Local Scour and Protection of F3 Offshore GBS Platform

In 1992 a GBS (Gravity Based Structure) was placed in the F3 block in the Dutch sector of the North Sea. Gabion mattresses around the GBS were applied to prevent scour. The potential scour and the required dimensions of the gabion mattresses were determined before the installation in small-scale physical model tests. After the installation of the GBS, annual surveys have been carried out to provide information about the scour development around the GBS and the status of the protection system. This paper presents field observations of local scour development around the F3 GBS platform and an evaluation of the performance of the scour protection system. The objectives were to assess the maximum scour around the GBS and to determine a cost-effective survey strategy. To assess the maximum scour around the GBS, metocean conditions for the F3 GBS location were hindcasted for the period between the platform installation and the latest survey. The observed scour depths were analysed and compared with predictions both from the model tests and from existing empirical formulas. The results showed that the predicted scour depths based on the model tests agree reasonably well with the field observations. The empirical formulas give larger scour depths than the surveys. Based on the field observations in combination with predictions of the models, the maximum scour depths under the design extreme environmental conditions have been determined. To determine a cost effective survey strategy, the behaviour and performance of the gabion mattresses were evaluated based on the annual surveys and video inspections. It was concluded that the protection system has functioned well and no damage was observed. On the basis of the maximum expected scour and the performance of the gabion mattresses, it was considered that the survey frequency may be reduced. Surveys and inspections may be carried out every three years instead of every year, and after a severe storm.