Modeling and fatigue analysis of an hybrid riser system submitted to vortex-induced motion

The large Vortex Induced Motion (VIM) due to current acting on a circular-shaped monocolumn platform induces low-frequency stress variations on the SCRs (Steel Catenary Risers) connected to it. These stresses together with stress variations associated to wave effects must be accounted for in the fatigue analysis of these risers. Normally, the joint statistics of waves and currents show that these environmental variables may be considered as statistically independent. Therefore, the number of global riser analyses necessary for the SCRs fatigue analysis becomes extremely high in order to consider a suitable number of combinations (including intensities and directions) of waves and currents. This paper describes a methodology for computing the fatigue damage in SCRs (Steel Catenary Risers) due to wave-frequency and VIM (Vortex Induced Motion) load effects based on a combination damage formula presented in DnV-OS-F204 [1]. The wave-frequency and VIM fatigue damages are calculated separately (by a time-domain rainflow approach) and the combined damage is evaluated by means of the DnV formula. This methodology reduces considerably the number of global riser analyses and consequently the computational burden associated to the fatigue analyses of SCRs connected to monocolumn-type platforms.

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Fatigue Analysis Technique of Deepwater Steel Catenary Risers

29th International Conference on Ocean, Offshore and Arctic Engineering: Volume 5, Parts A and B ◽

10.1115/omae2010-20832 ◽

2010 ◽

Cited By ~ 2

Author(s):

Jing Cao

Keyword(s):

Vertical Motion ◽

Fatigue Analysis ◽

Design Criterion ◽

Steel Catenary Riser ◽

Start Up ◽

Analysis Technique ◽

Induced Motion ◽

Steel Catenary Risers ◽

Stress Concentration Factors ◽

Sour Service

With the advances in deepwater offshore industries, Steel Catenary Riser (SCR) offers advantages over others and has been widely deployed worldwide including Gulf of Mexico (GoM), West of Africa (WoA), and Offshore Brazil. SCR concept has been applied to all types of deepwater floaters. Among analyses and design criteria, fatigue is still the number one governing design criterion of the deepwater SCR regardless the floater types (SPAR, TLP, SEMI, and FPSO). Typical SCR fatigue analyses include first order and second order floater motion induced fatigue; vortex induced vibration (VIV), deep draft floater vortex induced motion (VIM) fatigue, floater vertical motion induced VIV, slugging fatigue, start-up and shut-down, installation, and other kinds of dynamic loading imposed to the SCR. This paper presents the state-of-art fatigue analysis requirement and technique of different kinds of fatigue analysis for deepwater SCRs. Detailed discussion is given to different kinds of fatigue damage of SCRs. Focus of fatigue analysis of each type of deepwater floater is pointed out. Detailed discussion is also given to the selection of SN curves, stress concentration factors, influence of sweet and sour service, and post-processing of fatigue sensitive welds. Several examples of SCR fatigue analysis are given to demonstrate the analysis techniques and practices.

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