Fatigue life predictions have been undertaken for many structures, including complex castings and welded structures.
The approach used is dependent upon the type of structure, and the complexity of the loadings to be considered. Eatec has significant experience of assessing structures using FE-Safe, BS 7608, Eurocode 3 and Eurocode 9. Brief details of three projects are given below.
Assessment of ship berthing fenders
As part of a full design assessment, a fatigue life prediction had to be undertaken for ship berthing fenders. The fenders were fabricated steel structures, and the assessment was performed in accordance with BS 7608. Working closely with the client, fatigue loading spectra were determined to represent the berthing and moored conditions. Finite element analyses of the fenders and supporting structures were performed, along with detailed sub-modelling of regions where the peak stress ranges occurred.
As with most welded structures, the lowest fatigue lives were in welds. Using the results from the initial assessment, design changes were made to the geometries in the regions where the lowest fatigue life had been predicted.
Following the optimisation of the design, the required fatigue life was achieved, along with the strength needed to withstand the extreme loadcases.
Fatigue life prediction for a bolted joint in an offshore skid
In order to reduce production costs, a client intended to split a one-piece skid into two pieces which would be bolted together. The skid was to be used offshore on an FPSO, and hence subjected to accelerations due to ship motion as well as operational loads. The joint flanges were reinforced by webs and stiffeners, resulting in a number of welds in the regions of the joint. An assessment was required to determine the fatigue life of the welds and the regions around the joint.
A finite element analysis was performed to determine the stress levels, and the fatigue life prediction assessment was performed using FE-Safe and the Verity method, and also BS 7608, Code of practice for fatigue design and assessment of steel structures. The assessment identified the fatigue life of the joint, and the welds which had the shortest life. The FE analysis provided an understanding of how the joint behaved, and based on this it was possible to identify design improvements to increase the fatigue life.
Design and assessment of a test fixture
A fixture was needed to support a new suspension system whilst it was undergoing physical shock and vibration tests. To enable the testing to be performed using a particular actuator, the mass of the fixture was limited to a relatively low value.
Eatec undertook the design of the fixture, and performed finite element analyses to confirm that the fixture had an adequate fatigue life such that it would not fail during the vibration testing, and had sufficient strength to withstand the shock loads.
The fixture was designed as a welded structure, and hence particular attention had to be paid to the fatigue life of the welds under the vibration inputs. The fatigue assessment showed that the design had a life well in excess of the required test period.
Additional dynamic FE analyses of the fixture and suspension system were performed to determine the response to shock inputs, and these showed that the peak stresses were significantly below the yield stress of the material.