Research Papers: Structures and Safety Reliability

Lazy-Wave Buoyancy Length Reduction Based on Fatigue Reliability Analysis

[+] Author and Article Information
Vinícius Ribeiro Machado da Silva, Luis V. S. Sagrilo, Mario Alfredo Vignoles

Civil Engineering Program/COPPE,
Federal University of Rio de Janeiro/COPPE,
Rio de Janeiro 21945-970, Brazil

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received June 15, 2017; final manuscript received December 14, 2017; published online February 13, 2018. Assoc. Editor: Carlos Guedes Soares.

J. Offshore Mech. Arct. Eng 140(3), 031602 (Feb 13, 2018) (7 pages) Paper No: OMAE-17-1090; doi: 10.1115/1.4038937 History: Received June 15, 2017; Revised December 14, 2017

The current downturn of the oil and gas industry force managers to take hard decisions about the continuity of projects, resulting in delays, postponements, or even their cancellation. In order to keep with them, the rush for cost reduction is a reality and the industry is pushing the involved parties to be aligned with this objective. The Brazilian presalt region, characterized by ultra-deep waters, faces this scenario where flexible risers in lazy-wave configurations are usually adopted as a solution to safe transfer fluids from sea bed until the floating unit. The smaller the buoyancy length, the cheaper the project becomes, reducing the necessary amount of buoys and the time spent for its installation. This paper investigates the possibility of buoyancy length reduction of lazy-wave configurations by using structural reliability methods on fatigue failure mode. The application of the fatigue reliability approach considers four 6 in flexible riser configurations: an original lazy-wave, a lazy-wave with less 30% of buoyancy length, another one with less 50% of buoyancy length and a free-hanging. Failure probabilities and safety factor calibration curves are shown for each configuration and compared among themselves. The results indicate the possibility of defining a lazy-wave configuration with smaller buoyancy lengths, reaching 75% of reduction without changing the preconized high safety class. Structural reliability analysis is available to help engineers understand the driving random variables of the problem, supporting the actual scenario of cost reduction for better decision-making based on quantified risk.

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Fig. 1

Flexible risers' configurations of previous study [8]

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Fig. 2

Normalized function for Xi [8]

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Fig. 3

Flexible risers' configurations

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Fig. 4

Fatigue calculation schematic sequence [12]

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Fig. 5

Importance factors for the last year in operation [8]

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Fig. 6

Failure probability behavior due to different amount of random variables

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Fig. 7

Selected random variables

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Fig. 8

pf versus Toper for the four configurations

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Fig. 9

Importance factors for the last year in operation

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Fig. 10

Safety factor behavior of the four configurations



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