Abstract

Wave-in-deck model tests were carried out with vertical load measurements, pressure measurements and a two camera high speed video setup. With the undisturbed surface elevation at every time step, the load transfer to a flat deck was calculated by analyzing the rate of change of the added mass in a von Karman approach. This approach was applied with three different methods of estimating the added mass. The comparison with the model test data showed that the main criterion to estimate the vertical impact load on a horizontal platform deck properly, is an accurate derivation of the added mass (wetted area). This is a problem, if no detailed pressure measurements or high speed videos are available. Therefore, a correlation analysis was conducted of the wetted area of the undisturbed wave and the wetted area derived from the pressure cell measurements, to see whether an empirical correction factor of the wetted deck area can be determined. This would than allow to estimate the vertical loading on a platform deck based on measured wave data. In this paper the results of the simplified loading model in comparison with the model test data are presented and discussed.

Introduction

To ensure margin against overload failures, design recipes for offshore structures typically requires to control two limit states:

  • Ultimate limit state (ULS) and

  • Accidental limit state (ALS).

ALS is traditionally meant to cover accidental type of loading, i.e. loads caused by explosions, fires and collisions. The Norwegian rules, however, require that ALS is applied also for environmental loads. Characteristic ALS loads are defined as loads corresponding to an annual exceedance probability of 10⁻4.

Newer platforms will usually be designed with an air gap sufficient to avoid wave impacts with a 10⁻4 annual probability crest. The challenge lies in older platforms designed prior to 2000, where a negative air gap may be experienced. The loading from a wave impact event can be very large and is very difficult to predict numerically.

The vertical upward force can be critical for local structural details. The wave force acts over a small area, leading to high local pressures. However, in case of e.g. a TLP it can also lead to tether overload and slack and therefore be a threat to structural integrity. In platform design it can therefore be essential to have a proper estimate of extreme wave impact loading.

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