Michael Tognarelli

Actual VIV Fatigue Response of Full Scale Drilling Risers: With and Without Suppression Devices

In an effort to more effectively understand and manage vortex-induced vibration (VIV) fatigue integrity of its drilling risers, BP has instrumented several of them on a number of mobile offshore drilling units (MODUs) and offshore production platforms worldwide. This paper presents several aspects of the findings from those monitoring campaigns, with particular emphasis on the relatively more densely populated MODU data sets. In-situ monitoring has practical use as a realtime quantifier of accrued VIV fatigue damage to both drilling riser and wellhead casing over the course of a given monitoring period, a fundamental indicator of structural integrity. At present, this can be very useful to operators given that the gap between predicted and measured VIV fatigue damage can be very large. In this paper, the measured data are used to expose some of the physical details of full-scale riser response whose omission from predictive design tools and methods may contribute to this wide gap. To characterize the size of the gap, the data are compared to calculations using the most commonly used industry VIV analysis software. This demonstrates the inherent level of analysis over conservatism with respect to full-scale, unsuppressed drilling risers in the field when typical analysis parameters are utilized. A means of adjusting the parameters to reduce the over conservatism is then implemented. Finally, the data are used to reveal some performance indicators for VIV suppression devices that are presently being utilized in drilling operations.

Offshore Drilling Riser VIV Suppression Devices: What’s Available to Operators?

VIV suppression and drag reduction are key issues for improved operation in offshore drilling. Properly designed helical strakes are effective in the mitigation of VIV fatigue damage for many riser applications. However such strakes tend not to be applicable to offshore drilling riser applications. This is due to increases in drag force due to increased apparent diameter as well as workability problems for drilling operations. For these reasons, effective devices are sought that would mitigate VIV and reduce, or at least not increase, drag for drilling applications. Along with yielding good hydrodynamic performance, a drilling riser VIV suppression device must be compact and robust enough to be used in a drilling-rig environment. It needs to be deployable and recoverable in declared operational sea states. It must also be easy to store and assemble. Finally, and most importantly, it must be efficient to deploy and recover during normal riser operations. This last point is vital to drilling operations in deepwater in hurricane-prone areas. Weather conditions can change quickly and even a non-faired deepwater riser takes 2 to 3 days for a full retrieval. BP continues to research and document suppression device types and to assess their practical performance. A supply choice in the market place is important so that the correct device can be used for particular situations. To this end, we have recently worked with cooperative partners to demonstrate the hydrodynamic performance of a handful of the most promising devices. This paper is a tailored synopsis of previous suppression concepts and the philosophical pathway toward what is available on the market today. At its core are recent circumstances which precipitated a need to quantify and qualify for operational acceptance the performance of two commercially available short aspect ratio fairing devices. (i.e. a dual-fin splitter and an airfoil-shaped fairing). This paper discusses the results of the large-scale model acceptance tests over prototype Reynolds number for these devices. In addition to rigid devices, a relatively newer suppression product that “inflates” in the direction of the relative flow was also assessed by BP for expected hydrodynamic performance. This device shows particular promise for the mitigation of VIV during drilling operations surprises in high currents along with appearing potentially commercially viable.Copyright

The effect of rotational friction on the stability of short-tailed fairings suppressing vortex-induced vibrations

Experiments have been carried out on a free-to-rotate short-tail fairing fitted to a rigid length of circular cylinder to investigate the effect of rotational friction on the stability of this type of VIV suppressor. Measurements of the dynamic response are presented for models with low mass and damping which are free to respond in the cross-flow and streamwise directions. It is shown how VIV can be reduced if the fairing presents a rotational friction above a critical limit. In this configuration the fairing finds a stable position deflected from the flow direction and a steady lift force appears towards the side the fairing has deflected. The fluid-dynamic mechanism is very similar to that observed for a free-to-rotate splitter plate of equivalent length.Copyright

On the Vortex-Induced Vibration Response of a Model Riser and Location of Sensors for Fatigue Damage Prediction

This study is concerned with vortex-induced vibration (VIV) of deepwater marine risers. Riser response measurements from model tests on a densely instrumented long, flexible riser in uniform and sheared currents offer an almost ideal setup for our work. Our objectives are two-fold: (i) we use the measured data to describe complexities inherent in riser motions accompanying VIV; and (ii) we discuss how such data sets (and even less spatially dense monitoring) can be used effectively in predicting fatigue damage rates, which are of critical interest for deepwater risers.

Lock-In, Transient and Chaotic Response in Riser VIV

We show using experimental data on a model riser that lock-in of long flexible risers placed in sheared or uniform cross-flows is a much richer phenomenon than lock-in of flexibly-mounted rigid cylinders under similar conditions. In particular, we find that the frequency content of the riser response may be either narrow-banded around a single dominant frequency (Type I response) or distributed along a relatively broad range of frequencies (Type II response). Distinct transition from Type I to Type II response, and vice versa, can occur several times within a single experimental record. Type I responses reveal features of a quasi-periodic oscillation, often accompanied by large 3rd harmonic components in the acceleration and strain signals, increased correlation length, stable riser trajectories, and monochromatic traveling or standing waves. Type II responses, on the other hand, are characterized by features of chaotic oscillation with small or negligible 3rd harmonic components in the acceleration and strain signals, reduced correlation length, and a continuous spectrum. We study how the fatigue damage differs in the two types of riser response.Copyright

Benchmarking of SHEAR7v4.5: Comparisons to Full-Scale Drilling Riser VIV Data and Legacy Analyses

SHEAR7 [4], industry’s most widely-used vortex-induced vibration (VIV) fatigue damage software analysis tool, has been recently revised to include a fundamental change in methodology based on observations from Deepstar’s high-aspect-ratio, slender pipe towing tests in the Gulf Stream near Miami, Fla. The key revision pertains to the way in which VIV excitation zones (or “power-in” zones) are calculated along the length of the riser. Namely, it is no longer assumed that multiple structural modes are excited simultaneously by a given current profile along the length of the riser in a manner known as multi-mode response. Rather, based on the towing tests, it is assumed that potentially excited modes participate in the response one-at-a-time in a time-sharing fashion. The fraction of the total event time accorded to each responding mode is proportional to the input power of its exciting force, or, may be set to be uniformly distributed among all responding modes. This fundamental change can have a significant effect on the fatigue damage calculated for a given riser and current profile and direct comparisons to previous analyses are not straightforward as the input parameters of the software have been altered along with the analysis methodology itself. In a recent paper [1], BP utilized measured data that it had collected from risers in the field during several of its worldwide drilling campaigns to calibrate SHEAR7v4.4 to yield an average safety factor of ten on fatigue damage when VIV occurs. Given the extent of the changes to the analysis method, it was important to revisit that study and ascertain whether the latest software version similarly captured the trends of the full-scale data and could be calibrated to maintain an appropriate factor of safety. Furthermore, where measured data were not available it was important for consistency to identify and rationalize differences between analyses with the current and penultimate versions of the software. This paper describes BP’s benchmarking of SHEAR7v4.5. Comparisons are made between predicted and measured VIV fatigue damage for several full-scale drilling risers to demonstrate the efficacy of a calibration for the latest version. In addition, comparisons are made between VIV fatigue damage predictions using SHEAR7 versions 4.4 and 4.5 for drilling risers as well as for a typical deepwater SCR in typical design Gulf-of-Mexico loop currents. The version-to-version differences are illustrated and explained. Finally, results of sensitivity studies conducted with respect to the new parameters in SHEAR7v4.5 are presented. A key finding is that while the predictions, on average, are similar from version to version; the scatter in predictions — which leads to requirements for large safety factors — is largely unimproved.Copyright

Forced Oscillation Model Tests for Determination of Hydrodynamic Coefficients of Large Subsea Blowout Preventers

Large scale model tests have been conducted in a towing tank facility for the determination of the hydrodynamic coefficients of subsea blowout preventers. A subsea blowout preventer (BOP) is a large, complex device 10–15 [m] tall, weighing 200–450 [ton]. The BOP stack consists of two assemblies, the ‘lower marine riser package’ (LMRP) connected to the riser string and the BOP itself, connected to the wellhead. Together they represent a large lumped mass, which directly influences the natural frequencies and vibration modes of the riser system, particularly those of the BOP-wellhead-casing assembly.Large uncertainties in the estimates of the hydrodynamic coefficients (added mass, lift and drag or damping) result in large uncertainties in the fatigue damage predictions of the riser and wellhead system. The trend toward larger and heavier BOPs, which could place BOP-wellhead-casing oscillation frequencies in the range of wave frequencies, has motivated Statoil and BP to start a new research project on this subject. The project involves a large scale model test for experimental determination of hydrodynamic coefficients.Two different BOP designs were tested in a towing tank at model scale 1:12. The models weighed about 50 [kg] in air and were about 1.2–1.5 [m] tall. A six-degree-of-freedom oscillator was mounted under the carriage of the towing tank for oscillation of the models in different directions. Static tow tests and forced oscillation tests with and in the absence of steady current were carried out. Keulegan-Carpenter (KC) numbers ranged between 0.2 and 2.0, while the Sarpkaya frequency parameter β was in the range from 4,000 to 50,000. The Reynolds numbers of the static tow tests ranged between 50,000 and 150,000. This paper focuses particularly on tests in the surge direction with and in the absence of a steady current. Results indicate that the hydrodynamic coefficients for BOP stacks are quite different from those of simpler geometries like a circular cylinder. In addition, they provide new insight for analytical modeling of global hydrodynamic forces on BOPs in many configurations and scenarios.Copyright

Empirical Procedures for Long-Term Prediction of Fatigue Damage for an Instrumented Marine Riser

Slender marine risers used in deepwater applications can experience vortex-induced vibration (VIV). It is becoming increasingly common for field monitoring campaigns to be undertaken wherein data loggers such as strain sensors and/or accelerometers are installed on such risers to aid in VIV-related fatigue damage estimation. Such damage estimation relies on the application of empirical procedures that make use of the collected data. This type of damage estimation can be undertaken for different current profiles encountered. The empirical techniques employed make direct use of the measurements and key components in the analyszes (such as participating riser modes selected for use in damage estimation) are intrinsically dependent on the actual current profiles. Fatigue damage predicted in this manner is in contrast to analytical approaches that rely on simplifying assumptions on both the flow conditions and the response characteristics. Empirical fatigue damage estimates conditional on current profile type can account explicitly even for complex response characteristics, participating riser modes, etc. With significant amounts of data, it is possible to establish “short-term” fatigue damage rate distributions conditional on current type. If the relative frequency of different current types is known from metocean studies, the short-term fatigue distributions can be combined with the current distributions to yield integrated “long-term” fatigue damage rate distributions. Such a study is carried out using data from the Norwegian Deepwater Programme (NDP) model riser subject to several sheared and uniform current profiles and with assumed probabilities for different current conditions. From this study, we seek to demonstrate the effectiveness of empirical techniques utilized in combination with field measurements to predict the long-term fatigue damage and the fatigue failure probability.

Traveling Wave Response in Full-Scale Drilling Riser VIV Measurements

The dominance of traveling wave VIV response is observed in full-scale measured drilling riser data for the first time. This paper presents the novel methods developed to identify the presence of traveling versus standing wave riser structural response in the full-scale data and the observations. This paper reports on some of the work conducted under the most recently completed phase of the DeepStar JIP (Phase 9). The paper uses four different novel methods to identify the presence of traveling versus standing wave structural response in data obtained from a full-scale Gulf of Mexico drilling riser during a loop current event. The techniques are: 1) Observation of RMS accelerations between synchronized accelerometers; 2) Observation of filtered displacements between synchronized accelerometers (using a new postprocessing synchronization technique); 3) Displacement versus angular rate phase diagrams; and 4) Derivation of upward/downward curvature components via an algorithm proposed by two of the co-authors. High level conclusions are drawn about the structural response types. Recommendations for future instrumentation campaigns are made.Copyright

An Approach to Include Observed VIV Likelihood in Drilling Riser Fatigue Analyses

Field measurements of the response of a number of drilling risers indicate that vortex-induced vibration (VIV) occurs significantly less often than predicted by the industry-standard fatigue analysis computer program SHEAR7 V4.4. Several comparisons to model tests and field data, including one published by BP and 2H in 2007 [1], demonstrate that this analysis program is generally quite conservative, given that VIV occurs. Furthermore, this conservatism does not take into account those situations in which VIV fatigue is predicted but none is observed in the field, which adds yet another layer of “hidden” conservatism to design analyses. In an effort to address this and reduce conservatism to a more appropriate level, the probability of occurrence of vortex-induced vibration (VIV) is examined using full-scale measured data. The data has been collected over the past several years from five drilling risers without VIV suppression devices. These risers are on rigs under contract to BP at high-current-susceptible sites worldwide. Collectively, the data correspond to 9,600 10-minute field measurements, equivalent to 0.18 years of continuous monitoring. The riser response is obtained from motion loggers placed at selected positions along the riser as described in [1]. Each logger measures 3D accelerations and 2D angular rates. Through-depth currents are measured via Acoustic Doppler Current Profilers (ADCP). By comparison of measurements to computer predictions based on the observed current profile, a relationship is developed between the intensity of the fatigue damage predicted and the probability that VIV is observed in the field. Subsequently, an approach is proposed for scaling analysis predictions to reflect the relative likelihood of VIV. The database of measured and SHEAR7 maximum predicted fatigue damage rates is statistically characterized to determine how it may be used to determine factors of safety (FOS) for VIV design. A worked example for a deepwater drilling riser in the GoM is used to show how the FOS methodology can be applied in the case of multiple design currents each with a different annual probability of occurrence.Copyright