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Research Papers: Offshore Geotechnics

Seabed Interaction Modeling Effects on the Global Response of Catenary Pipeline: A Case Study

[+] Author and Article Information
Hany Elosta

Technip Norge AS,
P.O. Box 603 Kjørboveien 14 & 16,
Sandvika, NO-1303, Norway

Shan Huang, Atilla Incecik

University of Strathclyde,
100 Montrose Street,
Glasgow, G4 0LZ, United Kingdom

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 18, 2013; final manuscript received March 9, 2014; published online April 15, 2014. Assoc. Editor: Dong S. Jeng.

J. Offshore Mech. Arct. Eng 136(3), 032001 (Apr 15, 2014) (8 pages) Paper No: OMAE-13-1058; doi: 10.1115/1.4027177 History: Received June 18, 2013; Revised March 09, 2014

A steel catenary riser (SCR) attached to a floating platform at its upper end encounters oscillations in and near its touchdown zone (TDZ), which results in interaction with the seabed. Field observations and design analysis of SCRs show that the highest stress and greatest fatigue damage occurred near the touchdown point where the SCR first touches the seabed soil. The challenges regarding the fatigue damage assessment of an SCR in the TDZ are primarily because of the nonlinear behavior of SCR–seabed interaction and considerable uncertainty in seabed interaction modeling and geotechnical parameters. Analysis techniques have been developed in the two main areas: SCR–seabed interaction modeling and the influence of the uncertainty in the geotechnical parameters on the dynamic response and fatigue performance of SCRs in the TDZ. Initially, this study discusses the significance of SCR–seabed interaction on the response of an SCR for deepwater applications when subjected to random waves on soft clay using the commercial code OrcaFlex for nonlinear time domain simulation. In the next step, this study investigates the sensitivity of fatigue performance to geotechnical parameters through a parametric study. It is proven that employing the improved lateral SCR–seabed interaction model with accurate prediction of soil stiffness and riser penetration with cyclic loading enables us to obtain dynamic global riser performance in the TDZ with better accuracy. The fatigue analyses results prove that the confounding results indicated by the previous research studies on the SCR in the TDZ are due to different geotechnical parameters imposed with the seabed interaction model. The main benefit of employing nonlinear seabed approach is to capture the entity of realistic soil interaction behavior in modeling and analysis and to predict the likelihood of the fatigue damage of the SCR with seabed interaction, thereby minimizing the risk of the loss of the containment with the associated environmental impact.

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References

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Figures

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Fig. 1

Soil model characteristics for different modes

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Fig. 2

Lateral pipe soil interaction using trilinear model

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Fig. 3

(a) SCR–seabed interaction problem, (b) schematic of TDZ attached with hysteretic nonlinear soil springs, and (c) riser–soil model

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Fig. 4

Dynamic SCR–soil contact resistance in far load case

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Fig. 5

SCR–seabed interaction response in the TDZ at an arc length 1410 m

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Fig. 6

Dynamic SCR lateral oscillation for beam seas in TDZ (3 h simulation)

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Fig. 7

SCR–seabed lateral interaction at an arc length 1225 m

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Fig. 8

Influence of lateral soil models on riser penetration

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Fig. 9

Y–displacement of SCR at the TDP

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Fig. 10

DNV S-N curve in seawater/cathodic protection

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Fig. 11

Fatigue life for SCR in the TDZ for different SCF values

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Fig. 12

Effect of linear soil stiffness on SCR cumulative fatigue damage in the TDZ

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Fig. 13

Normalized maximum stiffness effect on fatigue life in the TDZ at arc length 1217.5 m

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Fig. 14

Normalized maximum stiffness effect on trench deepening in the TDZ

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Fig. 15

Influence of maximum normalized stiffness and soil suction ratio on the predicted fatigue life

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