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.

Secondary Stabilisation Design

One of the means to provide stability to the unstable flowline, pipeline and umbilical is by laying the concrete mattress above it.

The concrete mattress sizing and required spacing will be determined based on the following assessments:

• Mattress vertical edge stability check - to ensure mattress edge is not lifting due to vertical hydrodynamic loads;
• Mattress lateral self-stability check - to ensure mattress is not wash-offed due to lateral hydrodynamic loads;
• Pipeline pull-out check - to ensure there is no mattress dislodging from pipeline due to lateral active pipeline forces active on the mattress;
• Mattress and pipeline sliding together (system stability check) - to ensure mattress submerged weight is sufficient to prevent lateral movement of mattress and pipeline;
• Pipeline local buckling check - to ensure integrity of the pipeline located between the mattresses throughout design life (FE dynamic analysis will be required).

The mattress stability assessments as described above are based on static force balance method. One of the references for the concrete mattress stability design is:

• SAER-5711, Submarine Pipeline Engineering Guidelines, July 2000.

On-bottom Stability

Submarine pipelines stability is very important to ensure that pipelines subjected to environmental loadings stay acceptably stable resting on/buried under seabed. In other words, pipelines submerged weight must be more than hydrodynamic force to avoid flotation or lateral movements.

******************************DNV-RP-F109******************************

Design Methods:

• Vertical stability in water
• Vertical stability in/on soil
• Dynamic lateral stability analysis
• Generalize lateral stability method
• Absolute lateral stability static method

Download:
DNV RP F109

******************************DNV-RP-E305******************************

Design Methods:

• Dynamic method ( dynamic simulation of pipelines under the action of waves and current)
• Generalized method (use of a set of non-dimensional parameters to generalize Dynamic Analysis Results)
• Simplified Static Stability method (quasi-static equilibrium approach) SUITABLE for most of the design cases
  

Download:
ON-BOTTOM STABILITY ANALYSIS AND DESIGN OF SUBMARINE PIPELINE : Mohd. Ridza (THESIS)

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