JPH0369404B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a mooring member for a tension leg platform, and is suitable for mooring a tension leg platform, etc., which is a type of offshore structure used for oil and gas development in the ocean. The present invention relates to a tubular welding member used. (Prior art) A tension leg platform consists of a platform floating on the sea, a seabed foundation that serves as an anchor installed on the seabed, and a "mooring member" that connects the two. This is a steel offshore structure designed to stabilize offshore platforms. In short, such a mooring member has the characteristic that only a tensile external force is always applied to it in the sea, and that the tensile external force always fluctuates in accordance with wave force and tidal force. For this reason, there are strict restrictions on internal defects and surface shape of the mooring member, especially in order to ensure corrosion fatigue strength in seawater. That is,
The pipe must be machined so that there are no notches on the inside or outside of the pipe that would cause stress concentration, and the pipe must be formed using an extremely clean steel material (for example, forged steel). Due to these limitations, the first tension leg platform in the world was built at Hutton Field in the North Sea, which did not require any welding, but instead used a one-piece mooring member made from forged steel by machining. (Paper presented at Offshore Technology Conference,
(See OTC 4428, May 3-6, 1982). Furthermore, in addition to the above-mentioned forged steel integral mooring member, welded assemblies manufactured by welding pipes and connectors have also been proposed, but there are many problems that need to be solved as listed below. Ta. (Problems to be solved by the invention) In welded structures such as mooring members used underwater, (a) ensuring the strength and toughness of the welded part; (b) ensuring the strength and toughness of the base metal part , (c) Undersea corrosion fatigue characteristics, especially when there is a geometrical discontinuity at the bead toe of a welded point, this becomes a notch that causes stress concentration, resulting in a decrease in fatigue strength. becomes. In particular, in the case of mooring members such as those targeted by the present invention, which are constantly subjected to fluctuating tensile external forces under the sea, minute geometrical discontinuities that exist perpendicular to the direction of action of the external forces (for example, discontinuities in circumferential butt joints) The bead toe) also becomes a stress concentration source, and even under low stress, repeated loading may lead to fracture. An object of the present invention is to provide a member that can ensure fatigue strength equivalent to that of the pipe base material even in the welded part with respect to the above-mentioned fatigue properties. (Means for Solving the Problems) The present invention solves the above technical problem by providing a tubular member in which a connector portion is circumferentially butt welded to both ends of a seam welded pipe, in which both the connectors and the seam welded pipe are welded together. The bead toe radius of the circumferential butt weld bead is 5 cm or more, and
The shape of the bead excess at the meeting point between the bead and the vertical seam weld bead of the pipe has a slope of 1/
3 or more, and the pipe vertical seam weld bead near the meeting point,
and for all circumferential butt weld beads,
To achieve this purpose, we proposed a mooring member for a tension leg platform, which is characterized by a ground surface in which the linear flaws caused by the grinding are oriented parallel to the tube axis. (Function) Generally, in the case of a mooring member that connects a tension leg platform, the external force that acts on it is:
Due to its structural characteristics, it mainly consists of a certain tensile force that balances the buoyancy of the floating platform, and a variable component added to this by wave force and tidal force. In other words, the only force that requires consideration in terms of strength for this member is the tensile force in the tube axis direction, and the effects of external forces in the direction perpendicular to the tube axis and bending moments are so small that they can be almost ignored. Therefore, in order to make the corrosion fatigue strength equivalent to that of the base material, it is sufficient to control only the stress concentration sources that exist in the axial direction of the tube that constitutes the mooring member, that is, at right angles to the direction in which external force acts. Therefore, the present invention provides a tubular mooring member having both a seam weld bead parallel to the pipe axis direction in the pipe part and a circumferential butt weld bead perpendicular to the pipe axis direction at the boundary between the connector part and the pipe part. , the bead toe is rounded so that it does not form a notch shape, and only the circumferential weld bead and the meeting area of the bead and seam weld bead are ground smoothly, and minor flaws on the bead surface due to bead grinding: Grinding By preventing linear marks from occurring in a direction intersecting the pipe axis direction, these welded parts are made to have the same corrosion fatigue strength as the pipe base material. The general strength and toughness of welded parts can be determined by appropriately selecting steel materials, welding methods, welding materials, heat input, and post-weld heat treatment.
In addition, the strength and toughness of the base material can be imparted with the required properties in addition to the above-mentioned corrosion fatigue strength by using appropriate steel materials, heat treatment, and the like. Next, the mooring member of the present invention was tested for the relationship between excess grinding and corrosion fatigue strength, which will be described below. Figure 4 shows the members used in the test. The test member has a weld bead 3 parallel to the load direction shown by
(corresponding to the vertical seam welding bead of pipes, hereinafter referred to as seam bead), and welding bead 4 perpendicular to the load direction (corresponding to the circumferential butt welding bead of pipes, hereinafter referred to as butt bead). It is something. The steel material to be tested is a tempered type with a thickness of 25.4 mm.
HT70 steel (C/0.09, Si/0.27, Mn/0.82, Cu/
0.24, Ni/0.55wt%). seam bead 3,
For welding butt bead 4, we used calcined basic flux and Ni-Cr-Mo wire (AWS F9A6-EG-
This was done by submerged arc welding using M2). The combinations of experimental parameters related to weld bead excess grinding are shown in Table 1. Note that bead grinding was performed using a disc grinder.

[Table] Tests were carried out using test materials in ASTM artificial seawater (ASTM
The test was carried out by holding in D-11419 and applying a repeated external tensile force (average stress 172 MPa) at a frequency of 0.3 Hz. No anti-corrosion treatment such as cathodic protection has been applied. S obtained by conducting tests on 5 to 7 bodies with different stress ranges (σ _R ) for each of test conditions No. 1 to 7.
-N curve is shown in FIG. Numbers 1 to 7 in the figure correspond to test condition numbers. Note that the results obtained using a smooth base material having the same shape as the test member shown in FIG. 4 are also shown in the same figure. The fatigue strength (σ _R ) at 10 ⁶ cycles obtained from the same figure is as shown in Table 2.

[Table] This table shows that fatigue strength varies greatly depending on the grinding conditions of the extra bead. That is,
Compared to the strength of the test member that is completely welded (condition 7), when the butt bead 4 is left welded (conditions 5 and 6), the shape and surface flaws of the seam bead 3 are increased due to stress concentration at the weld toe. Regardless of direction, the fatigue strength improves by only 6-8%. Furthermore, even if the butt bead 4 is ground smoothly to a radius of 5 mm ^or more, if the surface slope of the seam bead 3 due to the grinding is as steep as 1:1 (conditions 3 and 4), the meeting area of the seam bead 3 and butt bead 4 will still be stressed. Concentrated source, 120-125% compared to as-welded joints
The fatigue strength was improved to a certain degree (approximately 80% of the fatigue strength of the smooth base material). On the other hand, Butt Bead 4,
Both seam bead 3 have a shape according to the present invention (condition 1)
Therefore, the fatigue strength was found to be equivalent to that of the smooth base metal and 178% higher than that of the as-welded joint. Note that even if the shape of the bead toe is the same as that according to the present invention, if the linear scratches on the bead surface due to grinding are perpendicular to the tube axis direction (condition 2), the fatigue strength will be lower than that of a smooth matrix. The amount of wood decreased to 89%.
From the above results, among the characteristics of the mooring member according to the present invention, (a) the shape of the seam bead 3, (b) the shape of the butt bead 4,
It has been found that if either the shape of the pipe or (c) linear flaws on the bead surface are not satisfied, it is not possible to secure a pipe base material with corrosion fatigue strength equivalent to that of the pipe base material, which is the object of the present invention. In addition, in the present invention, cathodic protection or heavy corrosion protection coating may be applied in order to improve corrosion resistance and fatigue resistance. (Example) FIG. 1 shows an example of a mooring member for a tension leg platform according to the present invention. This mooring member consists of a welded pipe 1 having a vertical seam bead 3 extending in the axial direction, and a tubular connector 2 having a male or female thread at both ends.
It is a tubular member which is circumferentially butt welded. Here, the connector 2 is a structure provided to connect the mooring members with each other through a threaded joint, and to extend the mooring members to a required length corresponding to the operating depth of the tension leg platform. In addition, as a modification of the single welded pipe 1, a plurality of welded pipes 1,
1' and 1 may be butt-welded circumferentially at the ends of the tube and integrated into a desired length. In either case, as shown in Fig. 2, a circumferential butt weld bead 4 (hereinafter simply referred to as "butt bead") exists in the direction perpendicular to the tube axis (circumferential direction).
As shown in the bead, the radius of the toe 5 is 5 mm or more, and the surface is finished so that the fine linear irregularities generated after grinding on the bead surface are parallel to the tube axis direction. The reason why the radius of the bead toe 5 is set to 5 mm or more is because if the radius is smaller than this, the same portion becomes a notch and the corrosion fatigue strength decreases. Next, welded pipe 1 that exists parallel to the pipe axis direction.
The vertical seam bead 3 (hereinafter simply referred to as ``seam bead'') is ground only at the joint part with the butt bead 4 on both the inner and outer surfaces of the tube, as shown in FIG.
The slope of the meeting part caused by the grinding is made to be 1:3 or less, and the joint with the butt bead 4 is smoothly connected and flush with the shape. Note that the seam bead is left welded without being ground except for the meeting portion. However, if the above shape cannot be achieved with welding as is, the desired bead shape may be achieved by grinding the excess using a grinder or the like. However, the direction of fine linear flaws (grinding line marks) generated on the bead surface due to bead grinding is made to be approximately parallel to the tube axis direction. Here, the slope of the bead surface of the seam bead 3 at the meeting point of the seam bead 3 and the butt bead 4 is set to 1:3 or less, and the joint part between the seam bead 3 and the butt bead 4 is made flush with the ground surface of the butt bead 4. If this cannot be achieved, the same part will become a notch and corrosion fatigue strength equivalent to that of the base metal cannot be secured. Furthermore, even if the surface flaws of the butt bead 4 and the seam bead 3 due to excess grinding are not parallel to the tube axis direction, the fatigue strength may similarly be lowered compared to the base material due to the notch effect of the surface flaws. In addition, 6 in the figure indicates a circumferential weld bead excess removal allowance, and 7 indicates a vertical seam weld bead excess removal allowance. Next, a corrosion fatigue test was conducted using HT60 steel with a plate thickness of 50 mm assuming a mooring member having the shape shown in Figure 1. A test member similar to that shown in FIG. 4 (with two weld beads parallel to the load direction and perpendicular to the load direction) was used. The test was conducted by applying repeated external tensile force (average stress 172 MPa) at a cycle of 18 cpm in the direction shown in FIG. 4 in ASTM artificial seawater (ASTM D1141) to cause natural corrosion. The results are shown in FIG. The results shown with a circle in the figure are obtained by grinding the butt bead excess using a disk grinder until it is flush with the base material, and also by grinding the seam bead excess at the meeting point of the butt bead and seam bead as shown in Figure 9. The shape was prepared using a grinder. The results are shown in FIG. 9, and are equivalent to the values for the smooth base material (based on specimens of the same shape) shown by the solid line. In other words, it can be seen that the corrosion fatigue strength when the bead excess is removed as described above is equivalent to that of the base metal. On the other hand, when both the seam bead and the butt bead were tested with extra buildup (as welded), both fractured with lower strength than the solid line, and the fracture location was at the toe of the butt bead. Next, heat-treated HT70 steel (C/0.10, Si/0.25,
Mn/0.85, Cu/0.20, Ni/0.98wt%) was formed into a tubular shape as rolled, seam welded using submerged arc welding, and then QT heat treated (outer diameter
508mm, pipe thickness 25.4mm, length 12000mm) with 21/
A tubular member (outer diameter 508 mm, tube thickness 25.4 to 70 mm, length 1000 mm) with a threaded joint made of 4Cr-1Mo forged steel,
The mooring member was formed by circumferential welding using submerged arc welding. Here, using a disc grinder on both the inner and outer surfaces of the pipe, remove the excess portion of the circumferential weld bead until it is flush with the base metal, while for the HT70 steel pipe seam weld bead, remove the circumferential weld bead until it is flush with the base metal. The excess portion was ground so that the joining part with the bead had a slope of 1:5 and the joint with the circumferential weld bead was flush. The mechanical properties of the same member are
It was found that it has a tensile strength of YS > 550 MPa, TS > 700 MPa or more, and absorbed energy of 70 J or more as a result of the Sharpie V-notch test at -20°C, and has sufficient performance as a mooring member. Next, a 1/10 scale model with a shape completely similar to the same member was repeatedly tensile loaded in ASTM D1141 artificial seawater to investigate corrosion fatigue strength. Data shown in FIG. 10 as the fatigue strength (σ _R ) was obtained. When the fracture surfaces of these test specimens were observed after fracture, it was found that cracks had developed from corrosion pits in the base material of the steel pipes, leading to fractures. Furthermore, when a seamless pipe having almost the same composition and strength as the HT70 steel pipe was repeatedly loaded under the same test conditions, the data shown in FIG. 10 with a circle was obtained. Both have almost the same fatigue strength.
From the above results, it was found that the HT70 class mooring member according to the present invention can ensure corrosion fatigue strength comparable to that of the steel pipe base material even in the welded part. As explained above, the mooring member according to the present invention is
Although it is a welded structure, it can ensure corrosion fatigue strength equivalent to that of the base member. Furthermore, the mechanical properties are also good as shown in the examples. In addition, the mooring member by welding assembly according to the present invention can be manufactured in all sizes and shapes within the range that can be manufactured as welded pipes in terms of pipe diameter and pipe thickness, and there are no restrictions on dimensions in terms of machining equipment. It is possible to realize a member with a larger diameter and thicker wall than a mooring member with a forged steel integral structure. Therefore, as oil and gas development moves deeper into the ocean in the future, it will be extremely useful from a cost perspective as a large-diameter, thick-walled mooring member that will be required to ensure platform stability.

[Brief explanation of drawings]

Fig. 1 is a front view a of a three-piece mooring member using a welded pipe according to the present invention and a modification b of the welded pipe, and Fig. 2 is a sectional view of the circumferential weld bead shape of the mooring member according to the present invention. , Fig. 3 is a front view and a sectional view along line A-A' of the meeting point of the pipe vertical seam bead and the circumferential weld bead of the mooring member according to the present invention, and Fig. 4 is a front view of the corrosion fatigue test member used in the experiment. Figure 5 is a cross-sectional view of various weld bead shapes according to the present invention, Figure 6 is a cross-sectional view of a weld bead shape shown as a comparative example, and Figure 7 is a graph of corrosion fatigue test results showing the usefulness of the present invention. , FIG. 8 is a graph of the corrosion fatigue test results in an example of the present invention, FIG. 9 is a graph of the weld bead shape in an example of the present invention, and FIG. 10 is a graph of the corrosion fatigue test results in an example of the present invention. be. 1... Welded pipe used as a mooring member, 2... Connector with threaded joint, 3... Vertical seam weld bead (seam bead) of welded pipe, 4... Circumferential butt weld bead (butt bead), 5... Circle Circumferential butt weld bead toe, 6... Circumferential weld bead excess removal allowance, 7... Vertical seam weld bead excess removal allowance.

Claims (1)

[Claims] 1. In a tubular member in which a connector part is circumferentially butt-welded to both ends of a seam-welded pipe, the bead toe radius of the circumferential butt-welded bead part between both connectors and the seam-welded pipe is 5 cm or more, and At the meeting point between the bead and the vertical seam welding bead of the pipe, the shape of the excess bead is gentle with a gradient of 1/3 or more, and the vertical seam welding bead of the pipe near the meeting point, and a mooring member for a tensile leg platform, characterized in that all of the circumferential butt weld beads have a ground finish surface such that linear flaws due to grinding are oriented parallel to the tube axis direction. .