steep wave
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Author(s):  
Ning Xu ◽  
Honglin Zhao ◽  
Yufang Li ◽  
Yingying Wang ◽  
Shimin Zhang

The next-generation underwater production system (NUPS) is based on the suspension cluster manifold (SCM) as a new conceptual scheme. SCM mooring stability is essential for establishing NUPS. Therefore, comparing the SCM mooring stability in different mooring systems is vital for evaluating system adaptability. This paper detailed two mooring schemes designed for the SCM, including the steel catenary riser (SCR) mooring system and the new steep wave (NSWR) mooring system. OrcaFlex software was used to establish the mooring system model, analyzing the static motion response of the SCM under the current and fluid density. Furthermore, the mooring system adaptability in the cluster wellhead layout was also evaluated and compared. The results showed that the maximum offset of the SCM with the SCR mooring system was within 2 m under the current, while the deflection of the SCM with the NSWR mooring system was within 1.5° in extreme fluid densities. Furthermore, the SCM with the SCR mooring system displayed superior station-keeping capability in the current, while the NSWR mooring system exhibited better stability when transporting extreme fluid densities and was more adaptable in cluster wellhead layouts.


2021 ◽  
Vol 31 (1) ◽  
pp. 19-26
Author(s):  
Sriram Venkatachalam ◽  
Shagun Agarwal ◽  
Torsten Schlurmann

Author(s):  
Sébastien Fouques ◽  
Csaba Pákozdi

Abstract The design of structures at sea requires knowledge on how large and steep waves can be. Although extreme waves are very rare, their consequences in terms of structural loads, such as wave impact or ringing, are critical. However, modelling the physical properties of steep waves along with their probability of occurrence in given sea states has remained a challenge. On the one hand, standard linear and weakly nonlinear wave theories are computationally efficient, but since they assume that the steepness parameter is small, they are unable to capture extreme waves. On the other hand, recent simulation methods based on CFD or fully nonlinear potential solvers are able to capture the physics of steep waves up to the onset on breaking, but their large computational cost makes it difficult to investigate rare events. Between these two extremes, the High-Order Spectral (HOS) method, which solves surface equations, is both efficient and able to capture highly nonlinear effects. It may then represent a good compromise for long simulations of steep waves. Unfortunately, it is based on a perturbation expansion where the small parameter is the wave steepness, and consequently, simulations tend to become unstable when steep wave events occur. In this work, we investigate the properties of irregular waves simulated with a modified HOS method, in which the sea surface is described with a Lagrangian representation, i.e. by computing the position and the velocity potential of individual surface particles. By doing so, nonlinear properties of the surface elevation are simply captured by the modulation of the horizontal and vertical particle motion. The same steep wave is then described more linearly with a Lagrangian representation, which reduces the instabilities of the HOS method. The paper focuses on bi-chromatic waves and irregular waves simulated from a JONSWAP spectrum. We compare simulations performed with the standard HOS and the modified Lagrangian methods for various HOS-orders.


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