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Ocean Engineering

A Direct Design Procedure for FPSO Side Structures Against Large Impact Loads

[+] Author and Article Information
Lin Hong, Jørgen Amdahl

Department of Marine Technology, Norwegian University of Science and Technology, Otto Nielsens v10, N-7491 Trondheim, Norway

Ge Wang

 American Bureau of Shipping, 16855 Northchase Drive, Houston, TX 77077

J. Offshore Mech. Arct. Eng 131(3), 031105 (Jun 03, 2009) (12 pages) doi:10.1115/1.3124140 History: Received October 28, 2008; Revised March 05, 2009; Published June 03, 2009

The performance and consequence of FPSOs subjected to large impact loads such as collisions from supply vessels or merchant vessels are of great concern in the offshore industry, notably when they are located close to heavy traffic lanes. Due to the lack of operation experience for ship-shaped FPSOs, direct design procedures are needed to rationalize the structural design of FPSOs, which can mitigate the consequence of collision accident and avoid possible contaminated compartment flooding. In this paper, three collision scenarios between a FPSO and a bulbous supply vessel are analyzed through explicit nonlinear finite element analysis code LS-DYNA . Thereafter, a direct design procedure is proposed for ship-shaped FPSO side structure against accidental collision forces, which follows the principle of accidental limit state. The procedure comprises the determination of the impact forces, shell plating, and stiffener framing design, and the consideration of the acceptance criterion. The proposed method is especially useful in the preliminary design phase because the design procedure for plating and stiffener is based on analytical formulas derived from plastic method and appropriate collapse mechanism. The side structure decided by the proposed design procedure also complies with the strength design principle that has been adopted in NORSOK standard. The proposed approach is demonstrated by the design of the FPSO side structure against impact loads from a 7500 tons supply vessel and verified by means of integrated collision analysis. The procedure could also be served to estimate the damage due to accidental loads.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

A supply vessel fitted with bulbous bow

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Figure 3

West Venture-Far Symphony collision

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Figure 4

Structural drawing and FE model of supply vessel bow

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Figure 5

General layout of the reference FPSO

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Figure 6

FE model of FPSO tank side

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Figure 7

Damage as a function of normalized element size

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Figure 8

Collision impact scenarios

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Figure 10

Impact force history curves for bulb impact

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Figure 11

Total energy dissipation

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Figure 12

Energy absorption ratio for FPSO tank side

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Figure 13

Force-deformation curve for supply vessel bow

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Figure 14

Contact force evolution of bulb impact against rigid wall

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Figure 15

Pressure-area relationship for bulb impact

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Figure 17

Relationship between permanent set and stiffener spacing for different ultimate strain

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Figure 18

Maximum allowable deformations

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Figure 19

Correlation between s/t and p/sy

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Figure 20

Correlation of required plate thickness and stiffener spacing for various materials

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Figure 21

Assumed framing design model

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Figure 22

Centrally loaded frame and mechanism model: (a) central uniform load pattern, (b) combined high-low intensity load pattern, and (c) three hinge collapse mechanism model

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Figure 23

Frame cross section property: (a) standard model and (b) simplified model

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Figure 24

Damage of FPSO outer shell (contact area)

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Figure 25

Impact force history curve comparison

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Figure 26

Energy absorption ratio for FPSO tank side

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Figure 27

Plate deformation pattern

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Figure 28

Longitudinal stiffener deformation pattern

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Figure 29

Plate response under different conditions

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Figure 16

“Double-diamond” yield line model for plates under patch loading (4)

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Figure 2

Strength, ductility, and shared-energy design

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Figure 9

Impact process for collision scenarios 1 and 2

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