I, positive inverted cone combination of the design of ice-resistant structure
1. The main factors to consider
①Sea ice floats on the water surface, up and down with the tide level changes, ice force only acts on the tidal changes in the local height of the tidal range, the smaller the tidal range, the smaller the scope of the ice force; ②Sea ice mainly with the movement of the sea currents, so the main stream direction mainly acted on the structure, the force of large components is the first contact with the ice structure edge; ③Sea ice damage forms mainly include extrusion, bending and buckling. First contact with the edge of the structure ice; ③ the damage form of sea ice mainly includes extrusion, bending and buckling. A large number of experiments and research work show that, with sufficient thickness and strength of sea ice, its bending damage strength is about 1/3 of the strength of extrusion damage; ④ simple construction, small volume, additional weight as light as possible;
⑤ comprehensive evaluation of the platform's stress condition, additional ice-resistant structure should not be excessive increase in the platform of the additional wave and seismic forces.
2. Structural form of positive inverted cone anti-ice structure
Figure 14-9 Positive inverted cone combination structure diagram
Adding cone structure in the tidal difference section of the conduit rack leg column will change the destructive form of the sea ice from extrusion to bending damage, reduce the force of the sea ice on the platform, and change the platform's ice-resistant performance. Generally speaking, the slower the slope is, the easier it is for the sea ice to climb up, and thus it is easy to form bending damage, and with the gradual steepening of the slope, the sea ice will gradually transition from bending damage to squeezing damage, when the angle between the foot of the slope and the horizontal line reaches a certain value, the sea ice will completely form squeezing damage, and the force of the sea ice will also increase, which is quite the same as that of the straight force column. Therefore, the angle of the cone should not be too large, usually should not exceed 700. in turn, reduce the slope of the cone, will inevitably increase the size of the cone, not only to the construction of the inconvenience, but also incidentally increase the structure of the ice force and wave loads, and resistance to seismic action is also harmful. Since the ice-resistant cone is added to the already installed conduit rack, the convenience of construction must also be considered. After a variety of options for selection, the final choice of positive inverted cone combination of structural form (Figure 14-9).
The combination of 4.0m high, cone large cross-section diameter of 4.0m, elevation +0.58m, positive cone height of 2.5m, the top elevation of +3.1m, the cone angle of about 650, inverted cone height of 1.5m, the bottom elevation of -1.3m, cone angle of about 52 °, the entire cone height of the range of about 4.4m, and the region of the differential tide and the change of tide difference in this area is basically compatible. Considering that the density of sea ice is lighter than seawater, part of the sea ice floats on the surface of the water, and most of the sea ice is submerged in seawater, the elevation of the bottom of the cone is appropriately lowered, so as to make the sea ice work better on the cone. The positive and negative cone is designed as 2 pieces, assembled on site and fixed by bolts at the top and bottom. Cone surface panel thickness of 30mm, the interior along the circumference into 6 equal parts, the establishment of 6 200mm × 120mm × 12mm angle steel reinforcement, close to the center of the height of the cone, add a 400mm × 200mm × 20mm × 25mm "T" ring-shaped reinforcement beam.
Two, positive inverted cone anti-ice structural strength accounting
(a) the ice force acting on the cone
It is assumed that the ice force is acting on the cone, according to APIBuL2N, the ice force acting on the cone is calculated according to the following formula:
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The formula: RH is the horizontal force acting on the cone; σ f is the bending damage strength of sea ice; t is the thickness of sea ice; ρwg is the unit mass of seawater; D is the diameter of the cone at the waterline; DT is the diameter of the cone's small head; tR is the climbing height of the sea ice; A1, A2 and the coefficients related to ρwgD2/σft and μ; A3, A4 and the coefficients related to the cone inclined line and the horizontal angle α; μ is the coefficient of friction of the sea ice with the surface of the steel structure.
Taking A1=1.6,A2=0.29,A3=0.55,A4=3.74 and substituting Eq. (14-3), the vertical force acting on the cone is:
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Eq. B1 and B2 are related to the values of α and μ.
B1=0.37,B2=0.0226, substituting into Eq. (14-4), then
Rv=165.55t
Combined force
(ii) Cone Shell and Shell Calibration
It is assumed that the ice force is uniformly distributed on the surface of half of the cone.
Surface area of half cone F = π × 250 × (200 + 83.4) = 222582cm2 (14-5)
Uniformly distributed load
At the cross section of the positive cone R = 1767mm, the unit-width sector shell plate between the two longitudinal reinforcing bars (Fig. 14-10) is calibrated as a curved arch.
In Fig. 14-10:
Fig. 14-10 Diagram of the sector shell plate of a cone
OD=Rcos11.25° = 1733mm
AD=Rsin11.25° = 344.7mm
Arch span L=ADx2 = 689.45mm
Sagittal height f=R-OD =34mm
The height-to-span ratio f/L≈0.05According to the Manual of Static Calculation of Building Structures, the value of the coefficient of influence of axial force K is:
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The coefficient of variability of the arch cross-section m is:
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The two formulas: Ic is the moment of inertia of the section of the arch top; Ia is the moment of inertia of the section of the foot of the arch; Ic=Ia=2.254cm4; half-arc center angle α=11.25°, cosα=0.981; n3 is the coefficient related to f/L, n3=11.3; A. is the cross-sectional area of the arch plate, Ac=3cm2; f2=11.56cm2.
According to the "Handbook of Calculation of the Static Forces of Building Structures", substituting the above K, m and qL2 values into the formula for calculating the bending moment at the foot of the arch, then:
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The sectional modulus of the arch plate w=32/6=1.5cm3
The bending stress σ=M/W=478.2kg/cm2
The force on the arch plate is shown in Figure 14-11.
Figure 14-11 Schematic diagram of the force on the arch plate
Figure 14-12 Longitudinal and vertical reinforcing material
(C) Reinforcing material checking
The position of the reinforcing material is shown in Figure 14-12.
The reinforcing material is considered as a simply supported beam at both ends Figure 14-13, and the load is distributed equally.
Q=295kg/cm
q=69×4.28=295kg/cm
Maximum moment Mmax=qL2/8=496190kg-cm.
Combined cross-section characteristics are shown in Fig. 14-14. effective area A=f×b×tp=87.71cm2
Eq. 69cm,tP=30mm,L=1159mm,f=0.3(1/b)2/3=0.424324.
Moment of inertia of combined section J=7911cm4;
Section modulus W=J/16.856=496.3cm3;
Bending stress σ=M/W=1057.3kg/cm2;
Allowable stress Fb = 0.66Fv = 1584kg/cm2, > σ, to meet the strength requirements.
Figure 14-13 Force diagram of cylinder support beam
Figure 14-14 Combined cross-section
The above calculation method of cone structure design is only an example for reference, for different engineering facilities, it is also necessary to use the calculation mode that meets the actual situation according to the different sea-ice and structural data for the design and accounting of cone structure.
Three, the cone ice structure indoor model test
JZ20-2 in the north platform after the installation of the positive inverted cone ice structure, the field observation has been fully proved that the cone of the anti-ice effect. However, the field observation is limited to the specific sea ice and environmental conditions at that time, which has great limitations. In order to make up for this defect and to mutually verify the theoretical and practical ice-breaking effects, an indoor model ice-breaking test was carried out for the prototype of the ice-breaking cone of the Zhongbei wellhead platform, simulating the same environmental conditions.
(I) Positive cone ice force
66 tests were conducted for different working conditions*** consisting of different ice speeds and different ice thicknesses, and the summarized results are shown in Table 14-8.
(ii) Inverted cone ice force
Inverted cone *** was conducted for 34 sets of different conditions consisting of different ice thicknesses and different ice speeds, and the summarized results of the tests are shown in Table 14-9.
Table 14-8 Results of orthoconical cone ice force tests
Table 14-9 Ice force test results for f inverted cones
(iii) Cylindrical ice force tests
Single Cylinder*** 31 groups of different ice thickness and different ice speed conditions were carried out, and the test results are shown in Table 14-10.
Table 14-10 Ice force test results of cylinder
(IV) Analysis of cone ice-breaking effect
1. Ice force acting on actual platform conduit rack piling columns
The comparative benchmark for the ice-breaking effect of cones is the ice force acting on the leg columns of actual conduit racks before installation of ice-resistant cone
The comparative benchmark for ice-breaking effect of cone is the ice force acting on the leg columns of actual conduits before installation of ice-resistant cone. Before the actual conduit leg column on the ice force, but the column model used in this test is taken at the waterline surface diameter of the inverted cone, which is different from the prototype conduit, but larger than the prototype conduit dimensions, therefore, should be first in Table 14-10 on the column model of the ice force converted to the original conduit on the ice force. The value of the ice force on the actual conduit after conversion is shown in Table 14-11.
Table 14-11 actual conduit on the ice force
2. cone of ice-breaking effect
According to Table 14-8 shown in the cone of the ice force and Table 14-10 shown in the conduit by the ice force, can be obtained under the same conditions of the two ice force of the ratio of the two, as shown in Table 14-12.
Table 14-12 Positive Cone Ice Forces/Conduit Ice Forces
From Table 14-12, it can be seen that the ice forces on the positive cone are equivalent to 40 to 70 percent of the ice forces on the conduit without the cone. It should be noted that the tabulated ratios correspond to the situation at the waterline surface of the positive cone, and when the water level changes, the ice force on the positive cone will increase or decrease with it, and the ratios in the table will increase or decrease as well.
3. Inverted cone of ice-breaking effect
Based on the inverted cone shown in Table 14-9 and the ice force on the conduit shown in Table 14-10, the same can be obtained under the same conditions of both the ice force of the ratio, as shown in Table 14-13.
Table 14-13 Ice force on inverted cone/ice force on conduit
From Table 14-13, it can be seen that the ice force on the inverted cone is only equivalent to 12% to 30% of the ice force on the conduit without the cone, which is better than that of the orthocone for ice breaking. It should also be noted that the ratios in the table correspond to the waterline surface of the inverted cone, and when the waterline changes, the ratios in the table will also change.
Fourth, the installation of positive and negative cone structure on the other performance of the platform
Through the field measurements and indoor model test, have confirmed that the cone anti-ice structure has a significant effect on the reduction of platform force and improve the vibration performance of the platform. On the other hand, the placement of the cone ice structure increases the structural weight of the platform and the wave loads on the structure. In order to clarify the effects of these additional weights and wave loads on the platform structure, indoor wave force tests and related theoretical analyses were conducted for the South Wellhead Platform.
(I) Indoor wave force test after adding cone structure to the platform
1. Test content
According to three kinds of water level, one kind of wave height and two kinds of structural form, there are 9 kinds of combinations, as shown in Table 14-14.
2. Test results
The wave forces obtained from the model test are shown in Table 14-15. The wave force converted to the prototype is shown in Table 14-16.
Table 14-14 Wave force test conditions
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Table 14-15 Wave force in different water depths and different models (kN)
Table 14-16 Actual wave force on cones and conduits in different water depths (kN)
From the table 14-16, it can be seen that the maximum horizontal wave force on single cone is 39t. The maximum horizontal wave force on a single cone is 39t, while the maximum horizontal wave force on a single conduit leg is 24t.
3. Analysis of results
The difference between the wave force on the cone and the wave force on the conduit in Table 14-16 is the additional wave force on the cone, and the values of the additional wave force at different water levels are shown in Table 14-17.
Table 14-17 cone additional actual wave force (kN)
From Table 14-17, it can be seen that each cone increases the horizontal wave force of about 17t, the whole platform 4 cones *** increased wave force of about 70t, equivalent to the platform to withstand the extreme wave force of 370t of the 19%, and the added additional vertical wave force of about 100t, is equivalent to the platform only! The additional vertical wave force is about 100t, which is only 8% of the maximum design weight of the platform of 1200t.
(2) Effect on seismic force
In addition to the environmental forces generated by waves and ice, seismic loads should not be ignored. The main factor that determines the size of the seismic force is first of all the size of the seismic level, while the structure in the seismic action of its own weight and weight distribution along the direction of height is also an important factor.JZ20-2 platform structure design, the weight of the upper block and conduit rack structure is more than 2,000t, while the weight of the four cones is only 40t, and the location in the vicinity of the waterline, the additional seismic force is not very large, not enough to pose a threat to the structure. The additional seismic force generated is not large enough to pose a threat to the structure. In the seismic force design, the conduit rack has sufficient safety margin.
V. Application effect of inverted cone structure
(I) Field observation test
In order to verify the effect of inverted cone structure, from late January to early February, 1991, in the JZ20-2 north central wellhead platform, the inverted cone structure of the anti-ice performance field observation test. The positive inverted cone assemblies were installed on the A1 and A2 leg columns in the high tide direction of the conduit rack, and the rubber ship parts on the B1 and B2 leg columns in the low tide direction were removed. From the viewpoint of ice condition in that year, it belonged to a lighter year, and only about 40cm overlapping ice and about 20cm thick leveling ice were observed. At low tide, the sea ice acts directly with the conduit legs, producing extrusion damage, forming a neat waterway behind the conduit legs, and the extruded and broken ice rolls up to the ice boards on both sides of the waterway; while at high tide, the sea ice acts on the positive inverted cone combination, and when the sea ice is close to the positive inverted cone, the circumferential cracks and longitudinal cracks appear successively, and when climbing along the cone to a certain height, the sea ice produces curvature damage and the sea ice behind the conduit leg columns The broken edge forms an unsmooth jagged shape with floating ice on the waterway. The displacement, velocity, acceleration of the conduit rack structure and the force of sea ice on the structure were recorded by using a displacement meter, a velocity meter, an accelerometer and an ice pressure box, respectively. During the field test, in order to obtain synchronized meteorological and sea ice data, a meteorological instrument, a current meter and a radar ice meter were installed on the platform, and it can be seen through observation and analysis:
- The self-resonance frequencies of the platform itself for the first few orders are 1.7Hz, 1.8Hz and 1.9Hz, and the structural damping ranges from 0.02 to 0.04;
- When the sea ice acts from the direction of the ebbing tide on the cone-less When one side, the platform has obvious vibration, and the conduit rack produces a general acceleration of 0.100g
for gravity acceleration.
, the maximum acceleration of 0.17g; and sea ice from the opposite direction, that is, the direction of the rising tide acts on the cone structure, the acceleration produced by the conduit frame in 0.02g or so, the maximum acceleration value of no more than 0.036g. Only from the instrument analysis of the platform there is a slight vibration, the human body is almost no feeling;- with a cone than the cone when the ice force generated by the cone is reduced by 1/3 ~ 2 /5;
-The ice force generated by the positive cone is about 1/5 smaller than that generated by the inverted cone.
(II) Comparison of vibration of the two platforms
In January 1990, the maximum ice thickness measured on the platform of the south wellhead of the JZ20-2 was 70-80cm, and the thickness of the single layer of ice was 14-40cm, and the maximum acceleration value obtained was 0.32g. At this time, sea ice Acting on the rubber ship components, the destructive force of the sea ice extrusion coupled with the rebound of the rubber material, the platform produced a strong vibration, resulting in the TV set are thrown to the floor, the chandelier swing amplitude of 45 ° or more, even double beds are moved more than 1m away, the staff obviously produce a sense of fear. Comparison of the vibration of the two platforms is shown in Table 14-18.
Compared with the two, although the ice condition in 1991 was slightly lighter than that in 1990, it has been clearly shown that the cone structure can significantly reduce the ice force of the platform and improve the vibration performance of the platform. It is clear from the observation that the installation of highly elastic shipboard parts in the ice area not only increases the total ice force on the platform, but also causes strong vibration of the platform structure, which is extremely unfavorable to the personnel and equipment. By removing the rubberized shipboard parts, the vibration acceleration of the platform can be reduced from 0.32g to 0.17g. According to the research results, this kind of positive inverted cone combination ice-resistant structure has been installed on three platform conduit racks which have already been built in JZ20-2, and good results have been received.
Table 14-18 Comparison of seismic measurement results between the north platform and the south platform in JZ20-2
(III) Conclusions
In summary, the following conclusions can be drawn:
a. Without the installation of the upper block, the two wellhead platforms of JZ20-2 had a strong vibration of acceleration of 0.32g under the action of sea ice. After installing the anti-ice cone, the acceleration value of the platform can be reduced to below 0.1g, which greatly improves the vibration performance of the platform, basically meets the requirements of the platform, and ensures the safe production of the oilfield.
b. Theory and practice have proved that the use of cone structure, the extrusion of sea ice damage into bending damage, so as to reduce the structure of the sea ice force is a well-established program. From the test data, the inverted cone anti-ice performance is better than the cone, whether to choose the cone or inverted cone, or inverted cone combination form, should be based on the specific form of the structure and the local sea ice conditions for comprehensive consideration.
c. It is observed that when the sea ice acts on the combination of positive and negative cones, or when the high tide level acts directly on the conduit, it will still cause part of the sea ice extrusion damage, which will have unfavorable effects on the platform. For the covering height and structure form of the cone, further research and improvement are still needed in the future engineering design.
d. In the sea ice is more serious, the platform is not suitable for rubber ship components. Rubber material and sea ice interaction, not only will exacerbate the vibration of the platform, by the ship itself is also easy to damage.
e. The cone should have enough strength and rigidity, and should be firmly connected with the platform structure.
f. JZ20-2 platform conduit rack original design maximum ice resistance nearly 1900t, is the platform design control load, and wave force is only 370t, seismic force is 1000t. Due to the increase of ice cone and the increase of additional wave force and seismic force and sea ice force, compared with the platform will not constitute a safety impact.