Reliability analysis of Tension-Leg Platform Tendon with Respect to Fatigue Failure under Environmental Condition of Caspian Sea




The primary objective of this paper is probabilistic quantification of the fatigue life of tension-leg platforms (TLP) using reliability methods. The need for such methods stems from the significant uncertainty in the loads exerted on offshore structures. The scope of this paper is limited to the study of fatigue in TLP tendons. For this purpose, nonlinear time-history of force response of the TLP tendon under random-wave load is computed via MOSES software and the damage due to fatigue is estimated in accordance with the Palmgren-Miners rule. Assuming a Rayleigh distribution for stress variation and eight different sea states, the ultimate fatigue damage is computed by accumulating the damage over all individual sea states. This cumulative damage enters the limit-state function that is based on the Palmgren-Miners rule. Prevailing sources of uncertainty in this problem are those in the estimation of fatigue stresses, fatigue strength, and the Palmgren-Miners rule. Finally, reliability analysis is carried out for four different service lives using the first- and second-order reliability methods (FORM and SORM) and Monte Carlo sampling. The results indicate that FORM computes the probability of failure sufficiently accurate. It is concluded that the probability of failure increases drastically with the service life. The importance vector from the sensitivity analysis in FORM reveals that the model error is the most influential source of uncertainty on the probability of failure.


[1]Chatterjee PC, Das PK, Faulkner D. (1997). A hydro structural analysis program for TLPs. Ocean Engineering; 24(4):313–34.
[2] Lotsberg I. (1991). Probabilistic design of the tethers of a tension leg platform. J Offshore Mech Arctic Eng.; 113:162–9.
[3] Banon H, Harding SJ. A methodology for assessing the reliability of TLP tethers under maximum and minimum lifetime load. Proceedings of the fifth international conference.
[4] Amanullah M, Siddiqui NA, Umar A, Abbas H. (2002). Fatigue reliability analysis of welded joints of a TLP tether system. International Journal of Steel and Composite Structure. 2(5):331–54.
[5] Khan RA, Siddiqui, N.A., Ahmad, Suhail, (2006), Reliability Analysis of TLP tether under Impulsive Loading, Reliability Engineering and System Safety 91 73-83.
[6] Siddiqui, N.A., Ahmad, Suhail, (2001). Fatigue and fracture reliability of TLP tethers under random loading. Marine Structure. 14, 331–352.
[7] Gerhard E., 2005. Assessment of existing offshore structures for life extension, Doctoral Thesis, University of Stavanger.
[8] Golafshani, A.A., Gholizad, A., (2009). Friction damper for vibration control in offshore steel jacket platforms. J. Constr. Steel Res. 65 (1). January, Elsevier.
[9] Tabeshpour M.R., (2007). Optimization and nonlinear dynamic analysis of tension leg platforms. Doctoral thesis, Sharif University of technology.
[10] Wirsching PH. 1984. Fatigue reliability for offshore structures. J. Struct Div ASCE; 110(10): 2340-56.
[11] Siddiqui, N.A., and Ahmad, S., (2000). Reliability analysis against progressive failure of TLP tethers in extreme tension, Reliability Engineering and System Safety, Vol. 16, pp. 195–205.
[12] Mazaheri, S., Haji Valiei, F., (2012). Wave Atlas Preparation for Persian Gulf, Oman Sea and Caspian Sea, Iranian Institute of Oceanography and Atmospheric Science.
[13] Golshani, A.A., Chegini, V., Taebi, S., (2005), Analysis of extreme wave and wind with different directions for Caspian Sea, Persian Gulf and Oman, Technical report, Tehran, Iran,.
[14] Tan S., and Gie, (1981). The wave induced motion of a tension leg platform in deep water, 13th annual OTC Houston, USA.
[15] Mahmoodi, M.R. (2017) Stability and dynamic response of tension leg platform in damaged condition (tendon removed) (experimental and numerical). Master thesis, Sharif University of Technology. Iran.
[16] Mahsuli, M., (2012). Probabilistic models, methods, and software for evaluating risk to civil infrastructure. Doctoral thesis, University of British Columbia.
[17] DNV Classification Notes 30.6, (1992), Structural Reliability Analysis of Marine Structures.