Carlos Guedes Soares

A 40 Year Hindcast of Wind, Sea Level and Waves in European Waters

The paper describes the initial efforts in a project whose objective is to obtain a 40-year hindcast of wind, sea level and wave climatology for European waters. The 40-year global atmospheric re-analysis carried out by the National Centre for Environmental Prediction, Washington, USA (NCEP) and the National Centre for Atmospheric Research, Boulder, Colorado, USA (NCAR) will be used as forcing of limited area atmospheric models. The fine grid atmospheric fields will be used to force state-of-the-art wave models (WAM) and sea level models (HAMSOM and TELEMAC) in regional areas around Europe so as to produce climatic information on waves, sea levels, and currents in a very large extend of the European waters, including the Mediterranean, North East Atlantic and North Sea. The available satellite data, including wind, wave and sea-level data, will be collected and will be used to be compared with the hindcast results, so as to yield uncertainty measures related to the data. Statistical analysis of the produced atmospheric, sea level and wave hindcast and remote sensed data will be performed in order to provide information about the climatological trends in the European Waters and Coastal Seas.Copyright

Bending Moments of an FPSO in Rogue Waves

The increasing numbers of reported rogue waves with extreme crest and wave heights and unusual group pattern with the consequence of severe damages raise the question if such exceptional events have to be considered routinely for the design of ships and offshore structures. For the investigation of the effects of rogue wave impacts time domain simulation methods are required in addition to traditional frequency domain methods which may not be sufficient to consider these extreme events. In this paper the vertical bending moments at the midship section of an FPSO are investigated using state of the art numerical simulation tools in combination with experiments. For the seakeeping tests the extremely high New Year Wave (registered in the North Sea) is generated in the wave tank, and motions and structural forces are analyzed at model scale. For validation the results are evaluated deterministically and compared to numerical simulations. The time domain calculation allows to artificially change local wave characteristics. The steepness of the selected rogue wave is varied and the influence on wave induced loads is studied. A comparison with standard procedures of seakeeping analysis and classification rules closes the paper.Copyright

Chapter 6 Probabilistic models of waves in the coastal zone

Publisher Summary This chapter summarizes the wave fields that are described by stochastic models. It is possible to describe them both in the frequency and in the time domains. The focus is mainly on Gaussian waves in deep water emphasized that the characteristics of waves in finite water depths are substantially different than those in deep water where the probability distribution is non-Gaussian. Refinements to the various models continue to provide solutions to different problems, and to meet required design and engineering considerations. The models account for the correlation among sea states and are useful for planning maritime operations, including construction of coastal structures.

Effects of Common Structural Rules on hull-girder reliability of an Aframax oil tanker

Abstract This paper aims at quantifying the changes in notional reliability levels that result from redesigning an existing Aframax tanker to comply with the Common Structural Rules (CSR) for double-hull oil tankers. The probability of structural failure is calculated using the first-order reliability method. The evaluation of the wave-induced load effects that occur during long-term operation of the ship in the seaway is carried out in accordance with the International Association of Classification Societies (IACS)-recommended procedure, while transfer functions are calculated using the sink–source 3D linear method. The still-water loads are defined on the basis of a statistical analysis of loading conditions from the loading manual. The ultimate collapse bending moment of the midship cross section, which is used as the basis for the reliability formulation, is evaluated by progressive collapse analysis and by a single-step procedure according to CSR. The reliability assessment is performed for “as-built” and “corroded” states of the existing ship and a reinforced ship complying with CSR. It is shown that the hull-girder failure probability of an Aframax tanker is reduced several times due to the reinforcements according to CSR. Sensitivity analysis and a parametric study are performed to investigate the variability of results with the change of parameters of pertinent random variables within their plausible ranges. Finally, differences between load combination approaches by Ferry-Borges and Castanheta method and Turkstras rule are investigated.

Whipping Response of Vessels With Large Amplitude Motions

The paper presents a time domain method to calculate the ship responses in heavy weather, including the global structural loads due to whipping. Since large amplitude waves induce nonlinear ship responses, and in particular highly nonlinear vertical structural loads, the equations of motions and structural loads are solved in the time domain. The “partially nonlinear” time domain seakeeping program accounts for the most important nonlinear effects. Slamming forces are given by the contribution of two components: an initial impact due to bottom slamming and flare slamming due to the variation of momentum of the added mass. The hull vibratory response is calculated applying the modal analysis together with direct integration of the differential equations in the time domain. The structural dynamic characteristics of the hull are modeled by a finite element representation of a Timoshenko beam accounting for the shear deformation and rotary inertia. The calculation procedure is applied to a frigate advancing in regular waves. The contribution of whipping loads to the vertical bending moments on the ship structure is assessed by comparing this response with and without the hull vibration.Copyright

Calculation of Second Order Drift Forces on a FLNG Accounting for Difference Frequency Components

The usual procedure for calculation of slow drift forces is to simplify the quadratic transfer function by representing the difference frequency components in terms of the zero difference results. In this way the second order problem is much simplified as well as the computational effort. However this approximation has some limitations and in particular for the slow drift oscillations problem it may be important to consider correctly the difference frequency components. The paper presents an analysis of the slow drift exciting forces on a FLNG (Floating Production Storage and Offloading system for production of LNG). The complete second order forces are computed by WAMIT V6.1s which is based on a boundary element method. Results of the horizontal second order drift forces are presented for several difference frequency values. Deep water and two shallow water depths are considered, since water depth seems to have an important effect on the quadratic transfer functions.Copyright

Sensitivity of the Expected Ships Availability to Different Seakeeping Criteria

The paper presents a methodology to calculate the seakeeping performance of ships, which is given as an operability index, and discusses the sensitivity of the results to the use of different seakeeping criteria. The calculation of the operability index, which represents the percentage of time during which the ship is operational, depends on the wave climate of the ocean area where the ship operates, the dynamic response of the ship to the waves, and the ship mission. The relation between the ship operability and the mission characteristics is established through the seakeeping criteria. The calculation of operability indexes and the sensitivity analysis are carried out for a containership operating in the North Atlantique between Europe and the United states, and a fishing vessel operating near the Portuguese west coast. These are two ships with different mission profiles, which permits assessment of the sensitivity of the estimated operability index to different ship types.Copyright

Parameter Identification of Ship Manoeuvring Model Based on Particle Swarm Optimization and Support Vector Machines

The method of system based manoeuvring simulation provides an effective way to predict ship manoeuvrability. Accurate determination of the hydrodynamic derivatives in the mathematical model of ship manoeuvring motion is vital to the prediction accuracy. A support vector machines (SVM) based approach is proposed in this paper. By analyzing the data from free-running model tests of KVLCC2 ship, the hydrodynamic derivatives in an Abkowitz model are identified. To diminish the parameter drift in the identification, a difference method is adopted to reconstruct the sample for identification. To obtain the optimized structural parameters in SVM, particle swarm optimization (PSO) method is incorporated into SVM. Predictions of manoeuvring motion are presented based on the regression model. Comparisons between the predicted results and the test results demonstrate the validity of the proposed methods.© 2013 ASME

A Boundary Integral Equations Method for Computing Inertial and Damping Characteristics of Arbitrary Contours in Deep Fluid

Abstract A modification of Yeung’s boundary-integral method of solving the linear boundary-value problem related to the frequency-domain hydrodynamic characteristics of arbitrary smooth contours intersecting the free surface of the fluid has been further developed. The modification relates to the integral equation’s discretization step and consists in substituting the orthodox one-point collocation with another collocation method satisfying the integral equation in the mean over each rectilinear segment in the polygonal approximation of the contour. Relatively complicated in fluence functions were evaluated analytically. Numerical tests demonstrate that the suggested method shows better accuracy at equal number of panels (segments) and is robust and reliable enough to be recommended for practical calculations.

Global Loads on an FPSO Induced by a Set of Freak Waves

The paper presents a systematic study of the structural loads induced by abnormal waves on a FPSO. This work is a follow-up of a previous investigation that explored the possibility of using freak, abnormal or episodic waves as additional wave load conditions to be considered in the design of ships and offshore platforms. In the previous work, a procedure was developed and implemented to adopt deterministic time series of wave elevation, which may include abnormal waves, as reference design conditions to calculate the wave induced structural loads on ships. An application example was presented for a FPSO subjected to the well known New Year Wave trace that was measured during a severe storm in Central North Sea. In the present paper, the same procedure is applied to obtain the wave induced structural loads on a FPSO, but a systematic investigation is carried out by using a large set of wave traces. These wave traces have been measured at different occasions in the North Sea, additionally at one location in the Central Gulf of Mexico, and they all include episodic freak waves. In this way it is possible to assess the influence of realistic rogue wave characteristics on the wave induced structural loads. Finally, and based on the platform responses to all wave traces, some statistics are produced, regarding the platform responses and structural loads induced by rogue waves.Copyright