Heather Saucier speaks with Mark Randolph and Susan Gourvenec, professors at the University of Western Australia’s Centre for Offshore Foundation Systems, about deepwater geotechnical challenges, such as complex carbonates found off the coast of Australia, and new developments for effectively exploiting deepwater oil and gas.
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Seabed drill being recovered (at night). The upper A-frame and the circular carousels of drill rods, etc., are visible. Photos taken by Chris Atwell, Seafloor Drill Superintendent, from the Fugro-McClelland office in Houston. |
As offshore exploration moves into deeper waters and into frontier basins that are carbonate in nature, challenges in geotechnical engineering have inspired a handful of advancements that, in some ways, are revolutionizing the offshore industry.
Many are taking place at the University of Western Australia’s Centre for Offshore Foundation Systems (COFS), which maintains the highest concentration of geotechnical researchers in the world, not to mention a carbonate-based “backyard” highly conducive to researching soil sampling, geohazards, anchoring systems, and welcoming the world’s first floating liquefied natural gas (FLNG) facility.
“We have got 60-70 research staff and students, partly because of the problematic seabed soils off the coast of Australia, and we need a concerted effort to understand them,” said Mark Randolph, a geotechnical engineer and professor and founding director of COFS, which is funded by the government and industry. “In many ways, we are on the leading edge of research.”
Unlike areas such as the Gulf of Mexico and the North Sea, which have established deepwater drilling programs, areas off the coast of Australia, India and China must deal with the complex carbonates that comprise their seabeds to effectively exploit oil and gas.
“Carbonates vary in nature and strength as you go down vertically through the seabed,” Randolph explained. “If you move laterally by 1km, you might find completely different stratigraphy.”
“Australia is just now looking over its continental shelf,” said Susan Gourvenec, a professor of geomechanics at COFS and co-author, with Randolph, of Offshore Geotechnical Engineering, a book that initially started out as a student’s guide.
She added that Australia’s first truly deepwater offshore project, the Gorgon gas project, extends 1100m deep in one field.
Soil sampling
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A coastline scene taken during a recent site investigation off the northwest coast of Australia, just outside Dampier. |
As the industry moves off the continental shelf, it becomes more difficult to investigate the seabed. “It’s not just a matter of taking scoop samples,” Randolph said. “Quantitative measurements of soil strength must be made.”
In the last decade, “enormous” developments in seabed robotic drilling systems have been made, Randolph said. Today, small and modular systems can be lowered into the water column and opened like a moonlander on the seabed. They are connected to remotely operated vehicles (ROVs) with umbilical cords, and all operations take place on the seafloor.
“Clever robotics drill through the layers of the seabed down to about 40-50m,” Randolph explained. “It’s a revolutionary way to take samples. You end up with a higher quality sample for less than the traditional cost of a vessel.”
A joint industry project led by COFS kicked off earlier this year with Benthic Geotech, Fugro, Shell, Total and Woodside. All are working to advance site investigation by delivering new sensors, tools, and engineering design methods that will improve intelligent and efficient geotechnical seabed surveys.
One goal is further advancing seabed penetrometers, including the T-bar and piezo ball devices, developed by Randolph and colleagues in the 1990s to measure soil strength. This new phase of research includes additional penetrometer devices, such as the hemiball and toroid, which will provide more detailed measurements of soil response through seabed interactions that are more directly relevant to engineering design.
“We still take samples and test them in a lab, but there has been an increasing amount of sophistication in testing offshore, and penetrometers are good examples of that,” Randolph said. “The more you can do offshore in situ, the more confidence you have in the numbers you get.”
Geohazards
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A view of Fugro’s seabed drill being lowered into the water from the vessel. |
As geotechnical engineers strive to better understand the makeup of the seafloor, understanding geohazards is crucial. “Geohazards have seen a significant increase in attention by the industry as it moves into deeper waters,” Randolph said.
Seabed slides, caused by continental shelf breaks, can inflict severe damage to fixed platforms, pipelines, submarine cables and other seafloor installations.
“Most deep waters are riddled with historical, or relic, slides,” Randolph said, adding that slides can be tens and even hundreds of kilometers long. “We must be able to accurately assess the risks of new slides to protect subsea infrastructure.”
Assessment of risk from seabed slides involves not only estimating the probability of a new slide being generated, but modeling the likely runout path of the resulting debris flow and determining whether or not it will impact subsea infrastructure.
Anchoring systems
Now that floating production storage and offloading (FPSO) vessels are replacing many fixed platforms, innovative anchoring systems are being developed to better secure FPSOs and simultaneously reduce production costs. The dynamically embedded plate anchor (DEPLA), developed by COFS Associate Professor Conleth O’Loughlin and Mark Richardson, a former PhD student at COFS, is one of the cutting-edge developments.
The DEPLA is a rocket-shaped anchor that relies on its weight and kinetic energy from free-fall to penetrate to a target depth in the seabed. Once it is embedded, its shaft is removed, leaving the anchor’s flukes vertically embedded initially, but then rotating under the action of the mooring line load to serve as a plate anchor.
Also relatively new is the OMNI-Max anchor, which is a gravity-based, dynamically penetrating anchor capable of being loaded in any direction around its axis. Developed by Delmar, the anchor penetrates more deeply into the soil under extreme loading and uplift angle conditions, according to the company’s website.
“It’s a clever design,” Randolph said, commenting on its minimal size yet great holding capacity. “It can be installed to a moderate depth, and when it is pulled it further penetrates the seabed floor and becomes stronger.”
The first FLNG
The need for effective anchors couldn’t be more obvious than on the developing FLNG project in Australia’s Prelude field, 200km off the continent’s northwest coast. “This is a revolutionary technology developed by Shell,” said Neil Gilmour, the company’s vice president of Integrated Gas Development. “It has the potential to change the way we produce natural gas.”
Rather that send natural gas to a processing facility through large and costly pipelines, the FLNG will liquefy the natural gas on the vessel so that it can be offloaded directly onto tankers for export, Randolph explained.
“The FLNG vessel itself is a gigantic tanker, nearly half a kilometer long. The loads on each mooring are high – extremely high,” he said, adding that each corner will be anchored by four lines that are attached to vertically driven piles 50-60m in the seabed.
Projects like the FLNG are necessary to reduce the cost of producing gas from deep waters, Randolph explained. “The export pipelines are colossally more expensive and hard to engineer. Especially in shallow waters, you must deal with the waves and the currents. The cost to stabilize a pipeline becomes enormous. The big driver is to avoid these large export pipelines – which are typically more than 1m in diameter – by liquefying the gas offshore.”
Making the seabed your friend
In the wake of such technical advancements, Gourvenec reminds of another possibility to make offshore geotechnical engineering more effective: making the seabed your friend.
“A lot of offshore operations can actually improve the strength of the soil with the loads they place on the seabed,” she said.
For example, when a subsea mat is set down on the seabed, the load of the foundation and structure it supports applies a load to the seabed, Gourvenec explained. This causes water in the soil voids beneath the foundation to drain away. As the water drains, the voids reduce in volume and the soil beneath the foundation becomes denser and stronger. The same holds true of lateral loads applied to a foundation, derived from thermal expansion of the attached pipelines.
“If these increases in seabed strength can be ‘banked,’ then efficiencies in foundation design can be realized,” Gourvenec said. “Since operational loading – rather than environmental loading – causes the lateral loading of subsea infrastructure, the operational processes are known – unlike unpredictable and uninterruptable environmental loading – and can be included into design processes.”
Ramping up research
Randolph recognizes that the industry is experiencing a downturn. However, he insists that now is an ideal time to develop more economical ways to exploit deepwater fields in challenging environments. Ironically, it also is a time when funds for research are often cut.
“The industry is down at the moment, but there are still new fields to be developed in next 10-20 years,” he said. “It’s harder to get research funding, and companies need to cut costs by using less expensive technology. But, the way to develop smaller gear requires research.”
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