Full-Scale Reynolds Number VIV Testing of Tri-Helically Grooved Drill Riser Buoyancy Module

A newly developed Tri-Helically Grooved drilling riser buoyancy module design was tested in the towing tank of SINTEF Ocean in June 2017. This new design aims to reduce riser drag loading and suppress vortex-induced vibrations (VIV). Objectives of the test program were two-fold: to assess the hydrodynamic performance of the design allowing for validation of previous computational fluid dynamics (CFD) studies through empirical measurements, and, to develop a hydrodynamic force coefficient database to be used in numerical simulations to evaluate drilling riser deformation due to drag loading and fatigue lives when subjected to VIV. This paper provides the parameters of the testing program and a discussion of the results from the various testing configurations assessed. Tests were performed using large scale, rigid cylinder test models at Reynolds numbers in the super-critical flow regime, defined as starting at a Reynolds number of Re = 3.5 × 105 – 5.0 × 105 (depending on various literatures) and continuing until Re = 3 × 106. Towing tests, with fixed and freely oscillating test models, were completed with both a bare test cylinder and a test cylinder with the Tri-Helical Groove design. Additional forced motion tests were performed on the helically grooved model to calculate lift and added mass coefficients at various amplitudes and frequencies of oscillation for the generation of a hydrodynamic force coefficient database for VIV prediction software. Significant differences were observed in the hydrodynamic performance of the bare and helically grooved test models considering both in-line (IL) drag and cross-flow (CF) cylinder excitation and oscillation amplitude. For the helically grooved model, measured static drag shows a strong independence from Reynolds number and elimination of the drag crisis region with an average drag coefficient of 0.63. Effective elimination of VIV and subsequent drag amplification was observed at relatively higher reduced velocities, where the bare test model shows a significant dynamic response. A small level of expected response for the helically grooved model was seen across the lower range of reduced velocities. However, disruption of vortex correlation still occurs in this range and non-sinusoidal and highly amplitude-modulated responses were observed.

Non-Destructive Inspection of Surface Defects in Cylindrical Structures

Cylindrical structures have been applied in various pressure vessels and weapon systems, which may be subjected to harsh environmental conditions such as large mechanical stresses and thermal stresses. As a result, non-destructive evaluation of such structures is critical in quality control. Among the various defects that may be generated during fabrication, transportation, operation/firing, and so on, surface crack is a critical one and needs to be quantitatively and accurately evaluated. In this study, both ultrasound phased array technique and eddy current technique are applied for the inspection of 120 mm steel test cylinder. In the cylinder, a total of nine sets of surface defects of various size, depth and orientation are fabricated and quantitatively evaluated. In ultrasound phased array evaluation, simulations and experiments on standard aluminum block were carried out first to calibrate the system parameter setup. During this calibration, ultrasound propagation and its interaction with defects were simulated and studied. The dependence of ultrasound field on the ultrasound parameters and on the characteristics of defects was analyzed and optimized. Then simulations and experiments on steel test cylinder were carried out for the detection of the smallest detectable defects. Results showed that the optimization of the number of active elements can improve the distortion of defect images; the steering angles and the beam focusing laws may change the ultrasound beam intensity and uniformity, which has a significant influence on the sensitivity and resolution of the phased array technique; the geometry and material properties of cylindrical structures could distort the ultrasound beam, and as a result, calibration is necessary and important during practical inspections. Frequency is a key factor for phased array technique to improve its sensitivity. In eddy current evaluation, a prototype for wireless eddy current system was designed, and an eddy current probe interface and a main unit interface were developed. The main advantages of such wireless probe are safety, economic benefits and maneuverability when compared to conventional wired probe. During testing, the signal at the probe interface was activated, measured, digitized and transmitted wirelessly to the main unit interface. Experimental results showed that the eddy current signals can be wirelessly communicated with main unit, and the results are comparable with the wired eddy current tester. Testing results also showed that the wireless signal is about 8 dB lower compared to wired signals and phase difference exists between the wired and wireless signals.

The Design of Hydraulic Cylinder for Measuring the Contact Stress of the Bud Type Seal Ring

At present, the bud type rings are widely used in the hydraulic cylinder piston rod seal, but there is no corresponding detection method to measure the contact stress when the seals are in the dynamic sealing condition. However, the contact stress is very important for analyzing the performance of the seals. A method for measuring the contact stress is presented in this paper, and the design of major parts of the test cylinder and the selection of seals are emphatically introduced.

Modeling and Simulation of Servo-Hydraulic Cylinder Systems for Multi Axis Test Control

Multi-axial mechanical testing with servo-hydraulic cylinders is used as essential tool within the development and manufacturing process of mechanical components and structures, enabling the experimental validation of the fatigue behavior and related mechanical endurance limits. In this paper we derive the analytical model of servo-hydraulic cylinders feasible for fatigue tests to enable the incorporation of the derived actuator dynamics within multi-axis test control strategies. Our derived cylinder model includes the test cylinder with attached position sensor, and a state-of-the-art servo valve. Based on the obtained cylinder dynamics we propose a simplification to a low order cylinder model, highly desirable for reducing overall system complexity in order to develop ease-of-use controllers of high performance for multi-dimensional test rigs. We compare the simulated output of the derived actuator models with the measured data from a real world test cylinder system. The obtained results show that the obtained system model accurately describes the dynamic properties of a real world test cylinder, and furthermore validates the process of model simplification for efficient control of such cylinders as part of low-bandwidth multi input multi output servo-hydraulic test systems.

Experimental Investigations of the Efficiency of Round-Sectioned Helical Strakes in Suppressing Vortex Induced Vibrations

Vortex induced vibrations (VIVs) may cause a large amount of damage to deep water risers. Helical strakes are used as a mitigating measure to suppress these vibrations. The purpose of this paper is to verify the efficiency of round-sectioned helical strakes in suppressing VIV. It is believed that round-sectioned helical strakes can be more readily mounted on risers for intervention and maintenance compared with sharp-edged strakes that may have to be welded onto the risers. Systematic experimental investigations including 28 configurations of round-sectioned helical strakes were tested in an attempt to find the most suitable strake configuration. The experiments were performed in a steady flow flume with an elastically mounted rigid circular cylinder of 500 mm in length and 50 mm in outer diameter. The test cylinder was spring-supported in both the inline and cross-flow directions. The measurements were limited to mapping the displacement of the cylinder. First, the cylinder was tested without strakes as a reference case. The best configuration among the tested round-sectioned helical strake configurations was found to reduce the amplitude of oscillation relative to the bare cylinder case by 96% in the cross-flow direction and by 97% in the inline direction. The main features of this configuration are the number of starts (3), the pitch (5D), and the diameter of the strake (0.15D), where D is the outer diameter of the test cylinder. Additionally, this paper investigates the effects of varying pitch, the effects of surface roughness, and the effects of the ratio between the cross-flow and inline natural frequencies of the test rig on the efficiency of the suggested configuration of round-sectioned helical strakes.

Effect of the Bottom and Top Configurations on Pool Film Boiling Around a Vertical Finite-Length Cylinder

The bottom configuration of a vertical finite-length cylinder is an important factor to examine the convective heat transfer by film boiling around a vertical finite-length cylinder, as the vapor generated under the bottom surface grows thicker during flowing upward along the vertical lateral surface and finally leaves the top surface as bubbles. In this study, four types of silver cylinder with a vertical lateral length equal to the diameter of 32mm were prepared for the possible combinations of bottom and top configurations: with a flat bottom and a flat top, with a flat bottom and a curved top, with a curved bottom and a flat top, and with a curved bottom and a curved top, where “flat” refers to “horizontal” and “curved” to “convex hemispherical”. Quenching experiments have been carried out for the test cylinders for saturated and subcooled water at atmospheric pressure. The initial temperature in the measurement is 600 °C. Boiling curves were obtained from the cooling curves measured using a K-type thermocouple inserted near the center on the axis of the test cylinder and the film boiling process was observed by still and high speed video cameras. The following results were obtained from the experiments using four types of test cylinder. 1. For saturated water, the test cylinders are entirely covered with a thick continuous vapor film, however, the effect of bottom configuration on film boiling heat transfer is appeared within 18% in terms of the wall heat flux averaged over the entire surface depending on the vapor fluid flow on the bottom and vertical lateral surfaces. 2. For the cylinders with a flat bottom surface, the wall heat flux averaged over the entire surface increases significantly with an increase in liquid sub cooling. This is attributed to that the convective heat transfer and the surface area ratio on the vertical lateral surface are predominant and govern the total heat transfer. 3. The effects of the cylinder top configurations on the film boiling heat transfer are small as the heat transfer on the top surface is small compared with that on the vertical lateral surface. 4. The differences between film boiling characteristics due to the bottom and top configurations are explained by examining the average heat transfer coefficient composed of the heat transfer coefficient and the surface area ratio on each surface. 5. The minimum wall superheat corresponding to the vapor-film-collapse is almost constant at 133K for four types of test cylinder in saturated water. In subcooled water, the minimum wall superheat for the cylinders with a flat bottom surface is larger than that for the cases with a convex hemispherical bottom surface.

Experimental Investigation on Effects of Surface Roughness Geometry Affecting to Flow Resistance

Experimental investigation on effects of surface roughness geometry affecting to flow resistance has been carried out. The concentric cylinder device composed of outer cylinder and inner test cylinder was employed to the experiment. We prepared 24 different roughness models having various skewness of roughness profile as test inner cylinders. Surface of test cylinder has ridge and valley roughness whose shapes are isosceles right triangle V-shape. These ridge and valley are arranged at equal intervals. Therefore, RMS roughness of the surface and skewness of the surface roughness profile can be evaluated. In the experiment, inner cylinder is rotated but outer cylinder is stationary, torque of rotating inner cylinder was measured. Based on the torque measurement, we investigated the effect of skweness of the surface roughness on flow resistance. As a result, when the roughness profile has Gaussian distribution (skewness = 0), friction coefficient increases with increasing RMS roughness. Moreover, friction coefficient also increases with increasing skewness of surface roughness under same RMS roughness. In order to predict the friction coefficient from the geometric information of the surface, we estimated the equivalent sand grain roughness from surface roughness parameters. Results showed that it was clarified the relation among skewness of roughness profile, equivalent sand grain roughness and the root mean square of surface roughness.