CN117787123A - Method for predicting fatigue life of steel catenary riser contact section through groove shape - Google Patents
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Abstract
The invention provides a method for predicting the fatigue life of a steel catenary riser contact area through a groove shape, which comprises the following steps: (1) Establishing a riser-flat seabed model according to actual riser parameters; (2) performing a static analysis on the stereo-flat seabed model; (3) simulating the actual longitudinal section of the groove; (4) Simplifying the shape of the groove into a three-section line form, and establishing a three-section line groove model on the seabed; (5) Selecting the most obvious environmental factors affecting fatigue, and simplifying load conditions for qualitative assessment; (6) performing a dynamic response analysis on the model; (7) According to the S-N curve formula, taking the magnitude of the cyclic stress as an index for evaluating the fatigue life; (8) Analyzing the influence of the change of a certain groove shape parameter on the fatigue of the contact section of the vertical pipe; and (9) monitoring the shape change of the groove of the contact section of the in-service riser. The invention has reasonable conception, can effectively explore the rule of influence of the change of the shape of the groove on the fatigue performance of the pair of conductors, and is suitable for popularization and application.
Description
Technical Field
The invention relates to the field of marine oil gas development, in particular to a method for predicting the fatigue life of a contact section of a steel catenary riser through a groove shape.
Background
Steel catenary risers represent a significant advantage in deep sea hydrocarbon development and have become the preferred riser type for deep sea hydrocarbon production. Under the action of environmental load, the vertical pipe in the ground contact area continuously vibrates and repeatedly contacts with the seabed, so that the bending stress of the vertical pipe is obviously changed, and fatigue damage is easily caused. Numerous studies and field surveys show that long-term cyclic vibration of the riser can cause the softer-soil seabed to gradually form grooves, and the shape of the grooves can have a significant effect on the fatigue life of the steel catenary riser in the touchdown region. During the service period of the vertical pipe, the position of the grounding point of the vertical pipe can move under the influence of the movement of the upper floating body, the direct hydrodynamic load of the vertical pipe, the mass change of the vertical pipe and the like, so that the shape of the groove is changed. In order to quickly simulate the shape of the groove, researchers try to construct a shape parameter equation of the groove through the existing nonlinear soil resistance theory, wherein a polynomial groove shape equation proposed by Aubeny and an exponential groove shape equation proposed by Shi r i are more commonly used. The influence of the shape of the groove on the amplitude of the grounding point is studied by simplifying the shape of the groove into a three-section line form according to the theory of curved surfaces by Shoghi and Shiri.
At present, the influence of the grooves on the fatigue life of the vertical pipe is not definite, the research on the fatigue of the vertical pipe is mostly focused on factors such as a pipe soil action model, the form and the size of a circulating load, the simulation duration and the like, and the relation between the shape change of the grooves and the fatigue life of the vertical pipe is still freshly researched.
In current riser fatigue recommendation practices, in-service riser fatigue assessment is typically performed using in-situ nondestructive testing, which has the following drawbacks:
(1) Only the current residual fatigue life of the vertical pipe can be detected, and the fatigue life change trend cannot be predicted;
(2) The effect of groove shape variations on riser fatigue is not considered.
In view of the foregoing, there is a need for further innovations in the art.
Disclosure of Invention
Aiming at the problems in the background technology, the invention provides a method for predicting the fatigue life of a steel catenary riser contact section through a groove shape, which is reasonable in conception, simplifies the groove shape into a three-section line shape by establishing a riser-groove model, changes three-section line groove shape parameters, takes the circulating stress of the cross section of the riser contact section as a fatigue evaluation index, and explores the rule of influence of the groove shape change on the riser fatigue performance.
In order to solve the technical problems, the method for predicting the fatigue life of the contact section of the steel catenary riser through the shape of the groove mainly comprises the following steps:
(1) Establishing a model of the steel catenary riser, the floating body and the seabed system, namely a riser-flat seabed model according to actual riser parameters;
(2) Under the condition of only considering gravity, carrying out static analysis on the vertical pipe-flat seabed model, determining the catenary shape of the vertical pipe at the equilibrium position after the two ends of the vertical pipe are respectively fixed on the upper floating body and the seabed, and determining the position of a static touchdown point, namely the starting point of the groove;
(3) Simulating the actual longitudinal section of the groove according to a groove fitting formula;
(4) Simplifying the shape of the groove into a three-section line form according to a curved surface theory, extracting shape parameters of the groove for qualitative assessment, taking a static touchdown point as a groove starting point, and establishing a three-section line groove model on the seabed;
(5) Selecting the most obvious environmental factors affecting fatigue, and simplifying load conditions for qualitative assessment;
(6) Carrying out dynamic response analysis on the model, selecting a maximum point of stress amplitude of a pipeline cross section as a stress extraction key node according to DNV related specifications, selecting a stress change stabilization period as a stress extraction period, and extracting a result of the change of the key node tensile stress and bending stress along with time in one cycle period as a stress performance evaluation index of a riser ground contact section;
(7) According to an S-N curve formula, taking the magnitude of the cyclic stress as an index for evaluating the magnitude of the fatigue life so as to further obtain the influence of the shape of the groove on the fatigue life of the riser contact section;
(8) Changing the shape parameters of the grooves by a controlled variable method, repeating the step (6) and the step (7), analyzing the influence of the change of the shape parameters of a certain groove on the fatigue of the contact section of the vertical pipe, and summarizing the influence rule of the shape change of the groove on the fatigue life of the contact section of the vertical pipe;
(9) And monitoring the shape change of the groove at the contact section of the in-service riser, so as to predict the fatigue life change trend of the riser.
The method for predicting the fatigue life of the steel catenary riser contact area through the groove shape comprises the following steps of:
X z-max ＝X z-sur f/5 (4)；
in the above formulae (1) to (4), c 1 、c 2 Is the coefficient, Z max Is the depth of the groove, X z-max X is the horizontal distance from the starting point of the groove to the deepest part of the groove z-surf Length of the groove
The method for predicting the fatigue life of a steel catenary riser contact area through the shape of a groove comprises the following steps: the load conditions in step (5) include environmental load, seabed stiffness and upper float movement; the upper float motion may be reduced to heave motion only.
The method for predicting the fatigue life of the steel catenary riser contact area through the groove shape comprises the following steps:
N＝aS -m (5)；
in the above formula (5), N is the number of stress cycles; s is the cyclic stress amplitude; m is the fatigue index, a is a typical fatigue strength constant, and is determined according to a specific selected S-N curve.
The method for predicting the fatigue life of a steel catenary riser contact area through the shape of a groove comprises the following steps: the trench shape parameters in step (8) include trench depth, trench length, and trench position.
By adopting the technical scheme, the invention has the following beneficial effects:
the method for predicting the fatigue life of the steel catenary riser touchdown region through the shape of the groove is reasonable in conception, the shape of the groove is simplified into a three-section line form by establishing a riser-groove model, three-section line groove shape parameters are changed, the circulating stress of the cross section of the touchdown point of the riser is used as a fatigue evaluation index, and the influence rule of the shape change of the groove on the fatigue performance of the riser is explored.
The method considers the influence of the grooves on the basis of the existing riser fatigue life assessment method, adopts the three-section linear grooves to facilitate the extraction of the shape parameters of the grooves, further explores the influence of the shape parameters of each groove on the riser fatigue life, and obtains the influence rule of the shape change of the grooves on the riser fatigue performance; by carrying out sensitivity analysis on various groove shape parameters, identifying key groove shape parameters with great influence on the fatigue life of the three-dimensional groove; on the basis of the method, the change situation of the shape of the groove of the contact section of the in-service vertical pipe is detected, and the fatigue life change trend of the vertical pipe of the contact section can be predicted by combining the influence rule of the shape parameters of the groove on the fatigue life of the vertical pipe; compared with the existing method, the method is more concise and efficient, the method can rapidly judge the influence of the three-dimensional fatigue life after the shape of the groove is changed, and the defect that the existing method can only evaluate the fatigue life but can not predict the trend of the change of the fatigue life is overcome.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings which are required in the description of the embodiments or the prior art will be briefly described, it being obvious that the drawings in the description below are some embodiments of the invention and that other drawings may be obtained from these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic diagram of an upper float-riser-trench model involved in the method of predicting steel catenary riser contact patch fatigue life through trench shape of the present invention;
FIG. 2 is a schematic view of a longitudinal section of an actual trench involved in the method of predicting fatigue life of a steel catenary riser touch section by trench shape of the present invention;
FIG. 3 is a schematic illustration of a practical trench simplified to a three-wire trench involved in the method of predicting fatigue life of a steel catenary riser touchdown region by trench shape of the present invention;
FIG. 4 is a schematic diagram of a cross-sectional stress check point of a steel catenary riser given by DNV specifications involved in the method of the present invention for predicting steel catenary riser contact patch fatigue life via trench shape;
FIG. 5 is a schematic illustration of a simplified three-segment line trench and fitted trenches of different depths involved in the method of predicting fatigue life of a steel catenary riser touchdown region by trench shape of the present invention;
FIG. 6 is a graph of results of bending stress changes at different check points of a cross section of a steel catenary riser involved in the method of predicting fatigue life of a steel catenary riser by groove shape according to the present invention;
FIG. 7 is a graph of stress results of the depth of the groove involved in the method of predicting fatigue life of a steel catenary riser in a touchdown section by groove shape with respect to touchdown section riser stress performance;
FIG. 8 is a schematic diagram of a groove model for changing groove shape parameters involved in the method of predicting steel catenary riser contact patch fatigue life from groove shape according to the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made apparent and fully in view of the accompanying drawings, in which some, but not all embodiments of the invention are shown. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The invention is further illustrated with reference to specific embodiments.
The method for predicting the fatigue life of the steel catenary riser contact section through the groove shape provided by the embodiment comprises the following steps:
s100, as shown in FIG. 1, establishing a model of a steel catenary riser, a floating body and a seabed system, namely a riser-flat seabed model according to actual riser parameters;
s200, under the condition that only gravity is considered, carrying out static analysis on the model, determining the catenary shape of the two ends of the vertical pipe which are respectively fixed on the floating body and the seabed and reach the balance position, and determining the position of a static touchdown point (a groove starting point);
s300, simulating an actual groove longitudinal section according to a groove fitting formula, wherein the shape of the actual groove longitudinal section is an irregular curve, so that the groove shape parameters are inconvenient to extract; the actual longitudinal section of the groove is shown in fig. 2, and the fitting formula is as follows:
X z-max ＝X z-surf /5 (4)；
in the formulae (1) to (4), c 1 、c 2 Is the coefficient, Z max Is the depth of the groove, X z-max X is the horizontal distance from the starting point of the groove to the deepest part of the groove z-surf Is the length of the groove;
s400, as shown in FIG. 3, in order to facilitate the extraction of the shape parameters of the groove, the shape of the groove is simplified into a three-section line form according to the curved surface theory, and the three-section lines are named NOZ, MPZ, FOZ respectively, and the simplification can affect the result numerical value but not the change trend, so that the method can be used for qualitative assessment, and a three-section line groove model is established on the seabed by taking a static touchdown point as a groove starting point;
s500, for simplifying calculation and analysis, selecting the most obvious environmental factors affecting fatigue, simplifying load conditions such as environmental load, seabed rigidity, upper floating body movement and the like, simplifying the upper floating body movement into heave movement only, and for ensuring that the groove does not deform greatly in the simulation process, the seabed rigidity is large enough, and the simplification can affect the result numerical value but does not affect the change trend, so that the method can be used for qualitative evaluation;
s600, carrying out dynamic response analysis on the model, as shown in fig. 4, selecting a maximum point of the cross section stress amplitude of the vertical pipe at the grounding point as a stress extraction key node, selecting a stress change stabilization period as a stress extraction period, and extracting the results of the change of the tensile stress and the bending stress of the key node along with time in one cycle period as a stress performance evaluation index of the vertical pipe grounding section in the cross section stress checking points given by DNV related specifications;
s700, according to an S-N curve formula, the cyclic stress amplitude S is increased, the cyclic frequency N is reduced, namely the fatigue life is reduced, so that the cyclic stress amplitude can be used as an evaluation index for evaluating the fatigue life, the influence of the groove shape on the fatigue performance of the riser contact section can be obtained, and the S-N curve formula is as follows:
N＝aS -m (5)；
in the formula (5), N is the number of stress cycles; s is the cyclic stress amplitude; m is a fatigue index, a is a typical fatigue strength constant, and is determined according to a specific selected S-N curve;
s800, changing groove shape parameters such as groove depth, groove length, groove position and the like by a controlled variable method, repeating the step S600 and the step S700, analyzing the influence of the change of the certain groove shape parameter on the fatigue of the contact section of the vertical pipe, and summarizing the influence rule of the change of the groove shape on the fatigue life of the contact section of the vertical pipe;
s900, monitoring the shape change of the groove of the contact section of the in-service riser, and predicting the fatigue life change trend of the riser.
The method is further illustrated by the following examples with reference to the accompanying drawings, wherein the specific procedures are as follows:
example 1
S101, establishing an upper floating body-riser seabed model, wherein the diameter of the riser model is 0.45m, and specific model parameters are shown in the following table. The total length 2050m of the vertical pipe is 1218m, the projection of the suspension point at the top end of the vertical pipe on the seabed is taken as the origin of coordinates, and the distance between the seabed anchoring point and the origin is 1500m, as shown in fig. 1;
s102, under the condition that only gravity is considered, carrying out static analysis on the model, determining the catenary shape of the two ends of the vertical pipe which are respectively fixed on the floating body and the seabed and reach the balance position, and determining the position of a static touchdown point (a groove starting point);
s103, according to a groove fitting formula, the actual longitudinal section of the groove is shown in fig. 2, and the fitting formula is as follows:
X z-max ＝X z-surf /5 (9)；
wherein, c 1 、c 2 Is the coefficient, Z max Is the depth of the groove, X z-max X is the horizontal distance from the starting point of the groove to the deepest part of the groove z-surf For the length of the groove, the shape of the groove with different depths can be fitted under the condition of the given length of the groove as shown in fig. 5 (a);
s104, simplifying the groove shapes with different depths into a three-section line form according to a curved surface theory, wherein the simplification influences the result numerical value but does not influence the change trend, so that the method can be used for qualitative evaluation, and a three-section line groove model is established on the seabed by taking a static touchdown point as a groove starting point;
s105, for simplifying calculation and analysis, selecting the most obvious environmental factors affecting fatigue, simplifying load conditions such as environmental load, seabed rigidity, upper floating body movement and the like, wherein the riser and the upper floating body are subjected to the environmental load to generate coupling movement, the coupling movement is relatively complicated to consider, and the coupling movement can be simplified into that the riser is not affected by wave load and only moves along with the upper floating body; the dynamic load can be simplified into simple heave motion with the amplitude of 2m and the period of 10s of the upper floating body, the simulation time length is 100s, the specific parameters are shown in the table below, and the vertical pipe moves under the drive of the upper floating body;
s106, simplifying the seabed rigidity, and analyzing the influence of a certain parameter on the stress performance of the steel catenary riser contact section in order to perform variable parameter analysis, namely changing the shape parameter of the groove, wherein the shape of the groove is stable under the action of cyclic load, and the shape of the groove can only be changed by modifying the shape parameter. In order to meet the condition, larger seabed rigidity needs to be set, and the seabed rigidity set by the method is 100kpa/m;
s107, as shown in FIG. 4, in cross-section stress checking points given by DNV related specifications, bending stresses of different checking points are extracted, as shown in FIG. 6, a stress maximum point (0 DEG point) is selected as a checking key node, a stress change stabilization period is selected as a stress extraction period, the results of the change of the key node tensile stress and the bending stress along with time in one cycle are extracted, and the cycle stress which should be considered in pipeline fatigue calculation is the pipeline cross-section normal stress and is a linear combination of the pipeline tensile stress and the bending stress:
σ t ＝σ a (t)+σ m (θ,t) (10)；
wherein:
σ t -the positive stress of the cross section of the pipe,
σ a (t) -the tensile stress of the cross section of the pipeline,
σ m (θ, t) -pipe cross-section bending stress.
S108, according to an S-N curve formula, the cyclic stress amplitude S is increased, the cyclic frequency N is reduced, namely the fatigue life is reduced, so that the cyclic stress amplitude can be used as an evaluation index for evaluating the fatigue life, and the S-N curve formula is as follows:
N＝aS -m (11)；
wherein N is the number of stress cycles; s is the cyclic stress amplitude; m is a fatigue index, a is a typical fatigue strength constant, and is determined according to a specific selected S-N curve;
s109, respectively extracting stress change results of the vertical pipe touchdown points with the groove depths of 1.5D, 3D and 5D (D is the diameter of a pipeline), evaluating fatigue life of the vertical pipe touchdown points by taking the cyclic stress as an evaluation index, wherein the stress change conditions of the vertical pipe touchdown points of different groove depth models are shown in a graph 7, and the influence of the change of the groove depth on the bending stress of the vertical pipe touchdown points is remarkable; the increase of the depth of the groove can reduce the positive stress and obviously increase the stress amplitude, and the increase of the depth of the groove can increase the fatigue damage of the grounding point of the vertical pipe according to the fatigue theory;
s110, analyzing the variable parameters, as shown in fig. 8, changing the shape parameters of the grooves, namely MPZ length, NOZ angle, FOZ angle, position of the grooves relative to the ground contact point and MPZ angle, establishing groove models with different shapes, and repeating the steps to obtain the effect result of the change of the shape parameters of the grooves on the fatigue life of the ground contact section.
S111, the change condition of the fatigue life of the grounding point of the riser can be estimated by monitoring the change condition of the shape of the groove of the grounding section of the in-service riser, if a plurality of groove shape parameters develop towards the direction beneficial to the fatigue life of the riser, the fatigue damage of the riser can be temporarily not considered, otherwise, the fatigue crack expansion condition of the riser should be timely monitored, and the fatigue life of the riser is re-estimated.
According to the invention, the shape of the groove is simplified into a three-section line form by establishing the riser-groove model, three-section line groove shape parameters are changed, the circulating stress of the cross section of the grounding point of the riser is used as a fatigue evaluation index, and the influence rule of the groove shape change on the fatigue performance of the riser is explored.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and not for limiting the same; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some or all of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit of the invention.
Claims (5)
1. A method for predicting the fatigue life of a steel catenary riser in a contact area by means of a groove shape, comprising the steps of:
(1) Establishing a model of the steel catenary riser, the floating body and the seabed system, namely a riser-flat seabed model according to actual riser parameters;
(2) Under the condition of only considering gravity, carrying out static analysis on the vertical pipe-flat seabed model, determining the catenary shape of the vertical pipe at the equilibrium position after the two ends of the vertical pipe are respectively fixed on the upper floating body and the seabed, and determining the position of a static touchdown point, namely the starting point of the groove;
(3) Simulating the actual longitudinal section of the groove according to a groove fitting formula;
(4) Simplifying the shape of the groove into a three-section line form according to a curved surface theory, extracting shape parameters of the groove for qualitative assessment, taking a static touchdown point as a groove starting point, and establishing a three-section line groove model on the seabed;
(5) Selecting the most obvious environmental factors affecting fatigue, and simplifying load conditions for qualitative assessment;
(6) Carrying out dynamic response analysis on the model, selecting a maximum point of stress amplitude of a pipeline cross section as a stress extraction key node according to DNV related specifications, selecting a stress change stabilization period as a stress extraction period, and extracting a result of the change of the key node tensile stress and bending stress along with time in one cycle period as a stress performance evaluation index of a riser ground contact section;
(7) According to an S-N curve formula, taking the magnitude of the cyclic stress as an index for evaluating the magnitude of the fatigue life so as to further obtain the influence of the shape of the groove on the fatigue life of the riser contact section;
(8) Changing the shape parameters of the grooves by a controlled variable method, repeating the step (6) and the step (7), analyzing the influence of the change of the shape parameters of a certain groove on the fatigue of the contact section of the vertical pipe, and summarizing the influence rule of the shape change of the groove on the fatigue life of the contact section of the vertical pipe;
(9) And monitoring the shape change of the groove at the contact section of the in-service riser, so as to predict the fatigue life change trend of the riser.
2. The method for predicting steel catenary riser contact life with trench shapes as set forth in claim 1, wherein the fitting formula in step (3) is:
X z-max ＝X z-sur f 5 (4)；
in the above formulae (1) to (4), c 1 、c 2 Is the coefficient, Z max Is the depth of the groove, X z-max X is the horizontal distance from the starting point of the groove to the deepest part of the groove z-surf Is the trench length.
3. The method of predicting steel catenary riser contact life with trench shapes of claim 1, wherein: the load conditions in step (5) include environmental load, seabed stiffness and upper float movement; the upper float motion may be reduced to heave motion only.
4. The method for predicting steel catenary riser contact life with trench shapes as set forth in claim 1, wherein the S-N curve equation in step (7) is:
N＝aS -m (5)；
in the above formula (5), N is the number of stress cycles; s is the cyclic stress amplitude; m is the fatigue index, a is a typical fatigue strength constant, and is determined according to a specific selected S-N curve.
5. The method of predicting steel catenary riser contact life with trench shapes of claim 1, wherein: the trench shape parameters in step (8) include trench depth, trench length, and trench position.
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