Seakeeping Tests Research Articles

The Taming of the Shrew: Tailoring Freak Wave Sequences for Seakeeping Tests

If you want to build a ship, don't drum up people together to collect woodand don't assign them tasks and work but rather teach them to long for the endless immensity of the sea.___Antoine de Saint-Exupery (1900–1944)___The basic law of the seaway is the apparent lack of any law.___Lord Rayleigh___Based on the wave focusing technique for the generation of task-related wave packets, a new technique is proposed for the synthesization of tailored design wave sequences in extreme seas. The physical wave field is fitted to predetermined global and local target characteristics designed in terms of significant wave height, peak period, as well as wave height, crest height, and period of individual waves. The generation procedure is based on two steps: First, a linear approximation of the desired wave train is computed by a sequential quadratic programming method that optimizes an initially random phase spectrum for a given variance spectrum. The wave board motion derived from this initial guess serves as a starting point for directly fitting the physical wave train to the target parameters. The subplex method is applied to improve systematically a certain time frame of the wave board motion, which is responsible for the evolution of the response-related design wave sequence. The discrete wavelet transform is introduced to reduce significantly the number of free variables to be considered in the fitting process. Wavelet analysis allows the efficient localization of the relevant information on the electrical control signal of the wave maker in time and frequency domain. As the presented technique permits the deterministic generation of design rogue wave sequences in extreme seas, it is well suited for investigating the mechanism of arbitrary wave-structure interactions, including capsizing, slamming, and green water as well as other survivability design aspects. Even worst case wave sequences, such as the Draupner New Year wave, can be modeled in the wave tank to analyze the evolution of these events and to evaluate the response of offshore structures under abnormal conditions. This procedure is illustrated by investigations of an offshore lift operation, of the behavior of a semisubmersible and an FPSO in tailored freak waves as well as the analysis of ship capsizing in deterministic wave sequences at selected target positions.

Some methods to obtain the added resistance of a ship advancing in waves

Added resistance in waves is an important part of ship dynamic due to its economical effect on ship exploitation. Can the ship sustain speed in a rough sea state? If not, this will produce delays and economical losses if added resistance is not taken into account in the propulsion design. There are not many simple methods to obtain the added resistance in waves of a ship, and the validity of the results is not always good enough for different ships. In this paper, some theories that can be used to predict the added resistance of a ship are studied and validated against seakeeping tests of some monohull models. Tests and results focus in head seas, which are the most severe for the added resistance. Experimental results are compared with numerical calculations and conclusions about the range of application of the presented theories are obtained.

Strip theories applied to the vertical motions of high speed crafts

The motions of a high speed craft are highly influenced by speed and dynamic forces that begin to be important for high Froude numbers. Classical ship motions theories and some seakeeping programs do not include the effect of these dynamic forces that mainly affect to the damping of vertical motions, and have to be corrected to model high speed crafts. In any other way, the use of these theories or programs would be unrealistic. In this paper, some theories that can be used to predict the seakeeping behaviour of high speed crafts, considering dynamic forces, are studied and validated against seakeeping tests of some fast monohulls models. Tests and results focus on vertical motions in head seas, which are the most severe for these fast crafts. Experimental results of vertical motions are compared with numerical calculations and conclusions about the range of application of the presented theories are obtained.

An optimal controller of an irregular wave maker

Laboratory seakeeping test under irregular wave load is essential for studying seakeeping performances of marine and offshore structures. One of the key techniques in such a sort of test is how to assure the quality of wave load. By assuring the quality of wave load, we mean that man-made irregular wave series exerting on a test model should have a given power spectrum. From a view of optimal control, the goal to assure the quality of wave load is to manage wave maker such that produced irregular wave series exerting on a test model has a given power spectrum in the optimization sense. This paper gives a control model to optimize wave maker to achieve the mentioned goal. Its application to wave maker control in a seakeeping model test of an offshore structure is demonstrated.

Investigation of Slamming Loads Using Slam Patches On A Scale

A model of an Open60’ class boat has been instrumented with slam patches in order to measure hydrodynamic forces acting on the bottom of the hull as a result of slamming. Drop tests and seakeeping tests have been performed to investigate the relative influence of different variables on the magnitude of slamming loads. Results are presented together with a discussion of the issues involved with this particular measurement technique.

Fundamental Characteristics of Motion in Waves of a Fishing Vessel with Bulges

In order to improve the stability and to increase the working area for fishing vessel, bulges are often added on ship sides from underwater part up to upper deck. From the view point of seakeeping quality, it is important to clarify the effect of bulges on ship dynamic response as well as static stability. A fisheries research vessel“Ushio-maru”has been rebuilt (elongated and bulges fitted) recently. In this paper, we present the results of investigation into its stability improvement and study on the influence of bulge on its motion in waves by model experiments and theoretical calculations. It was shown that stability improvement is considerably achieved by fitting bulges. In addition, resonant roll motion was slightly reduced due to damping effect of bulges. It was also found that there are no significant difference in heave and pitch due to the existence of bulge. On the other hand, roll motion in beam and quartering long waves increase. Furthermore, at-sea seakeeping tests of the ship with bulges were conducted and then motion characteristics in real ocean waves is described. Finally, authors point out the merits and demerits of fitting bulge to fishing vessel.

Measurement of Global Loads on a Full-Scale SES Vessel Using Networks of Fiber Optic Sensors

A method for measurement of global loads on a vessel, using strain measurements from a network of strain sensors and extensive Žnite-element analyses (FEA) with idealistic load cases, is presented. The sensors are attached at carefully selected locations at a cross section amidships, where the most signiŽcant loads have their maximum values. The Žnite-element (FE) strain values are used in precalculations to reduce the computational effort to solve a set of linear algebraic equations in real time. The method has been successfully tried out on the full-scale surface effect ship (SES) KNM Skjold (47 m, 270 tons, and 50 knots) instrumented with a network of Žber optic Bragg strain sensors. The seakeeping tests were performed in the North Sea (1999) in sea states 3, 5, and upper 6. Other parameters that were varied during the test program were the heading, forward speed, and on/off-cushion. The loads measured during the tests were the sagging/hogging moment, the horizontal bending moment, and the longitudinal twisting moment. The measured loads were compared with respective dimensioning loads based on DNV rules (HSLC) (1996). The results showed that when the vessel is subjected to bow flare slamming impacts, the sagging moment exceeded the DNV dimensioning value by a factor of 1.6, indicating that the HSLC rules do not provide conservative global loads when these extreme events occur. However, the peak loads were well within the design limits.

Ship hull structure monitoring using fibre optic sensors

We discuss the need for ship hull monitoring and the roles such asystem may fill during the different stages of a ship's lifetime. We havefound that fibre optic sensors are well suited for this application andpresent the fibre Bragg grating technology that has been employed in thecomposite hull embedded sensor system project (CHESS). Signal processing isthe key to real-time structure monitoring, and we comment on the modularsignal processing system that is being developed at FFI. Finally we presentthe CHESS installation on a new Norwegian naval vessel and show some resultsobtained with this system in systematic sea-keeping tests. The CHESSmeasurements on the new Norwegian fast patrol boat have been instrumental inthe design verification of the vessel.

Nonlinear effects in 2D transient nonbreaking waves in a closed flume

The nonlinear effects deriving from the focussing of unidirectional waves propagating in a wave flume are analysed by direct fully nonlinear numerical simulation. The main aim of this study is to evidence the features of the wave–wave interaction in the spatial region, where the maximum elevation of the component waves of a given wave group is almost in phase. It will be shown that in the frame of the inviscid fluid hypothesis, the nonlinear interaction of waves leads to an evident space variation of the amplitude spectrum with a large magnification factor of the focused wave elevation compared to the result from linear superposition, a phase shift of the component waves occurs and part of the energy content is shifted to frequencies well above the highest frequency in the input spectrum. These features have basic consequences on the safety and design aspects of ships and marine structures: for instance, this regards the ringing response of large offshore structures. At the same time the availability of kinematic data from the simulation of large waves can be useful for the evaluation of the forces exerted on slender surface piercing bodies in inertia-drag regime method without semi-empirical stretching methods. Moreover the controlled generation of deterministic wave groups with a given frequency content, aimed at the development of new techniques for seakeeping tests, is one of the subjects of interest in ITTC (23rd International Towing Tank Conference).

Task-related wave groups for seakeeping tests or simulation of design storm waves

The chaotic wave field of a natural seaway can be decomposed into an infinite number of independent harmonic waves, and its spectrum follows from the associated wave amplitudes and frequencies. If superimposed with random phase we register the well-known irregular sea, which is characterized by its significant wave height and zero-up-crossing period. As rare events very high waves are observed accidentally. Since RAOs of wave/structure interactions are independent of the random phase shift between superimposing component waves this parameter can be selected arbitrarily to compose an optimum and short-duration transient wave train which allows the precise determination of all response amplitude operators within the relevant spectral range. Applications of the wave group technique are presented for: (i) standard seakeeping tests of stationary or moving (self-propelled) marine structures; (ii) simulation of design storm waves for the investigation of coastal and offshore structures. The paper illustrates the generation of task-related wave packets, the determination of the associated acceleration, velocity and pressure fields, as well as the related energy flux. Based on the dispersion relation the propagation behavior is exactly predictable. Consequently, the kinematics and dynamics of the wave field can be determined at any position and time. If the converging wave group approaches its concentration point the associated particle motions are analyzed by a nonlinear procedure using coupled Lagrangian expansion equations. The efficiency and the limitations of the transient wave technique are demonstrated by presenting typical test examples. These include the determination of the RAOs of stationary offshore structures and towed or self-propelled ships as well as the investigation of coastal structures in 100-year waves. As the entire process is deterministic, the action/reaction chains can be evaluated in detail. The paper demonstrates that the wave group technique is a reliable and efficient tool for all standard investigations related to wave/structure interactions, and opens a new area for the analysis of transient processes in the sea, e.g. dynamic stability of floating vessels or design wave impacts on coastal or offshore structures.

Design and Hydromechanic Aspects of the Amphibious Transport Vessel for the Royal Netherlands Navy

In order to meet an increasing demand of amphibious lift capacity and logistic support, the Royal Netherlands Navy decided to procure an amphibious transport vessel (ATS), designed as a Landing Platform Dock (LPD). In the first part of this paper an outline of missions and tasks is provided, followed by a description of the preliminary design which has been produced by the Directorate of Materiel of the Royal Netherlands Navy. The ATS, now in the final stage of the forward design phase, will be able to transport a number of landing craft, six LCVP MK HI of the R Nl Navy or four LCVP MK 9B's of the Royal Navy, in a well dock. A major requirement is that embarking and disembarking operations will be possible for sea conditions up to Sea State 4. These operations can only be possible with low wave motions in the well dock. An extensive research program was carried out to investigate the hydromechanic aspects of the well dock design, in order to assess the safety of embarking and disembarking of landing craft and to minimize wave motions in the dock. After performing reference seakeeping tests with a model of a proven design (Fearless class, RN) at MARIN, a preliminary design of the ATS was tested: motion and wave measurements as well as observations with radio controlled models of landing craft. A number of options for improvement of the well dock configuration were investigated. The research provided insight in the relation between design and environmental parameters and wave motions in the dock, and resulted in a well dock design optimized for embarking and disembarking operations within design constraints.

Modelling techniques for dynamics of ships

I discuss techniques for the modelling of the dynamics of ships in an experimental tank. Advances in computational fluid dynamics have made the numerical tank a real possibility, and I discuss the relation between the numerical and the experimental tank. Current techniques for modelling rigid ships, i.e. the seakeeping test, and flexible ships are investigated. Finally, the future prospects of these techniques are discussed.

Gaussian wave packets — a new approach to seakeeping testsof ocean structures

The transient wave train techniques has been widely used in testing ocean structures. The methodpresented here is based on a special Gauss-modulated amplitude spectrum. These wave groups of limited length can be superimposed, the actual surface elevation being a function of packet characteristics and initial time lag. The application of this technique is demonstrated in three cases presenting a semisubmersible,an articulated tower and a floating oil skimmer. It is shown, that a particular problem needs a tailored wave packet containing sufficient energy all over the relevant frequency range. These packets are designed by superimposing individual Gaussian wave groups. The new seakeeping test techniques has the following advantages u - The wave train is well defined at any location of the tank. - The wave elevation and spectrum can be standardised or easily adapted to any specific problem. - The duration of the test is very short; reflections (beach) do not interfere the results. At theculmination point the length of the wave train is small. This facilitates short duration seakeeping and manoeuvring tests. - The results show high resolution and are in good agreement with regular wave test data. Summarising, the Gaussian wave packet method is a versatile technique yielding precise and highly resoluted results in a short time.

M/V Bernier-A Unique Canadian Seismic Vessel

Abstract Petro-Canada's new seismic vessel, the "M/V Bernier" is a world-class geophysical data acquisition vessel designed and built in Canada. Since its commissioning in July 1983, the vessel has operated in the Scotian Shelf and Grand Banks areas, offshore Barbados, and is presently shooting 3-D seismic in the Gulf of Thailand. The expense of offshore exploration programs demands that the highest quality seismic data be available to geologists and geophysicists. For this reason, great emphasis was placed on designing a vessel which was optimized for seismic data acquisition using state-of-the-art equipment and technology. In order to meet the needs of both Petro-Canada's offshore frontier exploration programs and those of Petro-Canada International Assistance Corporation (PCIAC) world-wide, special features were incorporated into the vessel design to enable extended operations to be carried out both in arctic and tropical waters. To ensure high-quality data collection a hull form was developed, which was hydrodynamically quiet and acoustic transmission was minimized by extensive vibration isolation of machinery. Petro-Canada required a high degree of safety and survivability. For this reason, the vessel was designed to the same stringent stability standards as passenger vessels and is ice-strengthened to fulfill the requirements of the Canadian Arctic pollution prevention regulations for a ‘Type B’ vessel. A maximum crew complement of 41 persons including company representatives, scientists, seismic and marine operating personnel can be accommodated. The design was carried out by Evan's, Yeatman and Endal (Associates) Limited, of Dartmouth, Nova Scotia who worked closely with Petro-Canada staff. Detailed design commenced in June 1981. By October, the design was complete and construction bids were being sought from Canadian shipyards. Hull Design The performance criteria set by geophysicists required the ability to tow a 5 km long streamer cable and a 61,800 cm3 (3772 in.3) air gun array, at 6 knots, firing the air guns at 10-second intervals. This requires approximately 80 m3/min. (2800 ft3/min.) of compressed air at 14 MPa (2000 psi). Extensive optimization of the hull was carried out by computer analysis and model tests at the Netherlands Ship Model Basin (N.S.M.B.). Conventional resistance, propulsion, seakeeping and maneuvering tests were conducted in towing tanks using a 1:25 scale model. A 1:10 scale model was used in a depressurized towing tank to study wake distribution and propeller cavitation inception. Main Machinery and Propulsion Considerable detailed study of suitable propulsion and power management systems was undertaken. Seismic vessels have widely varying power requirements due to the high compressor load at slow ship speed while shooting, and a requirement for rapid transit, in this case 12 – 14 knots, between survey areas. The final choice, based on flexibility, reliability and efficiency was a fully integrated A.C./D.C. diesel electric propulsion system. It is commonly considered that diesel electric vessels area complex and expensive, however, using proven off-the-shelf equipment originally developed for the drilling and mining industries, the cost penalty for a modern diesel electric system is small and easily amortized.

Methodology and results of full scale seakeeping tests performed by using a fully on board instrumentation set1

Description d'une methode d'analyse des signaux en provenance du detecteur d'ultrasons et de la plateforme stabilisee pour obtenir le spectre de la mer ainsi que le spectre des reponses des mouvements et leurs operateurs d'amplitude de reponse. Resultats obtenus pendant les essais de la fregate de la marine italienne, comparaison en terme d'operateurs d'amplitude de reonse entre ces donnees, resultats theoriques et resultats d'essais sur modele

Sea-induced motions of the OTTO HAHN’s nuclear steam generator

Seakeeping tests were run with the nuclear ship OTTO HAHN in a stationary wind-generated seaway measuring the wave height and the ship’s responses speed, pitch, roll, and accelerations for course increments of about 22 degrees. The measured motion sp

Stern Trawler Designs

This paper describes generally the various design phases required in the development of stern trawlers, and covers in particular development of the first of a new series of stern trawlers suitable for operations off the Canadian Eastern Atlantic. It is based on an extensive study, initiated by the Department of Fisheries of Canada, and on a comprehensive series of resistance, propulsion, and seakeeping tests carried out on three different models by the National Research Council of Canada. The paper also deals with the techno-economic factors necessary to be considered by the owner before he makes the decision to build.

On the Analysis of the Ship Oscillations among Waves (Part 4)

The general aspects of the data processing system, to get the response character of the ship statistically from the results of the seakeeping tests of actual ships, as well as of the model tests in irregular waves in the experimental tanks were discussed. A data processing system that is now under operation in the Ship Research Institute was introduced together with the descriptions on each elements of that system and the computing programs for the electronic computor to do the spectrum and correlogram analysis as a link of this system. An example of the application of this system to the actual ship performance test is reported, together with an attempt of the application of the results. Summary of the process to design the experiments, suitable for this kind of analysis, and some thoughts on the future development of this system were also mentioned.

The Design of Hull Forms for Supertankers and Bulk Carriers

This paper deals with the results of model resistance, propulsion and seakeeping tests conducted on a form which has a block coefficient to 0.835 and a large bulb at the fore-foot, intended for use in tankers and bulk carriers. If details, also, the results of similar tests conducted on a special design of hull form for a Great Lakes vessel, the object of which is to determine whether or not satisfactory performance can be achieved at sea, as, today, vessels built for service on the Great Lakes are no longer landlocked owing to the completion of the St. Lawrence Seaway.

Some seakeeping tests with a victory model12

Seakeeping experiments with a model of a Victory ship have been carried out in the Netherlands Ship Model Basin. The motions of the model as well as the power increase in waves were measured. Experiments have been carried out in waves having differen