Integrating Beam Modelling with Solid 3D Modelling
To maintain the supply of gas to a Power Station whilst the modifications were implemented, a temporary by-pass arrangement was to be constructed including 2 off 900mm x 200mm split-tees, 1 off 900mm x 900mm split-tee, and 1 off weldolet connection within close proximity to each other. The AGI also had two existing 900mm x 450mm and 900mm x 100mm split-tees in close proximity of the proposed split-tees.
Due to the nature of the installation process, the non-standard geometry of the tees, and the close proximity of the existing tees, it was necessary to perform a detailed 3-dimensional finite analysis of the system to determine whether the stress levels would be acceptable.
Consequently, AFAA were commissioned to undertake a finite element analysis (FEA) of the pipework system taking account of the loads that will be present during:
Installation, taking into consideration the drilling machine and sandwich valve weights
Short-term operation, taking into consideration the temporary by-pass pipework
Long-term operation, taking into consideration the system loads and fatigue duty
The pipework beam model and 3D solid split-tee models were created separately within the pre-processor MSC.Patran. The 3D finite element models and pipework stress model were then assembled within a single model, and analysed together in the general finite element package ABAQUS.
Based on the soil restraint data available, the effect of the soil on the pipework was modelled using 3D Pipe-Soil Interaction (PSI) elements.
A single concrete support underneath the 900mm x 900mm split-tee had been proposed.
The support was shaped at the top to match the curvature of the split-tee, and located centrally on top of a concrete slab. Borehole samples close to the excavation site had confirmed that the soil conditions were very soft, and there was a possibility of the concrete slab displacing during the installation of the split-tees.
In order to mitigate any displacement of the slab during the installation process due to soft soil conditions, 16 off 100mm diameter steel piles were used.
The central concrete support, slab and pile foundations were modelled to determine whether the proposed supporting arrangement was adequate, and to determine how much displacement/settlement could be expected from the installation loads.
The piles were modelled using beam elements with the appropriate stiffness assigned.
The British Standards Institute document PD5500 was used to assess the split-tees for global, local, and incremental plastic collapse, cyclic fatigue, and the requirement for post-weld heat-treatment. In addition, the suitability of the concrete support was assessed.