JNTU B.Tech III PIPELINE ENGINEERING Previous Question Paper (November/December - 2020 - R15)

Offshore pipeline laying differs significantly from onshore operations due to the challenging marine environment, which involves hydrostatic pressure, hydrodynamic forces from currents and waves, soil instability on the seabed, and the requirement for specialized vessel-based operations. Unlike onshore laying, where equipment operates on stable ground, offshore laying requires dynamic positioning or anchoring systems to maintain vessel stability and precise pipe placement while managing the tension and bending stresses imposed on the pipeline as it transitions from the vessel to the seabed.

The primary methods used for offshore pipeline laying include:

• S-Lay Method: The most common technique where the pipe is welded horizontally on the vessel and leaves the vessel in an 'S' shape. A stinger (a support structure) is used to control the curvature of the pipe as it enters the water to prevent buckling.
• J-Lay Method: Used for deepwater applications where the pipe is assembled and lowered in a near-vertical position, forming a 'J' shape. This minimizes the bending stress on the pipe, making it ideal for deep-sea environments.
• Reel-Lay Method: The pipeline is pre-fabricated and spooled onto a large horizontal or vertical reel at an onshore base. The vessel then unspools the pipe at sea. This method is highly efficient for smaller diameter pipelines and rapid installation.
• Tow-in Methods: Pipelines are fabricated onshore in long strings and then towed to the offshore location. Variations include Surface Tow, Sub-surface Tow, Off-bottom Tow, and Bottom Tow, depending on the buoyancy control and seabed conditions.
• Directional Drilling (Shore Approach): Often used for the landfall section, where a pipeline is pulled through a pre-drilled tunnel under the seabed to protect the pipe from surf zone wave action and coastal erosion.

The general flow equation for compressible fluids (gases) in a pipeline under steady-state conditions is derived from the mechanical energy balance equation. For a horizontal pipeline of length L and diameter D, the differential form of the energy balance is given by:

Assuming the fluid follows the ideal gas law, PV = ZRT / M, where Z is the compressibility factor, R is the universal gas constant, T is the temperature, and M is the molecular weight. Substituting V = ZRT / (MP) into the energy balance equation:

Using the continuity equation, the velocity v can be expressed in terms of the mass flow rate w and the cross-sectional area A (v = w / (ρA)). Since ρ = MP / (ZRT), we have v = wZRT / (AMP). Substituting this into the energy equation:

Rearranging the terms to separate the variables P and L:

Integrating from the inlet (P1, L=0) to the outlet (P2, L=L):

Solving for the mass flow rate w:

This equation relates the mass flow rate to the pressure drop, pipeline geometry, and fluid properties. For practical pipeline engineering, the friction factor f is often replaced by the transmission factor or calculated using the Weymouth or Panhandle equations, which account for the variation of the friction factor with the Reynolds number.