FPSO mooring chain ultimate breaking reliability analysis method

Abstract

The application discloses a FPSO mooring chain (cable) limit breaking reliability analysis method, which is characterized by comprising the following steps: S1, establishing a 3*3 safety level matrix; S2, measuring theta'; S3, calculating mooring chain (cable) tension T(theta'); S4, establishing a limit state equation; S5, calculating a reliability index beta; and S6, performing reliability analysis. The analysis method is in line with engineering practical application, fully considers and measures the measurement data in engineering, guarantees reliable and safe operation of the FPSO mooring system, can clearly and specifically manage the FPSO mooring chain (cable) risk, accurately and timely grasps the technical state of the FPSO mooring chain (cable), and scientifically detects and analyzes potential problems.

Description

Technical Field

[0001] This invention relates to the field of reliability analysis technology for FPSO mooring systems, specifically to a reliability analysis method for ultimate breakage of FPSO mooring chains (cables). Background Technology

[0002] During service, FPSO mooring chains (cables) are prone to failure due to ultimate breakage, resulting in difficult repairs and significant losses. FPSOs typically operate in deep-sea areas, exposed to extreme wind, wave, and current loads from harsh sea conditions, corrosion, and long-term wear and tear. The slow drifting motion of the FPSO also increases the tension on the mooring chains (cables), potentially leading to ultimate breakage and system failure. Therefore, analyzing the reliability of FPSO mooring chains (cables) during ultimate breakage is crucial for the reliable and safe operation of FPSOs.

[0003] Existing reliability analysis methods for FPSO mooring systems are mostly based on structural reliability analysis under extreme sea conditions, and are not closely integrated with measurement and monitoring data in actual engineering. There is a need to develop more scientific, accurate and efficient reliability analysis methods for FPSO mooring systems. Summary of the Invention

[0004] The purpose of this invention is to provide a reliability analysis method for the ultimate failure of FPSO mooring chains (cables). Addressing the advantages and disadvantages of traditional methods, this invention, based on existing theories, develops a reliability analysis method for the ultimate failure of FPSO mooring chains (cables) by considering and measuring measurement data from actual engineering projects. This method provides a more scientific, accurate, and efficient assessment of the failure risk of FPSO mooring chains (cables).

[0005] To achieve the above objectives, the present invention provides the following technical solution: a reliability analysis method for the ultimate breakage of FPSO mooring chains (cables), characterized by comprising the following steps:

[0006] S1. Establish a 3×3 security level matrix;

[0007] Measurement of S2 and θ′;

[0008] S3. Calculation of mooring chain (cable) tension T(θ′);

[0009] S4. Establish the limit state equations;

[0010] S5. Calculation of reliability index β;

[0011] S6. Reliability Analysis.

[0012] The preferred method for establishing a 3×3 security level matrix in S1 is as follows:

[0013] Taking the limit breaking failure consequence level of the FPSO mooring chain (cable) as the abscissa and the reliability index β as the ordinate, establish a 3×3 matrix of the safety level of the FPSO mooring chain (cable);

[0014] Classify the reliability index level and the failure consequence.

[0015] In the preferred method, use β to represent the reliability index level, and the classification is as follows:

[0016] When 0 < β ≤ 1.0, it is the reliability level 1;

[0017] When 1.0 < β ≤ 3.0, it is the reliability level 2;

[0018] When β ≥ 3.0, it is the reliability level 3.

[0019] In the preferred method, use cof to represent the failure consequence, and the classification is as follows:

[0020] When it causes 1 - 2 days of production loss and related maintenance economic loss, and 0mms < cof ≤ 0.1mms, it is the failure consequence level 1;

[0021] When it causes 1 - week production loss and related maintenance economic loss, and 0.1mms < cof ≤ 1mms, it is the failure consequence level 2;

[0022] When it causes 1 - month production loss and related maintenance economic loss, and cof ≥ 1mms, it is the failure consequence level 3;

[0023] Where mms: million US dollars.

[0024] The measuring method of θ′ in S2 is as follows:

[0025] As Figure 3 shown, θ is the angle between the top point of the mooring chain (cable) and the horizontal direction, and θ′ is the angle between the mooring chain (cable) at a distance s from the top and the horizontal direction; place an inclinometer device at a certain distance s from the top point of the mooring chain (cable), with a measuring range of ±80° and an error less than 0.2°; the inclinometer can complete the synchronous acquisition of the mooring chain inclination parameters and ultra-long standby, and the real-time and reliable transmission of the inclination parameters of multiple mooring chains at different locations; monitor the change of the static gravitational acceleration through the measuring sensor in the inclinometer, measure the angle θ′ between the mooring chain (cable) at the inclinometer and the horizontal direction, and thus realize the monitoring of the mooring chain (cable) attitude.

[0026] The calculation method of the mooring chain (cable) tension T(θ′) in S3:

[0027] Adopt the catenary method to calculate the static configuration of the mooring chain (cable), as Figure 4As shown, the mooring chain (cable) is discretized into n cable units, including concentrated mass points and massless spring units;

[0028] Given a tension F and a pretension direction with an angle θ between it and the horizontal direction, the tension T of the cable unit at a distance s from the top of the mooring chain (cable) can be obtained by equations (1) to (5) according to the catenary theory:

[0029]

[0030]

[0031] T z =T x tan(θ) (3)

[0032]

[0033]

[0034] Through derivation, the tension value on the mooring chain (cable) is related to θ′, where θ is the angle between the top point of the mooring chain (cable) and the horizontal direction, θ′ is the angle between a point on the mooring chain (cable) at a distance s from the top and the horizontal direction, and x and z are the horizontal and vertical distances of a point on the mooring cable from the top point of the mooring cable, respectively; T x T represents the component of the tension on the cable unit along the x-axis. z ρ is the component of the tension on the cable unit in the z-axis direction, l is the length of the mooring chain (cable), and ρ1 is the weight per unit length of the mooring chain (cable).

[0035] In S4, the limit state equation can be expressed as:

[0036] g = RS = T m (σ s )-λα1T(θ′)=0 (6)

[0037] When g > 0, it indicates that the mooring chain (cable) is reliable; when g < 0, it indicates that the mooring chain (cable) is unreliable and in a failure state; when g = 0, it indicates that the hull structure is in a limit state, which is the limit state equation.

[0038] Among them, the ultimate breaking tension value T of the mooring chain (cable) mThis is the LDBF (Design Breaking Force), which is 100%-105% of the MBL (Minimum Breaking Load) of a ship. It is usually confirmed experimentally. Based on the material's σ-s stress-strain diagram, the maximum stress σ of an undamaged mooring chain (cable) link under a given tension T0 is calculated. Then, a load is applied to a damaged mooring chain (cable) link. The loading force T corresponding to when the stress value of the link reaches σ is the ultimate breaking tension, and its yield stiffness σ is also considered. s Related, denoted as T m (σ s );

[0039] T(θ′) represents the tension on the mooring chain (cable), which is related to θ′. θ′ is measured by an inclinometer and follows a normal distribution over a period of time, making it a random variable. For example... Figure 4 As shown, the FPSO will be subjected to harmonic displacement excitation x=Asinωt, which will increase the tension on the mooring chain (cable). The impact amplification factor is taken as α1 to correct the tension value, and the safety factor is taken as λ.

[0040] In S5, the reliability index β can be expressed as:

[0041]

[0042] in,

[0043]

[0044]

[0045]

[0046]

[0047] By combining equations (7) to (11), the reliability index β of the mooring chain (cable) can be obtained:

[0048]

[0049] The calculated reliability index β is substituted into the 3×3 safety level matrix established in step 1, and the safety level of the mooring chain (cable) is determined in combination with the failure consequence level, thus completing the reliability assessment of the mooring chain (cable).

[0050] Compared with the prior art, the beneficial effects of the present invention are:

[0051] (1) The analytical method of this invention is closely aligned with practical engineering applications, fully considering and evaluating measurement data in engineering projects to ensure the reliable and safe operation of the FPSO mooring system. This allows for clear and targeted management of FPSO mooring chain (cable) risks. It enables accurate and timely understanding of the technical status of the FPSO mooring chain (cable) and scientific detection and analysis of potential problems.

[0052] (2) The method of the present invention can perform reliability assessment of FPSO mooring chains (cables) within a specified time period and represent them in a safety level matrix, thereby achieving dynamic monitoring. It can clearly observe the safety level zone of the FPSO mooring chain (cable) within a specified time period, and at the same time, it can clarify the overall risk value of a system, thereby achieving risk management based on system risk.

[0053] (3) The method of this invention provides a scientific and quantitative reliability assessment and optimizes monitoring costs. It can improve the management efficiency of the FPSO mooring system, and also focus on high-risk mooring chains (cables) for repair or replacement, ensuring the safe operation of the FPSO. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the process of the present invention;

[0055] Figure 2 This is a schematic diagram of the 3×3 safety level matrix structure of the FPSO mooring chain (cable) of the present invention;

[0056] Figure 3 This is a schematic diagram showing the position and included angle of the inclinometer used for measuring θ′ in this invention;

[0057] Figure 4 This is a schematic diagram of the mooring chain (cable) unit of the present invention. Detailed Implementation

[0058] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0059] Example

[0060] Please see Figures 1-4, the present invention provides a technical solution: a method for analyzing the ultimate breaking reliability of FPSO mooring chains (ropes). Aiming at the advantages and disadvantages of traditional methods, on the basis of the original theory, the present invention develops a method for analyzing the ultimate breaking reliability of FPSO mooring chains (ropes) by considering and measuring the measurement data in engineering practice, so as to evaluate the failure risk of FPSO mooring chains (ropes) more scientifically, accurately and efficiently. Specifically as follows:

[0061] I. Establish a 3×3 safety level matrix

[0062] The present invention establishes a 3×3 safety level matrix of FPSO mooring chains (ropes) with the ultimate breaking failure consequence level of FPSO mooring chains (ropes) as the abscissa and the reliability index β as the ordinate. The reliability index level is represented by β. When 0 < β ≤ 1.0, it is the reliability level 1; when 1.0 < β ≤ 3.0, it is the reliability level 2; when β ≥ 3.0, it is the reliability level 3. The determination of the reliability index level is as shown in Table 1 below:

[0063] Table 1 Determination of reliability index level

[0064]

[0065] Use cof to represent the failure consequence. When it causes 1-2 days of production loss and related maintenance economic loss, about 0 < cof ≤ 0.1 mms, it is the failure consequence level 1; when it causes 1 week of production loss and related maintenance economic loss, about 0.1 < cof ≤ 1 mms, it is the failure consequence level 2; when it causes 1 month of production loss and related maintenance economic loss, about cof ≥ 1 mms, it is the failure consequence level 3 (mms: million US dollars). The determination of the failure consequence level is as shown in Table 2 below:

[0066] Table 2 Determination of failure consequence level

[0067]

[0068] As Figure 2 shown, a 3×3 safety level matrix is finally formed, and its reliability is evaluated.

[0069] II. Measurement of θ′

[0070] As Figure 3As shown, θ is the angle between the top point of the mooring chain (cable) and the horizontal direction, and θ′ is the angle between the mooring chain (cable) at a distance s from the top and the horizontal direction. An inclinometer is placed at a certain distance s from the top point of the mooring chain (cable), with a measurement range of ±80° and an error of less than 0.2°. The inclinometer can synchronously acquire mooring chain inclination parameters and has ultra-long standby time, enabling real-time and reliable transmission of inclination parameters from multiple mooring chains at different locations. By monitoring the change in static gravitational acceleration through sensors within the inclinometer, the angle θ′ between the mooring chain (cable) and the horizontal direction at the inclinometer location is measured, thereby achieving the monitoring of the mooring chain (cable) attitude.

[0071] III. Calculation of Mooring Chain (Cable) Tension T(θ')

[0072] The catenary method is used to calculate the static configuration of the mooring chain (cable), such as... Figure 4 As shown, the mooring chain (cable) is discretized into n cable units (including concentrated mass points and massless spring units).

[0073] Given a tension F and a pretension direction with an angle θ between it and the horizontal direction, the tension T of the cable unit at a distance s from the top of the mooring chain (cable) can be obtained by equations (1) to (5) according to the catenary theory:

[0074]

[0075]

[0076] T z =T x tan(θ) (3)

[0077]

[0078]

[0079] Through derivation, the tension value on the mooring chain (cable) is related to θ′, where θ is the angle between the top point of the mooring chain (cable) and the horizontal direction, θ′ is the angle between the point on the mooring chain (cable) at a distance s from the top and the horizontal direction, and x and z are the horizontal and vertical distances of a point on the mooring cable from the top point of the mooring cable, respectively. T x T represents the component of the tension on the cable unit along the x-axis. z ρ is the component of the tension on the cable unit in the z-axis direction, l is the length of the mooring chain (cable), and ρ1 is the weight per unit length of the mooring chain (cable).

[0080] IV. Establishing Limit State Equations

[0081] The limit state equation can be expressed as:

[0082] g = RS = Tm (σ s )-λα1T(θ′)=0 (6)

[0083] When g > 0, it indicates that the mooring chain (cable) is reliable; when g < 0, it indicates that the mooring chain (cable) is unreliable and in a failure state; when g = 0, it indicates that the hull structure is in a limit state, which is the limit state equation.

[0084] Among them, the ultimate breaking tension value T of the mooring chain (cable) m This is the LDBF (Design Breaking Force), which is 100%-105% of the MBL (Minimum Breaking Load) of the ship. It is usually confirmed experimentally. Based on the material's σ-s stress-strain diagram, the maximum stress value σ of the undamaged mooring chain (cable) link under a given tension T0 is calculated. Then, a load is applied to the damaged mooring chain (cable) link. The loading force T corresponding to when the stress value of the link reaches σ is the ultimate breaking tension value, along with its yield stiffness σ. s Related, denoted as T m (σ s ).

[0085] T(θ′) represents the tension on the mooring chain (cable), which is related to θ′. θ′ is measured by an inclinometer and follows a normal distribution over a period of time, making it a random variable. For example... Figure 4 As shown, the FPSO will be subjected to harmonic displacement excitation x=Asinωt, which will increase the tension on the mooring chain (cable). The impact amplification factor is taken as α1 to correct the tension value, and the safety factor is taken as λ.

[0086] V. Calculation of Reliability Index β

[0087] The reliability index β can be expressed as:

[0088]

[0089] in,

[0090]

[0091]

[0092]

[0093]

[0094] By combining equations (7) to (11), the reliability index β of the mooring chain (cable) can be obtained:

[0095]

[0096] VI. Reliability Analysis

[0097] The calculated reliability index β is substituted into the 3×3 safety level matrix established in step 1, and the safety level of the mooring chain (cable) is determined in combination with the failure consequence level, thus completing the reliability assessment of the mooring chain (cable).

[0098] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A reliability analysis method for ultimate breakage of FPSO mooring chains, characterized in that, It includes the following steps: S1. Establish a 3×3 safety level matrix; S2. Measure θ′; S3. Calculate the mooring chain tension T(θ′); S4. Establish a limit state equation; S5. Calculate the reliability index β; S6. Conduct reliability analysis; The method for calculating the mooring chain tension T(θ′) in S3: The catenary method is used to calculate the static configuration of the mooring chain. The mooring chain is discretized and divided into n chain elements, including concentrated mass points and massless spring elements; Under the condition of a given tension F and the angle θ between the pre-tension direction and the horizontal direction, according to the catenary theory, the tension T of the chain element at a distance s from the top of the mooring chain can be obtained through equations (1) to (3): T z =T x tan(θ) (1) Through derivation, the tension value of the mooring chain is related to θ′, where θ is the angle between the top point of the mooring chain and the horizontal direction, θ′ is the angle between a point on the mooring chain at a distance s from the top and the horizontal direction, and x and z are the horizontal and vertical distances of a point on the mooring chain from the top point; T x T represents the component of the tension on the chain unit along the x-axis. z Let ρ be the component of the tension on the chain unit in the z-axis direction, l be the length of the mooring chain, and ρ1 be the weight per unit length of the mooring chain.

2. The reliability analysis method for ultimate failure of FPSO mooring chain according to claim 1, characterized in that: In S1, the method for establishing a 3×3 safety level matrix is as follows: Taking the FPSO mooring chain ultimate breaking failure consequence level as the abscissa and the reliability index β as the ordinate, establish a 3×3 safety level matrix for the FPSO mooring chain; Classify the reliability index level and failure consequence.

3. The reliability analysis method for ultimate failure of FPSO mooring chain according to claim 2, characterized in that: Using β to represent the reliability index level, the level classification is as follows: When 0 < β ≤ 1.0, it is reliability level 1; When 1.0 < β ≤ 3.0, it is reliability level 2; When β ≥ 3.0, it is reliability level 3.

4. The reliability analysis method for ultimate failure of FPSO mooring chain according to claim 3, characterized in that: Using cof to represent the failure consequence, the level classification is as follows: When 0mms < cof ≤ 0.1mms, it is failure consequence level 1; When 0.1mms < cof ≤ 1mms, it is failure consequence level 2; When cof ≥ 1mms, it is failure consequence level 3; where mms: million US dollars.

5. The reliability analysis method for ultimate failure of FPSO mooring chain according to claim 1, characterized in that: The method for measuring θ′ in S2 is as follows: θ is the angle between the top point of the mooring chain and the horizontal direction, and θ′ is the angle between the point at a distance s from the top of the mooring chain and the horizontal direction; An inclinometer device is placed at a certain distance s from the top point of the mooring chain, with a measurement range of ±80° and an error less than 0.2°; The inclinometer can complete synchronous acquisition of the mooring chain inclination parameters and ultra-long standby, and real-time and reliable transmission of the inclination parameters of multiple mooring chains at different locations; By monitoring the change of the static gravitational acceleration through the measurement sensor in the inclinometer, the angle θ′ between the mooring chain at the inclinometer and the horizontal direction is measured, and then the monitoring of the mooring chain attitude is realized.

6. The reliability analysis method for ultimate failure of FPSO mooring chain according to claim 1, characterized in that: In S4, the limit state equation can be expressed as: g=RS=T m (s s )-λα1T(θ′)=0 When g > 0, it means the mooring chain is reliable; when g < 0, it means the mooring chain is unreliable and is in a failure state; when g = 0, it means the hull structure is in a limit state, which is the limit state equation; Among them, the ultimate breaking tension value T of the mooring chain m This is the design breaking force, which is 100%-105% of the ship's minimum design breaking load; it is usually confirmed experimentally. Based on the material's σ-s stress-strain diagram, the maximum stress value σ of the undamaged mooring chain link under a given tension T0 is calculated. Then, a load is applied to the damaged mooring chain link. When the stress value of the link reaches σ, the corresponding loading force T is the ultimate breaking tension value, along with its yield stiffness σ. s Related, denoted as T m (σ s ); T(θ′) is the tension on the mooring chain, which is related to θ′. θ′ is measured by the inclinometer and follows a normal distribution within a certain period of time and is a random variable. The FPSO is subjected to a harmonic displacement excitation x = A sinωt, resulting in an increase in the tension on the mooring chain. Take the impact amplification factor as α1 to correct the tension value and take the safety factor as λ.

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