Pragmatic Riser VIV Analysis

Vortex-induced vibration (VIV) suppression devices minimize pipeline excitation during offshore current events, thereby reducing accelerated fatigue failure of pipeline components and minimizing current-related disruption on producing operations. Ideally VIV suppression devices are installed as the pipeline is being built onshore or as the riser is being assembled offshore onboard a pipe-lay vessel. Retrofitting VIV suppression devices is often a necessary pipeline/riser intervention. New methods and tooling are bringing down the price of VIV suppression hardware and associated installation times. This paper follows Shell Global Solutions (US) Inc. VIV suppression hardware retrofit tooling and method evolution from first requirement through theircurrent tooling configurations and installation methods. A case study was conducted, and our customer elected to install additional suppression fairings for vortex-induced vibration on two export pipeline steel catenary risers (SCRs). The VIV suppression hardware installed on the risers was designed and fabricated by Shell Global Solutions (US) Inc. (Shell GSUS) and was in the form of six-foot long fairing sections separated by load bearing collars. For hardwareinstallation, our customer teamed with Shell GSUS to develop a method and associated tooling to allow the fairing assemblies to be installed. The method of installation included a TLP-based work-class ROV and VIV suppression device installation tooling developed by Shell GSUS. The method and equipment was utilized without affecting flow line operations and with minimal impact on TLP resources. This case study reviews the planning, procedures and results of a deepwater, vertical SCR VIV suppression retrofit project. Introduction Vortex-induced vibration (VIV) can best be described as follows: As fluid passes a tubular under tension, vortices are shed on the down-current side. These vortices create fluctuating lift and drag forces causing the tubular to move perpendicular to the current. This perpendicular movement is the phenomenon known as vortex-induced vibration. VIV has become a significant challenge to structure and pipeline designers as offshore energy development moves into ever deeper water. Suppression Strategies VIV can be limited by using several methods. One method shortens the vortices correlation length by installing helical fins known as strakes. The helical fin arrangement shortens the correlation length of the vortex, reducing the lift and drag forces on the tubular. The strake has been used successfully in deep water and is a simple, robust method of suppressing VIV. There are disadvantages to using strakes, though. Strakes increase the diameter of the tubular by the combined height of the vortex interrupting fins. Additional strength must be factored into a riser system design to account for the additional drag force caused by the greater diameter of the tubular as a result of adding strakes. Strakes require periodic cleaning as marine growth creates drag that shortens or bridges the fins.

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.

A rigid jumper is an important part of the subsea production system, it may experience significant vortex induced vibrations (VIV) if subjected to current. It has normally non-straight geometry shape in three-dimensional space. Consequently, the response of a rigid jumper under VIV is much more complicated compared to straight pipeline structures. Currently, there are very limited studies and design guidelines including methods on how to assess the fatigue damage of rigid jumpers under VIV. The methodology used for straight pipelines is often applied by ignoring the non-straight geometry characteristics and the multi-axial stress states (coexisting of flexural and torsional stress). However, both experimental and numerical results show that the torsional stress does exist besides the flexural stress for rigid jumpers under VIV. On the other side, the response of the rigid jumper under VIV is also challenging. The objective of this study is to do a fatigue assessment practice based on state-of-the-art calculation methods to a rigid jumper on model scale. The VIV response is inherited from experimental tests and numerical calculations by either force or response model methods. The influence of torsional stress on fatigue assessment is demonstrated. Two approaches have been investigated. In the first method, the flexural and torsional stresses are evaluated separately. The second method uses the 1st principle stress to calculate the fatigue damage, thus the flexural and torsional stresses are evaluated together. It is shown that the use of the 1st principle stress gives higher fatigue damage if the torsional stress contribution is significant. Further, the principle stress method is also less time-consuming on processing the results. Detailed discussions based on results have been performed, which could be also applied to general real scale rigid jumpers.

Drilling risers are regularly deployed in deep water (over 1500 m) with large sections covered in buoyancy modules. The smooth cylindrical shape of these modules can result in significant vortex-induced vibration (VIV) response, causing an overall amplification of drag experienced by the riser. Operations can be suspended due to the total drag adversely affecting top and bottom angles. Although suppression technologies exist to reduce VIV (such as helical strakes or fairings), and therefore reduce VIV-induced amplification of drag, only fairings are able to be installed onto buoyancy modules for practical reasons, and fairings themselves have significant penalties related to installation, removal, and reliability. An innovative solution has been developed to address this gap; LGS (Longitudinally Grooved Suppression)1. Two model testing campaigns were undertaken; small scale (sub-critical Reynolds Number flow), and large scale (post-critical Reynolds Number flow) to test and confirm the performance benefits of LGS. The testing campaigns found substantial benefits measured in hydrodynamic performance that will be realized when LGS modules are deployed by operators for deepwater drilling operations.

The method of pulling a steel catenary riser (SCR) through a steel tube (termed as a pull tube) is common practice for deepwater riser tie back applications. Vortex-induced vibration (VIV) of such a system is complex. VIV analysis programs, such as Shear7 [1], are suitable only for a single, chain-like structure. The application of such a software tool in VIV design of the SCR-pull tube system requires careful consideration of a number of structural and hydrodynamic factors. This paper presents a methodology for VIV analysis of the combined structural system of the SCR with the pull tube. Firstly, the entire SCR-pull tube system is modeled with the finite element program Flexcom [2]. The modes are then calculated for the entire structure with program Modes [3]. Afterwards, the structural nodes are rearranged for VIV analysis with Shear7. The pull tube is secured on the platform through a number of guides on the truss structure of the hull. The diameter of the pull tube is much larger than that of the SCR, and the pull tube is much stiffer in bending than the SCR is. If the entire structure is analyzed with Shear7, the mode for the pull tube (a mode involving a large motion of the pull tube section), which is very high in order, would be embedded in the analysis. It makes sense to single out the pull tube mode for study as if it is the first mode. A computer program, named as V-Span [4] for subsea span VIV analysis, is used to analyze both in-line and cross-flow VIV of the pull tube. A numerical example is presented to demonstrate this methodology. This is a deepwater SCR, which has a diameter of 9 inches. The water depth is 6,300 ft. The pull tube is 640 feet long and 20 inch in diameter. Both the loop-eddy and background currents are analyzed. The fatigue damage resulted from both in-line and cross-flow VIV is estimated.

Our customer elected to post-install suppression fairings for vortex-induced vibration (VIV) on the suspended gas export pipeline in Green Canyon Block 113 due to recent data acquired on the depth and magnitude of loop/eddy and cold core current profiles in the Gulf of Mexico. VIV suppression fairings minimize pipeline excitation during high current events, thereby preventing accelerated fatigue failure of pipeline components and minimizing current-related disruption on producing operations. VIV suppression hardware installed on the span was designed and fabricated by Shell Global Solutions (US) Inc. (Shell GSUS) and was in the form of short (6-foot) fairing sections. For hardware installation, our customer teamed with Shell GSUS to develop a method and associated tooling to allow the fairing assemblies to be installed. The method of installation included a deepwater construction vessel using a work-class ROV and heave compensation techniques. This method and equipment can be utilized without affecting flow line operations. This paper reviews the planning, procedures and results of a deepwater, horizontal pipeline span VIV suppression installation project. This paper will be presented by the author at Offshore Technology Conference 2004 in Houston, Texas. Case for Action Span Description. During an as-built pipeline survey of a deepwater offshore gas pipeline, a span of over 300 feet was discovered. The span is in the Green Canyon area of the Gulf of Mexico (Figure 1). The depth of the span at location is approximately -2300 fsw. The span length is over 300 feet long and at times over 10 feet above the sea floor. The Gulf of Mexico has recently verified currents that can potentially harm suspended pipelines in deepwater. Fig. 1 - Approximate Span Geographic Location(Available in full paper) To limit vortex-induced vibration (VIV) during deepwater current events, suppression hardware is required. Shell GSUS was tasked with installing the VIV suppression devices. Fairings were the specified device due to their low drag properties and low cost. VIV suppression fairings have in the past been installed on vertical pipeline steel catenary risers (SCR) and top tensioned risers (TTR) by Shell GSUS. This was our first attempt at a horizontal span retrofit. Installation Method Development VIV Suppression Device Decision. Previously Shell GSUS developed and patented VIV suppression fairings. Fairings were found to be superior to commonly used helical strakes due to their light-weight, low-cost and low-drag properties. VIV suppression fairings developed by Shell GSUS (Figure 2) weathervane to keep the vortex-interrupting fin on the downcurrent side of the tubular that they are attached to. Unlike helical strakes, VIV suppression fairings do not increase the diameter of a pipeline substantially. Most deepwater facilities utilize VIV suppression in some fashion. Shell Exploration and Production Company (SEPCo) chose VIV suppression fairings for their tension leg platform (TLP) production risers in the high-current Mississippi Canyon region of the Gulf of Mexico. The Ursa TLP production risers were installed with VIV suppression fairings attached. Mars TLP production risers were also installed with VIV suppression fairings as well as strakes in the splash zone, and additional VIV suppression devices have been installed since arriving on location, due to a deeper understanding of the current profile.

Most deepwater tubulars experiencing high currents frequently require vortex-induced vibration (VIV) suppression to maintain an acceptable fatigue life. Helical strakes and fairings are the most popular VIV suppression devices in use today. Marine growth can significantly affect the VIV of a bare riser, often within just a few weeks or months after riser installation. Marine growth can have a strong influence on the performance of helical strakes and fairings on deepwater tubulars. This influence affects both suppression effectiveness as well as the drag forces on the helical strakes and fairings. Unfortunately, many VIV analyses and suppression designs fail to account for the effects of marine growth at all, even on a bare riser. This paper utilizes results from both high and low Reynolds number VIV test programs to provide some design considerations for managing marine growth for VIV suppression devices.

Vortex-Induced Vibration (VIV) has emerged as a crucial problem that may arise during a drilling operation, which could result in riser failure. Engineers focus on drilling riser's flow characteristics and controls with the intention of improving the drilling environment. Auxiliary lines or control rods are usually placed around the main drilling riser to suppress the VIV. However, the issue related to the flow past drilling riser system with auxiliaries has yet to be resolved. The flow characteristics around a riser system with auxiliaries were analysed in this research. The simulations were conducted using Computational Fluid Dynamics (CFD) software, Altair AcuSolve. This work focuses on the influence of various gap ratios (G/D) and diameter ratios (d/D) on the vortex interaction. A main cylinder with six auxiliary lines was modelled with G/D of 0 to 2.0 and d/D of 0.10 to 0.60 to simulate the riser system. The simulations were carried out at Reynold Number of 200 in the laminar flow regime. The results revealed that the hydrodynamic forces decreased when d/D and G/D increased. The vortex shedding was significantly reduced for auxiliary lines with G/D between 0.3 and 1.4. The numerical simulation results indicated that the vortex interaction in the wake region was, and the hydrodynamic forces were reduced due to the auxiliary lines configurations. The findings of this study are intended to contribute a new CFD simulation result for a better prediction of VIV on a drilling riser with auxiliary lines.

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.

Vortex-induced vibration of flexible pipe fitted with helical strakes in oscillatory flow

Time-domain analysis of vortex-induced vibration of a flexible mining riser transporting flow with various velocities and densities

Numerical investigation on the suppression of vortex-induced vibration system by assembling different rotors based on quasi-active control method

A numerical study on the vortex-induced vibration of flexible cylinders covered with differently placed buoyancy modules

Experimental exploration of unpowered propellers for vortex-induced vibration suppression of flexible riser

Towards an understanding of marine fouling effects on VIV of circular cylinders: Aggregation effects