Vikas Jhingran

Subsea Wellhead and Riser Fatigue Monitoring in a Strong Surface and Submerged Current Environment

A drilling campaign was recently undertaken by Shell Oil Company in a region with high surface and submerged currents. The water depth ranged from 5500-7000 ft at the various well sites in the region. Strong surface currents with maximum speeds of 4.5-5.0 knots were measured using an Acoustic Doppler Current Profiler (ADCP). In addition, submerged currents with maximum speed of around 1.5 knots were recorded. High fidelity Subsea Vibration Data Loggers (SVDLs) were used to monitor the in-situ riser and BOP stack vibrations due to the arduous current environment, as well as wave and vessel-driven motions. A semi-analytical method was developed to estimate wellhead fatigue damage directly using the measured BOP stack motion data. High quality vibration data from the SVDLs were used in conjunction with analytical transfer functions to directly compute stress time histories and S-N fatigue damage at any location of interest in the conductor/wellhead/BOP system. The method was utilized in a larger fatigue reconstruction scheme that was applied to subsea wellhead and riser fatigue monitoring activities during drilling operations in the region. ADCP data was correlated to the SVDL data to determine the source of vibrations at low and high frequencies. Simultaneous ADCP and SVDL data were also used to calibrate SHEAR7 v4.2 parameters. In between SVDL deployments, wellhead and riser stress and fatigue values were determined using the calibrated SHEAR7 models, driven by the measured current profiles. Wellhead motions were tabulated from ROV video and used to validate vibration reconstruction from the SVDL data and predictions from SHEAR7 simulation. Using these methods, stress and fatigue life consumption estimates are robust to unavailability of ADCP data and/or ROV video and/or data from one or more SVDLs. Normalized vibration, stress and fatigue consumption are presented over the riser deployment period. It was found that moderate speed submerged currents, which extend over a broad range below typical fairing depths, lead to significantly higher wellhead stress and fatigue life consumption rate compared higher speed surface currents. The sensitivity of a typical wellhead and BOP stack to lower-frequency vibrations was examined. It is shown that because the submerged currents are of a lower speed, they excite modes that are closer in frequency to the “flagpole” mode of the casing/wellhead/BOP subsystem, leading to higher wellhead motion and stress. The methods introduced herein provide rapid turn-around of raw data to fatigue consumption, enabling informed decisions to be made in adverse conditions. The methodology is easily extendable to real-time fatigue monitoring using a cabled system or acoustic modem to transmit data to the surface. In addition, the significance of regional submerged currents for wellhead stress and fatigue is highlighted, as well as considerations for vibration mitigation.

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

The Effect of Exposure Length on Vortex Induced Vibration of Flexible Cylinders

This paper addresses a practical problem: “What portion of fairing or strake coverage may be lost or damaged, before the operator must take corrective measures?” This paper explores the effect of lost fairings (the exposure length) on Vortex-Induced Vibration (VIV) of flexible cylinders. The source of data is a recent model test, conducted by SHELL Exploration and Production. A 38m long pipe model with varying amounts of fairings was tested. Response as a function of percent exposure length is reported. Unexpected results are also reported: (i) the flexible ribbon fairings used in the experiment did not suppress VIV at speeds above 1 m/s; (ii) Above 1 m/s, a competition was observed between VIV excited in the faired and bare regions of the cylinder, (iii) Unusual traveling wave behavior was documented—waves generated in the bare region periodically changed direction, and exhibited variation in VIV response frequency.The results of these tests showed that (1) the excitation on the bare and faired regions could be identified by frequency, because the faired region exhibited a much lower Strouhal number; (2) as expected, the response to VIV on the bare region increased with exposure length; (3) the response to VIV on the faired region decreased with exposure length.Copyright

VIV Excitation Competition Between Bare and Buoyant Segments of Flexible Cylinders

This paper addresses a practical problem: “Under which coverage of buoyancy modules, would the Vortex Induced Vibration (VIV) excitation on buoyant segments dominate the response?” This paper explores the excitation competition between bare and buoyant segments of a 38 meter long model riser. The source of data is a recent model test, conducted by SHELL Exploration and Production at the MARINTEK Ocean Basin in Trondheim Norway. A pipe model with five buoyancy configurations was tested.The results of these tests show that (1) the excitation on the bare and buoyant regions could be identified by frequency, because the bare and buoyant regions are associated with two different frequencies due to the different diameters; (2) a new phenomenon was observed; A third frequency in the spectrum is found not to be a multiple of the frequency associated with either bare or buoyancy regions, but the sum of the frequency associated with bare region and twice of the frequency associated with buoyancy region; (3) the contribution of the response at this third frequency to the total amplitude is small; (4) the power dissipated by damping at each excitation frequency is the metric used to determine the winner of excitation competition. For most buoyancy configurations, the excitation on buoyancy regions dominates the VIV response; (5) a formula is proposed to predict the winner of the excitation competition between bare and buoyant segments for a given buoyancy coverage.Copyright

On the Occurrence of Higher Harmonics in the VIV Response

Vortex Induced Vibrations (VIV) can lead to fast accumulation of fatigue damage and increased drag loads on slender marine structures. A cylinder subjected to VIV can vibrate in both in-line (IL) and cross-flow (CF) directions. The CF response is dominated by the primary shedding frequency and the IL response frequency is often two times of the primary CF frequency. In addition, higher harmonics can also be present. The third order harmonics are more pronounced when the motion orbit of the cylinder is close to “figure 8″ shape and cylinder is moving against the flow at its largest transverse motion. Recent studies with flexible beam VIV tests have shown that higher harmonics can have significant contribution to the fatigue damage in addition to the loads at the primary shedding frequency. However, there is a lack of understanding of when and where higher harmonic loads occur. The fatigue damage due to the higher harmonics is not considered in the present VIV prediction tools.In the present paper, the test data of selected cases subjected to linearly sheared flow profile from two test programs, the Shell high mode VIV test[11] and the Hanoytangen test[5] have been studied. The factors that may influence the occurrence of the higher harmonics, such as the bending stiffness, reduced velocity and orbits stability, have been studied. The importance of higher harmonics in VIV fatigue has also been investigated. Finally, a method to include higher harmonics in the fatigue calculation is presented.Copyright

Comprehensive Riser VIV Model Tests in Uniform and Sheared Flow

Despite of considerable research activity during the last decades considerable uncertainties still remain in prediction of Vortex Induced Vibrations (VIV) of risers. Model tests of risers subjected to current have been shown to be a useful method for investigation of the VIV behavior of risers with and without suppression devices.In order to get further insight on VIV of risers, an extensive hydrodynamic test program of riser models subjected to vortex-induced vibrations was undertaken during the winter 2010 by Shell Oil Company.The VIV-model test campaign was performed in the MARINTEK Offshore Basin Laboratory. A new test rig was constructed and showed to give good test conditions. Three different 38m long riser models were towed horizontally at different speeds, simulating uniform and linearly varying sheared current. Measurements were made In-Line (IL) and Cross-Flow (CF) of micro bending strains and accelerations along the risers. The test program compromised about 400 tests, which give a rich test material for further studies.In the present paper the test set-up and program are presented and selected results are reported.Copyright

Modeling VIV Supression Using Negative Lift Coefficients

Vortex-Induced Vibration (VIV) is a complex, non-linear fluid-structure interaction problem with important consequences for offshore risers, tendons and other tubulars. The prevalent approach in the industry is to use semi-empirical formulations to estimate VIV amplitudes, frequencies and the resulting fatigue damage. These semi-empirical techniques estimate VIV response amplitude by considering the balance of power input into the pipe due to vortex-shedding and the loss of power from the pipe due to damping. At the heart of this method are lift coefficient curves, which are used to estimate power input into the pipe. A key difficulty of this method is modeling the response of pipes with mitigation devices (strakes and fairings). This paper discusses the use of negative lift coefficient curves for estimating VIV response from mitigation devices. The author’s show how damping is handled in Shear7 using this approach. Results of comparisons between predicted and measured data show that negative lift curves are an effective means of modeling damping from VIV mitigation devices.Copyright

Lift Coefficient Curves for Predicting Response Using Shear7

Vortex-Induced Vibration (VIV) is a complex, non-linear fluid-structure interaction problem with important consequences for offshore risers, tendons and other tubulars. The prevalent approach in the industry is to use semi-empirical formulations to estimate VIV amplitudes, frequencies and the resulting fatigue damage. These semi-empirical techniques estimate VIV response amplitude by considering the balance of power input into the pipe due to vortex-shedding and the loss of power from the pipe due to damping. At the heart of this method are lift coefficient curves, which are used to estimate power input into the pipe. Local lift coefficients are difficult to measure or derive for a flexible pipe and hence most of the lift curves used today have been developed using experiments with rigid cylinders. This paper discusses the development of a new family of lift coefficient curves using experimental data. Results of comparisons between predicted and measured data show the lift curves to effectively predict VIV response.Copyright ?? 2010 by ASME

Non-Resonant In-Line VIV and Its Implications on Model Tests

Recently, small-scale experiments were conducted by [1] to study in-line VIV in pipe spans. The experiments were performed with six different pipes of varying stiffness and mass ratio, but with the same length-to-diameter ratio. The response of the pipe with the lowest mass and stiffness, made out of Acrylonitrile Butadiene Styrene (ABS), was surprising. The in-line RMS A/D response of the ABS pipe was larger and over a much wider reduced velocity range than shown in design codes like DnV F105. Since these codes are commonly used to design real pipelines, the authors were interested in understanding these observations. In the past, observations of VIV response over a wide reduced velocity range have been explained using added mass. This paper shows that though added mass could play an important role, observations of the in-line and cross-flow response mode and frequency content suggests that there could be other reasons for the response observed in the experiments. In particular, this paper investigates the observed large response away from the region of resonant VIV and proposes that this non-resonant in-line response could be different from what researchers typically call VIV. The paper also investigates when such a mechanism could contribute to substantial in-line VIV motion. The implications of this work could be significant, not just for pipe-span design but also for scaling pipes for in-line VIV model tests.Copyright