KR102486718B1 - System and method for monitoring fatigue damage of vessel structure - Google Patents

Abstract

A ship structure fatigue damage monitoring system and method are disclosed. Ship structure fatigue damage monitoring system according to an embodiment of the present invention, a stress calculator for calculating the global stress and local stress for the position to obtain the fatigue damage; a stress transfer function combination unit calculating a stress transfer function for a ship structure using the global stress and the local stress; and a fatigue damage acquisition unit for obtaining a real-time fatigue damage corresponding to the sea state by using the stress transfer function, a wave spectrum, and a fatigue diagram of the sea state.

Description

System and method for monitoring fatigue damage of vessel structure {System and method for monitoring fatigue damage of vessel structure}

The present invention relates to a ship structure fatigue damage monitoring system and method.

A ship's structure is designed to withstand the maximum environmental loads it is likely to encounter during its entire life (approximately 20 years). From this point of view, according to what has been known so far, the maritime environment in the North Atlantic region is known to be the roughest, and therefore, a normal ship structure whose operating area is not limited can encounter once every 20 years based on the North Atlantic maritime conditions. The design is made so that it can withstand under probable sea conditions.

From the point of view of the fatigue strength of the ship structure, it is also required to design it under the condition that no cracks will occur even if it is operated in the North Atlantic region for 20 years. However, most ships do not operate only in the North Atlantic Ocean, and most of the shipping companies operating in the North Atlantic region are operating while slightly modifying routes and schedules in consideration of the real-time maritime environment.

Therefore, even for ships built under the same design conditions and in the same type, several years after delivery, the accumulated fatigue damage to the ship structure depends on the sea conditions encountered by each ship (hereinafter referred to as accumulated fatigue damage). may vary, and ultimately the remaining structural life of the vessel may also differ.

However, the cumulative fatigue damage of the ship at a specific point in time cannot be visually confirmed. In order to know this, the cumulative fatigue damage should be calculated considering all the sea conditions encountered during the life of the ship.

As described above, since ships are designed assuming the most extreme sea conditions, theoretically, damage due to wave loads should not occur. Also, from the point of view of fatigue, since the S-N diagram has a survival probability of 97.72% (probability of no cracks occurring), the ship has a crack occurrence probability of less than 2.28% even if the ship operates in any sea area on the earth. In general, considering that an additional margin of fatigue life is given in the structural design of a ship, it can be said that cracks cannot occur in a ship theoretically. Nevertheless, there are still frequent cases in which cracks occur in ships due to various causes. In particular, there is still a possibility of cracks occurring in the welded portion during the lifetime of the ship due to complex reasons such as residual stress, welding bead shape, and welding defects.

When a claim is made due to a crack in the delivered ship, the designer identifies the cause of the crack. At this time, it is very important to understand the sea conditions in which the ship was operated. By understanding this, it is possible to determine whether it is a design defect or an operator's mistake in the manufacturing stage. If it is a design problem, measures must be taken for all similar ships, and if it is due to a worker's mistake, it may be finished at the level of confirming the working condition through inspection of similar ships.

Even when design changes are made and repairs are made, if there is a record of the sea conditions encountered by the ship, it can provide useful information on what level of reinforcement is required in combination with the lifespan in which cracks have been found.

From this point of view, a system that monitors the sea conditions encountered in real time during the ship's operation, calculates the stress generated in the structure according to the sea conditions, and evaluates the cumulative fatigue damage of the ship in real time from this can be usefully utilized. .

The most reliable way to evaluate the fatigue damage of a ship in real time is to directly measure the strain using a strain gage, extract the stress from it, and then evaluate the fatigue damage using the S-N diagram. However, in order to apply this, a strain gauge must be directly attached to the location to be measured and a system connecting it must be established. That is, if there are many locations to be evaluated, the investment cost for hardware increases significantly, and in some areas it is impossible to install strain gauges, so it is judged that a method for monitoring fatigue damage by measurement is not realistic. In addition, even if the strain gauge is installed, an error may still occur in the step of analyzing the measured value due to thermal deformation, deterioration in the reliability of the gauge that may occur due to long-term use, and the like.

Korean Patent Publication No. 10-2012-0047655 (published on May 14, 2012) - Hull Hazard Situation Recognition System

The present invention is to provide a system and method for monitoring ship structure fatigue damage based on structural analysis based on calculation that can secure high accuracy at low cost.

The present invention is to provide a structural analysis-based ship structure fatigue damage monitoring system and method that can be practically applied by calculating the stress occurring in the structure by dividing it into global stress and local stress so that fatigue damage can be monitored in real time. .

Other objects of the present invention will be readily understood through the following description.

According to one aspect of the present invention, as a system for monitoring the degree of fatigue damage to a ship structure, a stress calculation unit for calculating the global stress and local stress for the position to obtain the degree of fatigue damage; a stress transfer function combination unit calculating a stress transfer function for a ship structure using the global stress and the local stress; And a ship structure fatigue damage monitoring system comprising a fatigue damage acquisition unit for obtaining a real-time fatigue damage corresponding to the sea state using the stress transfer function, the wave spectrum and the fatigue diagram of the sea state. do.

The stress calculation unit calculates the global stress from a global moment and cross-sectional properties of the hull section, and the global moment may include a vertical bending moment (VBM) and a horizontal bending moment (HBM).

If the ship is a container ship, the stress calculator calculates the stress generated from the torsional moment by the linear superposition of the stress effect coefficient obtained from the unit torsional moment applied at multiple locations in the longitudinal direction by the torsional moment obtained from the hydrodynamic analysis. A stress influence factor due to a moment may be added to the global stress.

The stress calculator may calculate the sum of the external pressure applied to the stiffener between the web frames, the wave pressure, and the pressure of the ballast or cargo generated by the inertial force, which is the internal pressure caused by the acceleration, as the local stress.

Pressure generated by relative displacement of the web frame in front and behind the transverse bulkhead may be further included in the local stress.

Other aspects, features and advantages other than those described above will become apparent from the following drawings, claims and detailed description of the invention.

According to an embodiment of the present invention, there is an effect that can monitor fatigue damage of a ship structure based on structural analysis based on calculation that can secure a high degree at low cost.

In addition, by calculating the stress generated in the structure by dividing it into global stress and local stress, fatigue damage can be monitored in real time, so there is an effect that can be applied practically.

1 is a block diagram of a structural analysis-based ship structure fatigue damage monitoring system according to an embodiment of the present invention;
2 is a flow chart of a method for monitoring fatigue damage of a ship structure based on structural analysis according to an embodiment of the present invention;
Figure 3 is a cross-sectional view of the vessel for stress calculation.

Since the present invention can make various changes and have various embodiments, specific embodiments will be illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present invention to specific embodiments, and should be understood to include all modifications, equivalents, or substitutes included in the spirit and technical scope of the present invention.

Terms used in this specification are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, terms such as "include" or "have" are intended to indicate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, but one or more other features It should be understood that it does not preclude the possibility of the presence or addition of numbers, steps, operations, components, parts, or combinations thereof.

In addition, terms such as "...unit", "...unit", "...module", and "...group" described in the specification mean a unit that processes at least one function or operation, which is hardware or software or hardware and software. It can be implemented as a combination of

1 is a block diagram of a structure analysis-based ship structure fatigue damage monitoring system according to an embodiment of the present invention, and FIG. 2 is a flowchart of a structure analysis-based ship structure fatigue damage monitoring method according to an embodiment of the present invention. 3 is a cross-sectional view of a ship for stress calculation.

Structural analysis-based ship structure fatigue damage monitoring system and method according to an embodiment of the present invention calculates the stress generated in the structure by dividing it into global stress and local stress, enabling calculation and monitoring of fatigue damage reflecting real-time sea conditions characterized by one.

Referring to FIG. 1, the structural analysis-based ship structure fatigue damage monitoring system 100 according to this embodiment includes a stress calculator 110, a stress transfer function combination unit 120, and a fatigue damage acquisition unit 130. include

The stress calculation unit 110 calculates stress generated in the hull structure during ship operation. Stress generated in the hull structure can be divided into global stress, which is stress caused by global loads, and local stress, which is stress caused by local loads.

The global stress includes stress induced from vertical bending moment (VBM) and stress induced from horizontal bending moment (HBM) (step S210). If the ship is a container ship, the stress due to the torsional moment (TM) may be additionally included in the global stress.

Global stress can be obtained from the global force/moment and the sectional properties of the hull section. The global force/moment includes vertical bending moment and horizontal bending moment.

)은 수학식 1과 같이 수직굽힘모멘트(VBM)와 수평굽힘모멘트(HBM)의 조합으로 구할 수 있다. In the case of a general ship where torsional stress is known to be small, the global stress (

) can be obtained as a combination of the vertical bending moment (VBM) and the horizontal bending moment (HBM) as shown in Equation 1.

는 선체 수평방향 축에 대한 단면의 단면계수(section modulus)이고,

는 선체 수직방향 축에 대한 단면의 단면계수이다. here,

is the section modulus of the section about the horizontal axis of the hull,

is the section modulus of the section about the vertical axis of the hull.

In the case of a container ship where the stress due to the torsional moment (TM) is large due to warping caused by the torsional moment, as in the case of a container ship, the finite element analysis method is used as follows to determine the load acting on the finite number of cross sections of the hull. By calculating the stress at the position of , the Stress Influence Coefficient due to the torsional moment can be obtained.

는 j번째 station에서 단위 TM에 의하여 발생하는 i번째 위치에서의 응력으로 유한요소해석법 등의 구조해석을 통하여 추출될 수 있다.

는 j번째 스테이션에서 발생하는 비틀림모멘트이다. here,

is the stress at the i-th position generated by the unit TM at the j-th station and can be extracted through structural analysis such as finite element analysis.

is the torsional moment generated at the j-th station.

That is, the stress generated from the torsional moment in the container ship can be obtained as a linear superposition of the value obtained by multiplying the stress effect coefficient obtained by the unit torsional moment applied at multiple locations in the longitudinal direction and the torsional moment obtained from the hydrodynamic analysis.

For container ships, the global stress can be calculated as follows.

The local stress includes stress generated by external pressure (blue pressure) applied to stiffeners between web frames and internal pressure caused by acceleration (step S220).

In the case of a stiffener installed on the shell, the local stress can be obtained as the sum of the wave pressure and the ballast or cargo pressure generated by the inertial force.

Wave pressure can be obtained by directly mapping the pressure generated on the shell plate from hydrodynamic analysis. The pressure of the ballast or cargo due to the acceleration can be obtained from the fluid dynamics analysis of the acceleration at the center of the tank as follows.

,

,

는 각각 선체 길이방향, 횡방향, 높이방향 가속도이다.

는 파 주파수(wave frequency)이고,

는 선체 무게중심에서 계산 점까지의 길이방향 거리,

는 선체 무게중심에서 계산점까지의 폭방향 거리,

는 선체 무게중심에서 계산점까지 높이방향 거리이며,

는 피치 모션,

은 롤 모션,

는 요 모션을 나타내고,

는 중력 가속도이다.

,

,

는 각각 서지, 스웨이, 히브 모션을 나타낸다.

,

,

are the accelerations in the longitudinal, transverse, and height directions of the hull, respectively.

is the wave frequency,

is the longitudinal distance from the center of gravity of the hull to the calculation point,

is the widthwise distance from the center of gravity of the hull to the calculation point,

is the distance in the height direction from the center of gravity of the hull to the calculation point,

is the pitch motion,

silver roll motion,

represents the yaw motion,

is the gravitational acceleration.

,

,

represents surge, sway, and heave motions, respectively.

As in Equation 4, the pressure is calculated by multiplying the density of the stored cargo and the head by the obtained acceleration. When the pressure is obtained, the stress at the hot spot can be obtained by applying the beam theory therefrom.

The beam theory is as follows. The structure of a ship is a stiffened plate structure in which a stiffener is coupled to a plate, and when pressure is applied to the plate, a bending moment and a shear load are generated in the stiffened plate, and a bending stress is generated due to the bending moment. . In order to calculate such a behavior, finite element analysis using two-dimensional elements is generally performed. In this case, a lot of modeling time is required, so it is much faster to calculate by idealizing the reinforcing plate as a beam structure. The stresses occurring in the structure can be calculated. That is, if the reinforcing plate is idealized as a beam having equivalent bending stiffness and the calculation is performed, the stress can be obtained at a hot spot in a short time.

Additionally, stress (relative deflection stress) generated by relative deflection of web frames in front and behind the transverse bulkhead may be further included in the local stress. There are a plurality of transverse webs between the transverse bulkheads. Transverse bulkheads have the concept of separating spaces, and at the same time play a role in supporting vertical and lateral loads. The web frame serves to support vertical/transverse loads between the transverse bulkheads.

The stress transfer function combination unit 120 calculates a stress transfer function using the global stress and the local stress calculated by the stress calculator 110 (step S230).

)은 다음과 같이 글로벌 응력과 로컬 응력의 합에 응력 집중계수를 곱함으로써 계산할 수 있다. Hot spot stress at the location (calculation point) where the fatigue damage is to be obtained (

) can be calculated by multiplying the sum of the global and local stresses by the stress concentration factor as follows:

는 응력 집중계수,

는 글로벌 응력,

은 로컬 응력,

은 상대변위 응력이다.

is the stress concentration factor,

is the global stress,

is the local stress,

is the relative displacement stress.

In calculating the stress transfer function (Transfer function or RAO (Response Amplitude Operator)), when hydrodynamic analysis is performed based on the ship's displacement, draft, and weight information, the acceleration occurring in the ship and the shell plate at the bottom of the water The transfer function (RAO) of the pressure due to the wave can be calculated. When the structural analysis is performed by applying the pressure and inertial force (mass x acceleration of the solid weight element + mass x acceleration of the dead weight element) obtained from the hydrodynamic analysis to each node point of the ship structural model to the structural model, the stress transfer function can be obtained. This is a general spectral fatigue analysis method, which is applied in the design stage of many large ships and offshore structures. However, this method is not suitable for fatigue damage evaluation considering real-time sea conditions because it takes a lot of time for analysis and, in particular, when the weight information of the ship changes, the hydrodynamic analysis and structural analysis must be performed again. there is.

Therefore, in this embodiment, real-time fatigue damage can be monitored by calculating the stress generated in the structure by dividing it into global stress and local stress.

The fatigue damage acquisition unit 130 acquires the fatigue damage of the ship structure in real time using the stress transfer function (step S240). Fatigue damage in short-term sea conditions can be obtained as follows.

는 고려하는 기간 동안 발생하는 응력 범위의 개수이고,

는 응력범위(stress range)이며,

는 응력범위의 확률밀도함수(probability density function of stress range)이고,

는 피로선도(S-N 선도)에서

응력범위에 해당하는 응력범위 회수(the number of cycle which make failure at stress range of

)이다. here,

is the number of stress ranges occurring during the period under consideration,

is the stress range,

is the probability density function of stress range,

is in the fatigue diagram (SN diagram)

The number of cycles which make failure at stress range of

)to be.

The fatigue diagram (S-N diagram) is expressed as follows.

조건에서 피로크랙이 발생하는 반복횟수(실험에서 구함)이고, a는 S-N 선도의 상수(N축 절편)이며, m 은 S-N 선도의 기울기이다. where N is the specific fluctuating stress level

It is the number of iterations where fatigue cracks occur under the condition (obtained from the experiment), a is the constant (N-axis intercept) of the SN diagram, and m is the slope of the SN diagram.

Substituting Equation 7 into Equation 6, the fatigue damage degree D is expressed as follows.

Fatigue damage in short-term sea conditions (typically 3-hour wave conditions) represented by significant wave height (H _s ) and mode period (T _p ) or average period (T _z ) is determined by the Rayleigh Under the assumption that it follows the distribution, the probability density function of stress in the short-term sea state can be expressed as follows.

는 스펙트럼 모멘트, n은 스펙트럼 모멘트의 차수,

는 응력 스펙트럼,

는 파 주파수(wave frequency),

는 유의파고(significant wave height),

는 최빈주파수(peak frequency),

는 응력전달함수(Transfer function)이다. here,

is the spectral moment, n is the order of the spectral moment,

is the stress spectrum,

is the wave frequency,

is the significant wave height,

is the peak frequency,

is the stress transfer function.

When Equation 9 is substituted into Equation 6 and a gamma function is introduced, the fatigue damage using the S-N diagram having 2 slopes is calculated as follows.

는 파의 평균 주파수 (mean zero up crossing frequency, Hz)이고,

는 구조물의 사용 수명(요구되는 피로수명, sec)이며,

는 기울기가 2가지인 (2-slope) S-N 선도에서 기울기가 변하는 지점의 변동응력 값이다.

is the mean zero up crossing frequency (Hz) of the wave,

is the service life of the structure (required fatigue life, sec),

is the fluctuating stress value at the point where the slope changes in the 2-slope SN diagram.

과 같이 표현되는 함수로서, 크기가 다른 응력이 불규칙적으로 작용하는 해상상태에서 피로손상도를 하나의 적분식으로 표현할 수 있게 한다. Here, the gamma function is

As a function expressed as , it is possible to express the degree of fatigue damage in one integral equation in a sea state where stresses of different magnitudes act irregularly.

In this way, it is possible to calculate the fatigue damage corresponding to the sea state by using the transfer function of stress, the wave spectrum of the sea state, and the SN diagram.

Here, the spectrum may use a Pierson-Moskwitz (PM) spectrum and a JONSWAP spectrum, and has the following equation.

는 PM 스펙트럼,

는 피크 형상 파라미터(peak shape parameter),

는 스펙트럼 폭 파라미터(spectral width parameter),

이다. here,

is the PM spectrum,

is the peak shape parameter,

is a spectral width parameter,

to be.

The type of wave spectrum may be selected according to the wave characteristics of the sea encountered by the ship. In the ocean there is no confined space, so the wind can provide enough energy for the waves to form. Coastal areas are land-based, and there may be disturbances in the energy supply by wind. Therefore, the PM spectrum can be used when crossing the ocean and the JONSWAP spectrum can be used on the coast.

In addition, the significant wave height and the most frequent wave frequency (or wave period) input at this time use values measured from measuring equipment installed on the ship or extracted from sea state information provided from related companies (eg, WNI, AWT, etc.) can also be used

In this embodiment, if the loading conditions of the ship to be monitored are changed during operation, the results of the applied hydrodynamic analysis should also be changed. This can be applied by selecting a number of conditions that can occur in the design stage, analyzing them in advance, and selecting conditions similar to conditions during operation. At this time, displacement, draft, center of gravity, and GM can be criteria for selecting similar conditions.

According to this embodiment, in the design stage, an integrated system such as fluid dynamics analysis, section property calculation, stress analysis according to local pressure using beam theory, and fatigue damage calculation according to stress is prepared in advance, and the ship is loaded during operation. According to changes in loading and sea conditions, it is possible to calculate the fatigue damage of the ship in real time by selecting the hydrodynamic analysis result and receiving the sea state in real time.

The calculated fatigue damage can be checked by the driver in real time through a display device. In addition, it may be transmitted in real time to a control center (control system) installed on land (ground) to monitor the degree of fatigue damage of the ship structure at the control center.

In addition, the fatigue damage acquisition unit 130 may repeatedly calculate the real-time fatigue damage at regular intervals and accumulate the results to obtain the accumulated fatigue damage (step S250). Here, the calculation period is a time during which statistical characteristics (significant wave height (H _s ) and mode period (T _p )) of the sea state are kept constant, and may be about 3 to 5 hours.

The above-described ship structure fatigue damage monitoring method may be implemented in the form of a recording medium including instructions executable by a computer, such as an application or program module executed by a computer. Computer readable media can be any available media that can be accessed by a computer and includes both volatile and nonvolatile media, removable and non-removable media. Also, computer readable media may include computer storage media. Computer storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data.

The above-described ship structure fatigue damage monitoring method may be executed by an application basically installed in the terminal (this may include a program included in a platform or operating system basically installed in the terminal), and the user may use an application store server, It may also be executed by an application (that is, a program) directly installed in the master terminal through an application providing server such as an application or a web server related to the corresponding service. In this sense, the above-described ship structure fatigue damage monitoring method is implemented as an application (ie, a program) basically installed in a terminal or directly installed by a user, and may be recorded on a computer-readable recording medium such as a terminal.

Although the above has been described with reference to the embodiments of the present invention, those skilled in the art will variously modify the present invention within the scope not departing from the spirit and scope of the present invention described in the claims below. And it will be understood that it can be changed.

100: ship structure fatigue damage monitoring system 110: stress calculation unit
120: Stress transfer function combination unit 130: Fatigue damage acquisition unit

Claims (5)

As a system for monitoring the degree of fatigue damage to ship structures,
a stress calculation unit for calculating global stress and local stress for a location where fatigue damage is to be obtained;
a stress transfer function combination unit calculating a stress transfer function for a ship structure using the global stress and the local stress; and
A ship structure fatigue damage monitoring system comprising a fatigue damage acquisition unit for acquiring a real-time fatigue damage corresponding to the sea state using the stress transfer function and the wave spectrum and fatigue diagram of the sea state. According to claim 1,
The stress calculation unit calculates the global stress from the global moment and the cross-sectional properties of the hull section,
The global moment is a ship structure fatigue damage monitoring system including a vertical bending moment (VBM) and a horizontal bending moment (HBM). According to claim 2,
If the ship is a container ship, the stress calculation unit calculates the stress generated from the torsional moment obtained by linear superposition of the value obtained by multiplying the stress effect coefficient obtained by the unit torsional moment applied at multiple locations in the longitudinal direction and the torsional moment obtained from the hydrodynamic analysis. A ship structural fatigue damage monitoring system that adds to global stresses. According to claim 1,
The stress calculator calculates the sum of the wave pressure, which is an external pressure applied to the stiffener between the web frames, and the pressure of the ballast or cargo generated by the inertial force, which is an internal pressure caused by acceleration, as a local stress. Ship structure fatigue damage monitoring system. According to claim 4,
A ship structural fatigue damage monitoring system in which the pressure generated by the relative displacement of the web frame in front and behind the transverse bulkhead is further included in the local stress.

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