ABOARD THE URSA TENSION LEG PLATFORM: When the Guinness Book of Records was looking for the tallest structure in the world in 2009, it selected Shell’s Ursa platform, located about 130 miles southeast of New Orleans in the Gulf of Mexico.
The platform rests on the waterline, its visible structure measuring a mere 400 feet or so, but when the mooring tendons that hold it in place to the ocean floor are included, its total height from the seabed to the crown of the derrick is 4,285 feet – four times the height of Houston’s JPMorgan Chase Tower.
Shell made its decision to moor the Ursa with tension legs based both on the conditions at the platform site and on engineering expertise in the technology gained from previous Shell tension leg projects.
“Being the pioneers, you like to design something and get it right in all aspects,” said Bill Henry, Shell’s vice president for upstream development, noting that Shell built five tension leg platforms in close succession and the water depth for Ursa was appropriate for such mooring.
“It allowed us to focus on getting all the dimensions right, refining that design and improving on it,” Henry said.
Tension leg platforms are named for the steel tendons that reach straight down from the pontoon supporting the floating platform to the ocean bed, often more than a half a mile below.
Shell has used a tension leg system to moor five Gulf of Mexico floating deep-water platforms, and is using tension legs on its next platform, Mars B-Olympus, scheduled for completion in 2015.
Mary Grace Anderson, a deep-water development manager for Shell who led visitors on a recent tour aboard Ursa, said its engineers have made safety and efficiency refinements with each platform.
“We learn new things about distance, the placement of equipment, how different modules fit together and how they compensate for motion,” she said.
The 16 tendons – four on each corner of the pontoon – that keep Ursa moored to the ocean floor look like steel rods, each 32 inches in diameter. Earlier designs used four to eight tendons, but experience has shown that 16 provide greater stability.
While the tension legs that make it all possible are underwater, the benefits of the technology are visible on the Ursa. The buoyancy of the pontoon maintains the tension in the tendons, so that they never go slack, which minimizes vertical motion on the platform.
Reducing movement allows wellheads to be placed on the platform instead of the ocean floor. That makes it easier to monitor and operate the wellheads – assemblies containing production control valves and other equipment- said Tao Wang, a naval architect with Aker Solutions.
“Generally, the equipment itself is less expensive, because it is less sophisticated,” Henry said. “For subsea wellheads, we have to have subsea robots, equipment and lots of instrumentation that can only be remotely accessed.”
Ursa has six decks, each 300 feet by 300 feet, with enough total deck space for wellheads, drilling and processing equipment and crew quarters.
It produces 150,000 barrels of oil equivalent per day from eight wells.
The high volume helped justify the larger platform and pontoon necessary to support the tremendous weight of the tension legs.
Anderson said Ursa’s design allows for up to 14 wells, and tension leg platforms are preferred when multiple wells are clustered to serve a single platform.
The larger structure also can accommodate both drilling and production facilities, she said, making it possible to drill for additional wells even after production has begun.
“It might also change your plans on other reservoirs you want to go after,” she said. “You learn things about the production characteristics of the reservoir that may cause you to change your development plans.”
Shell introduced its tension leg platform design with Auger in 1993, followed by Mars, Ram-Powell, Brutus and Ursa, which was built in 1999. All of these discoveries are in the Gulf of Mexico and fall within the 4,000- to 7,000-foot water depth considered appropriate for tension leg platforms.
For 1,000-year storms
Henry said the water must be deep enough to justify the complicated, costly technology, but not so deep that the steel tendons are too heavy for the platform to support and too costly to manufacture and install.
Ensuring stability, even in severe hurricane conditions, has driven improvements in tension leg design by Shell and others.
Chevron’s Typhoon tension leg platform flipped over during Hurricane Rita in 2005 and had to be scrapped.
Its Bigfoot tension leg platform, now under construction, is designed to withstand 1,000-year storms, with improvements including 16 tendons.
“We are designing for a higher level of storm conditions and are confident that Bigfoot has the appropriate design,” said Joe Gregory, general manager for Chevron’s major capital projects in the Gulf of Mexico.
“There are a multitude of design alternatives,” Gregory said. “It depends on how we want to develop the reservoirs, what we are going to put on the facility and the way the facility behaves in the sea state.”
Ultimately, as with every vessel or structure that rests on the water, the most important consideration is how well it floats. “In the design, we start with the basics, and the basics that you have to consider are ballast and buoyancy,” said Wang, the naval architect.
Buoyancy becomes even more important in planning how to link the platform to future discoveries nearby through a process called subsea tieback. A platform’s ability to support tiebacks depends on its buoyancy.
“One of the important considerations in a platform is how big you build it and how much spare buoyancy you have for future tieback possibilities,” Henry said. “Everything you tie back has to be supported by the buoyancy of the platform. It is a consideration – how much you are willing to bet on future discoveries by extra buoyancy. Everything in deep water has to float. Everything you add to it has to be supported.”