This paper examines the effects of ocean currents on the vortex-induced vibration response of deepwater tubulars, such as umbilicals, production risers, tendons, and drilling risers, in proximity. This issue has received very little attention by the industry yet potentially has substantial importance to the design of deepwater tubulars and platforms.
Experiments have been conducted in a current flume on tandem flexible tubulars with diameter ratios of 2:1 and 4:1. The smaller diameter tubular was placed in both the upstream and downstream positions, with the top and bottom locations spaced apart at three different distances. The tension was kept constant, but the current speed was varied. While both uniform flow and sheared flow tests were conducted, only the uniform flow tests are reported herein.
The response modes varied from about 1-6, so that they were sufficiently high to easily allow both single mode and multiple mode responses, of which both were prevalent throughout the testing program. While all of the tests were conducted at low Reynolds numbers, the results can be thoughtfully translated to most field applications.
The results describe the vibration and response characteristics of both the upstream and downstream tubulars for various configurations that were tested. Since a myriad of diameter ratios, separation distances, and flow velocities are possible with tubulars in the field, these tests have attempted to provide general insights into the responses of tandem tubulars with unequal diameters and, in some cases, offer guidance on limitations to the responses that can be used for offshore field designs.