Description of random seas

This paper describes a hybrid computer simulation of two ships performing replenishment operations in random seas. Such operations present collision hazards due to the nonlinear interaction forces and moments which result from close proximity maneuvering while underway. Maneuvers are simulated to demonstrate automatic controller performance during station-keeping, station-changing, and the approach and breakaway phases of typical underway replenishment (UNREP) operations. Results indicate that automatic control should be considered as a possible solution to the UNREP collision avoidance problem.

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Non-Linear Fatigue Analysis for Jacket Offshore Structures

Volume 3: Safety and Reliability; Materials Technology; Douglas Faulkner Symposium on Reliability and Ultimate Strength of Marine Structures ◽

10.1115/omae2006-92142 ◽

2006 ◽

Author(s):

O. Gaidai ◽

A. Naess

Keyword(s):

Dynamic Analysis ◽

Direct Method ◽

Nonlinear Effects ◽

Offshore Structures ◽

Fatigue Analysis ◽

Offshore Structure ◽

Structural Fatigue ◽

Drag Forces ◽

Random Seas ◽

Fatigue Estimation

This paper presents different approaches for accounting for nonlinear effects in fatigue analysis. One approach is an application of the quadratic approximation method described in [3, 4] to the stochastic fatigue estimation of jacket type offshore structures. An alternative method proposed is based on a spectral approximation, and this approximation turns out to be quite accurate and computationally simple. The stress cycles causing structural fatigue are considered to be directly related to the horizontal excursions of the fixed offshore structure in random seas. Besides inertia forces, it is important to study the effect of the nonlinear Morison type drag forces. Since no direct method for dynamic analysis with Morison type forces is available, it is a goal to find an accurate approximation, allowing efficient dynamic analysis. This has implications for long term fatigue analysis, which is an important issue for design of offshore structures.

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Accurate Numerical Methods for Calculating the Response Statistics of Compliant Offshore Structures

24th International Conference on Offshore Mechanics and Arctic Engineering: Volume 2 ◽

10.1115/omae2005-67236 ◽

2005 ◽

Cited By ~ 1

Author(s):

A. Naess ◽

H. C. Karlsen ◽

P. S. Teigen

Keyword(s):

Numerical Methods ◽

Volterra Series ◽

State Of The Art ◽

Offshore Structures ◽

Second Order ◽

Stochastic Response ◽

Random Seas ◽

The Mean ◽

Extreme Responses ◽

Key Parameter

The state-of-the-art representation of the horizontal motions of e.g. a TLP in random seas is in terms of a second order stochastic Volterra series. Until recently, there has been no method available for accurately calculating the mean level upcrossing rate of such response processes. Since the mean upcrossing rate is a key parameter for estimating the large and extreme responses it is clearly of importance to develop methods for its calculation. The paper describes numerical methods for calculating the mean level upcrossing rate of a stochastic response process represented as a second order stochastic Volterra series. Since no approximations are made, the only source of inaccuracy is in the numerical calculation, which can be controlled.

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Extreme Waves and Stochastic Wave Groups

Volume 3: Safety and Reliability; Materials Technology; Douglas Faulkner Symposium on Reliability and Ultimate Strength of Marine Structures ◽

10.1115/omae2006-92527 ◽

2006 ◽

Author(s):

Francesco Fedele ◽

M. Aziz Tayfun

Keyword(s):

Numerical Results ◽

Resonance Interaction ◽

Experimental Results ◽

Wave Crest ◽

Extreme Waves ◽

Zakharov Equation ◽

Wave Groups ◽

Probability Of Exceedance ◽

Random Seas ◽

Crest Height

We introduce the concept of stochastic wave groups to explain the occurrence of extreme waves in nonlinear random seas, according to the dynamics imposed by the Zakharov equation (Zakharov, 1999). As a corollary, a new probability of exceedance of the crest-to-trough height which takes in to account the quasi-resonance interaction is derived. Furthermore, a generalization of the Tayfun distribution (Tayfun, 1986) for the wave crest height is also proposed. The new analytical distributions explain qualitatively well the experimental results of Onorato et al. (2004, 2005) and the numerical results of Juglard et al. (2005).

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