Oil & Gas Pipeline Survey

Dynamic Lateral Stability

The objective of a dynamic lateral stability analysis is to calculate the lateral displacement of a pipeline subjected to hydrodynamic loads from a given combination of waves and current during a design sea state. On-bottom stability is a highly non-linear phenomenon with a large degree of stick/slip response. This is particularly important to keep in mind for large values of current to wave ratios and large wave periods, and more so for stiff clay and rock than for soft clay and sand where the build up of penetration and passive resistance is more pronounced.

Current Condition

The steady current flow at the pipe level may have components from:

• Tidal current
  

• Wind-induced current
  

• Storm surge induced current
  

• Density driven current
  
  

  Short Term Wave Condition

The wave induced oscillatory flow condition at the pipe level may be calculated using numerical or analytical wave theories. The wave theory shall be capable of describing the conditions at the pipe location, including effects due to shallow water, if applicable. The short-term, stationary, irregular sea states may be described by a wave spectrum Shh(ω) i.e. the power spectral density function of the sea surface elevation. Wave spectra may be given in table form, as measured spectra, or in an analytical form.


For instance, the JONSWAP spectrum, the spectral density function reads:

• Hydrodynamic force, consists of drag force and inertia force (can be calculated using morrison formula), as well as lift force. Lift force is a vertical hydrodynamic force. This would happen with the concentration of streamline on the pipe.
  
  

• Soil friction force, is a horizontal force influenced by friction coefficient between pipe and seabed. Value of the friction coefficient depends on the seabed soil characteristics. For example, friction coefficient for clay-soil is 0.2 and friction coefficient for sandy-soil is 0.6.