No abstract

Abstract-Real time estimation of waves and ship responses using onboard measurements has been under investigation in recent years. This has been done using different methods, including parametric and non-parametric models. Since none of the methods are believed to be fully accurate, it is important to assign an uncertainty measure to the waves and responses that are being estimated. In this paper, a parametric model approach based on moments of responses is considered for wave estimation. A method based on linear error propagation is introduced to assess the uncertainty of wave estimations. The uncertainty of response calculation based on the estimated wave is also quantified.

An accurate estimation of the ocean wave directional spectrum at the location of an advancing ship is very useful for the ship master to improve operation and safety in a seaway. Research has been conducted to obtain sea state estimates by the Wave Buoy Analogy. The method deals with processing the ship’s wave-induced responses based on different statistical inferences including parametric and non-parametric approaches. This paper considers a concept to improve the estimate obtained by the parametric method for sea state estimation. The idea is illustrated by an analysis made on full-scale response measurements carried out on a 9400 TEU container ship. The process of fitting a standard JONSWAP spectrum takes into account an analysis of residuals formed by the difference between measured and estimated amount of energy of the response(s). A couple of motion responses and also bending moment are considered as the input of the estimation process. A comparison is made between the results and also with some inhand outputs from other estimation sources, e.g., wave radar measurements and sea surface elevation by microwave sensors. The discussed and analyzed procedure could also lead to an automatic selection of response combinations for sea state estimation.

Dynamic motions of Ships in severe conditions of sea may be undesired and should be controlled by some devices. The roll motion is much more significant than the other oscillations which can affect comfort, safety and efficiency of navigation at sea. This motion is controlled by some common stabilizers such as rudder, fins, etc. They may be divided into two categories of passive and active stabilizers or controllers. However controlling devices may be high in cost. Moving masses are employed to reduce the motions of floating vessels or offshore structures. This type of stabilizers consists of a mass with one or more springs and dampers. The movement of the mass produces an inertia force and moment to absorb wave excited motions. In this study the effectiveness of moving masses in vessels with large roll motion is investigated. A rolling vessel equipped with a moving mass can be modeled as a two-degree of freedom system. As in large amplitude roll motion the nonlinearities cannot be neglected, a mathematical model with second-order damping and fifth-order restoring moment is implemented as roll equation in beam waves. Applying to a trawler, this equation is solved simultaneously with the equation of movement of the mass in both frequency-domain and time-domain by analytical and numerical approaches. The computation shows that with an optimum passive control, a moving mass with 0.5% of the displacement of the ship is an effective device in roll minimization and enhance the vessel’s stability in high load conditions.

Abstract-This paper presents a trend analysis for prediction of sea state parameters onboard ships during voyages. Given those parameters, a JONSWAP model and also the transfer functions, prediction of wave induced ship responses are thus made. The procedure is tested with full-scale data of an in-service container ship. Comparison between predictions and the actual measurements, implies a good agreement in general. This method can be an efficient way to improve decision support onboard ships.