Page 20: of Marine Technology Magazine (April 2011)

specimens, one upstream from the other.

Oceanic’s Senior Consultant Paul

Herrington notes that in recent years, “more and more installations have multiple risers coming up through the water column that are getting closer and closer together. They’re trying to incorporate more produc- tion facilities in a smaller location.”

This requires mitigating the VIV gen- erated by multiple risers. In the recent research campaign, Oceanic evaluat- ed the performance of fairings and strakes on tandem-oriented cylinders at distances ranging from three-cylin- der diameters to 20-cylinder diame- ters apart. To do this, they used a high-Reynolds-number experimental rig capable of evaluating up to 23”- diameter cylinders in both single and tandem configurations. (‘Reynolds number’ is a non-dimensional meas- urement that takes into account three key parameters such as diameter, velocity and viscosity of the water.

Offshore installations typically involve flows at high Reynolds num- bers.) Oceanic built the rig in 2002 to evaluate 12”-diameter cylinders for their first DeepStar project. DeepStar is a Houston-based joint industry project between oil companies, ven- dors, academics and research insti- tutes whose focus is to develop deep- water technologies. While the rig was set up in the towing tank to conduct the recent experiments, Oceanic also investigated fairings for two other clients to reduce the set-up and take- down costs for each project.

Over the last five years, Herrington notes, Oceanic has seen a marked increase in research involving fairings because they reduce the drag more significantly than strakes. “With some fairing designs, the size of the fairing can be doubled for the same drag of a similar size strake,” he adds. “As a result, Oceanic has been able to investigate larger-diameter cylinders—up to 23 inches, com- pared to up to 18 inches with strakes.”

Owen Oakley, a research consultant with Chevron’s Energy Technology

Company in San Ramon, California, 20 MTR April 2011

The View from St. John’s

Dan Walker, President, Oceanic Paul Herrington, Senior Consultant,

Oceanic