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.