COFS).
Dutch drag anchor specialists Vryhof Anchors have gone down under in a move to develop efficient deepwater anchor systems. Elaine Maslin takes a look.
When Australia’s Centre for Offshore Foundation Systems (COFS) was established at the University of Western Australia (UWA) in 1997, it was in response to the new engineering challenges associated with building oil and gas platforms on the carbonate sediments of Australia’s North West Shelf.
Since 1997, COFS has developed into a leader in offshore geotechnical engineering research and the work is bearing fruit. A new anchor design from COFS, called a dynamically embedded plate anchor (DEPLA), has been bought by Dutch anchor specialists Vryhof Anchors.
DEPLA is a hybrid system able to sustain significant vertical load and requiring no external energy source or mechanical operation for installation.
Dr. Conleth O’Loughlin of COFS and inventor of the DEPLA explains: “The anchor resembles a dart, and is installed using gravity, similar to other dynamically-installed anchors, such as the torpedo pile. However, the main part of the ‘dart,’ which we call the follower, is removed after the anchor is embedded in the seabed and re-used for the next installation. This leaves the anchor flukes in the seabed, which then become the plate anchor. “Results indicate that the DEPLA exhibits similar behavior to other dynamically installed anchors during installation, but with much higher capacities and predictability than other dynamically installed anchors that resist load in friction,” Senol Ozmutlu, projects director at Vryhof, says.
The new anchor design, aimed at mobile drilling units and floating production systems in deep and ultra-deepwater will reduce installation time, costs, and materials, O’Loughlin says, who has been researching dynamically installed anchors for about 10 years.
Petrobras has been using a gravity-embedded design since the mid-1990s, O’Loughlin says. The torpedo-shaped design is dropped from about 100m above the seabed, reaching about 25m/s, burying itself to about three times its overall length in the seafloor when it lands—replicating a driven pile, without needing to pile it in, O’Loughlin says.
COFS’ centrifuge test facility.
“The rest of the world has been slow to follow,” he says. However, one of the limitations of the Petrobras design is that it is not the most efficient—it doesn’t have a lot of capacity relative to its dry mass, he adds.
Plates are the most efficient type of anchor. These may be dragged into the seabed or installed using a suction caisson. The caisson is then removed and the plate engaged. These can be big, expensive, and take time to install, O’Loughlin says.
After exploring various ideas, COFS came up with the DEPLA.
For a temporary anchoring, for example on a mobile offshore drilling unit, COFS suggests the DEPLA may be about 9m-high and have a 48-tonne total mass, of which the flukes (plate element) would be about 18-tonne.
For a permanent facility, such as a floating production storage and offloading unit, the DEPLA may be about 12m-high, with a 95-tonne total mass, of which the flukes would be about 34-tonne.
Compared to other systems, the DEPLA flukes are relatively light and small. The heavier element, the follower, can be reused, saving material and transportation costs. The plate anchor could potentially be left in the seabed, saving retrieval costs.
The DEPLA is also less susceptible to uncertainties on the seabed strength, O’Loughlin says. Embedments will be higher in weaker soils and lower in stronger soils, but with the same average strength and, in turn, anchor capacity at the final plate embedment depth.
|How the DEPLA works|
The system has been tested at model scale in the geotechnical centrifuge facilities at COFS. In these experiments, soil samples are spun at up to 200 times Earth’s gravity, creating stress conditions in the centrifuge sample that are equivalent to tens of meters of the seabed. The centrifuge data played a pivotal role in informing the final design concept. The centrifuge testing is now an accepted approach for obtaining performance data of geotechnical systems, COFS is a leader in this area, with both beam and drum centrifuge facilities, used by the worldwide offshore industry.
In 2004, a prototype DEPLA was successfully built and tested near the Woodside-operated Laminaria/Corallina project, where the FPSO Northern Endeavor is located off Darwin in the Timor Sea in 340m of water.
It was put through further testing in UWA’s centrifuge and more field trials, comprising more than 200 drops, in a soft clay lakebed and seabed using a 1:4 scale prototype.
COFS agreed a deal with Vryhof in September last year and it will be now Vryhof’s task to engineer and test a full scale prototype.
HitecVision recently bought 71.3% of Mooring Holding (Vryhof Anchors). The firm was founded in the early seventies and its anchors are primarily used for station keeping of on semisubmersible drilling rigs, FPSOs, and single point mooring buoys.
The firm’s new strategy is to expand/renew its drag anchor portfolio and systems; acquire/develop anchor jewelry and other mooring components to sell under the Vryhof brand; enter into the top-end of the low-end segments (anchors to pipelay barges, dredging, jackups, construction barges); and add engineering, design and project management capabilities with the aim to potentially become a full EPCI contractor, for permanent mooring systems.