This paper describes a series of model tests aimed at gaining insight in the tension variations in the export risers and mooring lines of a CALM buoy. The test result were therefore not only analysed carefully, but were also used as input and to validate a numerical tool that computes the coupled motions of the buoy and its mooring system. The tests were carried out at a model scale of 1 to 20. Captive tests in regular and irregular waves were carried out to investigate non-linearities in the wave forces on the buoy for example from the presence of the skirt. Decay tests were carried out to determine the damping of the buoy’s motions and to obtain the natural periods. Finally, tests in irregular waves were carried out. The dynamics of the mooring system and the resulting damping have a significant effect on the buoy’s motions. A numerical tool has been developed that combines the wave-frequency buoy motions with the dynamical behaviour of the mooring system. The motions of the buoy are computed with a linearised equation of motion. The non-linear motions of the mooring system are computed simultaneously and interact with the buoy’s motions. In this paper, a comparison is shown between the measurements and the simulations. Firstly, the wave forces obtained with a linear diffraction computation with a simplified skirt are compared with the measured wave forces. Secondly, the numerical modelling of the mooring system is checked by comparing line tensions when the buoy moves with the motion as measured in an irregular wave test. Thirdly, the decay tests are simulated to investigate the correctness of the applied viscous damping values. Finally, simulations of a test in irregular waves are shown to validate the entire integrated concept. The results show that: 1. The wave-exciting surge and heave forces can be predicted well with linear diffraction theory. However, differences between the measured and computed pitch moment are found, caused by a simplified modelling of the skirt and the shortcomings of the diffraction model. 2. To predict the tension variations in the mooring lines and risers (and estimate fatigue) it is essential that mooring line dynamics are taken into account. 3. The heave motions of the buoy are predicted well. 4. The surge motions of the buoy are predicted reasonably well. 5. The pitch motions are wrongly predicted.

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Recently, the phenomenon of out-of-plane bending (OPB) fatigue of mooring chain links emerged as an important parameter in the fatigue assessment of mooring lines. Vessel motions induce a bending moment at the top chain of a mooring line. This bending moment induces alternating local stresses in the link and thus contributes to fatigue damage of those links. High pretension mooring systems are particularly sensitive to this phenomenon, since a small vessel motion combined with a high tension results in a relatively large bending moment in the upper mooring chain links. In mooring systems with high pre-tensions, this damage is of much greater magnitude than the fatigue damage induced by tension-tension loading only. An extensive study has been executed to investigate the fatigue life of mooring chain in deep water systems. This paper presents the calculation procedure to include the effects of local chain bending in the overall mooring line fatigue analysis. It was concluded that despite the complexity of the OPB issue, it is a phenomenon that can be incorporated in the mooring analyses by means of numerical procedures. The developed method is based on extensive Finite Element Method (FEM) analyses of chain links. Models of multiple chain links have been used that take into account the plastic-elastic properties of the material and contact friction between chain links. The FE models are used to derive empirical relations, between load angles, interlink angles, bending moments and stresses. These calculations were made for different combinations of line tension, interlink friction and chain size. The results were stored in a database to gain insight in the out-of-plane bending phenomenon. This database provides empirical formulas to lead to the local stress in different points on a chain link. These empirical formulas are used to translate floater (vessel or buoy) motions into local stress variations and fatigue damages in chain links. The long-term motion behaviour of the floater is known, the long term tension and bending stress ranges can be obtained and thus a fatigue damage of the chain links can be calculated.

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.