SS: FPSOs and Floating Production Systems: The Monitas system for the Glas Dowr FPSO

Abstract

The paper is one of five papers about the Advisory Monitoring System (AMS) for controlling the fatigue lifetime consumption of FPSO hulls. The system has been developed within the Monitas Joint Industry Project (JIP) and is referred to as the Monitas system. The name Monitas stands for Monitoring Advisory System. The paper describes the Monitas system developed, installed and running for two years on-board the Glas Dowr FPSO operated by Bluewater Energy Services (BES) at the Sable field, offshore South Africa. The paper describes how the Monitas system advises on fatigue lifetime consumption based on the fatigue design data, fatigue design tool and the monitoring data of hull girder loads, global and local stresses, wave frequency motions, ship's loading condition and heading, and the environmental conditions. The paper concludes that contrary to conventional structural health monitoring systems, estimating the actual fatigue lifetime consumption, the Monitas system can explain potential deviations from the design assumptions. In this way the Monitas system provides designers with feedback about the quality of their design tools and made assumptions, and advises the FPSO operators on actions required to ensure a safe operation and, if needed, can be used to justify lifetime extension. Introduction Hull fatigue of FPSOs is an important design consideration which has been addressed in several Joint Industry Projects (JIPs) in the last two decades. In the late 90s, hull fatigue was investigated in the FPSO Integrity and FPSO Capacity JIPs. The FPSO Integrity I and II JIPs were headed by MARIN and their aim was to investigate the fatigue loading on an FPSO hull. For this purpose, BES's FPSO Glas Dowr (see Figure 1), at that time located at the Durward & Dauntless field at the UKCS, was instrumented with an extensive measurement system to collect data on hull loading. An extensive overview of this JIP and the monitoring system are given by Bultema (2000) and Boom van den (2000). The FPSO Capacity I and II JIPs were led by DNV and their main aim was to investigate and develop methodologies to describe the fatigue capacity of typical FPSO structural hull members. An extensive overview of these JIPs can be found in Lotsberg (2000). Although a lot of knowledge was acquired in these JIPs, additional development was needed in the FPSO Live JIP, which was led by BES in cooperation with MARIN and DNV. This JIP developed a hull fatigue calculation procedure in the time-domain and investigated a minimum monitoring system, i.e. the minimum amount of sensors required to derive fatigue loads on the hull of an FPSO.