JPH069752B2 - Fatigue strength assurance method for independent rectangular tank welds of liquefied gas tankers - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for guaranteeing fatigue strength of an independent rectangular tank welded portion provided in a liquefied gas tanker for loading LNG, LPG and the like.

[Prior Art] A liquefied gas tanker includes a rolling chock 3 and a support 4 in which an independent rectangular tank 2 is provided in a hull 1 as shown in FIG.
Is supported by the inner surface of the shell 1 with a gap, and when the LNG, LPG, etc. are loaded in the independent rectangular tank 2, contraction movement due to low temperature can be allowed. The independent rectangular tank 2 containing the low temperature liquid has a structure having sufficient strength against the deformation stress at the time of contraction.

This independent rectangular tank 2 is made of an aluminum alloy, for example, as shown in FIG. 2, a large number of tank members 5 such as brackets, ribs, mold members and skin plates are framed in the tank 2 and the joints of the tank members 5 are welded together. I'm building. By the way, in this type of liquefied gas tanker, the tanker belonging to Type B in the IMO Code is obliged to perform a detailed fatigue strength analysis by the IMO Code. The definition of this B-type tank is a tank in which the safety of the tank has been confirmed by accurate analysis and experiments, and a major tank failure does not occur at one time. Therefore, emphasis is placed on fatigue analysis and failure analysis. An independent rectangular tank has a bone structure in which many tank members are welded and has a high static indeterminate order, so complete analysis has been difficult, but recently it has become relatively easy to analyze by computer. .

[Problems to be Solved by the Invention] However, fillet welding or the like is inevitably used in the structure with bones, and the welded shape is irregular, so that it is necessary to individually perform a fatigue test on the welded portions. Conventionally, the fatigue strength analysis of this weld is based on the nominal stress generated in the member, and shows the relationship between the number of times fatigue and fracture occur and the stress when stress is repeatedly applied for each type of welded joint. The SN curve is experimentally obtained, and the fatigue resistance strength of the welded joint to the nominal stress is obtained based on the SN curve. However, since various fatigue resistances differ depending on the shape of the member, the shape of the joint, or the shape of the weld bead, various SN curves are required, so many tests are required. Virtually all the joints appearing in the independent rectangular tank were tested by SN
It is impossible to cover with curves.

Among the influential factors such as the joint shape and metallurgical characteristics of the welded portion and the residual stress, the fatigue strength of the welded joint member has the greatest effect on the fatigue strength of the welded joint member due to the welded joint shape. The present invention has been completed by finding out that the stress concentration factor at the fatigue crack occurrence portion, in particular, the stress concentration factor at the weld toe end portion.

An object of the present invention is to ensure fatigue strength of welds of independent rectangular tanks of liquefied gas tankers that facilitates fatigue analysis of various welded joints of the above-mentioned independent rectangular tanks and facilitates fatigue resistance design of various tank members. Is the way.

[Means and Actions for Solving the Problems] In the method for guaranteeing fatigue strength of an independent rectangular tank welded joint portion of a liquefied gas tanker that is composed of a large number of aluminum alloy tank members, and the joints of the members are welded, The stress concentration factor Kt at the weld toe is calculated from the functions of the flank angle θ, the toe radius ρ, and the bead height h with respect to the nominal stress generated in the plate material of the welded joint between the members, and the stress concentration factor is obtained. The local stress at the weld toe is obtained from Kt, and a fatigue fracture curve based on the local stress is prepared. From the fatigue fracture curve, the allowable local stress of each weld joint is obtained and the stress concentration coefficient Kt is a predetermined value. Fatigue strength assurance method for an independent rectangular tank weld of a liquefied gas tanker, characterized in that the flank angle θ, the toe radius ρ, and the bead height h are controlled to perform welding as follows. Located in.

As described above, in the welded joint portion, the fatigue resistance strength of all the welded joint portions can be guaranteed with one SN curve (fatigue fracture curve) including the base material.

[Embodiment] A preferred embodiment of the present invention will be described below with reference to the accompanying drawings.

First, the stress concentration coefficient Kt will be described by taking the cross welded joint of FIG. 3 as an example.

In FIG. 3, 6 is a vertical plate, 7 is a horizontal plate, and fillet welding 8 is performed on the vertical plate 6 and the horizontal plate 7. In this case, the excess embankment angle (hereinafter referred to as the flank angle) of the weld 8 is θ, the bead height is h, the bead toe radius of the toe 9 of the weld 8 is ρ, and the plate thickness of the lateral plate 7 is t.

Assuming that the lateral plate 7 is now subjected to the nominal stress σ _N , the stress distribution is 10 as shown in FIG.
Although the nominal stress σ _N remains the same, stress concentration occurs as it approaches the toe portion 9, and a substantially maximum local stress σ _L acts at the toe portion 9. Therefore, the concentration factor at the toe 9 is set to Kt.
Then, Kt is represented by the following equation.

Further, this Kt is a function of the above-described flank angle θ, toe radius ρ, and bead height h, and is represented by the following equation.

_Kt = [1 + f (θ) {g (ρ) −1}] C (a / t)… (2) where f (θ) is the influence of flank angle and g (ρ) is the weld toe. The influence of the radius, C (a / t), is a function due to the influence of the presence of the unwelded portion and is represented by the following equations, respectively.

Flank angle; Toe radius; g (ρ) = α _t · g _t (ρ) + α _b · g _b (ρ) (4) where gt (ρ) is given by the following equation in the case of tensile load.

Here, βt takes the following values according to the weld joint shape. Cross joint 2.2, butt joint 2.0, T joint 1.0.

Further, gb (ρ) in the case of bending load is given by the following equation regardless of the joint shape.

Here, βb takes the following values according to the welded joint shape. Butt joint = 1.5, T joint = 1.9, other joints = 1.
0.

Unwelded part; Here, W in Eqs. (3) and (5) is used as follows depending on the weld joint type.

However, in equations (2) to (8), θ is the flank angle, ρ is the toe radius,
t is the thickness of the member to be loaded, tp is the thickness of the member not to be loaded, h is the bead height (leg length), hp is the weld leg length, a is the length of the unwelded portion, and αt is the toe. The tensile stress coefficient (in the case of tensile load = 1, in the case of bending load = 0), αb is the bending stress coefficient at the toe (in the case of bending load = 1, in the case of tensile load = 0).

As a result of looking at the stress concentration coefficient Kt in various joints with Kt and flank angle θ, toe radius ρ and height h obtained from the above formulas (2) to (8) as parameters, the effect of flank angle θ and bead height h Was relatively small, and it was found that the stress concentration coefficient Kt mainly changes depending on the size of the toe radius ρ. The result of the T-shaped joint and the case of the cross joint are taken as an example
Shown in FIG.

FIG. 4 shows the relationship between the toe radius / plate thickness and the stress concentration factor in the T-shaped joint.

It can be seen from the graph of FIG. 4 that the stress concentration coefficient Kt increases as the toe radius ρ decreases when the plate thickness t is constant. Crank angle θ when the toe radius ρ is small
It can be seen that the change in stress concentration factor due to (100 to 170 °) becomes large, and the change becomes smaller as the flank angle θ increases.

Similarly, in the graph of FIG. 5, it can be seen that if the plate thickness t is constant, the stress concentration factor increases if the toe radius ρ is small.

In the above, by obtaining the stress concentration coefficient Kt,
From the above equation (1), the local stress σ _{L at the} toe 9 can be known.
That is, the local stress σ _L is obtained by the following equation.

[sigma] _L = Kt * [sigma] _N Therefore, for example, when creating the SN curve when the nominal stress [sigma] _N is repeatedly applied to the lateral plate 7 of the cross joint portion shown in FIG. If a SN curve is drawn with the local stress σ _L instead of σ _N , a unified SN diagram can be obtained. This SN diagram is shown in FIG. In the figure, the vertical axis represents the bending stress σ _L at the toe within the stress range, and the points at which the butt joint, cross joint, and T-joint were broken when each stress was repeatedly applied were plotted. The straight line shown shows the survival probability in the number of tests.

Since this SN curve is drawn based on the local stress σ _L , the fatigue strength of each joint can be easily obtained.

The stress concentration factor Kt is normally distributed over a wide range when welding is performed uncontrollably. Therefore, for example, when the Kt value is 7.0, the local stress σ _L applied to the toe portion becomes 7 times as large as the nominal stress σ _N , and the fatigue strength decreases as the stress increases. Therefore, in order to increase the fatigue strength, it is necessary to design the plate thickness to be large and the cross-sectional area to be large to reduce the total stress applied to the plate.
That is, in the welding of the joint portion, if the welding is carried out indefinitely, the welding cost will be low, but on the other hand, the Kt value tends to be large, so that a thick plate must be used and the material value thereof increases. Therefore, as shown in the graph of FIG. 9, the material cost increases as the Kt value increases and the welding cost decreases as the Kt increases. In this case Kt
Is less than a predetermined value, in this example 3.0 or less, the total cost can be kept low, and the welding of the joint portion can be performed extremely economically.

The present invention welds each joint while holding the stress concentration factor at the toe within a predetermined value, preferably within 3.0.

However, even if the welding conditions are controlled, the distribution of Kt values becomes the distribution shown in FIG. FIG. 7 shows the distribution of Kt values when a T-joint is welded under the same conditions, and shows the frequency at each Kt value. Therefore, when the stress concentration factor kt is suppressed within 3.0, the maximum Kt value (2.38) in the distribution of Kt values shown in FIG. 7 is 3.0.
It is necessary to control it as follows.

FIG. 8 shows a graph obtained by integrating the distribution curve of the Kt value from the maximum value to the minimum value of Kt for each value of Kt. In the figure, 1 is the distribution curve of the Kt value in the case of uncontrolled welding. A line is shown, a curve m is an assumed curve when welding is controlled according to the present invention, n is an average Kt value when uncontrolled welding is performed, and o is an average Kt value when controlled. When the control is not performed, the Kt value is distributed in a wide range from the maximum value to the minimum value as shown by the line l, but the Kt value max can be suppressed within 3.0 as shown by the curve m by performing the control.

Next, a welding method for controlling the stress concentration coefficient Kt within 3.0 will be described. FIG. 10 shows an example in which a T-shaped joint is welded. In the figure, 11 is a lower plate, 12 is a standing plate, and the joint 13 between the lower plate 11 and the standing plate 12 is a torch nozzle 14.
Shows the case of MIG welding. In this case, the Nerai position of the electrode 15 at the tip of the torch nozzle 14 is directed to the seam 13, and the torch angle θt is set to a constant value (for example, 45).
゜) keep it. From this state, the torch nozzle 14 is oscillated to the left and right. This oscillating width is the vertical plate 12 and the lower plate 1
The amplitude (usually ± 4 mm) is set so that sufficient welding strength can be obtained according to the plate thickness of 1.

The number of oscillations in normal MIG welding is 70-80 times /
However, the stress concentration factor cannot be controlled with this number of oscillating materials. The present invention has an oscillating number of 150 to
The stress concentration factor K of the toe part is increased by performing 250 times / min.
This makes it possible to control t to be 3.0 or less. In this case, as the number of oscillates increases, the toe radius ρ at the toe portion can be increased, and for example, the toe radius ρ can be 1.0 mm or more.

Also, instead of controlling the number of oscillates, the torch nozzle 14
50 helium gas in the argon shielding gas from
The stress concentration coefficient Kt can also be set to 3.0 or less by mixing at least%. That is, helium gas has a higher thermal conductivity than argon gas, so that the penetration of the filler material is improved and the Kt value can be lowered.

All of this welding is performed using automatic welding materials, and
It becomes possible to weld the mutual joints of the tank members 5 in the independent rectangular tank 2 shown in the figure with a stress concentration factor of 3.0 or less at the weld toe. Therefore, since the Kt value of each tank member 5 can be suppressed to 3.0 or less, the plate thickness can be economically matched with the plate thickness stress, and the fatigue strength can be sufficient.

[Effects of the Invention] As is clear from the above description, according to the present invention, the following effects are exhibited.

(1) Since the stress concentration rate Kt can be obtained by calculation and only one SN curve is required, the fatigue test of each joint can be greatly simplified.

(2) The stress concentration factor Kt can be confirmed by measuring the actual welded shape of the structure, and the fatigue strength guarantee with high reliability against the fatigue strength of the actually finished structure can be performed.

(3) Since the stress concentration factor Kt can be statistically grasped by measurement, it is possible to perform a true reliability analysis.

[Brief description of drawings]

1 is a sectional view showing an independent rectangular tank of a liquefied gas tanker according to the present invention, FIG. 2 is a perspective view showing an independent rectangular tank, and FIG. 3 is fatigue of a liquefied gas tanker independent rectangular tank welded portion according to the present invention. Sectional drawing of the cross joint for explaining the stress concentration factor of a toe part in a strength guarantee method, FIG. 4 is a graph which shows the relationship between a toe radius and a stress concentration factor in a T-shaped joint part in this invention, 5th FIG. 6 is a graph showing the relationship between the toe radius and the stress concentration factor in the cross joint portion according to the present invention.
FIG. 7 is a graph showing an S-N curve in the present invention, FIG. 7 is a graph showing a distribution of stress concentration factors in the present invention, and FIG.
FIG. 10 is a graph in the case of integrating the distribution of stress concentration factors according to the present invention, FIG. 9 is a graph showing the relationship between stress concentration factors and costs, and FIG. 10 is an independent rectangular tank weld of a liquefied gas tanker according to the present invention. It is a perspective view which shows the example which welds each joint part in a fatigue strength guarantee method. In the figure, 2 is an independent rectangular tank, and 3 and 5 are tank members.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masaki Kitagawa Masaki Kitagawa 3-1-15-1 Toyosu, Koto-ku, Tokyo Ishikawajima Harima Heavy Industries Ltd. Technical Research Institute (72) Inventor Kazuyuki Shimoda Shin-Nakahara-cho, Isogo-ku, Yokohama, Kanagawa Prefecture No. 1 Ishikawajima Harima Heavy Industries Co., Ltd. Technical Research Institute (72) Inventor Toshio Irisawa No. 1 Shin-Nakahara-cho, Isogo-ku, Yokohama-shi, Kanagawa Ishikawajima Harima Heavy Industries Co., Ltd. Technical Research Institute (72) Inventor Gogawa Yokohama, Kanagawa Prefecture Ishikawajima-Harima Heavy Industry Co., Ltd. (72) Inventor Hidenobu Amano 2-1, Showa-cho, Kure-shi, Hiroshima Prefecture Ishikawajima-Harima Heavy Industry Co., Ltd.

Claims (1)

[Claims] 1. A method for guaranteeing fatigue strength of an independent rectangular tank welded joint of a liquefied gas tanker, which comprises a number of aluminum alloy tank members and welds the joints between the members, wherein the welded joints of the members are mutually joined. The stress concentration factor Kt at the weld toe is determined from the function of the flank angle θ, the toe radius ρ, and the bead height h with respect to the nominal stress generated in the plate to be welded toe from the weld toe. In addition to obtaining the local stress in the above, a fatigue fracture curve based on the local stress is prepared, and the allowable local stress of each welded joint is obtained from the fatigue fracture curve, and the stress concentration coefficient Kt is set to a predetermined value or less. A method for guaranteeing fatigue strength of an independent rectangular tank welded part of a liquefied gas tanker, which comprises welding by controlling a flank angle θ, a toe radius ρ, and a bead height h.