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Suction

As part of Stride and Carisima JIPs, models have been developed to take into account suction effect on SCRs behavior. An example of suction effect on SCR bending moment is given in Willis and West (2001). Comparison of the lift and lay-down of a pipe is presented in the next Figure from Willis and West (2001).

Effect of suction on a pipe; comparison of the bending moment in lift and lay-down motion from Willis and West (2001).

For large displacements of the riser top, the suction can modify the bending moment. Some efforts have then been put into modeling the suction force. Bridge et al. (2004) suggests the representation of the force according to the schematic given in the next Figure.

Modeling of the suction force according to Bridge et al. (2004).

The maximum force is given by the following relationship: $$ \frac{F_{max}}{L}=\alpha_f v^{n_f} \quad and \quad \Delta_B=\alpha_D v^{n_D} $$

With :

  • \(v\) the pull out velocity

  • \(\alpha_f=\frac{1.12*D*C_u}{D^{n_f}}\). \(D\) is the line diameter and \(C_u\) is the soil cohesion.

  • \(\alpha_D=0.98*D\). \(D\) is the line diameter.
  • \(n_f\)=0.18 and \(n_D\)=0.26

The suction effect decreases for cyclic loadings and the dependency on the velocity suggests that it is more a load case for extreme events than for fatigue analyses

In Bridge et al. (2004), the behavior of a pipe submitted to large displacement is described in a full cycle of penetration and unloading with suction. The next Figure reproduces this cycle. The loading curve is a loading curve after breakout.

Loading and unloading of a pipe with suction as given in Bridge et al. (2004).

For the suction effect, six other parameters will be introduced, two parameters to compute the maximum suction force, two parameters to compute the suction depth, and two cut-off parameters. Both maximum force and depth are characterized by an type of law therefore two parameters for each quantity are required. The last two parameters will define the slope to reach the plateau as indicated in the next Figure.

Cut-off parameters \(x_{c1}\) and \(x_{c2}\) for suction.