Hull Flexibility Research Articles

A Fully Coupled Time-Domain BEM-FEM Method for the Prediction of Symmetric Hydroelastic Responses of Ships with Forward Speed

This paper presents a direct time-domain method for the prediction of symmetric hydroelastic responses of ships progressing with forward speed in small amplitude waves. A transient time-domain free surface Green function is used for the idealisation of the seakeeping problem using an Earth fixed coordinate system. Free surface ship hydrodynamics are idealised in the time domain by a Green function, and forward speed effects are idealised by a space-state model. Modal actions are accounted for by Timoshenko beam structural dynamics. Flexible fluid structure interaction (FFSI) coupling is enabled by a body boundary condition, and a direct integration Newmark- β scheme is used to obtain symmetric dynamic responses. The method is validated against available published numerical and experimental results. A parametric study for different container ship hull forms confirms that (i) forward speed effects should be taken under consideration as far as practically possible and (ii) hull flexibility effects accounting for hull shear deformation and rotary inertia are more notable for slender hull forms.

Springing Analysis of a Passenger Ship in Waves

Traditionally, the evaluation of global loads experienced by passenger ships has been based on closed-form Classification Society Rule formulae or quasi direct analysis procedures. These approaches do not account for the combined influence of hull flexibility, slenderness, and environmental actions on global dynamic response. This paper presents a procedure for the prediction of the global wave-induced loads of a medium-size passenger ship using a potential flow Flexible Fluid Structure Interaction (FFSI) model. The study compares results from direct long-term hydro-structural computations against Classification Society Rules. It is demonstrated that for the specific vessel under consideration: (a) the elastic contributions of the responses on loads are negligible as springing effects occur outside of the wave energy spectrum, (b) deviations of the order of 28% arise by way of amidships when comparing direct hydrodynamic analysis predictions encompassing IACS UR S11A hog/sag nonlinear correction factors and the longitudinal strength standard, and (c) the interpretation of the wave scatter diagram influences predictions by approximately 20%. Based on these indications, it is recommended that further parametric studies over a range of passenger ship designs could help draw unified conclusions on the total influence of global and local hydrodynamic actions on passenger ship loads and dynamic response.

Effects of Hull Flexibility on the Structural Dynamics of a Tension Leg Platform Floating Wind Turbine

AbstractDynamic analysis of floating wind turbines often considers the hull as a rigid body. This paper explores the consequences of modeling the pontoons of a tension leg platform (TLP) wind turbine as flexible beams. The analysis is based on numerical simulations of free decays, structural response to wave excitation, and short-term fatigue damage accumulation at tower base and tendons. In addition, the importance of hydroelastic effects due to the pontoons’ vertical deformations is evaluated. Pontoon flexibility changed the platform natural periods and motion amplitude significantly, and the adoption of flexible pontoons reduced the predicted fatigue damage in the tower base and tendons. On the other hand, hydroelasticity had negligible consequences for motion and load responses considered here.

Om bord på vikingetidens langskibe – En analyse af besætningsorganisation og kommunikation

On board Viking Age longshipsAnalysing crew organisation and communicationBy analysing the relations between man, ship and sea it is possible to gain a fundamental understanding of the sea voyages undertaken by Scandinavians during the Viking Age. This article examines crew organisation and communication on board Viking Age longships. Archaeological and written sources relating to Viking Age Scandinavian seafaring are scrutinised and combined with experience gained from sailing trials and voyages employing full-scale reconstructions of Viking Age longships: Conditions on board Viking Age longships and the seamanship of the period are investigated afresh (figs. 1-3).It is argued that sailing and rowing experiments are important components in the analysis of archaeological finds of ships and ship-related materials. These experiments function as catalysts for processes that may lead to new interpretations of the source material (fig. 4). The practical approach represented by participating as crew on a reconstruction of an archaeologically-found ship offers possibilities for examining aspects like the flexibility of the hull, the seaworthiness and speed of the vessel under different weather conditions and logistical limitations, as well as human factors such as the physical abilities and nautical skills required of the crew. This article presents examples of how sailing trials and voyages with the Viking Ship Museum’s Skuldelev 2 reconstruction The Sea Stallion from Glendalough have led to a new understanding of the hull flexibility and strength of Viking Age longships. It is also argued that studies of crew organisation and communication on board The Sea Stallion from Glendalough provide a fundamental insight into conditions on board Viking Age longships (figs. 5-8).A middleman is proposed as a key element in the communication on board a longship. The middleman is located close to the mast and is responsible for relaying commands and messages from the skipper, situated in the aft of the ship, towards the bow of the vessel, as well as relaying the look-out’s observations back to the skipper. In relation to this, it is interesting that sailing scenes depicted on the Bayeux Tapestry often highlight three people on board the vessels: One aft (the skipper or helmsman?), one amidships, close to the mast (the middleman?), and one close to the bow (the look-out?). These three people seem to communicate with each other, either verbally or using gestures (figs. 9-10).In conclusion, based on the experience gained from the sailing trails and voyages employing full scale reconstructions of Viking Age longships, it is evident that the large crew and limited space on board calls for a well-defined organisation with a clear and unambiguous command structure. When combined with the need to trust and protect each other while at sea in an open boat, this creates close-knit communities of practice, as also seen among soldiers in military units. It seems likely that the practices and living conditions on board the longships contributed greatly to the creation of the Vikings’ notoriously efficient and coherent military units (fig. 11).Morten RavnVikingeskibsmuseet i Roskilde

Two- and three-dimensional springing analysis of a 16,000 TEU container ship in regular waves

In recent years, the increase in world trade has resulted in a large expansion of sea traffic. As a result, market demands are leading to the development of Ultra Large Container Ships (ULCSs), with lengths of up to 400 m and increased flexibility of operational requirements. The multicellular open-decked thin-walled structural design of these ships means that flexible hull girder dynamics become important for the prediction of wave loads. This paper investigates the importance of various hydroelastic modelling approaches on the global symmetric and anti-symmetric response of a 16,000 twenty-foot equivalent unit (TEU) ULCS design. Two- and three-dimensional linear and weakly non-linear flexible fluid–structure interaction models that respectively combine Vlasov beam and three-dimensional finite element analysis (FEA) structural dynamics with a B-spline Rankine panel and Green's function hydrodynamics are assessed and compared. Comparisons between rigid body and hydroelastic predictions demonstrate the importance of considering the effects of hull flexibility on the dynamic response and the suitability of different idealisations at preliminary or detailed design stages.

Investigation into the nonlinear hydroelastic response of an 8600-TEU containership model advancing in regular waves

With the increase in the size of containerships, the lowest wet natural frequencies can be as low as 0.5 rad/s for vessels whose lengths are beyond 300 m. These lower frequencies make the estimation of wave loads when considering the hull flexibility quite challenging. Because no numerical tools are able to account for all of the nonlinear effects properly and there is not enough experimental data available for validation, the extent that current numerical tools can correctly predict the hydroelastic effects is a concern. This paper adopts a nonlinear hydroelastic code, WINSIR, to calculate the nonlinear hydroelastic response of an 8600-TEU containership model advancing in regular waves. The calculated results are compared with test data in terms of the first, second, third, fourth and fifth harmonics of the vertical bending moment amidships. Finally, the uncertainties in the experiments are discussed, and conclusions are given at the end of this paper.

High speed marine craft motion mitigation using flexible hull design

High speed marine craft motions can be severe and concerns regarding human performance and safety are widespread. The motions can reduce crew abilities to perform tasks and can result in injury. This is especially of concern for military and/or rescue operations and means to reduce or mitigate against motion exposures are required to protect the occupants. In this paper the motion mitigation provided by various ‘flexible’ hull systems during a slam event is examined numerically, following an extended introduction describing high speed marine craft motion effects and whole body vibration and repeated shock. Introducing hull flexibility to isolate the occupants from the externally induced disturbances can either exacerbate or alleviate the problem depending on the design.

Estimation of hull girder vertical bending moments including non-linear and flexibility effects using closed form expressions

A simple but rational procedure for prediction of extreme wave-induced hull girder bending moment in slender mono-hull displacement vessels is presented. The procedure takes into account main ship hull characteristics such as length, breadth, draught, block coefficient, bow flare coefficient, forward speed, and hull flexibility. The wave-induced loads are evaluated for specific operational profiles. Non-linearity in the wave bending moment is modelled using results derived from a second-order strip theory and water entry solutions for wedge-type sections. Hence, bow flare slamming is accounted for through a momentum type of approach. The stochastic properties of this non-linear response are calculated through a monotonic Hermite transformation. In addition, the impulse loading attributable to, for example, bottom slamming or a rapid change in bow flare is included using a modal expansion in the two lowest vertical vibration modes. These whipping vibrations are added to the wave frequency non-linear response, taking into account the rise time of the impulse response as well as the phase lag between the occurrence of the maximum non-linear load and the maximum impulse load. Previous results for the sagging bending moment are validated by comparison with fully non-linear strip theory calculations and supplemented with new closed form results for the hogging bending moment. Focus is on the extreme hull girder hogging bending moment. Owing to the few input parameters, this procedure can be used to estimate the wave-induced bending moments at the conceptual design phase. Another application area is for novel single-hull ship types not presently covered by the rules of the classification societies. As one application example the container ship MSC Napoli is considered. Further validations are needed, however, in order to select proper values of the parameters entering the analytical form of the slamming impulse.

Service Factor Assessment of a Great Lakes Bulk Carrier Incorporating the Effects of Hydroelasticity

This paper presents a summary of an investigation into the effects of hull flexibility when deriving an equivalent service factor for a single passage of a Great Lakes Bulk Carrier from the Canadian Great Lakes to China. induced bending moment predicted using traditional three-dimensional rigid body hydrodynamic methods is augmented due to the effects of springing and whipping by including allowances based on two-dimensional hydroelasticity predictions across a range of headings and sea states. The analysis results are correlated with full scale measurements that are available for this ship. By combining the long term "rigid body" wave-bending moment with the effects of hydroelasticity, a suitable service factor is derived for a Great Lakes Bulk Carrier traveling from the Canadian Great Lakes to China via the Suez Canal.

Experimental and numerical study of containership responses in severe head seas

An accurate determination of the global load effects in a ship is vital for the design of the vessel. This paper addresses an experimental and numerical study of containership responses in severe head seas. Experimental results were obtained using a flexible model of a containership of newer design. The experiments showed that, taking hull flexibility into account, the fourth and sixth harmonic of the vertical bending moments had a maximum value of between 25% and 50% of the first harmonic. We also demonstrated that hull flexibility can increase the vertical bending moment by up to 35% in sea states relevant for design. Comparisons of moments found experimentally with results from a nonlinear hydroelastic strip theory method showed that the effect of nonlinearities on the rigid body moments was slightly over-predicted in the aft body. The method also tends to over-predict the increase of the bending moments due to hull flexibility. In general however, the numerical results compared reasonably well with the experimental ones.

Sensitivity of extreme hydroelastic load effects to changes in ship hull stiffness and structural damping

A new hybrid method for the time-domain nonlinear simulation of the hydroelastic load effects and the peak over threshold (POT) method for the calculation of the short-term extreme responses are briefly described and applied to a flexible SL-7 class containership and a flexible liquefied natural gas (LNG) ship. Three stiffness levels, three stiffness distributions and three modal damping ratios are used to study the influence of the hull flexibility and structural damping on the short-term prediction of extreme vertical hydroelastic load effects. The results give justification for some simplified treatment of the first vertical flexible mode in early design stage when structural details are not available.

Stiffness characteristics and thermal deformations of the frame of high power marine engine

Stiffness characteristics and thermal deformations of the frame of high power marine engine In the subject-matter literature detail data on stiffness of the crankshaft foundation connected with the frame of marine main engine are still lacking. Thermal deformation models of the engine's casing, proposed by engine producers, are excessively simplified. However the parameters are crucial for the shaft-line alignment analysis as well as for the analysis of interactions between the shaft-line and engine crankshaft, especially in the case of high power engines. This paper presents a determination method of the marine engine body characteristics as well as results of example computations performed for a Sulzer 7 RTA 84 C engine installed on a ~3000 TEU container ship. It has been demonstrated that the producer's assumption about parallel displacement of the crankshaft axis in thermal working conditions is too rough. The thermal deformation of the engine is of hogging character, which results in significant change of the moment load exerted on the crankshaft and shaft line. The stiffness parameters recommended by the producers for the shaft-line alignment are estimated correctly, however they represent only engine's body flexibility, without taking into account ship's hull flexibility.

비선형 베어링 요소를 이용한 탄성 추진 축계정렬에 관한 고찰

The effects of hull flexibility on shaft alignment are growing as ship sizes are increased mainly for container carrier and LNG carrier. In order to consider hull flexibility on a propulsion shafting system, standardization of ship service conditions is necessary because hull deformation is continuously variable according to ship service conditions. How to summarize ship service conditions is suggested based on practically applicable four viewpoints : hull, engine, loading and sea status. Effects of the external forces acting on a ship propulsion shafting system are generally commented. Several design criteria regulated by classification societies are pointed at issue which seems to have Insufficient technical background. A qualitative verification is carried out to point out the invalidity of the assumption of effective supporting position. In this work, an elastic nonlinear multi-supporting bearing system is introduced as a key concept of the elastic shaft alignment. Hertz contact theory is proved to be more proper one than projected area method in calculation of the nonlinear elastic stiffness of the bearing, The squeezing and oil film pressure calculations in the long journal bearing like an after stern tube bearing are recognized as a necessary process for elastic shaft alignment design.

Long-Term Fatigue Damage of Ship Structures Under Nonlinear Wave Loads

Fatigue is a principal mode of failure in ship structures, especially when high tensile steels are applied. Although significant efforts have been made to predict fatigue damage, there are still uncertainties existing, e.g., in the stress histories that cause fatigue. This paper addresses estimation of fatigue damage in ships under wave loads, with an emphasis on containerships, which have large bow flare and low hull girder rigidity. Linear and nonlinear wave-induced loads as well as dynamic effects due to hull flexibility, i.e., whipping, are researched. With the direct analysis method of fatigue, the nature of the wave loading, hull rigidity, structural damping, stress range counting algorithm and SN curve on structural fatigue damage are investigated. In long-term fatigue damage estimates, the influence of different sea environments is numerically analyzed. The importance of nonlinearity of wave loads and especially the whipping on the structural fatigue damage is demonstrated by calculation for a large container vessel with large flare and lowest natural frequency of 0.749 Hz. Depending upon sea environments and SN curves used in long-term predictions, the fatigue damage based on nonlinear wave loads (excluding whipping) is 10–100% larger than that due to linear wave loads; the fatigue damage based on nonlinear combined loads (including whipping) may be 1–9 times larger than that of steady-state nonlinear wave loads.

Reduction of Hull Whipping in Slamming by Active Control System

The reduction of hull girder whipping in slamming by an active control system is investigated. Under the consideration of hull flexibility, a mathematical formulation is developed for the whipping of a hull, subject to slamming, and including an active vibration control system consisting of a tuned mass and hydraulic servo system. Using the optimal theory, the control law of the active system is determined. In addition, a closed-loop estimator is introduced to estimate the distribution of hull motion, which is required to compute the actuator output of the active system. Finally, a numerical example of an application to a 205-ton high-speed craft is described. The results show that the whipping will be reduced significantly. The whipping acceleration and induced stress in particular are reduced more than 95% within 2 sec by this scheme.

Deflections of Great Lakes Ore Carrier Hull Structure by Finite-Element Analysis

Calculation of hull deflections in way of machinery spaces of two Great Lakes bulk carriers is presented. Methods of modeling and analysis by the finite-element method are discussed, and some results are given. Calculated results are compared with measured results. Calculation of shaft bearing influence coefficients with hull flexibility included is discussed. Finally, an application is presented in which a proposed alteration to an existing ship was analyzed.

The influence of hull flexibility in the antisymmetric dynamic behaviour of ships in waves

The influence and significance of stiffness properties and shear centre on the dry and wet antisymmetric responses of a containership are examined. The ship travels at 26 knots in regular sinusoidal bow waves of unit amplitude. A comparison is included of response predictions derived from unified and rigid body theories since the latter theory excludes all structural properties such as flexural and torsional rigidity or stiffness, warping etc. The results of the investigation provide an excellent justification for adopting a unified dynamic analysis to a flexible ship structure and illustrate the versatility of the theory.

Bending Moments and Shear Forces in Ships Sailing in Irregular Waves

This paper presents some results concerning the vertical response of two different ships sailing in regular and irregular waves. One ship is a containership with a relatively small block coefficient and with some bow flare while the other ship is a tanker with a large block coefficient. The wave-induced loads are calculated using a second-order strip theory, derived by a perturbational procedure in which the linear part is identical to the usual strip theory. The additional quadratic terms are determined by taking into account the nonlinearities of the exiting waves, the nonvertical sides of the ship, and, finally, the variations of the hydrodynamic forces during the vertical motion of the ship. The flexibility of the hull is also taken into account. The numerical results show that for the containership a substantial increase in bending moments and shear forces is caused by the quadratic terms. The results also show that for both ships the effect of the hull flexibility (springing) is a fair increase of the variance of the wave-induced midship bending moment. For the tanker the springing is due mainly to exciting forces which are linear with respect to wave heights whereas for the containership the nonlinear exciting forces are of importance.

Prediction of propeller induced ship vibration: some structural aspects1

Described are some structural aspects of a finite element model used in the design stage to predict vibration levels of a 150,000 SHP triple screw, high speed trailership. The mode shapes of the model show that at blade frequencies of operating RPM the hull does not vibrate as a beam in bending, sh earing and twisting, but rather the decks and double bottom move with different wave lengths, amplitudes and phases. Hull flexibility can significantly affect the transverse vibration frequencies of propulsion shafts. The response of the model shows the importance of detuning of propulsion system resonant frequencies from operating RPM. The proper use of the finite element method in the calculation of propulsion system longitudinal vibration is suggested.