Fatigue Damage of Vertical Rigid Risers due to In-Line Vortex Induced Vibration in Nigeria Shallow Waters

Tobechukwu C. Ezeonwumelu; Chinwuba V. Ossia; Ibiba E. Douglas

doi:10.12691/ajme-5-2-1

American Journal of Mechanical Engineering

Vol. 5, No. 2, 2017, pp 33-40. doi: 10.12691/ajme-5-2-1 | Research Article

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Fatigue Damage of Vertical Rigid Risers due to In-Line Vortex Induced Vibration in Nigeria Shallow Waters

Tobechukwu C. Ezeonwumelu¹, Chinwuba V. Ossia^1,, Ibiba E. Douglas²

¹Offshore Technology Institute, Faculty of Engineering University of Port Harcourt, Port Harcourt, Nigeria

²Marine Engineering Department, Rivers State University of Science & Technology, Port Harcourt, Nigeria

	Abstract
1.	Introduction
2.	Materials and Methods
3.	Results and Discussion
4.	Conclusion
	Nomenclature
	References

Abstract

In-line and Transverse Vortex Induced Vibrations (VIV) pose potential Fatigue damage threat to Vertical Rigid Risers (VRR) even in the less volatile Nigeria Shallow Waters. In this paper, a typical VRR of 31.031m length clamped to a fixed jacket platform in 18.29m water depth was used while relevant metOcean data were used to simulate the environmental conditions. The process was statically and dynamically simulated using different wave spectra on Orcaflex platform. The results from JONSWAP spectra showed a fatigue damage value of 102.5 x 10^-5due to in-line VIV which is greater than 29.1 x 10^-5 due to transverse VIV. The results from the Ochi-Hubble spectra indicate a fatigue damage value of 96.2 x 10^-5due to in-line VIV which is greater than 7.03 x 10^-5 due to transverse VIV. Also, the in-line vortex force (VF) analysis on the VRR for the JONSWAP spectra showed VF range, beginning from the touchdown point (TDP) to the unstraked region (UR), of 0.16 - 0.34kN/m; 0.01 - 0.53kN/m; and 0 - 0.85kN/m at End A (0m); 8.74m and 21.00m, respectively. Whereas, for the Ochi-Hubble spectra, VF range, from TDP to the UR, of 0.16 - 0.302kN/m; 0.018 - 0.56kN/m; 0 - 1.42kN/m at End A (0m); 8.74m and 21.00m, respectively, were obtained. The results for both spectra showed zero in-line VF on the VRR at 23.69m, 26.21m and End B (31.031m). Hence, fatigue damage and VF due to in-line VIV is important from the TDP to most parts of the UR on the VRR irrespective of the wave spectra and requires proper analysis in riser designs.

Keywords: in-line vortex induced vibration, transverse vortex induced vibration, fatigue damage value, vertical rigid risers, Nigeria shallow waters

Cite this article:

Tobechukwu C. Ezeonwumelu, Chinwuba V. Ossia, Ibiba E. Douglas. Fatigue Damage of Vertical Rigid Risers due to In-Line Vortex Induced Vibration in Nigeria Shallow Waters. American Journal of Mechanical Engineering. Vol. 5, No. 2, 2017, pp 33-40. https://pubs.sciepub.com/ajme/5/2/1

Ezeonwumelu, Tobechukwu C., Chinwuba V. Ossia, and Ibiba E. Douglas. "Fatigue Damage of Vertical Rigid Risers due to In-Line Vortex Induced Vibration in Nigeria Shallow Waters." American Journal of Mechanical Engineering 5.2 (2017): 33-40.

Ezeonwumelu, T. C. , Ossia, C. V. , & Douglas, I. E. (2017). Fatigue Damage of Vertical Rigid Risers due to In-Line Vortex Induced Vibration in Nigeria Shallow Waters. American Journal of Mechanical Engineering, 5(2), 33-40.

Ezeonwumelu, Tobechukwu C., Chinwuba V. Ossia, and Ibiba E. Douglas. "Fatigue Damage of Vertical Rigid Risers due to In-Line Vortex Induced Vibration in Nigeria Shallow Waters." American Journal of Mechanical Engineering 5, no. 2 (2017): 33-40.

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1. Introduction

Marine risers are exposed to several environmental conditions which include wave, current, tides, wind, etc. When exposed to current, a stationary circular cylinder such as a rigid riser experiences flow around it and this flow is characterized by large zones of separation which causes the riser to build-up a surrounding boundary layer due to skin friction between the fluid and the body. Leonardo Da Vinci in 1497 was probably the first to pay attention to how flow separates from the surface of different objects and what types of pattern were formed in their wakes ^[1]. In 1883, Reynolds in investigating internal flow in pipes discovered that the transition between laminar and turbulence was a function of the density, viscosity and velocity of the fluid and of the diameter of the pipe ^[1]. Reynolds number (Re) tells us whether a flow is laminar or turbulent, and expresses the relationship between inertia and viscous forces in a flow.

Marcollo ^[2] deduced that at certain critical values, the flow transits from laminar to turbulence in which disturbances from other effects can affect this Re-value. Such effects might be roughness, turbulence and any cylinder oscillations as suggested by Zdravkovich ^[3]. Reynolds determined that the transition to turbulence happened within a range of Re. Reynolds number (Re) governs the effect of flow around circular cylinders which includes the transition states. Transition in the shear layer was discovered as Re was rising, the transition zone moved towards the separation points changing the length and width of the near wake region ^[4].

Also, Huera-Huarte ^[1] stated that there is an adverse pressure gradient in the direction of the flow which causes the boundary layer to separate from the surface of the cylinder generating a recirculation in the flow. The result of this phenomenon is the wake being formed around it. The result of the mutual interaction between two separating shear layers is the fundamentals of vortex shedding as emphasized by Gerrard ^[5]. Vortex shedding is the result of an interaction between the shear layers in the near body wake involving the inherent vorticity ^[1], but according to Marcollo ^[2], the phenomenon is an instability intrinsic to the flow itself. As further explained by Sarkpaya ^[6], it is a consequence of the interaction between shear layers, base pressure, diffusion and dissipation of vorticity. Separation begins at Re = 4, this continues until the Re = 47 where the near wake is observed to be unstable leading to vibrations of the shear layer at the neutral point. There is a common feature for circular cylinder flows with Re > 47, that is vortex shedding and the formation of the Karman-Bernard vortex street ^[1]; hence flow instability produces vortices that grow alternately on each side of the body ^[5]. At about Re = 47, the vortex shedding frequency or Strouhal frequency appears. For smooth cylinderical surfaces, Strouhal number S_t is found to be nearly constant at 0.2 to 0.25 for 5x10² < Re < 2x10⁵. HueraHuarte ^[1] showed, from experiments carried out by different researchers, that the frequency of the vortex shedding in the wake of a cylinder is a function of Re, the roughness of the cylinder surface, the turbulence intensity in the incoming flow, the aspect ratio, the blockage ratio, etc. The Strouhal number (S_t) is a function of the roughness and the incoming turbulence. The roughness on a cylinder has effect on the S_t with the largest effect being in the critical Re region ^[2]. Furtherrmore, Achenbach and Heinecke ^[7] suggested that the greater the roughness, the lower the S_t – value in the critical region, likewise for a smooth cylinder, the S_t is seen to more than double in this region.

These motions generated from the effects of vortices are known as Vortex Induced Vibrations (VIV). VIV phenomenon plays a significant role in marine riser analyses with respect to fatigue. In general, VIV is viewed in two aspects namely; the cross-flow VIV and in-line VIV which is due to lift and drag forces, respectively. In-line VIV is due to drag forces acting in the direction of flow. The drag is a measure of the resistance offered to flow by a body. Traditionally, focus has been on cross-flow VIV ^[8]. In-line VIV was considered less important because the displacements were smaller but Baarholm et al. ^[9] found that the fatigue damage due to in-line VIV can be equally important. This is mainly because in-line forces occur at twice the frequency of the cross-flow forces, causing the structure to vibrate in higher modes ^[8].

Aronsen ^[10] carried out experimental investigations between in-line and cross flow VIVs. Experiments were performed in a towing tank of 40m using a rigid cylinder of aspect ratio 20 under prescribed harmonic motions in uniform flow. The cylinder was installed in a yoke structure which in turn was suspended to an overhanging tow carriage. Model oscillations were achieved by oscillating the yoke on the carriage while the flow velocity was obtained by moving the carriage at constant speed in still water. All the experiments were performed at a Re = 2.4 x 10⁴. The aim was to investigate hydrodynamic coefficients for pure in-line oscillations as well as the interactions between in-line and cross flow VIV. From the first experiment on in-line oscillations, where the frequencies and amplitude were varied to obtain a detailed map of the forces acting on a cylinder in the pure in-line VIV regime, it was discovered that in-line oscillations will give rise to cross-flow forces that contribute to an early start-up of VIV, compared to conditions where the in-line motion is restrained.

Also, based on experiments carried out on a riser in Hanoytangen - Norway, Baarholm et al. ^[9] discovered that low current velocities tend to activate tension controlled modes (low order modes) which eventually cause fatigue due to In-line VIV. Similarly, the lowest modes (that is, from low current speeds), has significant contribution to fatigue damage because at this condition, in-line oscillations occur at double the frequency for the cross-flow. It was also discovered that In-line fatigue damage is due to a combination of water depth and current velocity (that is, fatigue due to in-line VIV will increase for a decreasing velocity and an increasing water depth). Also in-line fatigue damage was found to be significant at the bottom of the riser where current speeds are low. It can be shown that the fatigue damage is proportional to U⁷ (flow velocity) and U⁴ when the response is dominated by tension and bending stiffness controlled modes, respectively. The Hanoytangen riser fatigue damage was proportional to U⁷for the lowest velocities. It was also observed that the tension controlled case, corresponds to a mode with half the wavelength and since the curvature for the amplitude increases with mode wavelength squared; fatigue for in-line VIV tends to dominate for cases with tension controlled modes.

Thorsen et al. ^[8] showed that a number of models are available for predicting VIVs which include; computational fluid dynamics, wake oscillator models and semi-empirical models. Wake oscillator models use a Van de Pol oscillator to describe the fluid force in the time domain ^[11]. Ge et al. ^[12], presented a wake oscillator model for combined in-line and cross-flow VIV of a tensioned beam. While semi-empirical models such as VIVANA use coefficients from forced vibrations and apply iterative procedures to calculate the frequency, mode and amplitude of vibration ^[13]. Semi-empirical and wake oscillator models were originally able to calculate cross flow VIVs only, while some of the models have also been extended to include in-line VIV as indicated by Chaplin et al. ^[14]. Recently, Thorsen et al. ^[8], in their work, a time domain simulation method for a combined cross-flow and in-line VIV was presented. The key results were compared to experimental observations with an observation that the predicted curve was very realistic for the in-line direction, while slightly conservative for the cross-flow direction.

The time domain simulation process takes into consideration the wave effects in marine riser analyses; hence the inclusion of the JONSWAP and Ochi-Hubble wave spectra as comparative case studies in this work. Hasselmann et al. ^[15], showed that a JONSWAP spectra was developed where a peak enhancement factor was included as a modification to the Pierson-Moskowitz spectra as shown in Equation 1;

(1)

Where σ = σ_afor f < f_pand σ = σ_bfor f > f_p, f_prepresents the spectral peak frequency.

The JONSWAP spectrum contains 5 parameters f_p, α, γ, σ_aand σ_band the most probable values for some of these read: γ=3.3, σ_a= 0.07 and σ_b=0.09 .The values for α and f_pdepend on the wind speed, fetch and duration.

Similarly, Ochi and Hubble ^[16] formulated the double peak spectra model shown in Equation 2;

(2)

Where Γ is the gamma function.

This spectrum has 3 parameters for each wave system, significant wave height (H_sj), spectral peak period (T_pj) and the shape factor (λ_j).

In summary, relevant available literature show that initial focus was on the cross-flow VIV until the significance of in-line VIV was brought to limelight only recently ^{[8, 9]}, and ^[10]. Aronsen ^[10] carried out experiments in Norway to determine certain in-line oscillation parameters, while Baarholm et al. ^[9] compared his computer program results on fatigue damage to an existing experimental data measured in Norway. Thorsen et al. ^[8] then presented a time domain method for simulating both cross-flow and in-line VIVs. From the reviews it can be shown that there is insufficient experimental work and time domain simulations of in-line VIV in the tropical regions well impacted by the phenomenon. International Oil Companies are always on course in maximally applying best operational practices that maximize profit and minimize cost over a long period of time. In-line VIV is an anathema to the long time performance of VRRs in shallow waters. With major offshore projects to take place in West Africa, it becomes necessary to undertake adequate research on this phenomenon simulating this particular environment to ensure proper design of VRRs in this region at minimal costs. In-line VIV is important in low current offshore environments with shallow water depths as well as tensioned steel risers; hence, the reason for research into vertical rigid risers (VRR) for a fixed jacket platform in Nigeria shallow waters. This study therefore shows results from simulating a typical VRR for a shallow water Offshore platform using metOcean data of the region with JONSWAP and Ochi-Hubble wave spectra on OrcaFlex 8.47a ^[17] platform with a view to analyzing the VIV effects on fatigue damage.

2. Materials and Methods

2.1. Riser Material Properties

Data on a typical VRR for a fixed jacket platform in Nigeria Shallow water were collated. The relevant rigid Riser material properties include: Seamless Carbon Steel, Outer diameter (OD) 12.75 inch, inner diameter (ID) 11.278 inch, wall thickness (WT) 0.736 inch, and Specified minimum yield strength (SMYS) of 448MPa. These material data were imputed into the OrcaFlex platform. Figure 1, Figure 2 and Figure 3 are snapshots from the OrcaFlex platform showing relevant data imputed.

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Fatigue Damage of Vertical Rigid Risers due to In-Line Vortex Induced Vibration in Nigeria Shallow W...

Fatigue Damage of Vertical Rigid Risers due to In-Line Vortex Induced Vibration in Nigeria Shallow Waters

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Cite this article:

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1. Introduction

2. Materials and Methods

3. Results and Discussion

4. Conclusion

Nomenclature

References

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