Surveys reinforce engineering design decisions for offshore structures.

ConocoPhillips found no surprises when company engineers studied the offshore current survey for the Magnolia tension leg platform in 4,700 ft (1,434 m) of water in Garden Banks Block 783 in the Gulf of Mexico.

That's a good thing. That's the way the system is supposed to work. With hurricanes, loop-current eddies, jets from cold-core eddies and occasional strong near-floor currents in deep water, there's plenty of room for surprises that could be an engineer's worst enemy.

"We took year-long, near-bottom measurements to see if we had high bottom currents," said David Peters, leader, offshore engineering with ConocoPhillips in Houston. At that location, engineers didn't think they would find strong currents. It was helpful to know they were right. Other operators have found currents as strong as 1 m/second, or 2 knots, at those depths.

The survey did show periodic current spikes as high as 1/2 m/second.

The company also took current measurements while drilling in the Millennium Eddy - after thorough studies operators have named most of the known eddies - and found surface currents as high as 2.5 knots. Studies also projected a potential high of 3 knots at the site, so the engineers stayed with their original design concept for work in 3.2-knot currents, he said.

"There were no surprises," he said. "Our biggest concern was with what we would see with the near-bottom currents. We estimated high and found they were lower."

The Gulf currents have been well studied through a couple of joint industry projects, projects that were initiated because of the surprises that ambushed some operators.

Many operators still like to conduct site-specific surveys to assess the potential for extreme current conditions, said Rob Smith, sales and marketing manager and oceanographer with Fugro Geos, the company that handled the site-specific survey for Magnolia.

The metocean industry has considerable data and powerful predictive models for weather, he said, but far less information about ocean movement and particularly deepwater ocean currents. The oil and gas industry has a good understanding of the loop current and associated warm core eddies from satellite imagery, in-situ current measurements and most recently, hindcast models. However, intensified sub-surface currents have only been discovered since the industry moved into deeper waters.

Strong currents, termed sub-surface jets, have been identified but are still poorly understood. These currents may be linked to cold-core eddies within the Gulf of Mexico, but further work is needed to assess the oceanographic processes that cause them. The occurrence of these phenomena is unpredictable. Unlike the loop current and warm core eddies, the sub-surface jets cannot be identified from the surface. The best way to find out if a site is vulnerable is to undertake operational current measurements from a mobile drilling unit or production installation, or to deploy current meters on a single point mooring and measure throughout the water column at the site of interest for a long period of time (typically 1 year).
Even such a measurement program does not provide certainty that strong current conditions will be encountered, but it does increase the reliability of expectations. For example, a survey may show no unexpected current events for a year. An unexpected event might occur on the 366th day, but measurement over such a period reduces the chances that it will happen.

Operators conduct the surveys to ensure that the facilities they design to work offshore will be able to withstand the environmental conditions (current, wind and wave) that they are likely to encounter. This is required to ensure the safety of the people who work and live onboard these offshore structures, safeguard the marine environment and safeguard the investment in the offshore infrastructure, Smith said. "They need to reduce all uncertainty," he said. A good set of oceanographic data at a site provides oceanographers with the basis to derive extreme values. Engineers will then use these data as the design basis for offshore facilities.

Satellite pictures can show movements in the loop current and associated warm eddies, but they don't show the presence of sub-surface jets, with currents that can exceed 3 knots in a broad depth range around 500 ft. Satellites also don't see the strong currents of up to 2 knots that sometimes occur in a large part of the lower water column along the Sigsbee Escarpment. Last year, Fugro Geos conducted the Gulf of Mexico Lower Layer (GULL) Project to examine those Sigsbee Escarpment currents in a 15-month study at 19 locations.

There are basically two kinds of measurements: operational measurements, supporting drilling, development and production activity; and design criteria studies, conducted over a longer period of time. Even in a drilling campaign, the operating company typically bears the cost of the study, because it benefits from enhanced operational and environmental safety as well as maximizing the efficiency of the drilling activities.

These kinds of measurements take place throughout the world. The Gulf of Mexico has potentially harsh currents, as does the active deepwater area offshore Trinidad. The North Atlantic also has a complex and harsh current system. Currents in deep waters off Brazil can also be strong, particularly in the Amazon Basin. Ordinarily, West Africa has some of the most benign deepwater conditions in the world, but it can offer some surprise current conditions, too, he said. The industry is just starting to work in deep water off the Scotian Shelf, but some surveys already have been conducted off Newfoundland and Nova Scotia.
West of Shetlands, the North Atlantic current (Gulf Stream) splits at the Faeroes, and the Norwegian current moves along the coast of Norway, creating complex current conditions where the water masses meet in the Faeroes-Shetland channel.

In this area, the residual current in the warm top 1,640 ft (500 m) of the water column moves to the northeast while the current in the cold lower 1,640 ft (500 m) moves to the southwest. However, there is mixing of the warm and cold water masses producing eddies that can further intensify currents in the area. The warm North Atlantic waters also push up against the United Kingdom shelf slope, strengthening currents. In addition to these residual ocean currents, there are tidal currents superimposed that may offset that movement or amplify it twice a day.

All the fuss about currents concerns the reliability of platform integrity and drilling and production operations in terms of human and environmental safety.

Graham Feld, principal oceanographer with Fugro Geos, speaking at the 7th Annual Deepwater Technologies & Developments conference in Houston, said surveys should cover the full ocean column for as long as possible. If a survey is being conducted to support a drilling operation, then that survey should be conducted at the same time of the year as the drilling operation so it will pick up periodic events.

Currents also affect mooring behavior and the movement of moored rigs and platforms.
A full-water-column survey can show engineers whether the horizontal forces on a riser might be excessive and help them figure out what riser configuration will be most effective in operations and most resistant to fatigue. Those measurements can help predict the vortex-induced vibration effect on the riser.
At the same meeting, David Snell, vice president of marketing and sales with Atlantia Offshore Ltd., said deep currents in the Gulf of Mexico tend to bend risers more than companies might predict.

He also said his company uses step-diameter tendons instead of single-diameter tendons to keep the weight to a minimum, and it adds fairing to tendons and risers to minimize vortex-induced vibration in the tubulars. Critical analysis of current effects can help the company tailor the riser and tendon design to the platform requirements for each location.

"We're often surprised," Feld said. On one campaign, the survey went mostly as expected, but within 3 or 4 days of a certain date, currents hit an extreme. "If we didn't measure it, we could get a real surprise."
Accurate current measurements are often a requirement. In Indonesian deep water with a 2-knot current at the surface and a 1-knot to a half-knot current at depth, the drilling rig may have to start assembling the drilling riser nearly 25 miles (40 km) up-current in 5,000 ft of water to get the last riser section in place just as the rig floats down the current to the drill site.

That calculation assumes rig crews can run four joints of 75-ft (23-m) riser an hour, according to an SPE paper presented by James N. Brekke at Global SantaFe Corp.

In a worst-case scenario, if a company didn't have accurate current measurements for a drilling site, it might find that currents were so strong they wouldn't allow a drilling rig to be moored when it gets on site.

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