Defining a solid approach for designing a bridge landing against fatigue

Abstract

Offshore oil and gas projects usually require the presence of more than one facilities in the same location. These facilities need to be connected with each other in order to enable the transfer of personnel among each other. As a result, bridges are implemented for such purposes with their ends being positioned at extensions of the two connected platforms, known as the bridge landings.
Such a bridge should be able to follow the excitations that are imposed at its two ends from the response of the connected platforms due to the applied environmental loads. Thus, in its longitudinal direction, the bridge should be pinned-supported at one platform and sliding-supported at the other. Such a configuration enables the bridge to adapt to the continuously varying relative movement that is induced by the motion of the two connected platforms. This results in the generation of friction at the sliding end of the bridge.
Similarly to any other offshore structure, a bridge landing should be able to withstand the maximum operating loads and its configuration should be checked against the different limit states. Although a jacket substructure is commonly analysed against the serviceability, ultimate and fatigue limit states, a bridge landing is checked against only the first two states. However, the generated friction at the sliding bridge supports results in varying stresses at the corresponding bridge landing. This indicates that the fatigue limit state should also be examined and thus investigation is required in order to highlight its significance in the design of such a structure.
This is the motivation behind the certain thesis, which intends to clarify the sensitivity of a bridge landing into the varying dynamic load of the generated friction. In order to do so, a specific case is examined, with real information about the structure and the environmental details. The analysis comprises examining three limit states (SLS, ULS, FLS), concluding into the governing one for the case of the bridge landing. The structural analyses were performed using the SACS software, which enables performing all the SLS and ULS checks. Regarding fatigue, though, the whole analysis was conducted independently, using a simplified approach that enables to deal with the issue in a quick way. This comprises the base case approach, through which assumptions are made regarding the wave and friction main characteristics.
After verifying the significance of the fatigue limit state in the design, an assessment of the base case approach follows. This is performed through the examination of the main sensitivity parameters that influence the simplified approach through which the fatigue assessment was conducted. The results of the sensitivity analyses are then incorporated in order to review the method and conclude into any possible improvements.
Finally, enhancement of the structure is examined through four different ways, aiming to turn it to be sufficient against the fatigue requirements. The improvement actions consist of improving the existing weld details, modifying the existing structure and reinforcing of members.
It should be noted that the problem was also approached through a numerical approach that was generated using the Matlab software. Through this, it was intended to capture the behaviour of friction in a more realistic way before incorporating it in the fatigue assessment, something that was not possible to be done inside SACS. However, the model didn’t show rational results and thus it could not be used in the fatigue analysis. The whole procedure and theory, though, are described in detail since it is possible that they can set a useful background for further investigation.