Dropped Object Impact Analysis of Subsea Pipelines
Abstract
This thesis investigates the structural behavior of an impact between a subsea pipeline and atank with a handling frame. The primary focus has been on comparing the different thicknessesof the pipeline wall and different types of soil. Additionally, the results have been comparedwith practices given in design codes and methods proposed by other researchers to evaluate howthe simulations align with previously established models.
The approach for performing the simulations has been to use numerical analysis with the explicitand implicit nonlinear finite element software LS-DYNA. A total of 11 simulations were carriedout to capture the differences in the pipeline’s structural response due to different propertiesfor the seabed and pipeline configurations. Pipelines with nominal diameters of 50.0 mm, 25.4mm, and 12.7 mm were assessed. For the soil types, natural sand and clay have been chosen forevaluation and additionally, three simulations with a rigid seabed were carried out. In all cases,the nominal pipeline diameter has been 20 inches, which corresponds to 508mm.
The loading case has been the same for all simulations. A tank with a handling frame of mass7550 kg with an initial velocity of 6.3 m/s was utilized. This is equivalent to an impact energyof 149.8 kJ. In the simulations with natural sand and clay, the tank has been deformable. Inthe simulations with a rigid seabed, the tank has also been rigid. Two different tank positionarrangements have been executed. The dimensions of the tank were 2.95×1.85×2.3 meters.
The results from the rigid seabed simulations have revealed information about the impactcapacity of the pipelines, and the observations have been compared to data from DNV, whichindicate a good correlation to each other. The observations have also been compared to adeformation profile model proposed by T. Wierzbicki and M. S. Suh, and the model aligned wellwith the simulations, especially for the pipeline with a 50.0 mm wall thickness. The maximumdent depths obtained were 186.4 mm, 84.2 mm, and 25.2 mm for the 12.7 mm, 25.4 mm, and50.0 mm wall thicknesses, respectively.
In the simulations with natural sand and clay, a significant amount of energy has been dissipatedinto the soil and tank, resulting in small and negligible pipeline deformation. The vertical soildisplacement was largest for the clay and with an edge impact of the tank. In this case, thevertical displacement was estimated to be 140 mm. In terms of internal energy of the tank, themost critical case was obtained for the soil type natural sand with an edge impact.
In total, the work performed in this thesis has resulted in several findings and results thatare relevant for pipeline engineering. However, notable simplifications have been made, so theusability of the results on their own is limited. In addition, the alignment of the results to themodels and methods validates the results and makes the findings of the thesis reliable.