A method for evaluating the remaining fatigue life of an in-service unbonded flexible riser

Abstract

The present invention discloses a method for evaluating the remaining fatigue life of an in-service unbonded flexible riser. The method includes: S1: Establishing a sectional mechanical model of the unbonded flexible riser to obtain its mechanical properties such as axial tensile stiffness, torsional stiffness, and bending stiffness; S2: Measuring ocean environmental parameters and determining the dynamic parameters of the offshore production platform; S3: Establishing a global model of the unbonded flexible riser system; S4: Obtaining the time-domain loads at the most dangerous position of the unbonded flexible riser; S5: Combining the sectional mechanical model of the unbonded flexible riser to obtain the time-domain stresses of each layer at the most dangerous position; S6: Determining the S-N curves of each layer of the unbonded flexible riser; S7: Based on the fatigue cumulative damage criterion, calculating the remaining fatigue life of the in-service unbonded flexible riser according to the time-domain stresses of each layer of the riser and the S-N curves of each layer. The beneficial effect of the present invention is that it can effectively solve the most dangerous position of the unbonded flexible riser and update the S-N curve according to the annulus monitoring results to ensure the calculation accuracy of the remaining fatigue life.

Description

Technical Field

[0001] The present invention relates to the field of offshore oil and gas resource exploitation, and more specifically, to a method for evaluating the remaining fatigue life of an in-service unbonded flexible riser for offshore oil and gas exploitation. Background Art

[0002] Risers connect surface production platforms and subsea production systems and are known as the lifelines for offshore oil and gas resource development. Unbonded flexible risers have the advantages of easy installation, retrievability, heat resistance, flexibility, corrosion resistance, new materials, new structures, weak coupling with platforms, and large design space. Moreover, with the continuous development of domestic offshore oil and gas resource exploitation, the demand for unbonded flexible risers is gradually increasing. However, the increasing offshore exploitation depth and harsh marine environment pose higher requirements for the application of unbonded flexible risers.

[0003] The fatigue life assessment of unbonded flexible risers is a requirement in API standards and is also an essential part of the application of unbonded flexible risers. However, at present, China mainly relies on foreign countries for the fatigue life of unbonded flexible risers, and an effective assessment technology for the fatigue life of in-service unbonded flexible risers has not been formed, which is one of the bottleneck technologies in China's deep-sea offshore oil and gas resource exploitation.

[0004] Therefore, it is urgent to propose a method for assessing the fatigue life of in-service unbonded flexible risers, which is of great significance for ensuring the safe exploitation of offshore oil and gas resources. Summary of the Invention

[0005] The purpose of the present invention is to propose a method for evaluating the fatigue life of in-service unbonded flexible risers, evaluate the remaining fatigue life of in-service unbonded flexible risers, and ensure the safe exploitation of offshore oil and gas resources.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A method for evaluating the remaining fatigue life of an in-service unbonded flexible riser, characterized by comprising the following steps:

[0008] S1: Establish a cross-sectional mechanical model of the unbonded flexible riser to obtain the axial tensile stiffness, torsional stiffness, and bending stiffness of the unbonded flexible riser;

[0009] S2: Measure the marine environment where the unbonded flexible riser and the offshore production platform are located, and determine the dynamic parameters of the offshore production platform;

[0010] S3: Establish a global model of the unbonded flexible riser system according to the mechanical properties of the unbonded flexible riser, the riser design configuration, the marine environment, and the dynamics of the offshore production platform;

[0011] S4: The global model of the unbonded flexible riser system is used to calculate the static and dynamic responses of the unbonded flexible riser system, obtain the most dangerous position of the unbonded flexible riser, and obtain the time domain load of the most dangerous position;

[0012] S5: Combine the mechanical model of the non-bonded flexible riser section and the time-domain load at the most dangerous position to obtain the time-domain stress of each layer of the non-bonded flexible riser at the most dangerous position;

[0013] S6: Carry out non-bonded flexible riser annulus monitoring, obtain the non-bonded flexible riser annulus medium, and determine the SN curve of each layer of the non-bonded flexible riser according to the annulus medium;

[0014] S7: According to the time-domain stress of each layer of the riser and the SN curve of each layer, based on the fatigue cumulative damage criterion, the fatigue life of each layer of the riser is calculated, and the fatigue life of the first failed layer is taken as the fatigue failure life of the riser, and the remaining fatigue life of the in-service non-bonded flexible riser is calculated.

[0015] Furthermore, the non-bonded flexible riser comprises at least an outermost sealing layer, a middle layer and an innermost sealing layer, and the middle layer comprises at least two spirally wound metal bearing layers.

[0016] Furthermore, the non-bonded flexible riser cross-section mechanical model can also solve the stress and strain results of each layer under different load forms.

[0017] Furthermore, the marine environment mainly includes wind, waves and currents, and the dynamic parameters of the marine production platform mainly include motion displacement RAO, second-order average drift force coefficient, additional damping coefficient and additional mass.

[0018] Furthermore, the wave load in the global model of the unbonded flexible riser system is obtained using the random wave theory of the JONSWAP spectrum, and the current load is obtained using the Morison equation.

[0019] Furthermore, the static and dynamic responses mainly include the tension, bending moment and bending curvature of the riser.

[0020] Furthermore, the non-bonded flexible riser annulus is an annular space between the outermost sealing layer and the innermost sealing layer.

[0021] Furthermore, the annular medium mainly includes water, carbon dioxide and hydrogen sulfide.

[0022] Furthermore, the annular space monitoring is intermittent detection, and the time interval is 1 month, 1 quarter or 1 year. The beneficial effects of the present invention are:

[0023] 1) The present invention specifically proposes a method and steps for evaluating the fatigue life of a non-bonded flexible riser, which takes into account the actual marine environment and the dynamic parameters of the offshore production platform, and can effectively solve the most dangerous positions of the non-bonded flexible riser under static and dynamic loads;

[0024] 2) The present invention also innovatively proposes intermittent monitoring of environmental conditions for the non-bonded flexible riser to obtain the S-N curves of each layer corresponding to the changes in the annulus conditions of the non-bonded flexible riser, in view of the possible changes in the annulus environment of the non-bonded flexible riser over time, ensuring the accuracy of calculating the remaining fatigue life of the non-bonded flexible riser. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a flowchart of a method for evaluating the remaining fatigue life of an in-service non-bonded flexible riser according to the present invention;

[0026] Figure 2 is a schematic diagram of the global model of the non-bonded flexible riser system in a method for evaluating the remaining fatigue life of an in-service non-bonded flexible riser according to the present invention;

[0027] Figure 3 is a schematic diagram of the geometric structure of an in-service non-bonded flexible riser according to the present invention;

[0028] In the figure: 1 - offshore production platform, 2 - non-bonded flexible riser, 3 - sea level; 4 - seabed, 21 - outermost sealing layer of the non-bonded flexible riser, 22 - annulus of the non-bonded flexible riser, 23 - innermost sealing layer of the non-bonded flexible riser. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0030] As Figure 1 shown in the flowchart of a method for evaluating the remaining fatigue life of an in-service non-bonded flexible riser, the method includes the following steps:

[0031] S1: Establish a sectional mechanical model of the non-bonded flexible riser to obtain the axial tensile stiffness, torsional stiffness and bending stiffness of the non-bonded flexible riser; specifically, the sectional mechanical model of the non-bonded flexible riser can be established by theoretical methods or finite element methods, and the model can solve the stresses and strains of each layer under different load forms.

[0032] S2: Measure the marine environment where the non-bonded flexible riser and the offshore production platform are located, and determine the dynamic parameters of the offshore production platform; specifically, the measured parameters of the marine environment include the significant wave height, significant period and direction probability of the waves, and the dynamic parameters of the offshore production platform include the motion displacement RAO, second-order mean drift force coefficient, additional damping coefficient and additional mass.

[0033] S3: Establish a global model of the unbonded flexible riser system based on the mechanical properties, riser design configuration, ocean environment, and dynamics of the offshore production platform of the unbonded flexible riser; specifically, the global model of the unbonded flexible riser system is as shown in Figure 2 the following figure.

[0034] S4: Use the global model of the unbonded flexible riser system to calculate the static and dynamic responses of the unbonded flexible riser system, obtain the most dangerous position of the unbonded flexible riser, and obtain the time-domain load at the most dangerous position; specifically, in the global model of the unbonded flexible riser system, gravity, wave load, and ocean current load are assigned to the riser, and the change curves of the riser tension, bending moment, and bending curvature with the riser position can be obtained, so as to determine the most dangerous position of the unbonded flexible riser. Then, use the global model of the unbonded flexible riser system to solve the change curve of the load at the most dangerous position with time, that is, the time-domain load.

[0035] S5: Combine the cross-section mechanical model of the unbonded flexible riser and the time-domain load at the most dangerous position to obtain the time-domain stress of each layer of the unbonded flexible riser at the most dangerous position; specifically, the stress-load curves of each layer under tension and bending moment loads can be calculated using the cross-section mechanical model of the unbonded flexible riser, and then combined with the time-domain load at the most dangerous position to obtain the time-domain stress at the most dangerous position.

[0036] S6: Conduct annulus monitoring of the unbonded flexible riser to obtain the medium in the annulus of the unbonded flexible riser, and determine the S-N curves of each layer of the unbonded flexible riser according to the annulus medium; specifically, the annulus of the bonded flexible riser is as shown in Figure 3 the following figure, which is the annular space 22 between the outermost sealing layer 21 and the innermost sealing layer 23. Use the annulus monitoring equipment of the unbonded flexible riser to obtain the medium composition in the annulus, usually including water, carbon dioxide, and hydrogen sulfide. The different contents of these media in the annulus will affect the S-N curves of each layer of the unbonded flexible riser. Therefore, the S-N curves of each layer of materials are obtained corresponding to the annulus monitoring results; since the environment of the unbonded flexible riser is complex during service, the annulus conditions may change over time. Therefore, it is necessary to conduct annulus monitoring regularly to update the S-N curves of each layer of materials. The update period can be 1 month, 1 quarter, or 1 year.

[0037] S7: Based on the fatigue cumulative damage criterion, calculate the fatigue life of each layer of the riser according to the time-domain stress of each layer of the riser and the S-N curves of each layer, and use the fatigue life of the first failed layer as the fatigue failure life of the riser to estimate the remaining fatigue life of the in-service unbonded flexible riser.

[0038] The basic principles, main features and advantages of the present invention have been shown and described above. Those skilled in the art should understand that the present invention is not limited by the above embodiments, and what is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for assessing the residual fatigue life of in-service unbonded flexible risers. It is characterized in that The following steps are involved: S1: Establish the cross-sectional mechanical model of the unbonded flexible riser and obtain the axial tensile stiffness, torsional stiffness and bending stiffness of the unbonded flexible riser; S2: Measure the marine environment of the non-bonded flexible riser and the offshore production platform to establish the dynamic parameters of the offshore production platform; S3: establishing a global model of the non-bonded flexible riser system according to the mechanical properties of the non-bonded flexible riser, the riser design configuration, the marine environment and the power of the marine production platform; S4: The global model of the unbonded flexible riser system is used to calculate the static and dynamic responses of the unbonded flexible riser system, obtain the most dangerous position of the unbonded flexible riser, and obtain the time domain load of the most dangerous position; S5: Combine the mechanical model of the non-bonded flexible riser section and the time-domain load at the most dangerous position to obtain the time-domain stress of each layer of the non-bonded flexible riser at the most dangerous position; S6: Carry out non-bonded flexible riser annulus monitoring, obtain the non-bonded flexible riser annulus medium, and determine the SN curve of each layer of the non-bonded flexible riser according to the annulus medium; S7: According to the time-domain stress of each layer of the riser and the SN curve of each layer, based on the fatigue cumulative damage criterion, the fatigue life of each layer of the riser is calculated, and the fatigue life of the first failed layer is taken as the fatigue failure life of the riser, and the remaining fatigue life of the in-service non-bonded flexible riser is calculated.

2. The residual fatigue life assessment method according to claim 1, Features: The non-bonded flexible riser comprises at least an outermost sealing layer, a middle layer and an innermost sealing layer, wherein the middle layer comprises at least two spirally wound metal bearing layers.

3. The residual fatigue life assessment method according to claim 1, Features: The non-bonded flexible riser cross-section mechanical model can also solve the stress and strain results of each layer under different load forms.

4. The residual fatigue life assessment method according to claim 1, Features: The marine environment mainly includes wind, waves and currents, and the dynamic parameters of the marine production platform mainly include motion displacement RAO, second-order average drift force coefficient, additional damping coefficient and additional mass.

5. The remaining fatigue life assessment method according to claim 1, Features: The wave load in the global model of the unbonded flexible riser system is obtained using the random wave theory of the JONSWAP spectrum, and the current load is obtained using the Morison equation.

6. The remaining fatigue life assessment method according to claim 1, Features: The static and dynamic responses mainly include the tension, bending moment and bending curvature of the riser.

7. The residual fatigue life assessment method according to claim 1, Features: The non-bonded flexible riser annulus is the annular space between the outermost sealing layer and the innermost sealing layer.

8. The residual fatigue life assessment method according to claim 1, Features: The annular space medium mainly includes water, carbon dioxide and hydrogen sulfide.

9. The remaining fatigue life assessment method according to claim 1, Features: The annular space monitoring is intermittent detection, with a time interval of 1 month, 1 quarter or 1 year.

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