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Research Papers: Structures and Safety Reliability

Inverse Estimation of Local Slamming Loads on a Jacket Structure

[+] Author and Article Information
Ying Tu

Department of Civil and
Environmental Engineering,
Norwegian University of Science
and Technology,
Trondheim 7491, Norway
e-mail: ying.tu@ntnu.no

Thorvald C. Grindstad

Kongsberg Digital AS,
Trondheim 7040, Norway

Michael Muskulus

Department of Civil and Environmental
Engineering,
Norwegian University of Science
and Technology,
Trondheim 7491, Norway

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received March 17, 2017; final manuscript received June 22, 2017; published online August 8, 2017. Assoc. Editor: Xi-Ying Zhang.

J. Offshore Mech. Arct. Eng 139(6), 061601 (Aug 08, 2017) (12 pages) Paper No: OMAE-17-1035; doi: 10.1115/1.4037175 History: Received March 17, 2017; Revised June 22, 2017

Slamming loads from plunging breaking waves feature a high impulsive force and a very short duration. It is difficult to measure these loads directly in experiments due to the dynamics of the structures. In this study, inverse approaches are investigated to estimate the local slamming loads on a jacket structure using hammer test and wave test data from a model scale experiment. First, a state-of-the-art approach is considered. It uses two deconvolution techniques to first determine the impulse response functions and then to reconstruct the wave impact forces. Second, an easier applicable approach is proposed. It uses linear regression with the ordinary least square technique for the force estimation. The results calculated with these two approaches are highly identical. The linear regression approach can be extended to account for the loads transferred among different locations. This leads to lower and theoretically more accurate estimation of the loads compared to the previous two approaches. For the investigated case, the total impulse due to the wave is 22% lower. The estimated forces by the extended approach have a resolution at the millisecond level, which provides detailed information on the shape of the forces. The approach is an important tool for statistical investigations into the local slamming forces, and further on for the development of a reliable engineering model of the forces.

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Figures

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Fig. 1

Experimental setup and global coordinates

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Fig. 2

Four locations at the jacket and a transducer in a brace. Figure reprinted with permission from the WaveSlam project: (a) jacket model and (b) transducer.

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Fig. 3

Two approaches for load estimation

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Fig. 4

Input wave response forces

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Fig. 5

Input and output time series for hammer case 1.1

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Fig. 6

Estimated impulse response function (left) and original and calculated responses (right) using different stopping factors. Exemplary result using hammer test case 1.1: (a) α = 0.05, (b) α = 0.1, and (c) α = 0.5.

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Fig. 7

Estimated impulse response functions at the four locations: (a) location 1, (b) location 2, (c) location 3, and (d) location 4

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Fig. 8

Estimated impact force (left) and original and calculated responses (right) using different weighting factors. Exemplary result using hammer test case 1.1 and wave test response force at location 1: (a) ε = 0.001, (b) ε = 0.01, and (c) ε = 0.1.

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Fig. 9

Estimated wave impact forces from horizontal approach

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Fig. 10

Original and calculated responses for wave test (left) and estimated wave impact force (right) using different step factors: (a) δ = 2, (b) δ = 5, and (c) δ = 15

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Fig. 11

Estimated wave impact forces from vertical approach

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Fig. 12

Comparison of wave impact forces between horizontal and vertical approaches: (a) location 1, (b) location 2, (c) location 3, and (d) location 4

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Fig. 13

Original and calculated responses for wave test from extended vertical approach: (a) location 1, (b) location 2, (c) location 3, and (d) location 4

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Fig. 14

Estimated wave impact forces from extended vertical approach, compared to the results from vertical approach: (a) location 1, (b) location 2, (c) location 3, and (d) location 4

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Fig. 15

Comparison of the results from vertical approach and extended vertical approach: (a) peak force and (b) impulse

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