Life Assessment

Origen combines stress measurement, finite element analysis and fatigue/fracture mechanics principles to asses the remaining fatigue life of structures and how/whether the life of structures can be economically extended.

Origen combines stress measurement, finite element analysis and fatigue/fracture mechanics principles to asses the remaining fatigue life of structures and how/whether the life of structures can be economically extended. This is particularly relevant when future capacity and modifications are being planned.

As opposed to the limited and inaccurate Miners rule remaining life approaches which have underlying limitations in their approach,  are highly dependent on the historical loading condition which are seldom known with any certainty (specifically where the structure has been modified), are based on data with significant scatter, and assume a crack free structure - fracture mechanics approaches allow the future performance based on the current condition to be assessed. 

Fracture mechanics based approaches highlight critical areas to be identified. Once these are known minimum inspectable flaw sizes; crack growth rates; and maximum acceptable flaw size can be determined. Based on this data inspection frequencies, methods, and control plans can be developed allowing the future performance of the structure to be managed. Assessment of the effect of actual corrosion degradation can also be evaluated.

Note:

From a design perspective the Miners rule approach

  1. accommodates the inherent uncertainty related to fatigue (by being conservative) and
  2. is good at preventing failure.

However, when used to assess the remaining life in a structure the approach is crude (the summation approach used in the Miners rule introduces factors of uncertainty of an order of magnitude (° n/N = 1 can vary in practice from 0.3 - 5)).

Typical analysis on ship structures parallel those detailed in 'Fracture Toughness of a Ship Structure' and 'Fatigue Prediction' (from www.shipstructure.org) would include the:

  1. Development of detailed weight loading distribution for the vessel (typically done in conjunction with naval architects).
  2. Development of buoyancy loading distributions for discrete wave loading (for various period and wave length).
  3. Development of a detailed Finite Element model of the vessel for global and local stress determination.
  4. Correlation of the Finite Element analysis stress results with strain measurements vessel in a measured seaway (wave rider buoy used for situations where the vessel is moored).
  5. Experimental crack growth rate determination of steel samples from the vessel.
  6. Fracture Mechanics assessment of certain structural details based on the stress ranges calculated from the above processes of Finite Element analysis, loading, wave spectra analysis, and material characterization.

Analysis of industrial plant and structures is undertaken based on a similar methodology. In cases where modifications are being considered analysis of the structure in the modified and unmodified condition allows the effect of the modification on the structural performance to be evaluated.