**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

**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?**

**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?**

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

**6. When wave breaking occurs?**

**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?**

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.

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 –

_{1}

^{2}/2 +A

_{2}

^{2}/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.

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