Pipeline On Bottom Stability
Wednesday, February 17, 2016
Pipelines installed on the seabed are subjected to hydrodynamic forces. Waves and steady currents that are characteristics of all offshore areas subject the pipeline on the seabed to drag, lift, and inertia forces. For lateral stability, the pipeline resting on the seabed must resist these forces and at a minimum be at equilibrium.
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 DnVRP- F109,
On Bottom Stability of Offshore Pipeline Systems:
- Clay 0.2
- Sand 0.6
- Gravel 0.8
In general, the larger the
submerged weight, the higher the frictional resistance. However, later methods
for determining the stability include the depth of embedment of the pipeline.
Additional resistance is provided by the soil and, therefore, reduces the required
submerged weight of the pipeline.
As the pipeline is resting on the
seabed, soil characteristics play an important role in the lateral, as well as
the vertical, stability of the pipeline. The importance of vertical stability
of the pipeline is mainly in buried pipelines in soils with a high liquefaction
potential.
Software for pipeline on-bottom
stability analysis is AGA. 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 selfembedment.
- 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 deflections 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.
The following methods 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
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Sumber:
Lee, Jaeyoung. Introduction to
Offshore Pipelines and Risers. 2007.
Guo, Boyun, Shanhong Song, Jacob
Chacko, Ali Ghalambor. USA. 2005.
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