Estimating extreme tendon response using environmental contours

The environmental contour technique was used to estimate the extreme in-line responses of deep-water TLP tendons designed for the Gulf of Mexico. The simulated response estimates were then used to estimate failure probabilities and reliabilities utilizing a deterministic displacement limit state. The reliability of individual tendons and tendon systems is directly associated with their respective probabilities of failure. By designing for environmental contours identified using this technique the resulting design will be more likely to approach the intended target reliability. In this article the environmental contour theory is explained and then used to estimate the extreme tendon responses in two examples reflecting practical design uncertainties. Experimental data from large scale model tendon experiments was introduced in order to assess the numerical prediction. In the first example the problem of uncertainty associated with pretensioning of the individual tendons is investigated. Although the amount of uncertainty due to the change in tension is not known the use of contour inflation is illustrated as a means to compare the numerical prediction with the experimental data. The second example addresses the uncertainties associated with the fluid/structure interaction. The placement of tendons in close proximity results in the amplification of the tendon motions. At present, no adequate hydrodynamic model exists which can be used with confidence in design practice. Again contour inflation is explored as a means to compensate for this phenomena and to quantify in a global sense the impact of this uncertainty on design.