JP2019078032A - Tapered steel pipe pile and method for pulling out the same - Google Patents

Abstract

To provide a tapered steel pipe pile which can be easily pulled out at the time of removal and a method for pulling out the same.SOLUTION: A tapered steel pipe pile according to the present invention is a tapered steel pipe pile (4) installed on the underwater ground to support a floating structure, and has a straight portion (41) having a cylindrical shape that forms, at one end, a circular first opening (43) orthogonal to the central axis (O) of the tapered steel pipe pile and a tapered portion (42) which is connected to the other end of the straight portion (41), which extends to the opposite side of the first opening (43), which is reduced in diameter at a predetermined taper angle (θ) in the central axis (O) direction and which has a circular second opening (44) at the tip end orthogonal to the central axis (O).SELECTED DRAWING: Figure 2

Description

  The present invention relates to a tapered steel pipe pile and a method of drawing the same.

  In order to supply clean energy, installation of waterborne wind turbines at ports etc. is underway. The offshore wind power plant is often installed offshore where the wind is strong and the waves are high, and the steel pipe pile used to support the offshore wind power plant is strongly driven into the underwater ground. However, from the viewpoint of natural environment protection, it is required to remove the on-water wind power generation facility after the service life of the on-water wind power generation facility, for example, 20 years after installation, to restore the natural environment to its original state. Removal is required not only removal of facilities on water (towers, nacelles and blades) but also removal of steel pipe piles buried in the ground floor.

  In removing the steel pipe pile, first, the outer tube is driven by a vibrator and a water jet so as to surround the buried steel pipe pile, and the steel pipe pile buried by a vibrator or a crane is pulled out. Next, a method of pulling out the intubation tube with a vibro hammer or a crane is known as a conventional method. Further, Patent Document 1 discloses a method of drawing out a steel pipe pile while supplying compressed air to the inside of a steel pipe pile having a plurality of small holes and injecting the compressed air from the small holes to the outside.

JP, 2016-199874, A

  However, with the upsizing of waterborne wind power generation facilities, steel pipe piles to be buried are becoming larger in diameter. In order to pull out a large diameter steel pipe pile, a large vibro hammer or a crane is required. Moreover, the method of sending compressed air inside a steel pipe pile having a plurality of small holes requires sealing the head of the steel pipe pile, and further requires compressed air supply equipment.

  The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to propose a tapered steel pipe pile that can be easily pulled out when removing a water-based wind power generation facility and a method for pulling out the same.

  In order to achieve the above object, the tapered steel pipe pile according to the present invention is a tapered steel pipe pile installed on the underwater ground to support a floating structure, and is orthogonal to the central axis of the tapered steel pipe pile at one end. And a straight portion having a cylindrical shape that forms a circular first opening, and the other end of the straight portion connected to the other end of the straight portion and extended on the opposite side of the first opening and diameter-reduced at a predetermined taper angle in the central axis direction And a tapered portion at a tip end of which a circular second opening orthogonal to the central axis is formed.

  Furthermore, it is preferable that the length of the straight part is 1/3 to 1/10 of the length of the tapered steel pipe pile.

  Furthermore, the taper angle is preferably 1 to 4 (degrees).

Furthermore, it is preferable that the opening area of the second opening be smaller than the opening area of the first opening, and have an area of at least 0.28 (m ² ).

  The method according to the present invention is a method for pulling out a tapered steel pipe pile installed on the underwater ground to support a floating structure, wherein the tapered steel pipe pile is attached to the central axis of the tapered steel pipe pile at one end. A straight portion having a cylindrical shape forming a circular first opening, and the other end of the straight portion are connected to the other end of the straight portion and extend on the opposite side of the first opening, and the diameter is reduced at a predetermined taper angle in the central axis direction And a tapered steel pipe pile having a tapered portion forming a circular second opening orthogonal to the central axis at the tip.

  The tapered steel pipe pile according to the present invention can be easily pulled out after being buried.

It is a figure which shows the outline of an example of the conventional on-water wind power generation installation. It is a figure which shows an example of embodiment of a tapered steel pipe pile. It is a figure which shows the skeleton model used for a check. It is a figure which shows the flow to the member determination of a steel pipe pile. It is the figure which contrasted the shape of a straight steel pipe pile and a tapered steel pipe pile. It is a graph which shows the stress degree check result of an example of a straight steel pipe pile. It is a graph which shows the stress degree check result of an example of a tapered steel pipe pile. It is a figure which shows the variation of a steel pipe pile (the 1). It is a figure which shows the variation of a steel pipe pile (the 2). It is a figure which shows the variation of a steel pipe pile (the 3). It is a graph which shows the skin friction of a steel pipe pile. It is a graph which shows a stress degree check result. It is a figure explaining the method to withdraw a tapered steel pipe pile.

  Hereinafter, a tapered steel pipe pile and a method for drawing the same according to one aspect of the present disclosure will be described with reference to the drawings. However, it should be noted that the technical scope of the present disclosure is not limited to those embodiments, but extends to the inventions described in the claims and the equivalents thereof. In the following description and drawings, components having the same functional configuration will be assigned the same reference numerals and redundant description will be omitted.

(Summary of water and wind power plant)
FIG. 1 is a schematic view of an example of a conventional on-water wind power plant 1. A large diameter pile 2 installed by a so-called monopile method for driving one large diameter pile (monopile) having a diameter of several meters into the supporting ground of the water bottom supports the wind power generator 3 on the water. Usually, the large diameter pile 2 is a hollow pile having a cylindrical shape made of steel called a steel pipe pile 2. In the wind power generator 3, a tower 31 is installed on a steel pipe pile 2 which is a foundation, and a nacelle 32 and a blade 33 are provided on the tower 31.

  The steel pipe pile 2 supporting the wind power generator 3 has a bottom so as to withstand the vertical force Fz by the weight of the wind power generator 3, the blade 33 receiving the wind power, the horizontal force Fy by the wave, and the moment Mx. It is driven into the ground and installed.

  When the offshore wind power plant 1 is aged, the plant needs to be removed to protect the environment. Not only the wind turbine generator 3 above water but also the steel pipe pile 2 installed on the water bottom ground must be removed. In order to pull out the steel pipe pile 2 that has been squeezed into the underwater ground, facilities such as a large floating crane and a vibro hammer, which are the same size as or larger than when the steel pipe pile 2 is installed, are required. Therefore, the present invention has been invented which is a steel pipe pile that firmly supports a water-based wind power generator, and is a steel pipe pile that can be easily pulled out from the water bottom ground.

(Overview of Tapered Steel Pipe Pile According to the Present Invention)
FIG. 2 is a view showing an example of an embodiment of the tapered steel pipe pile 4 according to the present invention. 2 (a) is a perspective view of the tapered steel pipe pile 4, and FIG. 2 (b) is a cross-sectional view along the central axis O of the tapered steel pipe pile 4. As shown in FIG. On the side that supports the wind power generator 3 side, that is, on the water surface side, the tapered steel pipe pile 4 has a cylindrical straight portion 41 with a diameter D ₁ (m) and a steel pipe thickness t ₁ (m). In the direction in which the steel pipe pile 4 is driven into the underwater ground, it has a tapered portion 42 connected to the cylindrical straight portion 41 and extending and gradually reducing its diameter. The diameter of the tip of the tapered portion 42 is D ₂ (m), and the thickness of the steel pipe is t ₂ (m). Therefore, D ₁ > D ₂ . The total length of the steel pipe pile is L (m), the total length of the cylindrical portion 41 is L ₁ (m), and the total length of the tapered portion 42 is L ₂ (m). Therefore, a relation of L = L ₁ + L _2. The straight portion 41 forms a circular opening 43 orthogonal to the central axis (O) on the water surface side, and the tapered portion 42 is a circular perpendicular to the central axis (O) in the direction in which the tapered steel pipe pile 4 is driven into the underwater ground. Form the opening 44 of the D _1> is a D _2. Also, the thickness of the steel pipe thickness of the tapered steel pipe pile 4 is usually t ₁ = t ₂ . However, in order to reduce the weight of the steel pipe pile, it is possible to change the thickness of the steel pipe so as to reduce the thickness so that t ₁ > t ₂ . The angle between the vertical axis of the tapered steel pipe pile 4 and the side line of the tapered surface, that is, the taper angle is θ. Usually, the material of the steel pipe pile is SKK400, SKK490 or the like.

The reason why the tapered steel pipe pile 4 supporting the wind power generator 3 is partially tapered is to reduce the friction between the surface of the steel pipe and the ground. When pulling up a steel pipe pile vertically and pulling it out from the underwater ground, it is the gravity acting on the steel pipe pile and the frictional force between the surface of the steel pipe and the ground that is the resistance to the drawing force. Therefore to reduce the reduced frictional force the surface area of the steel pipe, it is tapered from the ground tip of tapered steel pipe piles 4 to a length L _2. In addition, reducing the weight of the steel pipe pile 2 by forming it into a taber shape also contributes to facilitating extraction. On the other hand, from the water surface with diameter D ₁ (m) and length L ₁ (m), in order to support the floating structure, to provide a solid foundation that can resist the stress received from the wind power generator 3 and waves. It is necessary to properly set the straight portion 41 extending straight to the ground. That is, the tapered steel pipe pile 4 according to the present invention has sufficient strength to support the on-water structure. It is a steel pipe pile that is easy to pull out when removing it.

(Study on design check of steel pipe piles)
In order to clarify the characteristics of the tapered steel pipe pile 4 according to the present invention, the supporting ground, the wind power generator 3, and the steel pipe pile 2 and the tapered steel pipe pile 4 were modeled and checked. The “Technical Manual of Offshore Wind Power Generation” 2001 for comparing the conventional steel pipe pile 2 without taper part (hereinafter referred to as “straight” steel pipe pile) and the tapered steel pipe pile 4 with taper part We referred to "3.2 Example of formula foundation design for monopile foundation" (hereinafter referred to as "design example") of the edition (author (Co) Development Center for Coastal Development). In the "Technical Manual for Offshore Wind Power Generation", offshore wind turbines are assumed, but for this inspection, the wind turbines are installed on land. It is for eliminating the influence of wave power etc. and simplifying the comparison with the straight pile 2 and the taper pile 4.

  FIG. 3 is a diagram showing a skeleton model used for control.

(Ground condition)
The ground conditions consisted of four layers of ground with the following characteristics. It is for simplification. In practice, since the ground on which the steel pipe pile is buried and installed has various geology and geological layer thicknesses depending on the installation place, it is necessary to conduct a boring survey and the like on site.

In addition, D. L. is an abbreviation of Datum Line (reference line, reference height), N is an N value, and is an index indicating the hardness of the ground.

(Windmill load)
In the above design example, the overturning moment of the wind turbine is applied because it models DL + 0.5m or less deep, but in this control, the overturning moment is not applied because it models up to DL + 60.00m with nacelle did. The wind turbine load shall be the load combination during a storm. In addition, the wave power and buoyancy that were loaded in the design example assuming installation on land were not loaded. The weight during a storm is shown below.

(Combination of load)
Load combination at the time of storm that became the member determination case in the design example. In addition, wave power and buoyancy that were loaded in the design example assuming installation on land shall not be loaded. Therefore, vertical load = self weight + windmill load (during storm).

(Additional allowable stress level)
The allowable stress level, which is the basis of design evaluation, was 1.0, as in the design example.

(Design verification method)
About member inspection of steel pipe pile, "Pile foundation design handbook" supervised by Ministry of Land, Infrastructure, Transport and Tourism 2007 edition, about ground supporting power "technical standard, commentary on facilities of harbor" 2007 edition (following, harbor standard) It checked according to. In addition, in order to conduct a simple study, it was decided not to study by the partial coefficient method but by the conventional allowable stress method.

  FIG. 4 is a diagram showing the flow until the member determination of the steel pipe pile. First, specifications of the pile on the ground where the steel pipe pile is driven are initialized (ST101). Next, the characteristic value β of the stake is calculated, and 3 / β is taken as the penetration length of the stake (ST102). The frame analysis is performed as a frame structure model in which the tower of the wind turbine is regarded as a long shaft, and the steel pipe pile is configured to be divided into several sections (ST 103). The stress check and the plate thickness are examined (ST104), and the supporting force check is performed (ST105). If the reference result is good, the design is over. If it is bad, re-examination of the penetration length of the pile is performed (ST106), the re-examined content is fed back to the frame analysis (ST103), the stress check and thickness are re-examined (ST104), and the supporting force is again An examination is performed (ST105). The feedback loop is repeated until the bearing check results are determined to be good.

The following points were noted for the straight steel pipe pile 2 and the tapered steel pipe pile 4.
(1) Insertion length of pile The insertion length of a tapered steel pipe pile was determined not by 3 / β of a tapered steel pipe pile but by 3 / β of a straight steel pipe pile based on pile specifications on the ground surface.
(2) Stress degree check It was decided that there is no margin in the stress degree ratio for test design.
(3) Pile thickness of piles The thickness of piles is 9 mm or more, and t / D / 1.0% according to the diameter of pile.
(4) Thickness change of pile Considering the influence of stress concentration of thickness change of pile, it was set to 7 mm or less.
(5) Since the allowable stress is reduced when the maximum thickness of the pile exceeds 40 mm, t ≦ 40 mm.
(6) Pile head displacement The displacement at the pile head (ground surface) was 1% or less of the diameter of the pile (15 mm for a diameter of 1500 mm or less).
(7) Plate thickness change point of pile In this trial design, the plate thickness change point of pile was set at any position.

  Regarding the performance check of underground piles, the check was conducted according to the pile foundation design manual. The stress generated in the pile due to the axial force and bending moment acting on the pile was calculated by the following equation.

here,
σ: Bending stress generated in pile (N / mm ² )
N: Axial force of pile (N)
A: Effective cross-sectional area of pile (mm ² )
M: Bending moment of pile (N · mm)
Z: Effective section coefficient of pile (mm ³ )
It is.

  It was checked that the degree of stress generated was equal to or less than the allowable stress of structural steels shown in Table 2.

  Based on the above design verification criteria, as shown in FIG. 3, we modeled the lateral spring in the ground and used the pin end as the pile tip for pin support, and studied in a two-dimensional framework model with beams on an elastic floor.

According to the above-mentioned harbor standard, lateral ground reaction force coefficient k _h (kN / m ² ) is a linear spring,
k _h = 1500N
Calculated by Here, N is an N value which is an index indicating the hardness of the ground.

  In order to make a comparative examination of a straight steel pipe pile and a tapered steel pipe pile, the wind turbine tower projecting to the ground part was the same as the design example in diameter 4,000 mm, thickness 40 mm, and material SM400.

(Calculation of pile penetration length)
To what extent the steel pipe piles are to be buried in the ground, that is, the pile penetration length is 3 / β or more according to the harbor standard. β is called a characteristic value of a pile and is obtained by the following equation.

here,
k _h ; lateral ground reaction force coefficient (kN / m ³ )
D: Diameter of pile (m)
E: Young's modulus of pile (kN / m ² )
I: Sectional second moment of pile (m ⁴ )
It is.

(Pulling force)
In the case of sand, the pull-out force of a pile driven into the ground by a striking method is determined by the following equation.

  In the case of sticky land,

here,
R _ut : Maximum pulling out force of pile (kN)

A _s ; Total surface area of pile circumference (m ² )

It is.

  FIG. 5 is a diagram comparing the shapes of two steel pipe piles to be checked, straight steel pipe pile 2 (model name T41T) and tapered steel pipe pile 4 (model name TB1-55T). For straight steel pipe pile 2 (model name T41T), the diameter D of the pile is fixed to 3400 (mm), and the thickness of the steel pipe is below 400 g (t = 39 (mm) and 400 (mm) from the ground side) The part of the pile is t = 34 (mm), and the total length of the pile is 24 (m). The tapered steel pipe pile 4 (model name TB1-55T) has the same total length 24 (m) as the straight steel pipe pile 2, the diameter of the pile on the ground side D = 3400 (mm), and the steel pipe thickness t = 39 (mm) . From the ground to the ground 3 (m) is a non-tapered cylindrical shape, and from the ground 3 (m), the diameter is reduced at a taper angle θ = 3.0 (degrees). Furthermore, from the ground surface to the underground 5 (m), t = 39 (mm), from the underground 5 (m) to the underground 11 (m), t = 36 (mm), and the underground 11 (m) to the underground 16 The distance up to (m) is t = 29 (mm), and the distance from underground 16 (m) to underground 24 (m) is t = 22 (mm). From the difference in the shape of the straight steel pipe pile 2 (model name T41T) and the tapered steel pipe pile 4 (model name TB1-55T), the skin friction resistance proportional to the surface area of the steel pipe pile is the straight steel pipe pile 2 (model name T41T) Compared to the above, tapered steel pipe pile 4 (model name TB1-55T) has a decrease of -39%. Also, the weight of the pile in the soil, which is proportional to the volume, decreases by -33%. Therefore, it is understood that the tapered steel pipe pile 4 (model name TB1-55T) is easier to pull out than the straight steel pipe pile 2 (model name T41T).

  FIGS. 6 and 7 are charts showing stress level check results for each of the straight steel pipe pile 2 (model name T41T) and the tapered steel pipe pile 4 (model name TB1-55T). The stress degree check result by frame analysis of straight steel pipe pile 2 (model name T41T) is shown in FIG. Here, the member number 100 of the frame is the position of the nacelle at 60 m above the ground. The member numbers 1, 2, ..., 24, 24 indicate the position of the straight pile 2 which descends every 1 m, with the ground surface as the member number 1, and the first member number 24 is the position of 23 (m) underground from the ground The last member number 24 indicates the position of the underground tip of the straight pile 2. The stress degree check result of the tapered steel pipe pile 4 (model name TB1-55T) is shown in FIG.

  Whether the steel pipe pile installed in the ground can endure supporting the aboveground structure is judged by the stress ratio. The stress level ratio must be 1.0 or less, which is calculated by stress level ratio = generated stress level / permissible stress level. The stress ratio in each member of the straight steel pipe pile 2 (model name T41T) and the tapered steel pipe pile 4 (model name TB1-55T) is all 1.0 or less, which satisfies the requirement. Therefore, a tapered steel pipe pile with a total length of 24 (m) can be supported even if it has a configuration with a length 3 (m) of the straight portion on the ground side and a length 21 (m) of the tapered portion on the ground side Some were obtained from the design check results.

8 to 10 show variations in which the length and taper angle of the straight portion of the steel pipe pile are changed, with the total length of the steel pipe pile being 24 (m) and the diameter of the ground side opening being 3400 (mm) in common. FIG. FIG. 8 shows a variation of taper angles θ = 1.0, 2.0, 2.5, and 3.5 (degrees) in a tapered steel pipe pile in which there is no straight portion with the steel pipe thickness 40 (mm) in common. In particular, the underground tip of the tapered steel pipe pile having a taper angle of θ = 3.5 (degrees) is blocked. If the tip of the steel pipe pile is closed, the sand can not pass through the steel pipe at the time of driving, and the steel pipe pile buckles. Therefore, the tip of the steel pipe pile is required to have an opening area of 0.28 (m ² ) or more.

FIG. 9 is a tapered steel pipe pile in which the thickness of the steel pipe is 39 (mm) and the straight portion 10 (m) is common, with variations of taper angles θ = 1.0, 2.0, 2.5, 3.5 (degrees). The reason why the straight portion is set to 10 (m) is because it is close to the reciprocal 1 / β of the characteristic value β (m ⁻¹ ) of the pile. The calculated penetration length of the tapered steel pipe pile is 3 / β = 24 (m), and it is considered that sufficient supporting characteristics can be obtained if it is close to 1⁄3 of 24 (m).

  FIG. 10 shows a variation of taper angles θ = 1.0, 2.0, 2.5, 3.0, 3.5 (degrees) in a tapered steel pipe pile having a steel pipe thickness of 39 (mm) and straight portions 2 to 3 (m). However, the tapered pile with model name TB1-55T changes the steel pipe thickness to 39, 36, 29, 22 (mm) in four stages at a taper angle θ = 3.0 (degree).

  FIG. 11 is a graph showing the circumferential surface frictional force of the steel pipe pile shown in FIGS. Here, what is indicated as straight is a conventional straight steel pipe pile having no tapered portion, and what is indicated as strike 10 is a tapered steel pipe pile whose straight portion has a length of 10 (m), strike 2 , 3 are tapered steel pipe piles having a straight portion length of 2 to 3 (m). The no strike is a tapered steel pipe pile without a straight part. It can be seen that the circumferential surface friction force decreases as the taper portion is longer than the total length of the steel pipe pile. Also, it can be seen that the surface friction force decreases as the taber angle increases. It should be noted that as described in FIG. 8, if the taper angle is too large, the tip of the steel pipe pile is blocked. The skin friction force of a tapered steel pipe pile with a straight part of 2 to 3 (m) can be linearly approximated as y = 2039.5x + 16067.

  FIG. 12 is a graph showing the result of checking the degree of stress of the steel pipe pile shown in FIGS. It can be seen that the tapered steel pipe pile (without strike) having no straight part has a stress ratio of 1.0 or more and is unsuitable. A taper pile with a taper angle of 1.0 to 3.4 (degrees) and a straight part of 2 to 3 (m) (str 2, 3), 10 (m) (str 10) has a stress ratio of less than 1.0 and is suitable as a support pile Know that

Therefore, the tapered steel pipe pile 4 according to the present invention has the following features.
(1) It has a straight part and a taper part, and the length of a straight part is 1/10-1/3 of the full length of a steel pipe pile.
(2) The taper angle is 1 to 4 (degrees).
(3) The opening area of the tip of the tapered portion is smaller than the opening area of the straight portion, and is at least 0.28 (m ² ).

  FIG. 13 is a view for explaining a method of pulling out the tapered steel pipe pile 4 after the wind turbine which is the water facility is removed. Conventionally, in order to weaken the circumferential surface friction force between the ground and the straight steel pipe pile, it is necessary to drive the outer intubation with a vibratory hammer, and to pull out the steel pipe pile with a large hoisting vessel etc. in two steps. Since the tapered steel pipe pile 4 according to the present invention has a tapered portion, the circumferential surface friction force and the self weight are reduced and it is easier to pull out than the conventional steel pipe pile, so that the driving of the outer tube is unnecessary and the turning type hoisting ship etc. It is possible to use it in one step. Further, with the enlargement of the water turbine, even when using a large diameter tapered steel pipe pile in which the diameter of the steel pipe pile exceeds 4 m, extraction and removal work using existing equipment becomes possible.

  It is to be understood that one of ordinary skill in the art can add various changes, substitutions, and alterations thereto without departing from the spirit and scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Water and wind power generation facility 2 Steel pipe pile 3 Wind power generator 31 Tower 32 Nacelle 33 Blade 4 Tapered steel pipe pile 41 Straight part 42 Tapered part 43 1st opening 44 2nd opening

Claims (5)

A tapered steel pipe pile installed on the bottom of the water to support floating structures,
A straight portion having a cylindrical shape that forms a circular first opening at one end orthogonal to the central axis of the tapered steel pipe pile;
A circular second opening which is connected to the other end of the straight portion and extends on the opposite side of the first opening, is reduced in diameter in the central axis direction by a predetermined taper angle, and which is perpendicular to the central axis at the tip Forming a tapered portion,
Tapered steel pipe piles with.   The steel pipe pile according to claim 1, wherein the length of the straight portion is 1/3 to 1/10 of the length of the tapered steel pipe pile.   The tapered steel pipe pile according to claim 1, wherein the taper angle is 1 to 4 degrees. The tapered steel pipe pile according to any one of claims 1 to 3, wherein the opening area of the second opening is smaller than the opening area of the first opening and has an area of at least 0.28 (m ² ). A method for pulling out a tapered steel pipe pile installed on the underwater ground to support a floating structure,
The tapered steel pipe pile is
A straight portion having a cylindrical shape that forms a circular first opening at one end orthogonal to the central axis of the tapered steel pipe pile;
A circular second opening which is connected to the other end of the straight portion and extends on the opposite side of the first opening, is reduced in diameter in the central axis direction by a predetermined taper angle, and which is perpendicular to the central axis at the tip Forming a tapered portion,
A method characterized in that it is a tapered steel pipe pile, having.

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