Shell technicians investigate the effect of vortex-induced vibration (VIV) on cylinders and discover some benefits along the way.

Don Allen and Dean Henning were working at the US Navy's Surface Warfare Center in Carderock, Md., when they came across a smooth-surface effect that suppressed VIV on tubulars.
While the US Navy has since sought practical applications for this discovery - on submarine conning towers and periscopes for example - the offshore drilling industry stands to save thousands of dollars in rig downtime by applying the same principle of VIV suppression to drilling and production risers, tendons and trusses on offshore structures.

"Our discovery was that smooth

surfaces can decrease VIV under the right conditions," said Allen, now business team manager for pipelines and offshore structures at Shell Global Solutions in Houston.

Referring to Reynolds numbers (used to quantify current strength acting against a cylinder) Allen and Henning were able to arrive at an optimum size of tubular to avoid VIV. "But the key is that the surface has to be smooth," says Allen, who with Henning discovered the smooth-surface effect in 1997, Shell's Changes magazine reported.

The technicians were testing strakes and fairings as suppression devices in a tank equipped with a 135-ft (45-m) rotating arm to simulate VIV effects.

One cylinder made of glass-reinforced fiber was tested first to obtain a baseline VIV response.
This cylinder was accelerated until rotation speeds reached "critical and super critical" Reynolds numbers and there was no VIV at all. The experiment was designed to allow current speed to reach 22 ft/sec (7.5 m/sec). In the absence of vibration, rotating arm speed was increased to 35 ft/sec (11.5 m/sec), tripling the cylinder loading. Again, no VIV was detected.

The super-smooth surface of the ground fiberglass cylinder had effectively killed vibration.
Henning and Allen concluded that the formula to achieve the smooth surface effect on larger diameter cylinders was governed by the ratio between the roughness height, divided by pipe diameter, where the roughness height is the average peak to trough height of the roughness, when traversed along a line.
"Our test pipe was only about 1 cm in diameter," Allen said. "So if I go out into the field and have a riser that is almost 1.25 m in diameter, my roughness height can be 20 times as great as on the test pipe. As long as you keep the ratio the same in the field as in the tests, you get the same result."

Subsequently, the discovery was tested during drilling with the Stena Tay semi-submersible offshore Trinidad where Shell fitted 30 of these super-smooth VIV suppression devices featuring fiberglass and gel-coat sleeves on a drilling riser.

"They drilled the well, and late in the program they encountered a 2-knot current. That's a level where you normally get quite a bit of riser vibration and some pretty significant deflection at the top. They saw no vibration and no deflection. It was as steady as a rock," Allen said.

Since then, practical applications have been found for this science. During installation of 16 tendons each on the Brutus and Ursa tension leg platforms in the US Gulf, technicians found smooth-surface tendons could tolerate high VIV levels without risk of damage.

In both cases, tendons were left hanging off an installation barge when a loop current was encountered. Because the tendons were vulnerable while not under tension, the usual procedure would have been to completely demobilize. For the Brutus tendon installation program, this meant two-week delays at a cost of $500,000/d.

Pre-testing had demonstrated that smooth-surfaced tendons could tolerate more VIV. "We said we could live with a much higher current than we initially thought," Allen said. "For Brutus, that meant the difference between pulling out [the tendons] and demobilizing and staying on station. A similar thing happened on Ursa, they could stay there with a lot less risk."

Tendon coating and materials are now the focus of new research by Shell Global Solutions. "Now we are looking at coatings that we can put on a production member which has to retard marine growth for its entire life," Allen told E&P.

Shell's group has also pioneered work on helical strakes and developed an ultrashort fairing half the diameter of drilling or production risers. Allen's group believes these fairings can provide super-low drag coefficients.

Whereas helical strakes can produce a coefficient of 2, a fairing on a 12-in. to 18-in. production riser can produce a drag coefficient of 1, while on tendons, a fairing can produce a figure of 0.5 or 0.6 and a smooth surface of 0.2 or 0.3.

A coefficient of 0.5 to 0.6 is quite sufficient, Allen said. "In deep water, we are thinking about risers with sleeves because they can stay smooth, there is less marine growth potentially and if we are going deeper and longer [risers], we need that strength."

References:

Judd, Steve. "Bonga: the challenges in Nigeria's first deepwater development," Oct. 2002.
Brekke, James N. and Wishahy, Momen A. "Simulation of the drifting process to assist riser deployment in high current," SPE 74486.
Shang, Jane; Magne, Eric; Morrison, Denby; Efthymiou, Mike; Leach, Colin and Lo, King H. "Pressurized drilling riser design for ultra-deepwater," SPE 74534.
Allen, Don. "Performance characteristics of short fairing," OTC 15285.

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