Flexible Line On-Bottom Stability Analysis

Objective:

The purpose of the on-bottom stability analysis is to ensure the flexible line is stable throughout its design life.

Summary :

Many Codes and Standards , and requirements have been developed to evaluate the stability of the flexible lines, the requirements/design standards to be used are determined by the many factors, the main ones are shown as below:

• Location of project (Country);
• Local authority requirements;
• Operator standards and requirements.

During FEED or BOD, the 2D on-bottom stability analysis is deemed sufficient. The commonly used design standard for on-bottom stability assessment is DNV-RP-F109 absolute stability approach. The absolute static stability design method is based on simple static equilibrium of forces. The flexible line is considered absolute stable if it fulfills all the following requirements:

• Vertical stability in water;
• Vertical stability on and in soil;
•  Lateral stability.

No further assessment is required if the flexible line is found to be absolute stable. 

On the other hand, if absolute stability of the flexible line is unable to achieve based on its mechanical properties, 3D on-bottom stability will be required during Detailed Design.   

Since absolute stability is a conservative requirement, the flexible line may not be necessary to be absolutely stable along entire route. It is acceptable as long as the 3D on-bottom stability conducted is able to demonstrate that the minimum bending radius (MBR) of the flexible line is not exceeded throughout its design lift under the most onerous hydrodynamic loadings.

Recommendations:

Secondary stabilisation is required if 3D on-bottom stability shows that the bending radius of the flexible line has exceeded its MBR under the design environmental load.

References:

DNV-RP-F109, On-Bottom Stability Design of Submarine Pipelines, October 2010.