Wave Measurements for the Monitas System

Abstract

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. Introduction Offshore installations such as FPSOs are continuously subjected to wave loading which results in fatigue damage leading to costly repairs. Often operators are unprepared for such failures because the actual conditions encountered during operations may be quite different than those used in the vessel design. The Monitas Joint Industry Project (JIP) aims at changing this situation through the development and demonstration of an Advisory Monitoring System (AMS) for FPSOs (L'Hostis et al., 2010; Kaminski, 2007). This system is further on referred to as the Monitas system (Monitoring Advisory System). Accurate assessment of the ocean surface wavefield climatology experienced by the FPSO is a critical component of the Monitas system. Robust wave analysis routines in the Monitas system have been developed to decompose the observed wave field into individual wind sea and swell wave components (Hanson, 2009). The isolated wave components are converted to conventional spectral forms such as the JONSWAP energy-frequency spectrum (Hasselmann et al., 1973) with a cos 2n directional distribution which are used in the design. The resulting sea state climatology is used by the Monitas system to perform a structural fatigue analysis for comparison with design lifetime estimates (Kaminski and Aalberts, 2010). As an endto-end demonstration of the Monitas system, the FPSO Glas Dowr was outfitted to obtain all required ship environmental and operational data to perform a lifetime fatigue analysis based on actual conditions (Aalberts et al., 2010). Observations were made while Glas Dowr was stationed in the wave environment off South Africa for a 16-month period (June 2007 – September 2008). Required directional wave observations were obtained with ship-mounted dedicated conventional navigational X-band radar. For validation purposes, a directional wave buoy was deployed nearby. Shipboard meteorological and GPS navigation systems provided the remaining required inputs for conducting the fatigue lifetime analysis. This paper is one of five papers that present the Monitas system and results of the Glas Dowr demonstration. The focus here is on the Monitas wave analysis module. Details are provided on the wave observation and analysis methods. A validation of the radar wave measurement system is also presented, including a summary of fatigue consumption based on both the radar and wave buoy measurement approaches. Finally, the paper shows how the use of different wave data formats describing the same wave data can affect the fatigue lifetime estimation.