Kjetil Skaugset

Effect of Marine Growth on an Elastically Mounted Circular Cylinder

The Riser and Mooring project of the Norwegian Deepwater Programme (NDP) has undertaken a parametric study on the influence of marine growth on the effectiveness of helical strakes to suppress vortex-induced vibrations on circular cylinders. Helical strakes are one of the most commonly used devices to suppress vortex-induced vibrations (VIV) on marine risers. The aim is provide guidance on the effect marine growth has on the dynamic response of a riser fitted with such VIV suppression devices. The tests were conducted at MARINTEK in a controlled laboratory environment. Artificial marine growth was modelled, manufactured and tested. Both hard and soft marine growth of various heights and coverages were tested. The present paper discusses some results obtained in this test campaign.Copyright

Modelling and Characterization of Artificial Marine Growth

The Riser and Mooring project of the Norwegian Deepwater Programme (NDP) has undertaken a parametric study of marine growth on circular cylinders fitted with helical strakes. The study was performed at MARINTEK. The aim is to provide guidance on the effect that marine growth has on the dynamic response of the riser. The tests were conducted in a controlled laboratory condition where organic material is not allowed. Hence artificial marine growth had to be modelled and manufactured. Both hard and soft marine growth have been modelled. Further it is vital to document the properties of the marine growth. The present paper will discuss issues in modelling artificial hard and soft marine growth, as well as characterizing the growth for reference after testing. Results from the NDP test campaign will be presented in the paper.Copyright

Experimental Studies of Hydrodynamic Properties and Screening of Riser Fairing Concepts for Deep Water Applications

The VIV oscillations of marine risers are known to increase drag, and lead to structural fatigue. One proven method of suppressing this vibration is the use of fairings and strakes. These coverings essentially modify the flow along the cylinder, tripping the production of Karman vortices so that they act less coherently or far enough downstream so they interact less with the body.The Norwegian Deepwater Programme (NDP) has conducted a project with the objective to develop and qualify effective low drag fairing concepts with respect to VIV mitigation and galloping. Furthermore, emphasis is put on easy handling and installation.This paper describes the work and findings in an early phase of the development. This includes small scale model test campaigns. In addition to the bare riser for reference, the behaviour and performance of a total of 10 different fairing concepts are evaluated. Free oscillation tests are performed in a towing tank, where 2D fairings were tested in a pendulum set-up. The set-up enables free vibrations in up to 3 DOF (in-line and cross-flow vibrations and yaw). Fix tests with the purpose of establishing hydrodynamic coefficients for the various fairings have been performed in a large cavitation tunnel.Clear differences in performance have been noticed; particular for drag and galloping responses. Based on the results from the 2D tests, a screening of the fairing designs has been performed and the findings have set the course for further development of the most promising candidates for real life applications.Copyright

Higher Order Modal Response of Riser Fairings

Riser fairings are designed to rotate freely about the riser axis and to passively align with the direction of incident flow so they will effectively streamline the flow and eliminate VIV (Vortex-Induced Vibrations). This rotational degree of freedom introduces the possibility of a complex dynamic phenomenon involving coupling between the hydrodynamic forces and the fairing / riser motions (e.g. cross-flow translation and rotation). Slocum et al. reported a scaled model test of a long flexible riser model with a freely-rotating riser fairing conducted at MARINTEK at OTC-2004. At low flow speeds, the test showed the fairings to be effective. However, at higher towing velocities they became unstable resulting in high displacements at its first bending mode. This paper presents the work related to a study of effectiveness of fairings and is one of several VIV research activities NDP (Norwegian Deepwater Programme) has conducted at MARINTEK in 2002–2007. The present work is partly a follow up activity to the work reported by Slocum et al. The purpose of the present work was to study possible higher order modal response of faired risers and in particular to find out if such response can occur in higher bending modes than the first. A vertically towed instrumented riser was tested with 2 different fairing designs in uniform current profile with different towing speeds. Fairing II was identical to the one used in Slocum et al (2004) while Fairing I represents an alternative design. The riser model was 9.32m long, had diameter of 20mm and was flexible. Both bare riser configuration and full coverage of the two fairing sets were tested. This study documented first-, second- and third-mode responses at high amplitudes (instability behavior) for Fairing II. Tests with Fairing I showed that the riser was stable, but the riser vibrations were found to be similar with respect to displacement amplitudes and frequencies to the bare riser VIV.Copyright

Full Scale Fairing Qualification Tests

Production risers as well as drilling risers are often subjected to Vortex-induced vibrations (VIV) when exposed to ocean currents. VIV have been observed in the field and can cause fatigue failure and excessive drag on the riser. In order to suppress VIV and reduce drag, fairings are often used. This paper presents hydrodynamic qualification tests for two types of fairings: the short crab claw (SCC) and a tapered dual fin design. The short crab claw fairing design is a novel design that was developed by the Norwegian Deepwater Programme (NDP). As will be detailed in this paper, the SCC design offers very low drag, completely suppresses VIV and reduces riser interference.In 2012, a model test campaign was undertaken to understand and qualify the hydrodynamic performance of fairings at prototype conditions. The program consisted of testing the three fairing geometries and a strake to understand the stand-alone performance in VIV and the performance in interference. This was accomplished by utilizing a single pipe setup for the standalone test and a two-pipe setup for the interference tests. The paper reports the results of the program and draws conclusions on the hydrodynamic performance of the VIV suppression devices tested.Overall, all VIV suppression devices tested were able to suppress VIV with the SCC fairing being the most effective. In all cases tested, the downstream fairings / strakes were very effective in suppressing VIV in an interference scenario where a fairing was placed upstream. Contrary to the well-documented case of two strakes in tandem, in this case the upstream fairings did not reduce the effectiveness of the downstream fairings/strakes.Copyright

Evaluation of Fairing Performance by Three-Dimensional Tests Using a Flexible Riser Model

Due to its low drag and suppression effectiveness, fairings are considered to be an attractive alternative to helical strakes for mitigation of riser Vortex Induced Vibrations (VIV). The Norwegian Deepwater Programme has conducted a project with the objective to develop and qualify effective low drag fairing concepts with respect to VIV and galloping. Furthermore, emphasis is put on easy handling and installation.By use of 2D small scale tests in a towing tank and in a large cavitation tunnel, a total of 10 different fairing designs have been evaluated and screened in terms of hydrodynamic performance [1]. Three fairing design candidates were promoted for further development. This paper describe the work and findings when studying the effectiveness of promoted fairings on a 3D flexible riser model with respect to galloping and VIV. Three different fairing profiles have been studied in the present model tests.The tests were carried out in the sub-critical Reynolds number regime with Rn up to about 53000. Four different fairing coverages were tested for each design: full coverage, 60 % coverage, 40 % coverage and full coverage of the lower half of riser.Differences in performance have been noticed for the fairing profiles, and the findings have laid the ground for further development of the most promising candidates for real life applications. Further, it is concluded that the combination of 2D and 3D tests, as well as combination of small scale and prototype scale tests, has been an efficient strategy in development of VIV suppression devices.Copyright

An Investigation of Riser Fairing Instability

An analysis of galloping of two different types of riser fairings is presented. The first is named “long fairing” (LF) and the other “Short Crab Claw” (Short CC). The first one has a traditionally winged formed shape with a cord-to-diameter ratio of 2.43. The other one is more truncated in shape, and has cord-to-diameter ratio of only 1.4.Results from two related experimental set-ups are included in the work; one 2D experiment with towing tests of fairings that are free to translate and rotate to investigate instability regions, and one 2D experiment with fixed fairings to obtain drag, moment and lift curves. The present analysis is based on two-degrees of freedom, linearized equations of motion, and predicts a range of velocities where instability occurs. Below and above this region, the fairing is stable. Damping complicates the analysis. An empirical damping model is included and discussed. The two fairing types inhibit appreciably different instability characteristics. In particular, the Short CC fairing has a narrower instability region than the long fairing, and is therefore less prone to instabilities.Copyright