Assessment of Marine Pipelines Subjected to Impact from Dropped Objects

Abstract

When damage assessment is needed for an operating pipeline due to impact with an accidentally dropped object, DNV standards treat this case with conservatism and thus fail to give a realistic estimation. Usually, damage is measured as the dent deformation that the pipeline will experience. Depending on the size of the dent, leakage, rupture or cease of production might occur. Thus, it is important to quantify how a pipeline behaves and interacts with its environment during an impact with a dropped object.
Initially, a simple finite element model has been developed in order to verify some laboratory experiments from Karamanos and Gresnigt that have been conducted under quasi-static conditions, where no inertia or velocity need to be taken into account. The falling object’s geometry, external or internal pressure and different material models have been investigated in order to derive preliminary conclusions regarding the stiffness of the system and the shape of the dent.
Next, velocity and mass of the indenter and of pipeline are taken into account in order to simulate the previous experiments dynamically. It has been observed that there are significant differences when inertia is taken into account in the denting behavior of a pipeline for low-velocity impact scenarios. Moreover, the effect of strain-rate sensitivities of steel have been incorporated by using the Cowper-Symonds law and their importance is stressed out in the results especially for mild steel pipelines.
In an effort to model closer the reality, simplified fluid models have been created using both the Lagrangian approach and the acoustic element formulation. This way, the partial incompressibility of the fluids, their inertia and their pressure can be modeled more accurately in order to reach valuable conclusions as to how they contribute in the system behavior.
All the aforementioned analyses have been conducted under the assumption that the bed upon the pipeline is resting is completely rigid. However, in reality the pipeline rests on a soil bed which is flexible and deformable. This is the most significant aspect of this thesis. Specifically, the energy dissipation due to the soil deformation and the pipe penetration into the soil is investigated. A soil – structure finite element model has been developed, considering a simplified Mohr-Coulomb elastoplastic model of failure which is adequate to obtain a good estimate regarding the soil contribution. It has been shown that for a range of different soil profiles of clay and sand, the energy dissipation is significant resulting in decrease of the dent deformation compared to a rigid bed case. Sensitivity analyses have been carried out regarding the impact velocity, mass and the initial embedment of the pipeline into the soil where it is shown that for the same kinetic energy input different results are being derived.
A final model is considered, where pressure, soil and strain-rate of steel are combined. The system behavior can be explained based on fundamental physics which give additional confidence in the interpretation of the results. Useful conclusions are derived in the end, showing that in many cases current practice is over conservative when assessing damage from dropped objects and thus a more detailed analysis and approach should be used in the future when conducting a risk or integrity assessment.