Joint team probes steel riser vibration

To test the vibration behavior of deepwater risers, 2H Offshore Engineering towed 200-m, 10-in. pipes at speeds of up to 3 m/sec. The tests were carried out near Stavanger, with the lead boat pulling a tow wire to adjust the angle of the riser model, which was weighted to hang beneath the rearward boat. Photo courtesy of 2H. [158,788 bytes]
Two model risers were used for the towing trials: one was an unmodified pipe, and the other, shown above in the photo on the left,
(right) fitted with vibration suppression strakes designed by 2H. The model risers were fitted with accelerometers, enclosed in yellow canisters (photo above on the right), fitted at 5-m intervals along the length of the pipe, to record movement during towing. Early analysis of the data suggests the vibration suppression brought about by the strakes more than compensated for the drag they induced. Photos courtesy of 2H. [17,030 bytes]

A group of offshore operators and contractors completed a joint project to analyze vibration problems in steel risers for deepwater developments.

The project was led by 2H Offshore Engineering Ltd., Woking, U.K., which conducted tests in Bokna Fjord near Stavanger in December to check the effects of vortex-induced vibration (VIV) on two towed risers.

In the tests, two 200-m, 10-in. welded steel pipe strings were towed at speeds of up to 3 m/sec to simulate deepwater currents flowing over a marine catenary riser (see illustration). The angle of the riser in the water was varied from near vertical to almost horizontal, and the tests were repeated at a range of towing speeds. One pipe was bare, and the other fitted with VIV suppression strakes designed by 2H.

Both pipes were fitted with a variety of accelerometers to record pipe movements during towing. The devices were fitted in canisters strapped to the pipe at 5-m intervals.

Neil Willis, 2H's project manager for the Steel Risers for Deepwater Environments (Stride) study, told OGJ the VIV suppressers are intended to be commercialized and brought to market.

Willis said the first part of the Stride study comprised a finite element study of installation operations for steel risers. The second phase was the tests, after which the Stride group hopes to raise funding from the project participants to build a full-scale riser for further trials.

The two towed pipes behaved very differently on trials. Willis said the benefits of the VIV suppression strakes were thought to more than compensate for the additional drag they induced.

Future work

The next step in the study will be to work out how the strakes can be applied on offshore installation vessels: "Reeling is likely to be impossible so they may have to be fitted behind the vessel."

Willis said analysis of the data from the Norwegian trials was expected to be completed during the next 2 months, after which 2H would ask the project sponsors which way they want further research to go.

One of the main requirements from the study was a validation of existing VIV prediction software. The project included benchmarking of existing programs and is intended to provide a better understanding of riser VIV response.

The £1.5 million ($2.5 million) Stride project participants are: ARCO British Ltd., BHP Petroleum Pty. Ltd., BP Amoco plc, Chevron U.K. Ltd., Conoco (U.K.) Ltd., Elf Exploration U.K. plc, Enterprise Oil plc, Exxon Production research, Mobil North Sea Ltd., Norsk Hydro AS, Saga Petroleum AS, Shell U.K. Exploration & Production, Statoil AS, Total Oil Marine plc, and contractors Brown & Root Energy Services, ETPM, Single Buoy Moorings Inc., Sofec, and Stolt Comex Seaway AS.