Pipeline On Bottom Stability

Pipeline laid on the sea floor should be stable during installation, after installation, and during operation. If the pipe is too light during installation, it will be hard to control the pipe since it behaves like a noodle due to waves and current and installation vessel’s motion. Most installation contractors require a minimum 1.15 pipe SG (Specific Gravity) to avoid pipe buckling which may occur due to pipe’s excessive movement during installation.

After installation, before the pipe is filled with water or product fluid, the pipe should be checked for 1 year return period waves and current conditions. If the pipe is laid as empty for a long period before commissioning, a 2-year, 5-year, or 10-year return period metocean data should be used. During operation, the pipe should be stable for a 100-year return period metocean data.

The soil data is very important to estimate the pipeline on-bottom stability. If no soil data is available, use the following data for the pipe-soil lateral friction coefficients per DnV-RP-F109, On Bottom Stability of Offshore Pipeline Systems:

• Clay       0.2
• Sand      0.6
• Gravel    0.8

To keep the pipeline stable, the soil resistance should be greater than the hydrodynamic force induced on the pipeline.

μ(Ws – FL) ≥ (FD + FI)

Where:
FL = 1/2 ρ D CL (V^2)    –> Lift Force
FD = 1/2 ρ D CD V |V|    –> Drag Force
FI = 1/4 (D^2) π ρ CM A –> Inertia Force

μ is the soil friction coefficient; Ws is the pipe submerged weight (lb/ft); ρ is the water mass density (64 lb/ft^3); V is the near-bottom wave & current velocity; and A is the water particle acceleration corresponding to the V. The recommended lift, drag, and inertia force coefficient (CL, CD, and CM) is 0.9, 0.7, and 3.29 respectively.

Figure 1. Force on Pipe

The AGA pipeline on-bottom stability program is widely used by industries. The program has three modules:

• Level 1- Simple and quick static analysis using a linear wave theory and Morison equations as above, without accounting for pipe movement or self-embedment.
• Level 2 – Reliable quasi-static analysis using a non-linear wave theory and numerous model test results considering pipe’s self-embedment.
• Level 3 – Complicated dynamic time domain analysis using series of linear waves and allowing some pipeline movements. Compare the computed pipe stresses and deflection with allowable limits.

Level 2 is recommended for most cases. Level 3 can be used to predict pipeline movements especially for dense sand or stiff clay where the pipe embedment does not take a big role. However, level 3 takes a long computer running time and it is difficult to estimate how far the pipeline will move over the design life. Therefore, level 3 is not recommended unless small savings of concrete coating can affect the project cost significantly.
In level 2 analysis, it is noted that the vertical safety factor in the output should be treated as a reference use only. This is because the lift force is already considered in the horizontal stability check and the lift force is calculated based on the pipe sitting on the seabed. Once the pipe is lifted off the seabed, the water will start to flow underneath the pipe. The underneath flow velocity is faster than the upper flow, thus the underneath pressure is less than the upper pressure. This pressure differential tends to push the pipeline back to the seabed and drastically reduces the lift force.
The following methods (see figure 2) can be adopted to keep the pipeline stable on the sea floor:

• Heavy (thick) wall pipe
• Concrete weight coating
• Trenching
• Burial
• Rock dumping (covering)
• Concrete mattress or bitumen blanket
• Concrete block 

 Figure 2. Some of Pipeline On-bottom Stability Mitigation Methods

Sources :

Lee, Jaeyoung. Introduction to Offshore Pipelines and Risers. 2007. Page 83
http://www.piping-engineering.com/offshore-pipelines-design-activities.html

https://anakkelautan.wordpress.com/2014/02/01/pipeline-on-bottom-stability-design/