Themistocles L. Resvanis

Effects of Strake Coverage and Marine Growth on Flexible Cylinder Vortex-Induced Vibrations

In this paper we present some results from the recent SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented flexible cylinders at Reynolds numbers up to 220,000. The main objective is to present the experimental results describing the effectiveness of different amounts of strake coverage and to explore the influence of simulated marine growth. The data is presented in terms of CF response amplitudes and rainflow-counted damage rates due to the combined CF and IL bending stresses. All results are compared with the bare cylinder cases which will be used as a reference to determine how effective the strakes are in suppressing VIV and how this effectiveness can be affected by marine growth. The results show that even small bare sections (missing strakes) can lead to significant VIV response. We also observe that moderate amounts of marine growth can quickly negate any suppression coming from the strakes. INTRODUCTION As oil and gas exploration and production moves into deeper and deeper waters, the fatigue damage accumulated due to Vortex-Induced Vibrations (VIV) is quickly becoming one of the most critical aspects of deep-water riser design. Helical strakes and other suppression devices are commonly used to try and minimize the effects of VIV. They work by ensuring that the flow separation points are not aligned along the structure or by moving the reattachment points further downstream of the cylinder, in any case they interrupt the coherence of the shed vortex sheet thus minimizing the power into the structure. Over the past years several strake and fairing designs have been proven to be extremely effective in disrupting VIV on rigid or fully covered flexible cylinders. The question that remains to be answered is how much strake coverage is actually necessary to minimize VIV on a flexible cylinder or conversely; how many strakes can one lose before VIV becomes a serious source of fatigue damage on the now partially straked riser. The other pressing issue that needs to be addressed once a strake design and coverage has been chosen is to determine how the suppression characteristics of the chosen device will be affected by the inevitable presence of marine growth. This might be done in order to determine a cleaning schedule or to simply account for the accumulated damage so that one has more confidence in the design or future operation of the riser. This work makes use of test data gathered on behalf of SHELL International Exploration and Production Co. by MARINTEK. The tests involved towing three different 38m long flexible pipes/cylinders in uniform and sheared currents with different suppressions devices, amounts of strake coverage and simulated marine growth. In the first part of this paper we propose a framework/methodology for experimentally determining the worst possible combination of Cross-Flow (CF) and In-Line (IL) damage, i.e. we calculate the position around the circumference of the cylinder’s cross-section that will experience the most severe loading due to the simultaneous motion in both CF and IL directions. The analysis shows that the damage due to the IL motion is comparable to that of the CF motion, and their combined effect results in a damage rate of the same order of magnitude as the pure CF signal and is usually situated at a position around the circumference of the cross-section that is very close to or coincides with the CF direction. The second part of this paper focuses on how effectively four different strake coverage amounts suppress VIV (always using the same strake design). The data is presented in terms of response amplitudes and damage rates, and it is demonstrated that the much larger reduction in damage rate is Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering OMAE2014 June 8-13, 2014, San Francisco, California, USA

Effects of Strake Coverage and Marine Growth on Flexible Cylinder VIV

In this paper we present some results from the recent SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented flexible cylinders at Reynolds numbers up to 220,000. The main objective is to present the experimental results describing the effectiveness of different amounts of strake coverage and to explore the influence of simulated marine growth.The data is presented in terms of CF response amplitudes and rainflow-counted damage rates due to the combined CF and IL bending stresses. All results are compared with the bare cylinder cases which will be used as a reference to determine how effective the strakes are in suppressing VIV and how this effectiveness can be affected by marine growth. The results show that even small bare sections (missing strakes) can lead to significant VIV response. We also observe that moderate amounts of marine growth can quickly negate any suppression coming from the strakes.Copyright

Ramp Tests: A Novel Approach to VIV Model Testing of Flexible Cylinders Using Continuously Varying Towing Speeds

In this paper we present a small portion of the results from the 2011 SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented flexible cylinders at Reynolds numbers up to 150,000 in order to study the Vortex-Induced Vibration (VIV) response in uniform current profiles.This paper presents the experimental results collected from a series of tests where the towing speed was continuously varied while the cylinder and carriage traversed the basin. As the cylinder is accelerated (or decelerated) the incident current speed is continuously changing which means that multiple modes can be excited consecutively in a single tow through the basin. These varying towing speed tests are collectively referred to as ‘ramp tests’.The response data collected in these ‘ramp tests’ are presented in terms of CF response amplitudes and strains and are carefully compared with the response data collected during conventional steady towing speed tests. The data shows that when the acceleration of the carriage is kept below a critical value the ‘ramp tests’ are then able to provide VIV response information which is equivalent to that obtained from many constant speed tests. One ramp test provides the equivalent response data of (up to) 10 constant speed tests in a single run.This paper also introduces a dimensionless parameter γ, which determines if the proposed acceleration for a ramp test is within acceptable limits and may be used as a substitute for many constant speed tests. The parameter can be used to determine the appropriate acceleration or deceleration rate in order to ensure that a ramp test will yield suitable VIV response data. The parameter also allows one to know quickly whether or not fully developed VIV is possible in a given set of unsteady flow conditions.Copyright