New, short drilling riser fairings lower material costs, cut installation time, reduce weight and do as good a job as older, heavier, longer fairings in most offshore drilling situations.
Strakes used to reduce vortex-induced vibration (VIV) have been around for a long time.

They do a good job of dampening vibration that reduces the life of production risers and

Lighter, plastic fairings lower costs by allowing faster installation on drilling risers. (Photos courtesy of Shell Global Solutions)
tendons on tension-leg platforms. Normally, however, drilling riser isn’t in the water long enough for that to be a problem. Contractors are more worried about strong currents that bend the riser so far they can’t drill or retrieve the riser string. Effective fairing is the answer to their problem.

Shell Global Solutions (US) Inc. is a pioneer in both technologies. Don W. Allen, regional manager for pipelines and offshore structures with Shell Global Solutions (US) Inc., said the VIV suppression program was well under way when he joined the group in 1986. Much of the VIV suppression equipment used on deepwater risers today stems from the company’s research.

In 1988, the group came out with two inventions. One was a tall helical strake to disrupt VIV. It started with strakes that were 10% of the diameter of the pipe because that was the combination that worked best in air. Shell later evolved the system to 25% diameter strakes, the height used on most deepwater tubulars.

Evolution

In water, the 25% strake height models work just as well, and they will accommodate more marine growth before they need cleaning.

The second invention was a fairing to reduce the impact of current on tubulars. The first models had a chord-to-thickness ratio of about 2.5 to 1. Later, the group found it could get nearly equal performance with ratios of 1.5 to 1 or less. The latest patented model includes a 1.12 to 1 ratio. “It’s just a little nub off the pipe,” Allen said.

The evolution doesn’t stop there. The group tested its short fairings by adding shag carpeting to simulate marine growth and found the shorter fairing doesn’t need cleanup as often as helical strakes, he said.

Weight reduction was another step in the evolutionary process. The earlier fairing models were made of fiberglass. They did the job, but they were heavy and hard to install. The oilpatch is used to tough jobs, but in this case, the time involved in the installation makes a big difference in costs and adds to the risk of slowdowns from external events such as a Gulf of Mexico loop current that could shut down drilling operations temporarily.

Around 2000, the group added a smooth sleeve, essentially adding a coating to smooth out the rough fiberglass. It helped with VIV and drag, but the fairings still were heavy and hard to install.

Then they got the idea that, if they could design the fairings right, they could make them out of plastic. That would ease the manufacturing process and result in a lighter but still durable fairing.

“Our goal was to create a fairing that was so light it could be installed without lifting equipment,” Allen said.

The design of the fairing has changed over time, too. At first, the attaching mechanism consisted of stiff straps that wrapped around the tops and bottoms of the fairing sections. Later, they found plastic straps could wrap around the pipe more easily. The design of the strap assembly allows the fairing to rotate around the pipe to weathervane with current.
Research showed the tailfin fairing was more effective with an upset or lip around the strap and fairing. Without the lip the vortex could correlate and strengthen from fairing to fairing along the riser. The lip prevents that correlation and amplitude growth. Now the upset goes onto every fairing and strap.

Contractor concerns

The short fairings have slightly less VIV suppression capability, but they have about the
A view from the top on the Deepwater Nautilus shows the faired riser as it’s lowered into the water.
same drag coefficient as the tail fairings. For drilling contractors more concerned about riser drag than riser fatigue, the drag coefficient is the key element. “They don’t want to drill through the riser,” Allen said.

The real cost for the drilling contractor is the time it takes to put the fairings on the riser and take them off. With the lighter plastic fairings crews can install each fairing in just a few minutes.

The concerns of the drilling contractor make a big difference in this market, he added. For awhile, the fairing market dropped nearly to zero because contractors hated to take the time to add the fairings. That was particularly true since many of the new offshore rigs could handle a lot more tension and pull riser without fairing in more difficult conditions.

Results

Last year, Shell put a joint of the newest system on the Deepwater Nautilus, then under contract to Shell. The team installed eight fairings on riser sections and nine collars in about 40 minutes the first time it attempted the installation. The normal installation time for the older models is about 20 minutes per fairing-collar pair.

An experienced crew can push that time for the older models down to about 8 minutes, Allen said, but an inexperienced crew can do the same job with the new fairings in about 5 minutes. With a little experience, they ought to be able to reduce that time to 3 minutes, he added.

Just as important, the new fairing comes off — with the help of a small, special crowbar — even easier than it goes on. That operation takes about a minute.

The key element of any drilling riser fairing is its ability to handle current. Unless it can make a difference, it doesn’t matter how fast the fairings make up or tear down.

One company, Allen said, was able to run the faired drilling riser in more than 3 knots of current offshore Brazil. Without fairings it would not have been able to run the pipe.
The short fairings have proven themselves offshore Gulf of Mexico and Brazil.

The company is analyzing the systems for work offshore Malaysia and Australia.
The acceptance rating among both contractors and operators is high for the lighter, shorter fairings in parts of the world with potential for significant delays, Allen said.

The cost of the fairings depends on the quantity needed to handle anticipated currents, the diameter of the riser and whether the company needs new tooling to manufacture a size that isn’t standard.

Water depth also makes a big difference. Some customers plan to run fairing top to bottom so they can run and pull the risers in any subsea condition. Other customers plan to use the riser on only the first couple hundred feet below the surface. Still others will use fairing on the top 300 to 400 ft (91.5 to 122 m). Those decisions depend on the force and depth of stronger currents anticipated at the drilling site.

“I’ve seen rigs down for a month waiting on currents,” Allen said. Still, “Even fairings are not a cure-all. They just enable retrieval under more challenging conditions.”

He also hinted at other products under development but not yet ready for disclosure, not tail fairings but products for production risers. “We continually try to push the envelope and work with test data,” he said.

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