Stern Quartering Seas Research Articles

Comparative Numerical Analysis Fatigue Damage Based On Variation In Quantities Mooring Line On Aker Smart 2 FPSO

The Aker Smart 2 FPSO is the second FPSO from the Norwegian FPSO operator: Aker Floating Production, and will operate for Reliance in India. This FPSO will be moored with a mooring system catenary to the seabed. In its operation, the FPSO will be influenced by dynamic loads such as sea waves, ocean currents and wind on the mooring rope structure periodically. This may cause damage to the structure mooring line which influences the performance of the operational structure. The aim of this research is to investigate motion trajectory (surge & sway), mooring line stress, and deterministic fatigue on the mooring system catenary by making comparisons using variations in quantities mooring line. Dimensions mooring line type chain on fairlead with varying amounts mooring line namely 4x1, 4x2, and 4x3 which will then be analyzed fatigue damage-his. Numerical observations of FPSO motion on following seas, stern quartering seas, beam seas, bow quartering seas, and head seas show Response Amplitude Operator from movement surge, sway, heave, roll, pitch, and yaw due to harsh environments with Hs = 2.0 meters and T = 12 seconds. Observation Motion Trajectory it is found that the greater the number mooring line then value DOffset or movement surge & sway will get smaller. Meanwhile, in observation Mooring Line Stress and Fatigue Damage obtained both are directly proportional where the greater the tension mooring line the greater the damage. This is because the mass of the rope used is not proportional to displacement boat. And in this numerical analysis simulation it only reaches the initial movement, namely at 100 seconds, where the ship's condition has not yet reached the stable condition it should be.

Experimental study of the water on deck effects on the transverse stability of a fishing vessel running in stern quartering seas

The trapped water on the deck affects the transverse stability of purse seiners running in stern quartering seas. In this paper, we conducted free-running model experiments in regular and short-crested irregular following and quartering waves to clarify the state of trapped water on the deck and its effect on transverse stability. In the model experiments, pressure sensors, and cameras are installed on the weather deck of the ship model to measure the state of trapped water on the deck. The influence of trapped water on the deck is quantitatively evaluated by conducting a phase difference analysis of the measured trapped water on the deck motion and the time series of the ship model's rolling motion. The results confirm that trapped water on the deck acts as a hydrostatic heeling moment in the range around the design speed.

Numerical study of the interaction between the pure loss of stability, surf-riding, and broaching on ship capsizing

Ship capsizing in stern quartering wave is very complicate with a strong correlation to the stability failure modes proposed in second-generation intact stability criteria. It is of great importance to shed some light on this complex phenomenon. In this study, the capsizing behavior of a free-running ONR Tumblehome model in stern quartering seas is investigated by an unsteady Reynolds-averaged Navier–Stokes (URANS) solver coupled with dynamic overset grid approach. Validation study of the URANS solver is performed for the prediction of extreme roll motion of the ship. The comparisons show satisfactory agreement, with an average difference less than 3% between the simulation and experimental results. The correlation between the occurrence of stability failure modes (surf-riding, pure loss of stability, and broaching) and ship capsizing is then assessed under different ship speed and heading angle conditions. It is concluded that the capsizing occurs as the free-running ship sailing with a high speed, which is caused by the interaction between the broaching and severe stability loss. In addition, influenced by the wave-induced moment, both the pure loss of stability and surf-riding modes may cause the broaching, indicating the similar occurring condition and strong relation between these stability failure modes.

Line detection techniques to pinpoint slamming impulses in time-frequency images of hull acceleration measurements

The detection and localization of slamming is investigated using a hull monitoring system on a polar vessel. The system comprises an accelerometer array, resulting in time histories in which slamming impacts must be detected. Proposed methods exploit the attribute that slamming impacts cause broadband excitation which result in line features in short-time Fourier transform spectrograms and Morlet wavelet scalogram images. The proposed methods are benchmarked against the MATLAB findpeaks function which requires a pre-defined detection threshold and falsely identifies several slams if consecutive peaks of the free decay exceed the threshold. An evaluation on simulated data investigates artefacts such as an unpredictable frequency of incidence, varying impulse magnitude and ever-changing noise floor. The likely wave impact site is pinpointed at the sensor which first displays an impulsive peak. Preliminary insights are extracted from full-scale experiments on the stationary vessel in stable wave conditions and a systematically varied relative heading. The Morlet wavelet transform delivers the most promising slamming detection capabilities and concurs that stern slamming is prevalent in following and stern-quartering seas. A threshold is beneficial to consistently identify significant slams despite varying ocean conditions.

Effect of stern appendages configurations on the course-keeping of ships in stern-quartering seas

Ships can experience serious difficulties in keeping a straight course when sailing in stern-quartering seas. Design modifications like the addition of stern passive fins, or the modification of active control surfaces, are common solutions to improve the ship course-keeping. However, the success of such design modifications depends on the delicate balance between the excitation forces induced by the waves on the appended hull, the stabilization forces provided by the lifting surfaces as appended fins, and the steering forces provided by the control surfaces. This research investigates which of these aspects of a ship design play a concrete role in improving the ship course-keeping in waves. The study is carried out with the intention of looking at the different behaviors of the ship originating from different stern appendages configurations. Three modifications of stern appendages on three different ship hulls were investigated in various mild-to-rough sea conditions. The behavior of the vessels were simulated using a time domain, boundary element potential method, with the addition of semi-empirical formulations for the modelling of the stern lifting surfaces. The simulations were carried out in long crested irregular waves at three different direction, using the JONSWAP spectrum. The results showed that although larger stern appendages improve the directional stability of relatively large and slow vessels, in most cases they worsen their course-keeping ability, increasing the yaw motions. For smaller and faster vessels instead, passive and active fins tend to improve the course-keeping, because at high speed the lift provided by the appendages stabilizes the vessel. This effect is compensated by the wave excitation force at lower speed. Similarly to yaw, the roll motions increases with larger stern appendages.

Estimation of added resistance of an LNG carrier in oblique waves

Despite recent moderate oil prices, the reduction in fuel oil consumption of voyaging ships has been of great interest. Ships experience additional resistance from wind and waves in actual seas during their voyages, and those added resistance contributes the increase of fuel consumption. The accurate prediction of the added resistance is, therefore, crucial for developing highly energy efficient and low fuel-consuming ships.Ships routinely navigate along pre-determined routes between ports and thus relative heading angle is typically not controlled. Therefore, ships can encounter wind and waves coming from any direction. However, added resistance in oblique direction has less been studied by researchers than that in head seas. To predict the operating performance of ships accurately, more focus should also be given to added resistance in oblique direction.In the present study, the motions and added resistance of an LNG carrier are estimated by experimental investigations and numerical computations. The scaled model ship is towed at a constant forward speed in a square basin and the resistance increase is measured in regular oblique waves. Potential flow method and Reynolds-Averaged Navier-Stokes (RANS) based computational fluid dynamics method are employed to compare the magnitudes of motion and added resistance to those obtained from model test. The global trend of motion responses agrees reasonably at all heading angles between the results from experiments and numerical computations. In regard to the added resistance, however the level of accuracy is less favorable for both numerical methods. The discrepancy is further investigated by checking the relevant parameters in potential method, roll damping and relative phases. The detailed results are discussed and key finding is addressed.In the experimental and numerical investigations, the magnitude in the transfer function of added resistance due to waves is significant at bow quartering seas. In addition, the resistance increase appears to be not negligible even at the following and stern-quartering seas. Therefore, operating efficiency of a ship can be evaluated appropriately when contribution of all heading angles is properly taken into account to the evaluation of added resistance.

Seakeeping time domain simulations for surf-riding/broaching: investigations toward a direct stability assessment

Seakeeping time domain simulations are carried out to capture the likelihood of a surf-riding and/or broaching event, with the aim to gain a further insight into the ship performance in stern quartering seas. The applied numerical tool, named PANSHIP, is a 6-DoFs time domain seakeeping software based on panel method, combined with semi empirical viscous models. The investigated ships are two yacht vessels and a patrol boat. Calculations are also carried out with a tool implementing the IMO second level vulnerability criterion for surf-riding/broaching. Similarities, differences and critical issues of both approaches are discussed, as a first step in the perspective of a comprehensive application of the IMO Second Generation Intact Stability criteria, inclusive of the so-called “Direct Stability Assessment”.

CFD prediction of wave-induced forces on ships running in irregular stern quartering seas

Surf-riding/broaching is a dangerous event for high-speed ships running in stern quartering seas. Theoretical methods for the prediction of wave-induced forces on ships in surf-riding and broaching situations are still of the qualitative level. Therefore, an unsteady Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) solver with overset grids was applied to predict the wave-induced forces on ships in irregular stern quartering seas. To validate the CFD method, a dedicated model experiment was conducted for slender ships, and the wave-induced forces were measured to be for a direct comparison with CFD predictions using the same irregular waves encountered. The comparison results indicate good agreement between the CFD results and the experimental fluid dynamics (EFD) results except for the wave-induced roll moment.

선미파, 선미사파를 받는 선박의 과도 운동 추정에 대한 연구

Recently, researches to find rational mathematical model for prediction of capsizing have been progressed by ITTC. Lee(1997) developed a mathematical model which describes 6 DOF transient motions, such as capsizing, of a ship in regular waves. In this study a mathematical model for prediction of capsizing in following and quartering seas is developed based on Lee's model. And factors affecting prediction of capsizing are analyzed through comparing simulation results with experimental results. Present simulation results are compared with ITTC bench mark test results. In rolling tests with beam seas and tree runs with stern quartering seas, capsizing events are predicted well. But calculated roll angle is larger than experimental one. It is found that nonlinear manoeuvring coefficients don't affect the prediction of capsizing events.

An investigation of hydrodynamic forcing on a semi-planing monohull in following and stern quartering seas

Navigating a ship in following and stern quartering seas is well known to seafarers for the possible troublesome situations that may arise. The issue has also been addressed by many researchers during the last 50 years. In 1964 Du Cane and Goodrich presented one of the first systematic studies on ships in following seas together with a compilation of accounts on real occurrences of dangerous situations for ships in following and stern quartering seas. Later research has confirmed the possibility for occurrence of several types of allegedly capricious behaviour for a ship travelling at a course within one third of the compass card, i.e. between −60° and 60° relative to the waves. The research included in the references is covering many different types of ships from fishing boats to fast naval vessels and merchant ships.

Reconciling experimental and numerical data: techniques of nonlinear seakeeping code validation

In this paper the authors apply qualitative and quantitative approaches in an effort to demonstrate techniques for validation of numerical nonlinear seakeeping codes. The work begins with a presentation of safe basin boundaries and integrity curves focusing primarily upon variations in wave steepness and roll/roll velocity state space variables for a realistic ship operating in stern quartering seas. The simulated results are compared to experimental data acquired in the Maneuvering and Seakeeping Basin (MASK) at the Naval Surface Warfare Center, Carderock Division. The study then discusses the use of analytical methods, such as Lyapunov exponents, to quantitatively validate a numerical simulation to experimental results.

An experimental study of global loads acting on an intact and damaged Ro–Ro ship model

Measurements of global loads acting on a Ro–Ro model have been carried out in regular waves for intact and damaged conditions. The stationary model was tested in different wave heights and wave frequencies for the head, beam and stern quartering seas in order to explore the effect of damage and wave heights on the global loads acting on the model. The analysis of the results indicates that damage has an adverse effect on the structural loading responses of the model depending upon the directionality of the waves and frequency range applied. This effect may cause structural damage on the ship and danger the safety of the ship and passengers on board.

An experimental study of motion behaviour with an intact and damaged Ro-Ro ship model

Six degrees of freedom motion response tests of a Ro-Ro model have been carried out in regular waves for intact and damaged conditions. The stationary model was tested in different wave heights and wave frequencies for the head, beam and stern quartering seas in order to explore the effect of damage and varying wave heights on the motion responses of the model. Analysis of the results indicates that damage has an adverse effect on the motion responses of the model depending upon the directionality of the waves and frequency range applied.