The air gap under offshore platform decks is an important design factor, and predicting the minimum air gap in harsh environments and the probability of wave crests impacting the underside of the deck is a challenging task.
The first application of the new method is TLPs (Tension Leg Platforms). The wave field under the platform deck is disturbed by the structure. Although the air gap under irregular wave conditions can be accurately determined through a well-designed model or CFD test, it is difficult to obtain reliable estimates for the extreme response. Due to nonlinear effects in steep sea states, data from many realizations of a given sea state are often required to obtain robust estimates. In many cases, therefore, only one or a few three-hour realizations are simulated in the model basin.
There is thus a need to develop new methods to improve the estimates obtained from a limited data set.
MARINTEK has adopted a new approach, based on utilizing the ACER (Average Crossing Exceedance Rate) method that allows accurate prediction at extreme response levels. Model test results of the air gap underneath the deck of a TLP in extreme random sea were used as case study data to benchmark the statistical method.
A second application is jacket-type platforms. Unlike TLPs, jackets are relatively transparent to the waves, so that the wave field can be assumed to be undisturbed and therefore simulated numerically. The ACER method was applied to Monte Carlo-generated data.
For both TLP and jacket-type data ACER performed extremely well, which makes them of high engineering importance. The new method efficiently utilizes experimental or Monte Carlo data, and it is highly competitive with regard to existing methods of estimating design values, such as Weibull, Gumbel or Pareto.
This project was presented in MARINTEK Review No 1 - 2011