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Tether analyses of offshore triceratops under wind, wave and current

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Abstract

Offshore triceratops is new-generation offshore platform, whose novel geometric form is advantageous and safe in ultra-deep waters. Deck is partially isolated from the buoyant legs by ball joints. Buoyant legs are anchored to sea bed using taut-moored tether with high initial pretension. Compliant nature of the platform induces degree of flexibility with large time periods; they fall within the range of wind-excitation frequency, making them vulnerable under aerodynamic loads. As tethers are crucial component in compliant structures such as triceratops, this study aims at investigating the effect of combined action of wind, wave and current on dynamic tension variation caused in tethers. It is followed by fatigue life prediction under different sea states. Statistical analyses of tether tension variation show that it is periodic in nature; maximum number of cycles is found to be in low sea state. Presence of current enhances energy of tension spectrum significantly. There is an increase in service life of tethers in the presence of wind under different sea states; but presence of current reduces the service life of tethers.

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Abbreviations

AC:

Aerodynamic centre

API:

American Petroleum Institute

CG:

Centre of gravity

FD:

Fatigue damage

H S :

Significant wave height

N :

Number of allowable cycles

n :

Number of counts from the histogram

ω:

Frequency

ω p :

Peak frequency

PM:

Pierson–Moscowitz

PSD:

Power spectral density

RMS:

Root mean square

\({\sigma _u}{(z)^2}\) :

Variance

T z :

Zero-crossing period

θ :

Frequency ratio

z :

Reference height (= 10 m)

z s :

Surface height (= 20 m) variance

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Authors and Affiliations

  1. Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, India

    S. Chandrasekaran & R. Nagavinothini

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  1. S. Chandrasekaran
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  2. R. Nagavinothini
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Correspondence to S. Chandrasekaran.

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Chandrasekaran, S., Nagavinothini, R. Tether analyses of offshore triceratops under wind, wave and current. Mar Syst Ocean Technol 13, 34–42 (2018). https://doi.org/10.1007/s40868-018-0043-9

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  • DOI: https://doi.org/10.1007/s40868-018-0043-9

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