Abstract

The objective of the Joint Industry project 'FPSO Integrity' is to provide insight in the fatigue loading of FPSO's and the accuracy and validity of computational models. The results should contribute to reliable life time prediction and site assessment studies for this relative new concept of production.

Within the frame work of this JIP, 'Glas Dowr', a new build double hull FPSO operated by Bluewater for Amerada Hess at the Durward and Dauntless fields in the North Sea, has been instrumented extensively.

The objective of the instrumentation was to measure sufficient signals to derive the fatigue loading on the vessel. The dedicated monitoring system comprised the instrumentation of two cross sections of the vessel with strain gauges, pressure transducers and wave height radar's. Also the deck and the turret were equipped with strain gauges. Both the wave induced motions and the low frequency positions and orientation of the vessel was recorded. For the analysis of the measured signals the wind, wave and current conditions have been recorded as well as the vessel parameters such as draft, trim and tank loads. Data from the Glas Dowr has been collected from October 1, 1997 to 1 August 1999. The data has been subjected to long term statistical analysis and for selected periods to more detailed spectral and statistical analysis as well as rain flow counts.

The results have been used for validation of existing fatigue load calculation methods used by five classification societies and for the development of new fatigue load prediction software.

INTRODUCTION

For design, engineering and site assessment of Floating Production Storage and Offloading (FPSO) units, a reliable fatigue life prediction is essential. A computational model of the fatigue life of an FPSO comprises a description of the environment, a model of the fatigue loads and a model of the fatigue response.

As the operation of an FPSO is completely different from a trading tanker, in 1996 a Joint Industry project named FPSO Integrity was initiated to provide insight in the fatigue loading of FPSO's and the accuracy and validity of computational models. The background, objectives and structure of the project are described in further detail in ref [1].

The FPSO Integrity JIP comprised four major tasks:

  1. Structural monitoring of FPSO 'Glas Dowr' operated by Bluewater for Amerada Hess on the Dauntless and Durward field in the North Sea.

  2. Analysis of the measured data to derive the fatigue loads on the vessel.

  3. Development of a fatigue load assessment method, which can be used for a fatigue lifetime prediction.

  4. Comparative Study on the existing fatigue analysis methods currently used by the five Classification Societies involved in the project.

As the ultimate goal of the project was not only development of new calculations procedures but also comparison against existing methods and real life data, the project demanded a strong input from all stakeholders in the FPSO industry.

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