Sea State Estimation Based on Ship Motions Measurements and Data Fusion

In-service monitoring can help to increase safety of ships especially regarding the fatigue assessment. For this purpose, it is compulsory to know the environmental conditions encountered: wind, but also the full directional wave spectrum. During the EU TULCS project, a full scale measurements campaign has been conducted onboard the CMA-CGM 13200 TEU container ship Rigoletto. She has been instrumented to measure deformation of the ship as well as the sea state encountered during its trip. This paper will focus on the sea state estimation. Three systems have been installed to estimate the sea state encountered by the Rigoletto: An X-band radar from Ocean Waves with WAMOS® system and two altimetric wave radars from RADAC®. Nevertheless, the measured significant wave height can be disturbed by several external elements like bow waves, sprays, sea surface ripples, etc… Furthermore, ship motions are also measured and can provide another estimation of the significant wave height using a specific algorithm developed by DCNS Research for the TULCS project. As all those estimations are inherently different, it is necessary to make a fusion of those data to provide a single estimation (“best estimate”) of the significant wave height. This paper will present the data fusion process developed for TULCS and show some first validation results.

Hydrodynamic Interactions in Multiple Body Array: A Simple and Fast Approach Coupling Boundary Element Method and Plane Wave Approximation

The hydrodynamic forces acting on an isolated body could be considerably different than those when it is considered in an array of multiple bodies, due to wave interactions among them. In this context, we present in this paper a numerical approach based on the linear potential flow theory to solve full hydrodynamic interaction problem in a multiple body array. In contrast to the previous approaches that considered all bodies in an array as a single unit, the present approach relies on solving for an isolated body. The interactions among the bodies are then taken into account via plane wave approximation in an iterative manner. The boundary value problem corresponding to a isolated body is solved by the Boundary Element Method (BEM). The approach is useful when the bodies are sufficiently distant from each other, at-least greater than five times the characteristic dimensions of the body. This is a valid assumption for wave energy converter devices array of point absorber type, which is our target application at a later stage. The main advantage of the proposed approach is that the computational time requirement is significantly less than the commonly used direct BEM. The time savings can be realized for even small arrays consisting of four bodies. Another advantage is that the computer memory requirements are also significantly smaller compared to the direct BEM, allowing us to consider large arrays. The numerical results for hydrodynamic interaction problem in two arrays consisting of 25 cylinders and same number of rectangular flaps are presented to validate the proposed approach.

Simulation of the Gap Resonance Between Two Rectangular Barges in Regular Waves by a Free Surface Viscous Flow Solver

This paper compares numerical and experimental results in the study of the resonance phenomenon which appears between two side-by-side fixed barges for different sea-states. Simulations were performed using SWENSE (Spectral Wave Explicit Navier-Stokes Equations) approach and results are compared with experimental data on two fixed barges with different headings and bilges. Numerical results, obtained using the SWENSE approach, are able to predict both the frequency and the magnitude of the RAO functions.

Application of Wave Devouring Propulsion System for Ocean Engineering

A Wave Devouring Propulsion System (WDPS) generates thrust directly from wave power while simultaneously generating a strong damping force. A simple WDPS design consists of hydrofoils mounted below the bow of a vessel. If a WDPS is integrated with the hull of a vessel, then it can power the vessel forward, even against the wave direction itself. One example of a successful WDPS was installed on the vessel named Mermaid II, which completed a trans-Pacific voyage in 2008, traveling approximately 7,800 km from Hawaii to Japan using wave power alone. This success indicates that the WDPS has potential for use in the field of ocean engineering. As described in this paper, we intend to apply the WDPS to the small autonomous boat and to conduct sea trials. We designed and built an autonomous WDPS boat, developed a data acquisition system, and experimentally investigated its performance in Orida Bay. The experimentally obtained results indicate that the autonomous navigation of the WDPS boat is possible when the wave height is greater than 5–10 cm.

Research on the Arrangement of the Mooring Lines of the Thruster Assisted Mooring System

As the offshore industry is developing into deeper and deeper water, station keeping technics are becoming more and more important to the industry. Based on the dynamic positioning system, the thruster assisted mooring system (TAMS) is developed, which consisted of mooring lines and thrusters. The main function of the TAMS is to hold a structure against wind wave and current loads with its thruster and cables, which is mainly evaluated by the holding capacity of the system. The arrangement of the mooring lines (location of anchor or the mooring line angle relative to platform) will directly affect the TAMS holding capacity because of the influence of the directions of the mooring forces. So finding out an optimum arrangement of the mooring lines is essential since the performance of the TAMS depends greatly on the arrangement of the mooring lines. The TAMS of a semi-submersible platform, which is studied in this paper, consisted of eight mooring lines. By fixing the layout of the thrusters and changing the location of each mooring line for every case, the performances of the TAMS are analyzed. The platform motions, mooring line tensions and power consumptions are compared to obtain the optimum arrangement of mooring lines, and thus a thruster assisted mooring system with a better performance can be achieved. Time domain simulation is carried out in this paper to obtain the results.

Research on Wind Wave Considering Nonlinear Dispersion Effect

Understanding of the offshore wind wave status plays a guiding role in surrounding marine engineering constructions, marine traffic, sea farming, etc. Further study is beneficial to marine economy development, as well as to the academic value of wave theory. This paper primarily introduces the deduction of new wind wave growth relations. Firstly, a new relation formula between wave steepness and wave age was deduced by combining the 3/2 power law developed by Toba with the nonlinear dispersion relation deduced by Li, and by ignoring the effect of water depth. And when the higher-order term was ignored, the relation formula can be simplified as that based on linear dispersion. Secondly, based on the combination of this new relation formula with the significant wave energy balance equation, new wind wave growth relation formulae including the wave non-linear dispersion effect were deduced. When the deduced growth relation formulae were applied in offshore area of Jiangsu incorporating with Mitsuyasu’s empirical formula about the open sea fetch and wind speed, accurate open sea wave parameters of Jiangsu can be formulated by only considering one parameter, such as wind speed. Overall, as this methodology avoided the uncertainty about the fetch of open ocean and operation error during the calculation process, results gained from this report had higher accuracy than other published formulae, and results were validated by NCEP reanalyzing data of Jiangsu offshore area and other researches.

Simulation of Strong Inertial Currents to Support Preliminary Engineering Offshore Namibia

The vertical profile of ocean current needs to be characterised for preliminary engineering applications such as exploration drilling. The acquisition of suitable current profile data can be a key challenge, especially in relatively unexplored deepwater frontier regions. Recent years have seen a dramatic increase in the use of ocean current models for this purpose, frequently involving freely available data from the HYCOM consortium. These data have proven reliable in some locations, but the accuracy remains questionable, or unproven, in many others. This paper describes a study in which it was clear from the outset that HYCOM would not represent dominant features of the local current regime. This concerned the region offshore Namibia, near 30°S, at which latitude the inertial period of natural ocean oscillation equals the diurnal period of 24 hours. This region is also subjected to relatively strong diurnal sea breezes, which can drive nearly resonant inertial responses in the ocean, which can, in turn, dominate the current regime. The spatial resolution of the wind field used to drive the global HYCOM model is insufficient to resolve this critical wind forcing, so the resulting model currents cannot represent the dominant features. Fortunately some relevant measurements were well documented in the public domain, from which a pragmatic inertial current simulation methodology was developed. Lack of inertial energy was actually beneficial for using HYCOM daily spot values to represent other, longer term, components of the flow. If inertial currents had been represented in the model, then more frequent values would be required to capture them. HYCOM data proved useful for representing long term inter-annual variability in the features it does represent, including meanders in the seasonal Benguala current.

Research on the Optimization of the Offset and the Power Consumption for a DP Assisted Mooring System

DP assisted mooring system is a new positioning system including mooring system and dynamic positioning system. In this paper, two measures are proposed to reduce the offset and the power consumption of a generic semi-submersible platform which is equipped with a DP assisted mooring system during operation. One is to tighten the windward mooring lines and slacken the leeward mooring lines, the other is to reduce or cancel the forbidden angles of the thrusters adjacent to the failed thrusters. Given that the two measures can both reduce the offset and the power consumption of the semi-submersible unit, it is suggested to combine the two measures together to obtain better positioning accuracy. This paper is helpful to engineering practice.

On the Intrinsic Mechanism of Wakes Interference Past Two Cylinders Towed Beneath a Free Surface

An experimental study of the flow past a set of two horizontal cylinders is presented. The cylinders are towed in a uniformly accelerated and decelerated motion in a visualization tank in order to enhance the vortex effects. The main cylinder (D = 0.04 m; L/D = 16) is placed in the flow past a front one (d = 0.002m; L/D = 16). They are towed beneath the free surface and the drag and lift forces are measured. The main cylinder wake pattern is visualized by an embarked CCD camera. The Reynolds number based on the maximum velocity is from 0 to 14000 and the Froude number based on the main cylinder immersion from 0.2 to 1.2 for an acceleration value of 0.15m.s-2. It is shown that the near wake is made of a combination of the main cylinder Von Karman vortices and those of the front cylinder. The interference phenomenon and the free surface effects are studied by varying the depth parameter and the two cylinders arrangements.