- In the design of tension leg platforms, the slender vertical structural members known as tendons play an important role in the dynamic response behavior of the platform. In this study a general method which allows one to address the uncertainties associated with key design variables is discussed during the estimation of the systems probability of failure. Contours are then constructed to account for the uncertainties in the environment. These contours are then inflated, which allows the uncertainty in these environmental loading variables to be combined with structural uncertainties. These inflated contours account for the system uncertainties as a whole and can be verified experimentally. To illustrate the method the variation of TLP tendon pretension is examined and discussed.