JPS6082495A - Independent square tank for liquefied gas tanker - Google Patents

Abstract

PURPOSE:To improve reliability for fatigue strength in a structure, by welding a weld joint part as holding a stree concentration factor in the weld stopping end part of a tank member down to below the specified value. CONSTITUTION:In the case where a joint 13 between an upright plate 12 and a lower plate 11 is welded by a torch nozzle 14 through MIG welding using a wire electrode, an aimed position of an electrode 15 at the tip of the torch nozzle 14 is directed at the joint 13, keeping a torch angle thetat in a constant value. Under this condition, when the torch nozzle 14 is oscillated in both directions 150-250 times per minute, a stree concentration factor Kt in a stop end part is controllable to below 3.0. With this constitution, the reciprocal joint of a tank member inside an independent tank can be welded at below 3.0 in the stress concentration factor, thus reliability in the tank is improvable.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a liquefied gas tanker that carries LNG, LPG, etc. as cargo, and more particularly to an independent rectangular tank installed in the liquefied gas tanker.

As shown in Fig. 1, the liquefied gas tanker has an independent rectangular tank 2 in a hull 1 supported by rolling chocks 3 and supports 4 with a gap between the inner surface of the shell 1 and the inner surface of the shell 1. It is designed to allow shrinkage and movement due to low temperatures when cargo is loaded. The independent rectangular tank 2 containing the low-temperature liquid has a 4Fj structure that has sufficient strength against deformation stress during contraction.

This independent rectangular tank 2 is made of an aluminum alloy, and as shown in FIG. It is built horizontally with welded seams. By the way, IMO Code for this type of liquefied gas tanker
Tankers belonging to [NoruT-ypc[3] are IM
It is mandatory to perform detailed fatigue strength analysis using OCode. The definition of a Type B tank is that the safety of the tank has been confirmed through accurate analysis and experimentation, and the tank will not undergo major destruction at any one time.Therefore, emphasis is placed on fatigue analysis and fracture analysis. An independent rectangular tank has a bone-solid structure with many welded tank parts inside, and has a high static constant order, so it was difficult to completely analyze it, but recently it has become possible to analyze it relatively easily with computers. There is.

However, in a bone-cut structure, fillets) and d-joints are often used, and since the welded shape is irregular, it is necessary to conduct a fatigue test on each welded part individually. Conventionally, this fatigue strength analysis of welded joints has been conducted based on the nominal stress of the member, and for each type of welded joint, we have shown the relationship between the stress and the number of times the welded joint will fatigue and break if stress is repeatedly applied. The fatigue strength of the welded joint against the nominal stress is estimated based on the S-N curve determined experimentally. However, since the fatigue strength differs depending on the shape of the member, the shape of the joint, or the shape of the weld bead, various S-N+TLt wires are required, and many tests are required.

In fact, it is impossible to cover all the joints that appear in an isolated rectangular tank with the S-N curve obtained in each test.

The present inventors found that among the influencing factors such as the joint shape, metallurgical properties, and residual stress of the welded joint, the fatigue strength of welded joint members is influenced by the shape of the welded joint. The inventors have discovered that this is the stress concentration factor of the fatigue crack occurrence area, particularly the stress concentration factor of the weld toe, which led to the present invention.

An object of the present invention is to provide an independent rectangular tank for a liquefied gas tanker in which fatigue analysis of various welded joints of the above-mentioned independent rectangular tank can be easily performed, and in addition, 11 fatigue settings of each tank member can be easily performed. It is.

The present invention consists of a large number of aluminum alloy tank members,
In addition, in an independent rectangular tank in an IC liquefied gas tanker by welding the joints between the members, the stress concentration factor K[J3 at the weld toe of the welded joint between the above parts shall be 3.0 or less. By welding while controlling the JUG stress concentration coefficient Kt to 3°0 or less at the toe, welding can be performed using a single S-N curve including the base I. This allows the fatigue strength of welded joints to be stepped down.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of an independent rectangular tank for a liquefied gas tanker according to the present invention will be described below with reference to the accompanying drawings.

First, the stress concentration coefficient Kt will be explained using the cross-welded joint shown in FIG. 3 as an example.

In Figure 3, there is J3, 6 is the vertical board, 7 is the horizontal board, and the vertical board 6
Assume that a fillet weld 8 is applied to the horizontal plate 7.

In this case, the reinforcement angle (hereinafter referred to as flank angle) of the weld 8 is θ, the bead height is 11, the heat toe radius of the toe 9 of the weld 8 is ρ, and the thickness of the horizontal plate 7 is t.

If a nominal stress σ is now applied to the horizontal plate 7, the stress distribution will be as shown at 10 on the horizontal plate 7 without welding.
Although the nominal stress σ remains the same, stress concentration occurs as it approaches the toe 9, and a substantially maximum local stress σ acts at the toe 9. Therefore, the stress concentration factor created in this toe portion 9 is K [where Kt is expressed by the following formula.

-71・ ...... (SσN Also, this Kt is the above-mentioned flank angle θ, toe radius ρ,
It is expressed by the above equation as a function of the bead height 1).

Kt-[1+f(g(ρ)-1):IIc(a/
L) --- (2) Here, f(θ) is the shadow of the flank angle] (1(ρ) is the influence of the weld toe radius, C(a/l
) are functions affected by the presence of the unmelted neck, and are expressed by the following formulas.

Flank angle: Toe radius; g (ρ) - α1・gt (ρ) 10 α,・gb (ρ) ・・
(4) Here, gt(ρ) is given by the following equation in the case of a tensile load.

P2.8(-)-2 Here, βt takes the following value depending on the shape of the welded joint. Cross joint 2.2, butt joint 2.0.181 hand 1.00 In addition, gb (ρ) in the case of bending load is given by the following equation regardless of the joint shape.

Here, βb takes the following value depending on the shape of the welded joint. Butt joint - 1, 5, tooth joint - 1, 9, square joint = 1.
0° Unwelded area: Here 'C-(31, (51) W in formula is used as follows depending on the type of welded joint.

However, in formulas (2) to (8), θ is the flank angle, ρ is the toe radius, [ is the plate thickness of the member that is loaded, t is the plate thickness of the member that is not loaded, and 11 is the bead height and leg length. ), tip is the weld leg length, a is the length of the 6 unmelted parts, αt is the tensile stress coefficient at the toe (-1 for tensile load, -0 for bending load m),
αb is the bending stress coefficient at the toe (-1 for bending load,
In case of tensile load -〇).

KL and flank angle θ calculated from equations (2) to (8) above
As a result of looking at the stress concentration coefficient Kt in various joints using the toe radius ρ and bead height 11 as parameters, the flank angle θ
The influence of the bead height 11 is relatively small, and the stress concentration coefficient Kt changes mainly depending on the size of the toe radius ρ. The case is shown in FIGS. 4 and 5 as an example.

Is figure 4 a king? It shows the relationship between the toe radius/plate thickness and stress concentration coefficient at J3 in the joint.

From the graph of FIG. 4, it can be seen that if the plate thickness deviates from a constant value, the stress concentration coefficient Kt increases as the toe radius ρ becomes smaller. When the toe radius ρ is small, the crank angle θ(
100 to 170°), the stress concentration factor increases as the flank angle θ increases, or the change decreases as the toe radius ρ increases.
It turns out that it is at e'.

Similarly, in the graph of FIG. 5, it can be seen that if the plate thickness is set constant and the toe radius ρ is decreased, the corresponding horn concentration coefficient increases.

In the above, by setting the stress concentration factor]<1,
The local stress σ at the toe 9 can be determined from the above equation (1). In other words, the local stress σ is determined by the following formula.

σL ”” KL ”σN Therefore, for example, when creating an S-N curve for the case where the nominal stress σ is repeatedly applied to the horizontal plate 7 of the cross joint shown in Fig. 3 until it breaks, the nominal stress σ If we draw the S-N curve at local stress 9 instead of
A line diagram can be created. This S-N diagram is shown in FIG. In the figure, the vertical axis indicates the local stress 9 at the toe in the stress range, and when each stress is repeatedly applied, the butt 'I! fittings,
The points where the cross joint and T-subtitle hand were destroyed are plotted, and the straight line showing the graph shows the probability of survival among the number of test pieces.

This S-N curve shows the local stress σ1. Since the drawings are based on , it is possible to easily determine the fatigue strength of each joint.

If the concentration coefficient + < 1, there is usually no fl, welding is performed in one group 4
For example, it is distributed over a wide range. Therefore, for example, when the Kt value is 7°0, the local stress σ1. is the nominal force σ
It becomes 7 times larger than that of N, and the fatigue resistance N strength decreases. Therefore, in order to increase the fatigue strength, it is necessary to design the plate to have a large thickness and a large cross-sectional area to reduce the overall stress applied to the plate. That is, when welding a joint part, if welding is performed without control, the welding cost will be reduced, but on the other hand, the value of K tends to increase, so a thick plate must be used, and the material value will increase. Therefore, as shown in the graph shown in FIG. 9, the material cost increases as the Kt value increases, while the welding cost decreases as the K[ increases. In this case, if Kl is 3.0 or less, the total cost can be kept low, and the welding of the joint can be performed extremely economically.

The present invention is to weld each joint while keeping the stress concentration coefficient of the toe within 3.0.

However, even if the welding conditions are controlled, the distribution of Kt values is as shown in FIG. 7. FIG. 7 shows the distribution of Kt values when T-shaped joints are welded under the same conditions, and shows the frequency at each Kt value. Therefore, the stress concentration factor! In the distribution of Kt values shown in Fig. 7, the maximum value of +<1 value (2,38>) is kept within 3°0.
It is necessary to control it so that it is 0 or less.

Figure 8 shows a graph obtained by integrating the distribution curve of ]<L value from the maximum value to the minimum value of 1<[ for each value of K[, and the text in the figure is the distribution of Kt value when welding without control. The curve shows the integrated line, and the curve... shows the control of welding according to the present invention1111
L shows the assumed curve when welding, n shows the average 1<[ value when welding is performed without control, O shows the average + < 1 value when welding is controlled, and $ shows the average + < 1 value when welding is not controlled, Although the Kt value is distributed over a wide range from the maximum to the minimum value, by carrying out step 11, it is possible to suppress the value of K to within 3.0 as shown by the curve (3).

Next, a welding method for controlling the stress concentration factor Kt to within 3.0 will be explained. Figure 10 shows an example of welding a 1-shaped joint, where J3 is shown in the figure, 11 is the upper plate, 12
1 is a vertical plate, and the joint 13 between the lower plate 11 and the vertical plate 12 is MIG welded using a torch nozzle 14. In this case, the position of the electrode 15 at the tip of the torch nozzle 14 is kept facing the seam 13 (1), and the torch angle θt is kept at a constant value (for example, 45 degrees. From this state, the torch nozzle 1
Avoid oscillating 4 to the left and right. This oscillation width is set to an amplitude (normally ±4 m+n!!i! degree) that can provide sufficient welding strength according to the thicknesses of the upright plate 12 and the lower plate 11.

Normally, the number of A oscillations in MIG welding is 70 to 80 times/minute, but the stress concentration factor cannot be controlled with this number of oscillations. The present invention has A-sillage 1-number of 15
By performing the test at a rate of 0 to 250 times/min, it is possible to control the stress concentration coefficient +<1 at the toe to 3.0 or less. In this case, by increasing the number of oscillations, it is possible to increase the toe radius ρ at the toe,
For example, it is possible to set the radius ρ of the toe to be equal to or greater than i, Q mm.

Also, instead of controlling the number of A shots, the torch nozzle 1
5 helium scum in the argon shielding gas from 4
0% or more” - Mixing also reduces the stress concentration factor K[to 3.0
You can do the following. Furthermore, helium gas has a higher thermal conductivity than argon gas, and therefore the filler metal is more easily incorporated into the melt, making it possible to set a lower value for K.

All of this welding is performed using automatic welding material, and the stress concentration coefficient of the weld toe shown in A is 3.0 or less at the mutual joint of the tank members 5 in the independent rectangular tank 2 shown in FIGS. 1 and 2. It becomes possible to weld it.

Therefore, the electric value of each tank member 5 can be kept to 1<3.0 or less, so it can be made into an economical plate commensurate with the stress of the plate thickness, and its fatigue strength can also be made sufficient. .

As is clear from the above detailed description, the present invention provides the following effects.

(1) Since the stress concentration ratio +<1 can be expressed as J in Kl'R and only one S-N curve is required, the fatigue test of each joint can be greatly simplified.

(2) The response/concentration coefficient Kt can be confirmed by measuring the welded shape of the actual structure. Highly sexual.

(3) The stress concentration factor Kt can be determined by measurement! Since it can be grasped visually, reliability analysis in the true sense becomes possible.

[Brief explanation of the drawing]

Fig. 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 a stopper in an independent rectangular tank of a liquefied gas tanker related to water source 11J. A sectional view of a cruciform joint to explain the stress concentration coefficient at the end, FIG. 4 is a J graph showing the relationship between the toe radius and the stress concentration coefficient at the T subtitle hand part in the present invention, and FIG. A graph showing the relationship between the toe radius and the stress concentration factor in the cruciform joint part in the invention, Figure 6 is a graph that does not show the S-N curve in the invention, and Figure 7 FIG. 8 is a graph showing the distribution of the stress concentration factor according to the present invention when integrated. FIG. 9 is a graph showing the relationship between the stress concentration factor and cost. FIG. 10 J to the independent rectangular tank of the liquefied gas tanker according to the present invention.
3 is a perspective view showing an example of welding each joint portion. In the figure, 2 is an independent rectangular tank, and 3.5 is a tank member. [Applicant: Ishikawajima Harima Heavy Industries Co., Ltd. Representative Patent Attorney Nobuo Kinutani Figure 1 Figure 2 Figure 3 Figure 6 Figure 4 Figure 5 Figure 7 C Figure 8

Claims (1)

[Claims] Stress concentration coefficient Kt at the weld toe of the welded joint between the members is located in an independent rectangular tank in an LNG tanker that is constructed from a large number of aluminum alloy tank members and the joints of the member set H are welded. An independent rectangular tank for a liquefied gas tanker, characterized in that the ratio of