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Research Papers: Ocean Renewable Energy

Preliminary Assessment of the Importance of Platform–Tendon Coupling in a Tension Leg Platform

[+] Author and Article Information
Meyer Nahon

Department of Mechanical Engineering,
McGill University,
Montreal,
Quebec H3A 2K6, Canada

Frederick Driscoll

National Renewable Energy Laboratory,
Golden, CO 80401

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received May 25, 2011; final manuscript received October 22, 2012; published online May 24, 2013. Assoc. Editor: Ron Riggs.

J. Offshore Mech. Arct. Eng 135(3), 031901 (May 24, 2013) (11 pages) Paper No: OMAE-11-1046; doi: 10.1115/1.4023795 History: Received May 25, 2011; Revised October 22, 2012

This paper presents performance metrics that can be used to evaluate the response sensitivity of a tension leg platform (TLP) to its tendons. An uncoupled TLP model ignores the intrinsic dynamics and environmental loads on the cables by treating each tendon as an ideal massless spring. A coupled TLP system, in contrast, considers the effects of distributed mass and drag along the tendon. Under certain operating conditions, an uncoupled dynamics model can produce results comparable to its coupled counterpart. This paper defines the conditions under which it is acceptable to model a TLP tendon as a linear spring, as opposed to one that considers the cable dynamics. The analysis is performed in the frequency domain and, for generality, the results are nondimensionalized. The findings indicate that a more elaborate set of conditions than the platform–to–cable mass ratio must be satisfied for the two models to provide similar results. To conclude this study, two simulations are performed and compared against the performance metrics derived in this paper.

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Figures

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Fig. 1

The analytical models derived in this work assume the TLP geometry and boundaries depicted in this figure. Variable Tx(t) is the dynamic tension due to transverse oscillations, and it is aligned with the x-axis. For clarity, the profile for one tendon is shown, but it is implied that remaining tendons oscillate in a similar manner.

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Fig. 2

In the coupled analytical TLP model, each tendon is permitted to deform in the manner pictured. u(z, t) represents the cable stretch in the longitudinal direction, and w(z, t) is a displacement in a direction normal to the z-axis and represents transverse motions.

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Fig. 3

The nondimensionalized TLP surge and heave responses of a TLP in (a) 600 ms depth and (b) 1200 ms depth. This figure depicts both the coupled analytical model G˜x,z(s˜) and the uncoupled analytical model H˜x,z(s˜).

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Fig. 4

When Λi is a small number, the platform senses the longitudinal/transverse tendon motions. In the opposite case, the mooring line provides additional damping in the platform.

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Fig. 5

The above figures illustrate results for weakly coupled TLP in 600 ms water depth. Although differences between the uncoupled and coupled are small when comparing the time histories, the PSD plots help to elucidate the source of the model differences. In the pitch PSD plot, a difference between models emerges at the pitch natural frequency since Ωθ has approached a critical threshold.

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Fig. 6

Simulation results for a TLP in 1200 ms water depth. In this example, the coupled TLP response is more receptive to the cable dynamics. Differences between the coupled and uncoupled models are attributable to (1) the stiffness condition being violated, (2) the platform sensing the mooring drag force, and (3) the cable natural frequencies being excited.

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