Kamaldev Raghavan

Reduction/Suppression of VIV of Circular Cylinders Through Roughness Distribution at 8×103 < Re < 1.5×105

Vortex Induced Vibration (VIV) of a circular cylinder in a steady flow is reduced using distributed surface roughness. VIV reduction is needed in numerous applications where VIV is destructive. Roughness is distributed to the surface of the cylinder in the form of sandpaper strips to achieve three goals: (1) Trip separation in a controlled manner so that some uncertainties are removed and the flow becomes more predictable. (2) Reduce spanwise correlation, which is strongly linked to VIV. (3) Select roughness grit size to achieve the first goal without energizing too much the boundary layer, which would induce higher vorticity and circulation, and consequently lift. Our experiments show that it is possible to reduce VIV amplitude and synchronization range. More tests are needed to achieve full suppression. Our experiments are conducted in the TrSL2 and TrSL3 flow regimes.Copyright

Improved SCR Design for Dynamic Vessel Applications

Design of Steel Catenary Risers (SCR) has seen increasing challenges in deep water applications due to higher pressures, vessels with more dynamic motions and severe weather conditions. In the Gulf of Mexico, the SCR is sometimes designed with little margin due to harsh environment. In West Africa, SCRs may be infeasible with turret moored Floating Production Storage and Offloading (FPSO) System, and are often limited to a narrow hang-off range near the Center of Gravity (CoG) with spread moored FPSO. Freestanding hybrid risers are feasible solutions for many fields in West Africa. However, the freestanding riser cost is generally considered much higher than that of an SCR. Hence, there is a need to improve SCR performance in dynamic vessel applications. This paper presents the development of Shaped SCR configuration that has significantly improved performance and minimal cost increase compared to an SCR. Compared with a conventional Lazy Wave SCR, the riser configuration has increased installation flexibility and decreased cost as it is installable via J-lay or S-lay. The configuration is developed and assessed for a spread moored FPSO in West Africa. The strength and fatigue response is compared with those of a simple SCR and Lazy Wave SCR. The key considerations in developing the configurations are discussed. This paper also presents the use of upset ends to improve riser fatigue performance. This is achieved due to reduced bending and tension stress ranges resulting from thicker pipe ends, as well as to reduced Stress Concentration Factors (SCF) resulting from forging process.Copyright

Using CFD to Study the Effects of Staggered Buoyancy on Dilling Riser VIV

This paper examines the potential benefit of the spaced arrangement of buoyancy modules to reduce the vortex induced vibration of deepwater drilling risers. Buoyancy modules are most often continuously applied to a riser to manage the top tension in the riser but they can also be spaced apart so that segments of bare riser are exposed. Field experience suggests this “staggering” of the buoyancy modules tends to suppress vortex induced vibration (VIV). However, the effects of staggered buoyancy are not well understood. In this paper we use CFD simulations to find the flow characteristics of various combinations of buoyancy modules and bare sections which include choke and kill lines. The effect of varying the percentage of buoyancy coverage is estimated based on the forced and free vibration characteristics of individual sections and combinations of the sections. Some direct reduction in VIV was observed but the benefits seem small.

Novel Technique for SCR Porch Location Screening

For a spread moored FPSO, the determination of feasible SCR hangoff locations is usually performed considering numerous load combinations including different riser hangoff locations, vessel drafts, wave heights, periods and directions and current profiles. Due to the riser response sensitivity to the random waves multiple three hour random sea realizations are also typically used to capture the worst riser response. Thus, large numbers of load cases have to be performed. The amount of computational resources needed to obtain the results is substantial and sometimes prevents the work from completion in a timely manner. Furthermore, significant effort needs to be invested in organizing and understanding the results. This paper introduces the concept of developing Porch Location Envelopes (PLE) which considerably simplifies the understanding of all results, and recommends an approach that drastically reduces the time in obtaining those results. The PLE for a given SCR is defined as the limiting riser hangoff locations for a range of vessel motions. The PLE is determined using rigid body vessel motion transformation and relationships between porch motions and riser response characteristics. By utilizing the approach provided in this paper, the feasible SCR hangoff locations can be determined for each riser using only a limited number of riser strength analyses.Copyright

HYDRODYNAMIC COEFFICIENT MAPS FOR RISER INTERFERENCE ANALYSIS

Riser wake interference analysis is conducted based on analytical / semi-empirical models such as Blevins’ and Huse’s models. These models are used for modeling the reduction in particle flow velocity due to the presence of a cylindrical object upstream in the flow path. However, these models are often too conservative and accurate only for circular cylinders. Many top tensioned risers (TTRs) use vortex induced vibration (VIV) suppression devices such as strakes or fairings. There is a need for alternate methods to obtain drag and lift coefficient datasets for circular cylinders with strakes and fairings. Two such approaches are to obtain data from Computational Fluid Dynamics (CFD) simulations or from experimental large-scale model test data. Interpolation and/or extrapolation methods are needed to obtain additional data points for global riser finite element analysis.This paper presents a methodology to obtain hydrodynamic coefficients for TTRs with VIV suppression devices. The proposed methodology uses a combination of empirical formulas based on Blevins’ model and numerical interpolation techniques along with experimental tow tank test data and CFD analysis. The resulting data is then input as user-defined drag/lift coefficients into a global riser finite element analysis to obtain a more realistic riser system response.Copyright

Tandem Riser Hydrodynamic Tests at Prototype Reynolds Number

Deepwater riser interference is an area of significant technical complexity and uncertainty in the design cycle due to the intricacies of wake hydrodynamics. Existing models, found in industry guidelines, are based on approximate theoretical models of bare cylinder wake and nominally checked against small scale tests at low Reynolds numbers. In actual conditions the Reynolds number is sufficiently higher and the risers are fitted with vortex-induced vibration (VIV) suppression devices. This raises questions on the applicability of the standard models and hydrodynamic coefficients used, especially if the geometry is different than a circular cylinder. A series of full scale tests, at supercritical Reynolds numbers, were conducted to address these uncertainties and obtain hydrodynamic coefficients for interference design. The tests were carried out utilizing two full scale cylinders fitted with actual VIV suppression devices and towed either in fixed or spring supported configurations. The paper discusses the experimental methodology and findings from the testing program, showing deviations from the standard models found in industry codes.Copyright

Interference Assessment Between Top Tensioned Risers Using a Comprehensive Screening Approach

Interference between top tensioned risers (TTRs) is a key design challenge. Due to TTR tensioner stroke limits combined with large vessel offsets, the space out of wellheads is limited. Therefore, riser-to-riser contact is more likely to occur in extreme current conditions.Riser clearance between adjacent risers is evaluated accounting for the effects of wake, vortex-induced vibrations, current directionality (including variation through-depth), vessel offset, riser configuration, and drilling sequence. Accounting for all of these effects simultaneously and in detail when assessing TTR interference can be challenging.The typical TTR array interference approach consists of a combination of riser deflection shape matching and detailed wake assessment. In this paper, a revised TTR interference analysis approach is discussed, with the inclusion of an intermediate step involving screening for critical riser pairs using a simplified wake model assessment. Riser deflection shape matching ensures that the likelihood of clashing is minimized. The riser interference screening process avoids detailed wake modelling of non-critical riser pairs. The screening analysis method emphasizes avoidance of false positives (unrealistic riser clashing pairs) and false negatives (missing riser clashing pairs). It employs a simplified conservative wake model using a stratified downstream current profile to determine which riser pairs are critical and warrant detailed wake modelling. To illustrate the efficiency of the screening approach, results from this approach are compared to results from analysis with detailed wake modelling.An implementation of this approach is presented for riser joints with fairings and strakes. Nominal drag coefficients for these joints are obtained based on experimental testing and/or computational fluid dynamics simulations. Drag amplification of the upstream riser is obtained from vortex induced vibration (VIV) analysis and is also incorporated in the analysis.Copyright

STRAKED RISER DESIGN WITH VIVA

VIVA is a vortex induced vibration (VIV) analysis software that to date has not been widely used as a design tool in the offshore oil and gas industry. VIVA employs a hydrodynamic database that has been benchmarked and calibrated against test data [1]. It offers relatively few input variables reducing the risk of user induced variability of results [2]. In addition to cross flow current induced standing wave vibration, VIVA has the capability of predicting traveling waves on a subsea riser, or a combination of standing and traveling waves. Riser boundary conditions including fixed, pinned, flex joint or SCR seabed interaction can be modeled using springs and dashpots. VIVA calculates riser natural frequencies and mode shapes and also has the flexibility to import external modal solutions. In this paper, the applicability of VIVA for the design of straked steel catenary risers (SCR) and top tensioned risers (TTR) is explored. The use of linear and rotational springs provided by VIVA to model SCR soil interaction and flex joint articulation is evaluated. Comparisons of the VIV fatigue damage output with internal and external modal solution is presented in this paper. This paper includes validation of the VIVA generated modal solution by comparing the modal frequencies and curvatures against a finite element (FE) model of the risers. Fatigue life is calculated using long term Gulf of Mexico (GoM) currents and is compared against the industry standard software SHEAR7. Three different lift curve selections in SHEAR7 are used for this comparison. The differences in riser response prediction by the two software tools are discussed in detail. The sensitivity of the VIVA predicted riser response to the absence of VIV suppression devices is presented in this paper. The riser VIV response with and without external FE generated modal input is compared and the relative merits of the two modeling approaches are discussed. Finally, the recommended approach for VIVA usage for SCR and TTR design is given.Copyright