Don W. Allen

Vortex-induced vibration current tank tests of two equal-diameter cylinders in tandem

Tests have been performed on equal diameter ABS cylinders in tandem subjected to uniform and sheared flows in a current tank at subcritical Reynolds numbers. An accelerometer on each cylinder was used to measure the vortex-induced vibration response, which primarily varied between the third and seventh transverse bending modes. The results reveal some interesting relationships between the responses of the two cylinders when plotted against the upstream cylinder displacements. In addition, it was discovered that the downstream cylinder consistently experiences bimodal response even when the upstream cylinder response is dominated by a single mode.

Design And Installation Of Auger Steel Catenary Risers

The export pipelines on the tension leg platform (TLP) Auger are connected by steel catenary risers (SCRs). This is believed to be the first time steel pipe has been used for catenary risers. Significant design effort was required to prove that the SCRs could safely withstand environmental loads and the effects of TLP motions. The design effort consisted of extensive dynamic analyses as well as full scale fatigue testing of both the riser joint welds and the flexible joint that connects the riser ro the TLP pontoon. Devices which suppress vortex induced vibrations were also tested. SCR installation is accomplished by lowering the riser on the abandonment and recovery cable from the J-Lay installation vessel and transferring the riser on a chain that is run through a chain jack hung from the TLP upper deck structure. A special installation and maintenance system was built for this purpose.

In-Line Motion of Subsea Pipeline Span Models Experiencing Vortex-Shedding

Subsea pipeline spans, when experiencing bottom ocean currents, are prone to vortex-induced vibration (VIV). Experiments and computational fluid dynamics (CFD) are conducted to evaluate the effects of the pipe stiffness on its first mode in-line VIV motion, primarily in the reduced velocity range from approximately 1.0 to 4.0. Experimental results also indicated that there was obliqueness in motion trajectories, which could have impacts on VIV design of the free spans. The main findings of this investigation are presented in this paper.Copyright

Drilling Riser Fairing Tests at Prototype Reynolds Numbers

Tow tests have been performed on flexible circular cylinders, with and without short weathervaning fairings, towed in a basin at critical and supercritical Reynolds numbers. The tests were conducted in the David Taylor Model Basin and the Rotating Arm Facility, at the Carderock Division, Naval Surface Warfare Center, in West Bethesda, Maryland. Measurements were made of both the drag and acceleration (due to vortex-induced vibration) of the cylinder. A 5-9/16-inch diameter PVC pipe was used to achieve Reynolds numbers ranging from about 7×105 to 1.5×106 , in uniform flow, for straight tow tests with the pipe experiencing first mode bending vortex-induced vibration. Fiberglass pipes with a 2.5 inch diameter were used to achieve high mode number vortex-induced vibration, in sheared flow, at Reynolds numbers as high as about 3.75×105 . The test results illustrate the importance of conducting tests at prototype Reynolds numbers for drilling riser as well as the importance of conducting tests in sheared flows and at higher mode numbers to fully understand the performance of a suppression device.Copyright

Damping Characteristics of Fairings for Suppressing Vortex-Induced Vibrations

Vortex-induced vibration (VIV) tests have been performed on long, flexible pipes with fairings in sheared flows in a circular towing tank at prototype Reynolds numbers for production risers [1]. It is discovered that there existed strong attenuation of the vibration responses for the test configuration with fairings placed in the strong current zone, compared to the pipe configuration without any fairings. Wave attenuation is typical when waves travel in systems with dissipation. Strong attenuation is an indication of large damping. Additional pluck tests of pipes with fairings in still water were conducted to determine damping in the system in a quantitative manner. The results, though scattered, provide evidence on the level of damping that could exist in structures with fairings. Furthermore, analytical models based on a Green’s function solution and mode superposition method for a taut string were also used to complement experimental data. A range of damping values was considered to find the damping value for which the level of attenuation matched that of the experiments. It is found that this damping value is close to that from the tests.Copyright

Blade Henning Devices for VIV Suppression of Offshore Tubulars

This paper discusses a new type of vortex-induced vibration (VIV) suppression devices, the blade Henning device, named after one of the primary inventors, Dean Henning. It consists of four blades, 90-degree apart, on a circular shaped sleeve. Tests of these devices on a flexible cylinder in uniform flows have been conducted. The results indicate that the amount of suppression required with this type of apparatus to effectively mitigate VIV is significantly reduced.Copyright

High Reynolds Number Flow Tests of Flexible Cylinders With Helical Strakes

Tow tests have been performed on flexible circular cylinders, with and without short helical strakes, towed in a basin at critical and supercritical Reynolds numbers. The tests were conducted at the Naval Surface Warfare Center’s David Taylor Model Basin in Carderock, Maryland. Measurements were made of both the drag and acceleration (due to vortex-induced vibration) of the cylinder. A 3-1/2 inch diameter ABS pipe was used to achieve Reynolds numbers ranging from about 2×105 to 5×105 , and a 5-9/16-inch diameter PVC pipe was used to achieve Reynolds numbers ranging from about 7×105 to 1.5×106 . Tests were also conducted with aluminum inserts (strong-backs), made to fit just inside the test cylinders, in order to obtain stationary (rigid) cylinder drag measurements for comparison purposes. The test results for cylinders fitted with triple-start helical strakes are presented in this paper.Copyright

The Effects of Mixing Helical Strakes and Fairings on Marine Tubulars and Arrays

Most deepwater tubulars experiencing high currents frequently require vortex-induced vibration (VIV) suppression to maintain an acceptable fatigue life. Helical strakes and fairings are the most popular types of VIV suppression devices in use today.It is quite common to use only one type of device (helical strakes or fairings) on a single tubular and, in fact, to use a single device type on an entire tubular array. The use of both styles of suppression devices on a single tubular has grown in popularity, but mixing them within an array is a relatively new concept. It is sometimes desirable to use one suppression device on one tubular and another suppression device on an adjacent or tandem tubular.This paper utilizes results from two different types of VIV experiments. The first consists of a long tubular at high Reynolds numbers with VIV suppression on the outer end where current speeds are the highest. The use of only fairings, only strakes, or a mixture of the two devices is examined.The second VIV experiment examines the use of helical strakes on one tubular and fairings on a tandem tubular. Results are compared to experiments with either helical strakes on both tubulars or fairings on both tubulars. This paper is intended to provide some direction, and in many cases assurance, for mixing helical strakes and fairings on deepwater tubulars.Copyright

Practical Design Considerations for Managing Marine Growth on VIV Suppression Devices

Most deepwater tubulars experiencing high currents frequently require vortex-induced vibration (VIV) suppression to maintain an acceptable fatigue life. Helical strakes and fairings are the most popular VIV suppression devices in use today.Marine growth can significantly affect the VIV of a bare riser, often within just a few weeks or months after riser installation. Marine growth can have a strong influence on the performance of helical strakes and fairings on deepwater tubulars. This influence affects both suppression effectiveness as well as the drag forces on the helical strakes and fairings. Unfortunately, many VIV analyses and suppression designs fail to account for the effects of marine growth at all, even on a bare riser.This paper utilizes results from both high and low Reynolds number VIV test programs to provide some design considerations for managing marine growth for VIV suppression devices.Copyright

An Approach to VIV Design of Marine Risers

This paper presents an approach to vortex-induced vibration (VIV) design of marine risers. It begins with a brief review of the issue and then proceeds to presenting the key elements needed to make VIV predictions for marine risers. Uncertainties and discrepancies between model prediction and field measurements are discussed. Examples are used to illustrate the approach presented herein. Conclusions are drawn at the end of this paper.Copyright