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Stability of TLP tether using a stochastic averaging technique

机译：使用随机平均技术的TLP系链的稳定性

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摘要

Tension Leg Platforms (TLPs) are popular deep water platforms for extracting oil and gas. TLPs are connected to the sea bed by pretensioned tethers. If fluctuating tension in tethers is sizable compared to the pretension, many dynamic stability problems may arise which also affects the platform motion. In this paper, stochastic stability of up tethers in transverse oscillations is investigated using stochastic averaging technique and the probability flow of the reduced Fokker-Plank equation. Stochastic averaging is used to obtain drift and diffusion coefficients independent of time. Onset of such transverse oscillations may be caused due to regular or random vortex shedding. The tension fluctuation of the tether due to random vertical wave forces on the platform induces multiplicative random excitation in the tether because of the presence of nonlinear drag. As a consequence, dynamic instability may become a significant problem. The tether is modeled as a simply supported uniform beam under initial tension assuming transverse oscillation to take place only in its fundamental natural mode. Thus, the problem is reduced to a single degree of freedom system. Fluid resistance is calculated by Morison's equation consisting of drag and inertia forces. A semi-analytical procedure combined with stochastic averaging technique is employed to obtain the drift and diffusion coefficients of the response in closed form. By examining the probability flow in terms of drift and diffusion coefficients at the two boundaries, the stability of the system is investigated. Stability of tethers of a UP in 300 m water depth, for different pretensions, is studied for sea states of 21 m, 16 m and 8 m significant wave heights represented by PM spectrum. It is shown that for low values of pretension, stochastic stability of tether in transverse direction may take place for large wave heights. (C) 2016 Elsevier Ltd. All rights reserved.
机译：张力腿平台（TLP）是用于提取石油和天然气的深水平台。 TLP通过预张紧的系绳连接到海床。如果与预张力相比，系链中波动的张力相当大，则可能会出现许多动态稳定性问题，这些问题也会影响平台的运动。在本文中，使用随机平均技术和简化的Fokker-Plank方程的概率流，研究了横向振荡中上系绳的随机稳定性。随机平均用于获得与时间无关的漂移和扩散系数。可能由于规则或随机涡旋脱落而引起这种横向振荡。由于存在非线性阻力，平台上的随机垂直波力引起的系链张力波动会在系链中引起乘法随机激励。结果，动态不稳定性可能成为一个重大问题。系绳建模为在初始张力下简单支撑的均匀梁，假定横向振动仅在其基本自然模式下发生。因此，该问题被减少到单一自由度系统。流体阻力通过由阻力和惯性力组成的莫里森方程计算。采用半分析程序与随机平均技术相结合，以闭合形式获得响应的漂移系数和扩散系数。通过检查在两个边界处的漂移和扩散系数方面的概率流，研究了系统的稳定性。对于不同预应力的UP系绳在300 m水深中的稳定性，研究了以PM谱表示的21 m，16 m和8 m有效波高的海况。结果表明，对于较低的预应力值，对于较大的波高，可能会在横向方向上出现系绳的随机稳定性。 （C）2016 Elsevier Ltd.保留所有权利。

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