Fatigue Testing
Overview
Although it is not common to carry out a fatigue analysis, it should be done. Fatigue can lead to major damage or even loss of the installation. The purpose of the fatigue analysis is to determine the fatigue life time of the construction. If the life expectancy is not enough the design has to be changed. Fortunately fatigue problems can mostly be solved with minor design changes.
A detailed schematic is given below.
Five steps
The normal general fatigue analysis is divided into five steps.
The first step is to choose which parts of the construction will be analyzed. It is impossible to check all construction parts so a choice of the most critical parts is made. Experience and regulations of class societies can play a major role. It is generally accepted that when critical points will have a positive result, all other construction parts will have a longer fatigue life time.
The next step is to determine which method will be used. There are three accepted methods used:
The third step is to determine the stress levels and the changes in the stress levels. Also the number of cycles during the expected life time of the ship has to be determined.
Following from this step is the choice of the right SN-curve. A general example is shown below.
The last step is to determine the fatigue life time of a construction part or the inverse: the taken damage in a period of time.
The methods
The first method which can be used is the crack failure method. This method is rarely used in design phases of constructions. It starts with a small initial crack. The origin of the crack is unknown and can be caused by different ways. If the energy of the stress level change is above a certain value the crack will grow. When the crack has reached a critical length the construction part will fail.
The most accepted method is the nominal stress method. This method is the most simplified of the three and so the most cheap method. For all normal construction parts SN-curves are made. This method will become more inaccurate if the construction parts differ from the standard construction parts with given SN-curves
The third method is the hot spot stress method. This method is mostly used for jacket and tower constructions. The most important aspect is to know where the spots with stress level concentrations are. This method uses a reference stress level for the hot spots to determine the crack grow.
Your system has been checked for fatigue now, but should be scale tested. The procedure that has been followed has been thorough, but model testing can provide even more valuable data that can not be gathered in any other way. It also should help bring the unforeseen to the surface.