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Glossary of Technical Terms Used in Oil & Gas Specific Terms: VIV (riser)

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VIV (riser)

VIV: A Silent Threat to Offshore Riser Systems

In the realm of offshore oil and gas exploration, risers play a crucial role in transporting hydrocarbons from subsea wells to surface platforms. These long, vertical pipes are susceptible to a phenomenon known as Velocity Induced Vibration (VIV), which can lead to fatigue damage and ultimately, catastrophic failure. Understanding VIV is paramount for ensuring the safety and longevity of offshore infrastructure.

What is VIV?

VIV occurs when a riser, exposed to ocean currents, experiences vibrations due to the interaction between the flow and its cylindrical shape. These vibrations, often unseen and silent, can be significant, inducing high stresses in the riser and leading to fatigue cracks over time.

Key Factors Influencing VIV:

  • Current Velocity: Higher velocities induce more intense vibrations.
  • Riser Diameter: Larger diameters amplify the effect of currents.
  • Riser Flexibility: Flexible risers are more prone to VIV than rigid ones.
  • Seabed Conditions: Rough terrain can contribute to VIV by creating turbulence.

Types of VIV:

  • In-line VIV: Vibrations occur in the direction of the current flow.
  • Cross-flow VIV: Vibrations occur perpendicular to the current flow.

Consequences of VIV:

  • Fatigue Damage: Repeated vibrations lead to micro-cracks that can propagate, weakening the riser.
  • Corrosion: Vibration-induced stress can accelerate corrosion, further compromising the integrity of the riser.
  • Increased Drag: VIV can increase the resistance to flow, requiring more energy to pump fluids.
  • System Failure: If left unchecked, VIV can result in riser failure, leading to costly downtime, environmental damage, and potential safety risks.

Mitigating VIV:

  • Design Considerations: Optimizing riser design, including materials, diameter, and flexibility, to minimize VIV.
  • VIV Suppressors: Using devices like strakes, fairings, or helical strakes to disrupt the flow and reduce vibration.
  • Active Control Systems: Employing real-time monitoring and active control systems to adjust riser tension and minimize VIV.

Conclusion:

VIV is a significant challenge in offshore engineering, requiring careful analysis and mitigation strategies. Understanding its mechanics, predicting its occurrence, and implementing effective solutions are crucial for ensuring the safety and operational efficiency of riser systems. By embracing advancements in design, monitoring, and control technologies, the industry can effectively combat VIV and secure a reliable and sustainable future for offshore oil and gas exploration.

Test Your Knowledge

Quiz: VIV - A Silent Threat to Offshore Riser Systems

Instructions: Choose the best answer for each question.

1. What is the primary cause of Velocity Induced Vibration (VIV)?

a) Strong winds blowing on the riser.

Answer

b) The interaction of ocean currents with the riser's cylindrical shape.

c) Seabed vibrations caused by earthquakes. d) Internal pressure fluctuations within the riser.

2. Which of these factors DOES NOT influence VIV?

a) Current velocity.

Answer

b) Riser material strength.

c) Riser diameter. d) Seabed conditions.

3. What is a major consequence of VIV?

a) Increased oil production.

Answer

b) Fatigue damage to the riser.

c) Reduced maintenance costs. d) Improved stability of the platform.

4. Which type of VIV occurs perpendicular to the current flow?

a) In-line VIV.

Answer

b) Cross-flow VIV.

c) Vertical VIV. d) Spiral VIV.

5. Which of these is NOT a method for mitigating VIV?

a) Optimizing riser design. b) Using VIV suppressors.

Answer

c) Increasing the current velocity.

d) Employing active control systems.

Exercise: VIV Mitigation

Scenario: A new offshore platform is being designed in an area known for strong ocean currents. The riser connecting the subsea well to the platform is expected to experience significant VIV.

Task:

  • Identify three key design considerations to minimize VIV in the riser.
  • Suggest two different types of VIV suppressors that could be used on the riser.
  • Explain how these design considerations and suppressors will help reduce the risk of VIV-related damage.

Exercise Correction

Here is a possible solution to the exercise:

Design Considerations:

  • Riser Diameter: Reduce the riser diameter as much as feasible while still accommodating the necessary flow rate. Smaller diameters experience less VIV.
  • Riser Flexibility: Choose materials and construction techniques that create a less flexible riser. This will reduce the amplitude of vibration.
  • Riser Material: Select a material with high fatigue strength to better withstand repeated stress cycles caused by VIV.

VIV Suppressors:

  • Strakes: These are small, rigid plates attached to the riser's surface. They disrupt the flow of water around the riser, reducing the formation of vortices that cause vibration.
  • Fairings: These are larger, more streamlined devices that cover a section of the riser. They smooth out the flow of water, minimizing the turbulence that contributes to VIV.

Explanation:

By implementing these design considerations and using VIV suppressors, the riser's susceptibility to VIV is significantly reduced. This is achieved by:

  • Reducing the surface area exposed to the current (smaller diameter).
  • Making the riser less prone to dynamic movements (reduced flexibility).
  • Increasing the riser's resistance to fatigue (stronger material).
  • Disrupting or smoothing the flow of water around the riser (strakes and fairings).

This comprehensive approach will minimize the risk of VIV-related damage, ensuring the long-term integrity and safety of the riser system.

Books

  • Offshore Structural Engineering: By Chakrabarti, S.K. (2010) - Covers a wide range of offshore structures, with a dedicated chapter on VIV and its effects.
  • Dynamics of Marine Structures: By Faltinsen, O.M. (2005) - Focuses on the dynamic behavior of offshore structures, including VIV analysis and mitigation.
  • Marine Hydrodynamics: By Newman, J.N. (2018) - A comprehensive text on fluid mechanics with sections on the physics of VIV and related phenomena.

Articles

  • "A review of the development and implementation of VIV analysis and mitigation techniques" by Larsen, C.M. et al. (2016) - Offers a detailed overview of VIV research and its practical application.
  • "VIV Mitigation of Riser Systems: A Review" by Li, Y. et al. (2022) - Provides an up-to-date review of various VIV mitigation strategies.
  • "The Effect of VIV on Offshore Riser Fatigue Life" by Trivedi, S. et al. (2014) - Examines the impact of VIV on riser fatigue life and the importance of fatigue analysis.
  • "A numerical study of VIV suppression by fairings for a riser" by Wang, X. et al. (2015) - Demonstrates the effectiveness of fairings in mitigating VIV through numerical simulations.

Online Resources

  • Offshore Technology Research & Development (OTRC): https://www.otrc.com/ - An online resource offering a wide range of articles, reports, and data related to offshore engineering, including VIV.
  • American Society of Mechanical Engineers (ASME): https://www.asme.org/ - Offers technical resources and publications on VIV and related topics, including standards and codes.
  • Society for Underwater Technology (SUT): https://www.sut.org/ - Provides information on underwater technology, including research and developments related to VIV mitigation.
  • The International Energy Agency (IEA): https://www.iea.org/ - Offers insights into the global energy sector, including reports on offshore oil and gas production and associated challenges like VIV.

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