Fatigue in Offshore Structures

Offshore structures are exposed to cyclic mechanical loading; forces and moments, both environment (wind, waves) and service (operations, machinery) induced. The response in terms of stress, strain or energy is cyclic by nature accordingly, meaning fatigue, a cyclic loading en response induced local, progressive, structural damage mechanism is a governing limit state. To be able to assess the offshore structure integrity en longevity in this matter, course contents include:

Fatigue fundamentals, i.e. physics and damage process phenomena in:

• materials at defect locations (stress concentrations at micro- and meso-scale); different sources and different stages in the fatigue en fracture process (crack initiation, growth and propagation, fracture).

• structures at notch locations (stress concentrations at macro-scale) as part of structural members or at the member connections; particular attention will be paid to arc-welded joints in that respect, since marine structures are predominantly assemblies consisting of plates, shells and tubular members.

Macroscopic (notch) stress concentrations, distributions and intensities:

• as part of structural members.

• at (welded) structural member connections, consisting of a cyclic mechanical- and quasi-constant thermal residual part.

Fatigue resistance:

• materials (plane geometries)

• structures (notched geometries: welded joints) at small-, large- and full-scale

• testing (measurement techniques)

• damage criterion life time relations in different ranges (low-, mid- and high-cycle fatigue); formulations and parameter estimates using regression analysis (least squares- and likelihood approaches).

• short- and long crack growth relations.

Fatigue assessment concepts:

• damage criteria using an intact geometry parameter (nominal stress-, structural hot spot stress- and effective notch stress/ strain concept).

• damage criteria using a cracked geometry parameter (crack tip stress intensity- and equivalent stress concept).

Particular attention will be paid to analytical formulations and numerical approaches; concept capabilities and limitations.

Fatigue influence factors: material and geometry (strength, surface integrity, volume, welding improvement techniques), environment (temperature, corrosion), loading en response (min/max ratio, mean stress, amplitude variability (time domain analysis, cycle counting, spectral approaches, scatter diagrams, cumulative damage hypotheses), multi-axiality including proportionality (in-phase, out-of-phase).

Fatigue reliability for (risk based) inspection and maintenance purposes: time dependent limit state functions, (joint) design variable statistics, level 1, 2 and 3 analysis, 1st and 2nd order analytical reliability methods and Monte Carlo simulation.

Structure integrity en longevity: fatigue strength calculation and life time estimation for assessment and design using different (design) philosophies, concepts, rules, standards and guidance.