Shell NZ Flexible Flowline Stability Design

Concrete mattress spacing and quantity were optimised whilst ensuring that the flexible flowline’s allowable axial forces, bending moments and stresses will remain within allowable limits.

Flexible flowlines usually have a low specific gravity and often require secondary stabilization to satisfy the absolute stability requirements of DNVGL-RP-F109. Concrete mattresses are commonly used at a designed spacing to achieve absolute stability requirements. However, using concrete mattresses can present another challenge as the flowline may deflect between the concrete mattresses under the design metocean conditions, resulting in localised high stress and bending moment concentrations. This may affect the integrity of the flowline as the axial forces, bending moment or flowline displacements between the concrete mattresses may exceed the allowable limits.