The investigation concerns integrity check of the pipeline, considering possible falling scenarios as a result of the geohazard risk assessment. Information related to possible scenarios of falling rocks have been provided by SAIPEM and are summarized here below:

• Size of the boulders as a function to the temporal probability of fall.
  
• Kinematic data of each boulder:
  
  • The position of altitude and distance from the pipeline.
  • Kinetic energy as a function of the position.
  • Height of bounce.

On the basis of such information, it has been possible to define a sensitivity study so to verify the pipeline safety conditions, in relation to the scenarios, varying the following parameters:

• Boulder speed.
  
• Angle of impact on the ground.
  
• Weight of the rock.

Fig. 3 - Geometry isometric view

Fig. 4 - Geometry - Section view

Fig. 5 - Mesh

A parametric 3D CAD model has been elaborated. The model consists of a primary volume that represents the ground where the steel pipe buried is buried and backfilled and protected at the top by a concrete slab. The model is completed by a cubic boulder close to the ground surface. In Figure 3 and Figure 4 two images shown the environment that has been simulated, while Figure 5 show the mesh. In agreement with Saipem, a completely parametric model was generated, so to be able to quickly modify, the boulder size, pipeline burial depth and the backfilling soil mechanical characteristics, with the aim of identifying the most critical conditions. In addition the impact configuration is the most burdensome possible; as it can be seen from the images (Figure 3), the rock impacts on the long edge, so to investigate the worst impact conditions. In the initial phase the parameterization was used to define the length and the minimum width of the simulation environment, which is necessary not to have edge effects. In the subsequent phases, the geometrical parameterization was used to verify the improvements on the results, due to the variation of the pipeline burial depth.

The materials used to model the soil and the concrete have been obtained from ANSYS Autodyn database. For the pipeline the StE 445.7 TM has been adopted. Materials used are listed in the following table:

Tab. 1 – Materials and materials colour legend

The Figures 3 and Table 1 clearly describe how the materials have been assigned to the single components of the model. Each color refers to a single material as shown in Table1. For the fall line 7, 8 and 9 the concrete layer is not present, the material of this layer is switched into “soil landstone” as for the remaining soil around the pipeline. The rock is modeled as rigid.

Boundary Conditions

A fixed constraint is applied around and under the assembly (blue color in Figures 6); furthermore on the same faces (red color in the Figures 7), the impedance boundary condition has been applied; this setting permits to avoid the birth of reflected waves at the borders of the model; in other words the continuous waves beyond the constraint without being reflected. The contacts between each components are frictional with friction coefficient equal to 0.3.
In each analysis the Standard Earth gravity acceleration is applied and the rock has an initial velocity derived from the energy provided by the geohazard risk assesment report and a specific impact angle (Figures 8).

Fig. 6 – Fixed surfaces loads

Fig. 7– Impedance boundary

Fig. 8 - initial velocity applied to the rock

Fig. 9 - Total Deformation environment section 1

Fig. 10 - Total Deformation environment section 2

Fig. 11 - Equivalent Von Mises stress on the pipeline