Determining Condition of the Ship-to Shore Container Crane
The goal of this project was to determine the possibility and conditions of further operation of the ship-to-shore crane manufactured in 1976,
The following work has been done:
- Inspection of the boom steel structure using coercive force method.
- Structural analysis for the crane boom for different trolley positions.
- Stress alteration measurements for the boom structure critical points.
- Residual life assessment of the crane boom using the information about the material properties degradations of metal (data in par. 1) and loading conditions (data in par. 2-3).
Inspection of the boom structure using coercive field strength method
In case of time-varying loads of steel structures, metal properties degrade. Metal becomes brittle and its structural performance degrades.
There is a threshold for degraded metal properties, below which the steel structure cannot longer resist operating stress and will be cracked, which leads to an accident.
Inspection of steel structures using coercive field strength method allows to control the degradation level of mechanical properties of metal.
Advantages: it is a non-destructive technique allowing measurements through paint coating.
Structural analysis for the boom in the case of crane operatingIn order to assess the boom residual life the following expression was applied: Initial life (according to design) = expired life + residual life.
The residual life was determined using coercive force approach, and made up 48% for base metal and 32% for reinforcement metal in the critical element.
However, the Customer wishes to know the number of working cycles left till the end of the crane’s residual life, but not the percentage value.
Thus, it is necessary to know stresses in the boom structure during the crane operation.For instance, a heavy loaded crane, the elements of which are under high stresses, will use up 32% of the remaining life in 4 years, whereas a crane subject to medium stresses might operate for up to 8-9 years.
To determine the stress altering in the crane critical elements during operation a model of the crane boom was created, and its static strength was calculated using the finite element method.
As an example, the figure shows stress distribution in the ship-to-shore container crane boom, when a trolley is located between the boom-girder joint and boom-forestay joint.
Elements with low stress level have blue colour.
Elements with high stress level have red colour.
To determine the accuracy of finite element boundary the boundary conditions the stress variation for the critical joints was measured during the crane typical working cycle.
Difference between results obtained using the structural analysis and experimental methods may be due to the crane mounting (uneven distribution of forces between the crane the elements), poor condition of rail connections (shocks during trolley movement), etc.
In the present case an error between calculation and measurements did not exceed 12.7%, which is an adequate result.
Residual life assessment for the Ship Loader structure
Residual life of the Ship Loader residual life is determined based on coercive force parameter.
As a result of the inspection the following elements have been found elements without residual life. In case of further operation of the Ship Loader there is a risk of intensive crack initiation and propagation in these elements with further accidents.
Life of the Ship Loader structure main elements has been determined.
- Based on results of the inspection using coercive force method, the following was determined:
- Base material of the ship-to-shore boom structure, which is in operation since 1976, is in “monitored operation” mode.
- At the same time, material of reinforcements installed 4 years ago prior to inspection, is in “restricted operation” mode.
- To determine the reason of this situation the given assembly was calculated. It turned out that the reason was poor reinforcement design.
- As a result of Structural Analysis it was determined that in case of crane operating with load up to 40 tones the stresses in the boom structure are so low that they do not cause accumulation of fatigue damage in critical areas.
Thus, in the given case the key damage mechanism is corrosion.