SS: FPSOs and Floating Production Systems: Application of Fatigue Damage Sensors in the Monitas system
Authors: Takaoka Yukichi , Nihei Kanta , Vargas Pedro , Aalberts Pieter , Kaminski Miroslaw Leck 
Published: July 2010

ABSTRACT
This paper is one of a series of papers presenting research results from the Monitas Joint Industry Project (JIP). The name Monitas stands for Monitoring Advisory System. This paper focuses on the application of the Fatigue Damage Sensor (FDS) for monitoring of fatigue life consumption of FPSOs. The sensor has been developed by Kawasaki. It is a passive sensor similar in size and installation to a conventional strain gauge. After the installation the sensor feels strains of the structure.
These strains develop a crack in the sensor. The crack length is a measure of fatigue lifetime consumption. This paper describes the sensor and compares its readings with conventional fatigue assessment methods based on the rain-flow counting of measured strains. This paper focuses on the first successful application of FDS to an FPSO. In general, the FDS is applicable to any structure. However, for the FPSO application the sensor has to be calibrated in order to take into account the characteristic stochastic nature of fatigue loading of marine structures subjected to action of waves. The beauty of the sensor is its simplicity and low cost. The disadvantage is that the crack length has to be read periodically. The paper demonstrates that the FDS gives reliable results and is very attractive for direct assessment of fatigue lifetime consumption of vital structural elements of FPSOs. The paper gives guidance on application of FDS including preferred locations, installation procedure, number of sensors per location and reading procedure.

Introduction
The structural health monitoring is a wide and fast developing area of engineering showing multiple developments and applications. One of them is the development and application of the Advisory Monitoring System (AMS) which is taking place within the Monitas Joint Industry Project (L'Hostis et al, 2010) led by MARIN. The AMS, referred to as the Monitas system, controls the fatigue lifetime consumption of FPSO hulls. Aalberts et al, (2010) describe how the Monitas system advises on fatigue lifetime consumption based on the fatigue design data, fatigue design tool, the monitoring data of global and local stresses, vessel motions, and the environmental and operational conditions. The Monitas system is an active system. It requires sensors, cabling, power, data acquisition systems and the Monitas software that translates Gigabytes of data into a concise advice including explanation of possible differences between the design and actual fatigue lifetime consumption. In this respect the Monitas system advances the conventional Hull Monitoring Systems (HMSs) which are specified in present rules of different Class Societies. This is because the information provided by a HMS can be at best restricted to the value of fatigue lifetime consumption without exploring possible differences from the design values.
A different approach has been followed by Kawasaki Heavy Industries Ltd. and Kawasaki Shipbuilding Corporation. Together, both companies have developed the Fatigue Damage Sensor (FDS) (Kawaguchi et al, 2003, Kobayashi et al, 2003, and Muragishi et al, 2004). The beauty of this passive sensor is its simplicity and low cost. The disadvantage is that it has to be read manually. In general, the FDS is applicable to any structure but it has to be calibrated in order to take into account the characteristic stochastic nature of fatigue loading for that structure. FPSOs as permanently moored marine structures are subjected to specific action of waves and, therefore, there was a need to calibrate the FDS for FPSO applications. This calibration was carried out within the Monitas JIP and is described in this paper.