Fatigue Analysis of Kizomba 'A' FPSO using Direct Calculation based on FMS
Authors: Oh M.H. , Sim W.S. , Shin H.S. 
Published: October 2008

This paper describes the fatigue analysis of Kizomba 'A' FPSO based on the FPSO Fatigue Methodology Specification (FMS) [1] which is developed on the direct stochastic calculation. The FMS provides designers with technical guidance for fatigue control of newly built FPSO and can be characterized as integrated fatigue analysis to ensure the structural safety of all critical FPSO details by means of full stochastic analysis as well as component based one.

At the cost of modeling complexity and computer resources, the full stochastic method gives the accurate fatigue damage for FPSO-specific details such as topside, piperack, flare etc., which are governed by hull-wave interactions. On the other hand, the component based one provides the cost-effective tools to screen the most fatigue prone area in topside supports for full stochastic analysis. Besides, the spectrum combination method that is suggested in order to consider both wave-line and wave-hull interactions, is adopted in fatigue damage calculation of the supports for FTL, OOL, riser and mooring. Apart from technical aspect, the interface management between other groups in charge of FPSO-hull, topside, riser and mooring system turns out very important due to tight project schedule.

This paper proves the practical possibility and appropriateness of full stochastic fatigue analysis for FPSO and envisions its usefulness for other floating structures such as TLP, semisubmersibles and etc. The considered Kizomba 'A' FPSO to be installed on Angola Block 15 is 285 meters long, 63 meters wide, and 32 meters high, and can store 2.2 million barrels of crude oil in its lower hull facilities [2].

Introduction

As the offshore oilfields shift into the deeper waters with little infrastructures, FPSOs have become the more popular solution. Currently one of the most active areas in FPSO installation is the West Africa where the weather is benign and the swell from the south is dominant. Whereas strength of the FPSO in such a calm site is mostly controlled on towing route, the fatigue control is directly dependent on the site-specific environment. It should be ensured that FPSOs could stay for decades without drydocking for maintenance.

ExxonMobil has developed FPSO Fatigue Methodology Specification (FMS) with DNV for providing designers with technical guidance for fatigue control of newly built shipshaped FPSOs [3]. The aim of the fatigue control is to ensure that all critical structural details meet the fatigue design requirements. Also, the calculated fatigue lives form the basis of FPSO design and are used as inputs to the inspection plans during fabrication and in-service.

The focus of the FMS is a full stochastic fatigue analysis method that gives the accurate fatigue damage for the FPSOspecific details like topside support, piperack, flare tower foundation etc. The structural behaviors of these supports are governed by wave-hull interactions and describable using linear analysis. The wave loads based on the metocean data of installation site are directly calculated and transferred to the structural model. Generally, full-ship structural model is utilized in the analysis. Figure 1 shows the general procedure of full stochastic fatigue analysis.