ASSESSMENT OF FATIGUE RELIABILITY FOR JACKET-TYPE OFFSHORE PLATFORMS CONSIDERING DYNAMIC BEHAVOIR

In order to traditionally investigate the strength of marine
structures, the structure is subjected to a maximum static load.
However, the marine structures are usually suffering
environmental forces varying with time. Wave forces are the
most important time dependent loading that causes fatigue in
structural elements and joints. In this paper different methods
base on S-N curve and linear elastic failure mechanics are
presented. The governing equations and theories that are used in
each method are expressed and the application of each method
will be discussed.
The two main methods of deterministic analyses are: stressbased
approach (S-N curve approach) and linear elastic fracture
mechanics (LEFM) approaches. These approaches are
applicable to different analyzing strategies, ie the first approach
is used for cases in which general form of fatigue is dominant,
but the latter involves the calculations of reliability as functions
of crack geometry and its boundary conditions.
The SPD12C jacket platform is also modeled as a case
study and the results of fatigue reliability analysis are presented.
In this paper a comprehensive method is presented to
accurately predict the reliability of offshore platforms. This
method is based on S-N curve and the results are compared with
the fatigue life of joints.
Due to nonlinear interaction of soil and piles and the other
affecting parameters such as flexibility of joints, non Gaussian
procedure of loading, and nonlinearity of reaction force, the
precise analyzing of stress levels will be impossible and a
complex numerical analysis could only give limited information
about the statistical properties of stress. In order to perform the
fatigue analysis and predicting the cycles of stress SACS was
used which is known as a powerful software in designing and
analyzing offshore structures.
In this paper the whole structure was modeled subjected to
different forces such as wave and sea currents. The effects of
parameters such as marine growth and interaction of soil and
piles are also included. The latter is shown to have a significant
effect on determination of fatigue life of the platform.