12 Most Wanted Hydro-Structural Engineering Related Short Questions Answered

In this section I have tried to answer the most wanted questions of offshore structural design and Hydro Dynamics.

1. What are the Offshore Environment Challenges?

• Hs > 2m, up to 30 ~ 35m during storm (oil rig in storm)
• 4s < Tp < 15 ~ 20s
• Extreme wind velocities of 40 ms-1
• Water depth up to 300m 
• Current speed variation along depth
• Stationary platforms

Here, 

Significant Wave Height, H_s:

    Average wave height of 1/3 of highest wave.

Wave Period, T_p:

The wave period is the time for a particle on a medium to make one complete cycle.

2. Why it is important to know the wave period for a structure?

        If the natural frequency of the structure and natural frequency of incident wave are close the structure will show large amplitude of response. So, it will face vibration. As a result, the structure will face structural deformation. The natural frequency is related with the wave period. 

T ∝ 1/ω

So, if the period is higher the natural frequency will be lower and vice versa.

3. Why slamming occurs?

A slamming is characterized by a sudden high force of relatively short duration imposed on a body. This event occurs when a body enters a fluid with a small relative angle between the body surface and the fluid surface (forefoot emergence) and generates impact pressure on the body surface.

4. What happens if we add viscosity to the fluid?

Viscosity is a measure of a fluid's resistance to flow. If we add viscosity it will increase the resistance of the fluid to flow. 

5. Why knowing water depth is important for offshore structure?

 It can,

• Affect choice of platform (jacket, TLP, semi, mooring issues etc.)
• Create environment load issues (wave breaking, wave, particle speed)

6. When wave breaking occurs? 

    In deep water the movement of water particles are circular and the waves are not affected by bottom. In intermediate depth water waves feel bottom and steepen. In shallow water, the movement of the particle becomes elliptical and the velocity of the water particle is maximum. Hence, a deformation forms at the wave crest and amplitude reaches such a critical level that causes large amounts of wave energy to be transformed into turbulent kinetic energy. As a result, the wave breaks. 

Fig: Wave Breaking due to limited water depths

7. Why wave breaking is important?

            Wave breaking presents one of the most interesting and challenging problems of fluid mechanics. This is a phenomenon of rapid release of wave energy, and therefore the main source of dissipation for waves. Breaking waves impact on structures and vessels, on the coast and bottom; facilitate momentum, energy, gas, moisture, and heat exchanges across the air–sea interface; and produce bubbles and aerosols. Wave breaking is an intermittent random process, very fast by comparison with other processes in the wave system. Distribution of wave breaking on the water surface is not continuous, but its role in maintaining the energy balance within the continuous wind–wave field is critical.

8. How can we compute force using potential flow theory?

            For applying potential flow theory assume an irrotational, inviscid and incompressible fluid medium where ϕ is the velocity potential function.

Now we can measure velocity by taking time derivative of ϕ i.e. ∂ϕ/∂t

Plotting this velocity in Bernoulli’s equation we can determine pressure. If we integrate pressure we can get force.  

9. What is P - Δ (P - Delta) Effect?

The P - Δ or P - delta effect refers to the abrupt changes in ground shear, overturning moment, and/or the axial force distribution at the base of a sufficiently tall structure or structural component when it is subject to a critical lateral displacement.

To illustrate the effect, consider a case in statics, a perfectly rigid body anchored on the ground subject to small lateral forces. In this example, a concentrated vertical load applied to the top of the structure and the weight of the structure itself are used to compute the ground reaction force and moment. Real structures are flexible and will bend to the side. The amount of bending is found through a strength of materials analysis. During this side displacement, the top has changed position and the structure is experiencing an additional moment, P×Δ, or near the middle, P×δ. This moment is not accounted for in a basic first-order analysis. 

    Fig: P - Δ Effect 

According to figure,  

      Over Turning Moment due to P - Δ effect will be, Mov = F . d + P . Δ

Here, Δ is horizontal distance of load acting line and structure line

d = distance of point of action of environmental force from base.

F = environmental force.

10. Why should we study buckling for structure?  

In structural engineering, the term "deflection" is usually reserved for a member's displacement due to bending forces. Deflection in this manner is predictable and can be calculated. On the other hand, the lateral deflection caused by buckling is unstable. Once a member begins to buckle, any further load will cause significant and unpredictable deformations. That’s why it is very important to study possible buckling of a structure.

11. What is Vortex Induced Vibration (VIV) ? How does it occurs? What is the mechanism of VIV? 

Fig: Vortex Induced Vibration of a Cylinder

    Vortex-induced vibrations (VIV) are motions induced on bodies interacting with an external fluid flow, produced by – or the motion producing – periodical irregularities on this flow.

        It can be seen in cylinder type structures. When a cylinder moves through the water in the direction perpendicular to its axis, the flow around the cylinder will be slowed while in contact with its surface, forming a boundary layer. At some point, however, this boundary layer can separate from the body because of its excessive curvature. Vortices are then formed changing the pressure distribution along the surface. When the vortices are not formed symmetrically around the body (with respect to its midplane), different lift forces develop on each side of the body, thus leading to motion transverse to the flow. This motion changes the nature of the vortex formation in such a way as to lead to a limited motion amplitude. The Strouhal number relates the frequency of shedding to the velocity of the flow and a characteristic dimension of the body (diameter in the case of a cylinder). It is defined as, 

In equation f_st is the vortex shedding frequency of a body at rest, D is the diameter and U is the velocity of the flow.

Lock in Phenomenon: The phenomenon of lock-in happens when the vortex shedding frequency becomes close to a natural frequency of vibration of the structure. When this happens large and damaging vibrations can result.

12. What are the sources of Non-linear forces in offshore structure? 

Sum frequency & Difference Frequency

A simple way to illustrate the presence of non-linear wave effects is to consider the quadratic velocity term in Bernoulli's equation for the fluid pressure. We can write this term as –

Let us consider (V = V1, V2, V3)  is the fluid velocity vector. Now consider an idealized sea state consisting of two wave components of circular frequencies ω1 and ω2. An approximation for the x – component of the velocity can be written formally as – 

This means that we have the presence of a constant term represented by -0.5p (A₁² /2 +A₂² /2) and a pressure term which oscillates with the difference frequency ω1 - ω2.  Equation (i) also tells us that non-linear effects can create excitation forces with frequencies higher than the dominant frequency components in a wave spectrum. This is due to terms oscillating with frequencies 2ω1, 2ω2, (ω1+ω2). Here, the first term represents mean force, the 4^th term represents sum frequency force and the last term represents difference frequency force. These are the sources of non-linear forces in offshore structures.