Fatigue properties of tension leg tendon: A full-scale approach

The tension leg platform is a typical compliant platform that is connected to the seabed through tension leg tendons. However, it is hard to characterize tension leg tendons due to the complexity of their force and motions as well as the lack of full-scale test methods. We performed a finite element analysis and full-scale four-point bending fatigue tests on tension leg tendons and connectors to study the fatigue properties of the tension leg tendons (made using 36in-X70 steel pipes) used in the Gulf of Mexico. The maximum deflection and the maximum stress of samples under complex loading were estimated through finite element simulation to ensure the testing requirements, including load intensity, load method, load path, and frequency. The maximum equivalent strain and the corresponding position were then determined through testing, which were further compared with simulation results to verify their accuracy and applicability. The maximum strain amplitude from simulations was 761.42 με, while the equivalent strain amplitude obtained through tests was 734.90 με, which is close to the simulation result. In addition, when the number of fatigue cycles reached 1.055 million, sample damage did not occur. It confirms that the fatigue performance of the tendon steel pipe weld is better than the C1 curve value shown in the DNV RP C203 specification. The proposed full-scale approach to study the fatigue properties of tension leg tendons can provide a reference for domestic engineering design and manufacture of tension leg tendons as well as promote the localization of test equipment.