Wang Zhanyong, Luo Ying, Zhang Fei, Zhang Weiwei, Li Gang

Glass fiber reinforced plastic pipe is a composite material composed of glass fiber and its products and resin. Its structure is from the inside to the outside, which is an anticorrosive layer, an anti-seepage layer, a structural layer and an outer protective layer. It is mainly manufactured by a fiber winding molding process. Compared with common carbon steel pipes and stainless steel pipes, it has the characteristics of good corrosion resistance, light weight, low fluid resistance and good thermal insulation performance. In addition, due to the easy installation and long service life of FRP pipes, it is currently widely used in various fields such as petroleum, chemical and water supply and drainage. In offshore projects such as offshore platforms and FPSOs, FRP pipes have also been widely used in seawater and fire protection systems. With the increasing application scope and scale of FRP pipelines, the problem of pipeline failure and damage is becoming more and more serious, but there is no specific standardized method for stress analysis of FRP pipelines at present, so the stress analysis method of FRP pipelines for the safe production of oil and gas fields And economic benefits are of great significance.

1 Destructive factors of FRP pipelines In the actual application process, the factors that cause FRP pipelines to break are mainly in the following aspects: In transportation, installation or use, FRP pipelines will be subjected to impact loads or accidental loads caused by wind, earthquakes, waves, etc. When the operating stress exceeds the allowable stress of the FRP material, it will lead to the destruction of the pipeline.

Although FRP has good corrosion resistance, under long-term cyclic loading, its material properties will gradually deteriorate, so it will cause fatigue stress damage of the product.

Any material will have a design life under stress. If the product is in an overstressed state, the life it can achieve will be less than the design life, resulting in pipeline damage. The cause of such damage is often due to improper design and manufacturing. , Installation or allow the material to use excessive stress and other factors.

This article focuses on the analysis of allowable stress check and fatigue stress check.

2 Stress check method of FRP pipeline The stress analysis method of ordinary steel pipe is based on a large number of tests and structural response of offshore platform.

Test research can be divided into secondary stress check and secondary stress check, and the characteristics of FRP stress are non-linear and anisotropic. According to the stress classification method of steel pipe, it is not suitable for stress analysis of FRP. The practice of this article is to use the test data published by the authoritative organization according to the actual materials and usage of the project and the test data published by the authoritative organization to make simulation adjustments and select appropriate calculation methods and parameter values ​​for stress analysis.

2.1 Design pressure specification of FRP pipes In UKOOA, the design pressure P of FRP pipes is: p f2xLOflxLTHPLCL-lower confidence limit pressure; fi-the ratio of the lower confidence limit pressure of 97.5% to the average value of LTHP, the default value is 0.85; The inferred long-term average static failure pressure for the component's 20-year life.

f2-Safety factor, the default value is 0.67, its size is related to the actual application failure results of the pipeline, and can be adjusted according to the accidental load conditions, but the adjusted size does not have a certain correspondence with the size and duration of the accidental load . This value must be confirmed by relevant authorities and persons in charge.

f No pressure causes load to affect conditions. The calculation formula of f3 is: LTHS-long-term hydrostatic test strength; r-axial strength ratio of FRP.

Long-term axial strength value under 1 condition; Long-term axial strength under condition 1 Short-term axial tensile strength value under 1 condition; Short-term axial strength value under 1 condition.

2.2 The formula for calculating the number of load cycles of FRP pipelines The conclusion of the report RE-200-016 given by the test and test of Ameron FRP samples shows that the failure of FRP pipelines begins with the base material first, and the axial tensile strength is slightly less than that of steel pipes. Low, the elastic modulus is about 1/16 of the steel pipe, indicating that the tensile strength of the FRP pipe is close to the steel pipe, but the flexibility is much greater than the steel pipe. The report's fatigue test results for FRP indicate that the number of load cycles has a greater impact on the allowable stress range. The SN formula obtained through the test is as follows: 4-8.527xlgSN-number of cycle loads; S total stress range.

Because this test report uses Ameron Bondstrand 3400 series epoxy resin glass fiber reinforced plastic samples, and the test results have been certified by DNV, so in most occasions, this report can be used as an important basis for the same type of glass fiber reinforced plastic stress check.

FPSO upper module glass fiber reinforced plastic pipeline stress analysis example This article uses CAESAR stress analysis software to take the stress analysis of the PYRENEESFPSO project upper module seawater system pipeline as an example to introduce the glass fiber reinforced plastic pipeline stress analysis process. The FPSO is located in the northwestern waters of Australia with a water depth of 200m. It includes a total of 10 upper oil and gas water treatment upper modules and has a design life of 25 years. The material is Ameron Bondstrand7000M series epoxy resin glass fiber reinforced plastic.

3.1 The revised allowable stress check formula and parameter values ​​The check of the allowable stress of FRP mainly determines whether the calculated stress under various working conditions is less than the allowable stress of the FRP material after the various parameters are revised. Check according to continuous load (SUS), operating load (OPE) and accidental load (OCC).

Enter the allowable stress of FRP into the calculation software. Since the FRP pipe does not need to check the secondary stress, the basic allowable stress S at the lowest expected temperature is not used. According to the calculation method of UKOOA, the basic allowable stress Sh at the highest temperature is expected to correspond to the allowable stress of the FRP. The calculation method is: LTHS——The strength of long-term hydrostatic test is determined according to the method of ASTM D2992, and the specific value corresponds to the product data (HDB value).

The default value of the safety factor f2 is 0.67, but for various working conditions, the safety factors corresponding to the three working conditions are determined to be 0.67 (SUS), 0.83 (OPE), and 0.89 (OCC) respectively. Check separately.

The strength value, the specific value corresponds to the product data (HDB value divided by 2); the long-term axial calculation under the condition of 1; Sas (2: 1)-the corresponding short-term axial strength value, according to the corresponding product circumferential tensile strength The value is divided by 2 to obtain; Sas (0: 1)-the short-term axial tensile strength value of the corresponding product. (4) Average temperature change coefficient K: According to the provisions of 7.2.1 in the standard BS7159, the liquid is 0.85, the gas is 0.8, and the ambient temperature is changed to 1. 3.2 Fatigue stress check method Fatigue check of the FPSO upper module FRP pipe, Using the calculation method of cumulative fatigue stress, according to the characteristics of the pipeline, it can be carried out as follows: First, calculate the number of allowable cycles during the entire life cycle, including three working conditions: allowable load / unload and thermal expansion load cycle times, allowable waves The number of load cycles, the number of wave cycles allowed for towing conditions; second, the total stress range calculated according to the above three load conditions, the actual number of cycle loads calculated according to the SN formula of the SLP report, and finally the calculation results and the full life cycle The corresponding pipeline failure cycle times are compared and accumulated. If the sum of the accumulated values ​​is less than 1, it can be considered that the FRP fatigue stress check is within the allowable range.

The FRP material is Ameron Bondstrand epoxy series. For the specific materials and calculation methods, the relevant parameter settings after revision are shown in Table 1, and the settings of the CAESAR configuration file are shown in Table 2 and Table 3. Table 1 UKOOA standard allowable stress analysis input parameters ( Temperature) / Yi (Ambient temperature change) Table 2 configuration file setting a material property characteristic data file stiffness rolling type density / coefficient of thermal expansion x axial elastic modulus / kPa ratio of shear modulus to elastic modulus axial elastic modulus Multiplied by the Poisson's ratio of the quantity and the hoop modulus of elasticity, set the Amelon 7000M design strain multilayer fiber Table 3 configuration file setting two parameter items default standard accidental load factor yield stress check basis B31.3 continuous load stress increase Coefficient setting The third strength theory 3.4 Establish a calculation model According to the ISO diagram data of the seawater system pipeline, enter the size, temperature and pressure of the pipeline, and the corresponding acceleration, deformation, concentrated load, accidental load and other calculation parameters in CAESAR to determine the corresponding See the calculation model for the combined calculation method of load conditions.

3.5 Stress analysis calculation results This article does not introduce the stress analysis working condition combination method of the FPSO upper module fiberglass pipeline. According to the specific conditions of the project, the continuous load under the normal operating condition of the FPSO (working condition 38) is checked; the normal operating condition Operating load (operating condition 60); accidental load under normal operating condition (operating condition 59); fatigue stress under normal operating condition under wave load (operating condition 71); loading / unloading with normal operating condition Fatigue stress of thermal expansion load (working condition 77); continuous load under towing condition (working condition 26); operating load under towing condition (working condition 29), during towing condition, due to FPSO Production is stopped, so it is also an accidental load; accidental load under towing conditions (condition 28); fatigue stress under towing conditions (condition 38). The analysis results are shown in Table 4 ~ 8. Table 4 Normal operating conditions / 2 working conditions-H-upper node number maximum stress / kPa allowable stress / kPa maximum stress ratio /% Table 5 Normal operating conditions / 2 working conditions- H-upper node number maximum stress / kPa allowable stress / kPa maximum stress ratio /% Table 6 Normal operating conditions / 2 working conditions-H-upper node number maximum stress / kPa allowable stress / kPa maximum stress ratio / % Table 7 Towing conditions / 2 conditions-H-maximum node number maximum stress / kPa allowable stress / kPa maximum stress ratio /% Table 8 Towing conditions / 2 conditions-H- Shangkou node number maximum Stress / kPa allowable stress / kPa maximum stress ratio /% allowable stress check result: under two working conditions, the maximum stress ratio of various check loads is less than 1.0, and the stress check is passed.

Fatigue stress check results: under two operating conditions, the fatigue stress ratio calculated under a single operating condition is less than 1.0, which satisfies the stress requirements. For fatigue damage, cumulative fatigue check should also be carried out. The allowable number of cycles for each working condition in the full life cycle is (designed 25-year life): the number of assumed loading / unloading times in the design cycle is 667 times, considering the fatigue coefficient of 3.0, the final allowable loading / unloading and thermal expansion The number of load cycles is 2 permissible wave load cycles (97% of design life); permissible towing wave load cycles (3% of design life): based on the previous single-case fatigue stress calculation results, the SLP report is used The corresponding calculated load cycle times calculated by the SN formula are: load / unload and thermal expansion load cycle times (NT): 4.42x106 times; wave load cycle times (NW): 6.82x108 times; towing wave load cycle times (NE) : 9.51x106 times. The cumulative fatigue stress calculation formula is: the cumulative stress calculation result is less than 1.0, so the fatigue stress check is passed.

Through the previous calculations, the stress ratio and cumulative fatigue stress of each working condition of the FRP pipeline are within the allowable stress range, so the overall stress check is passed.

4 Conclusion This article combines the actual FPSO design project and CAESAR stress analysis calculation software, based on the relevant standards and test results, using examples to introduce the allowable stress check and fatigue stress check method of FRP pipeline, and the cumulative The method of calculating fatigue stress is emphasized. The calculation methods and parameter values ​​selected in this article provide a basis for the standardized research on the stress analysis of FRP pipes, and are of great significance for the safe production of FRP pipes.

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