It is concerned with the fatigue strength of tendon porch found in TLP. Large-scale fatigue tests with models reduced to 30% of real structures have been carried out to investigate the fatigue behavior of API 2W Grade 50 steel recently produced by POSCO. Fatigue life for the present test models was obtained based on the concept of nominal stress. Comparison of the present test results with those obtained by numerical approach based on the structural analysis results has showed good agreement. The present results were also compared with the design curves in DnV RP-C203.
TLP (tension leg platform) is well known as the appropriate offshore structure type for deep sea due to its excellent motion characteristics, flexibility in operation and so on. Among structural members, tendon porch at which tendons (tension leg) are attached is one of the most critical members because their failure must cause disaster of the whole TLP structure. Due to this reason tendon porch is categorized as the special category of structural member. Typical example of TLP is illustrated in Fig.1, which is the pontoon extension type. Tendon porch is subjected to varying load due to various sources such as wave, current, wind, motion and so on under the initial tension due to excessive buoyancy.
Among these, since tendon porch is easily expected to be subjected to varying load, fatigue failure is the most important failure mode, which should be, therefore, primarily taken into account at the design stage. According to the structural design rule of DnV OS-C105 tendon porch is made of API 2W steel as seen in Fig.2. POSCO has recently developed API 2W Grade 50 steel. As a new material is developed, the fundamental mechanical properties of the base metal and its weldments are evaluated