No abstract

The challenging development of new platform concepts and their installation in deeper water in more remote areas and more severe weather conditions require a direct feedback from offshore experience to design and engineering. Moreover the platform operation itself can be enhanced by utilizing the actual behavior of the platform and its environmental conditions.

The effects of operational loads and wind loads on offshore monopile wind turbines are well understood. For most sites, however, the water depth is such that breaking or near-breaking waves will occur causing impulsive excitation of the monopile and consequently considerable stresses and displacements in the monopile, tower and turbine. To investigate this, pilot model tests were conducted with a special model of an offshore wind turbine with realistic flexibility tested in (extreme) waves. This flexibility was considered to be necessary for two reasons: the impulsive loading of extreme waves is very complex and there can be an interaction between this excitation and the dynamic response of the foundation and tower. The tests confirmed the importance of the topic of breaking waves: horizontal accelerations of more than 0.5g were recorded at nacelle level in extreme cases.

In this paper the development of a monitoring system for hull fatigue of FPSOs is discussed. Fatigue assessment is subject to many uncertainties. Asset integrity can be improved by limiting these uncertainties. Continuous monitoring reduces these uncertainties, but requires extensive data analysis. Automated analyses are required to ensure the monitoring data is used in an optimal and consistent way. A joint effort of industrial partners has resulted in a smart data analysis system that shows fatigue accumulation in an easy-to-understand format. Sources of accelerated fatigue accumulation can be identified which enables rational actions to reduce lifetime consumption.

The paper is one of the series of papers about the Advisory Monitoring System for controlling the fatigue lifetime consumption of FPSO hulls. The system has been developed within the Monitas Joint Industry Project (JIP). The name Monitas stands for Monitoring Advisory System. A key factor for proper lifetime prediction is an accurate assessment of the ocean surface wavefield. Therefore, a dedicated wave system analyses tool (XWaves) has been developed within the project that allows for online analysis of the measured wave data. The Monitas project recommends use of navigational radar for measuring waves. This paper compares the wave data obtained from such radar with that obtained from a wave buoy. The differences in obtained wave data from both instruments are illustrated and explained. The effect of these differences on fatigue lifetime consumption has been quantified. The paper also investigates how different wave data formats which are being used by the offshore industry affect the fatigue lifetime calculations. All comparisons and conclusions are based on real data collected from the Monitas system installed on board FPSO Glas Dowr. It has been concluded that navigational radar can be used as the instrument for wave measurements and that different wave data formats are acceptable providing the wave directionality data is preserved.