Remnant life assessment of ship unloader structure
The main goal is to perform study of the possibility of the further exploitation of the Ship Unloader crane manufactured in 1979.
The biggest problem was the absence of the drawings for the crane, so the usual Finite Element model of the whole crane was not be able to be used.
Because of this fact the unique approach has been used as follows Find the most critical elements of the crane structure that limit the service life of the whole crane. For this visual inspection and coercive force metods have been used For the found critical elements the FE models were created.
The Boundary conditions for the models are taken from the strain gauges. The stress was measured during crane operation of ship unloading.
Scope of work
Ship unloader, conveyors and hopper structural, mechanical survey (visual, vibromeasurements etc.)
Structure accumulated fatigue damage measurement based on coercive force method Forestay force distribution measurement
Stress measurement for the critical crane elements during its operating
Test of the crane structure samples (impact toughness test, fatigue test and tensile test)
FE analysis (crack growth modelling) of the critical elements where the measured stress was used as a boundary conditions and material properties are taken from the test. Remnant life calculation based on FE analysis.
Ship unloader, conveyors and hopper structural, mechanical survey
Structure accumulated fatigue damage measurement based on coercive force method
During normal exploitation of the material handling equipment the stress level is quite low. That is why the critical for them is the fatigue strength. For the fatigue the crack could appear on the very last stage and lead to the collapse of the whole structure. Before this the material properties change under the applied stress which causes the degradation of the material properties (i.e. material becomes more brittle and is not able to restrain the crack, density of micro defects increases etc.)
Coercive force is the method of the NDT that allows to find the material properties degradation and make a conclusion about the current state of the structure. Based on the measured values it is decided about the operational mode: safe mode, under control operation mode, critical operation mode.
Forestay force distribution measurement using strain gauge method
The consequence of crane operating with unbalanced forestay system causes the rise of the stress in one part of the structure in comparison with the design stress. Based on the level of unbalance the consequences could be as follow
- low value of unbalance (normally up to 10%)– due to the safety factors the increased stress are still within the permissible band. In this case no consequence occurs;
- average vale of unbalance – leads to decreasing of fatigue life;
- severe value of unbalance – crack appearance and propagation during short period of time, local plastic deformations of structure;
Stress measurement for the critical crane elements during its operating for using as the Boundary Conditions for FEM
- boom, between boom/girder hinge and boom/forestay hinge;
- girder, above the hoper;
- water side Trolley Girder Support Beam (TGSB), middle part.
Test of the crane structure samples (impact toughness test, fatigue test and tensile test) for using the real degraded metal properties in FEM
FE analysis (crack growth modelling) of the critical elements
Remnant life estimation for water side TGSB