A method for determining seabed cyclic liquefaction considering equipment settlement

Abstract

The application provides a determination method for seabed wave-induced cumulative liquefaction considering equipment settlement. The method comprises in-situ pore water pressure preprocessing, equipment settlement calculation, wave-induced cumulative pore pressure calculation and seabed wave-induced cumulative liquefaction determination, and can realize efficient elimination of equipment settlement, tide and other non-cumulative pore pressure elements and accurate determination of seabed wave-induced cumulative liquefaction. Through the technical scheme of the application, the problem that in-situ pore water pressure data is affected by equipment settlement, tide, wave and other factors and it is difficult to calculate key elements such as wave-induced cumulative pore water pressure for determining seabed wave-induced cumulative liquefaction is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of seabed exploration, in particular, and particularly relates to a method for determining seabed wave-induced cumulative liquefaction considering equipment settlement. BACKGROUND

[0002] Wave-induced cumulative liquefaction, as a common shallow-sea geological disaster, has been a hot issue in the field of marine engineering research in the past few decades. It mainly refers to the phenomenon that the strength of overlying sediments decreases or even loses due to the cumulative rise of pore water pressure in the seabed caused by wave cyclic loading, and the sediment particles are suspended in the pore water, and the whole presents a flow instability phenomenon. This potential liquefaction phenomenon of seabed sediments under wave action will greatly threaten the safety of offshore engineering facilities such as oil platforms, optical cables and marine ranches, and cause economic and social property losses. Therefore, in-situ monitoring of seabed pore water pressure and its data interpretation have become an important means for the research and prevention of wave-induced cumulative liquefaction.

[0003] At present, the in-situ monitoring of seabed pore water pressure mainly uses arrayed pore water pressure sensor probe equipment, that is, multiple pore water pressure sensors are integrated and arranged vertically on the seabed for synchronous measurement. For example, a beach and shallow sea pore water pressure in-situ observation system based on vibration liquefaction principle disclosed in application No. 201610289676.6, a monitoring device for in-situ pore pressure and influence depth of soil under wave action disclosed in application No. 03112050.4, and a pore pressure probe device capable of automatic folding and its working method disclosed in application No. CN202111158026.5. The above-mentioned disclosed patents mainly focus on the innovative design of in-situ pore water pressure observation device, and there is little mention of the data processing method of the obtained in-situ pore water pressure and the wave-induced cumulative liquefaction evaluation method. Especially, the in-situ obtained pore water pressure data is affected by equipment settlement, tides, waves and many other factors, how to eliminate the interference of these factors and obtain reliable wave-induced cumulative pore pressure value and its depth is of great significance for the accurate determination of seabed wave-induced cumulative liquefaction. SUMMARY

[0004] In order to make up for the shortcomings of the prior art, the present application provides a method for determining seabed wave-induced cumulative liquefaction considering equipment settlement. The method includes in-situ pore water pressure preprocessing, equipment settlement calculation, wave-induced cumulative pore pressure calculation and seabed wave-induced cumulative liquefaction judgment, which can realize efficient elimination of non-cumulative pore pressure factors such as equipment settlement and tides and accurate judgment of seabed wave-induced cumulative liquefaction.

[0005] The application is implemented by the following technical scheme: a determination method for seabed wave-induced cumulative liquefaction considering equipment settlement, comprising a reference pore water pressure sensor for measuring fluctuation of seabed surface water pressure and a pore water pressure sensor embedded below the seabed surface, the pore water pressure sensors are equidistantly distributed in the seabed depth direction, and the method comprises the following steps:

[0006] S1: pre-processing the in-situ obtained pore water pressure data to obtain the characteristic frequency range of the wave in the original data and filter out high-frequency noise signals;

[0007] S2: calculating the settlement amount of the arrayed pore water pressure sensor probe equipment (Δh) , and obtaining the equipment settlement amount (Δh) ) by subtracting the average absolute water pressure after settlement (p ) from the average absolute water pressure before settlement (p ):

[0008] (2)

[0009] wherein, ρ is the seawater density, and g is the gravity acceleration;

[0010] judging whether the seabed is wave-induced cumulative liquefaction; the expression is as follows:

[0011] (1)

[0012] wherein, p is the cumulative pore water pressure at the vertical distance of from the seabed surface; σ is the average vertical effective stress of the overlying sediment at the seabed position where the pore water pressure sensor is located; ρ is the horizontal effective force and vertical effective force ratio of the sediment;

[0013] S3: calculating the cumulative pore water pressure; the cumulative pore water pressure (p ) at different depths (h , h , …, h ) in the seabed:

[0014] = (3)

[0015] wherein, p to obtain the wave-induced pressure signal in the pore water pressure data inside the seabed by high-pass filtering, to obtain the tide-induced pressure signal by low-pass filtering;

[0016] S4: correcting the cumulative pore pressure calculation result; combining the equipment settlement amount (S4) with the cumulative pore water pressure (S3) calculated in S3 to obtain the corrected cumulative pore water pressure (S4):

[0017] (4);

[0018] S5: introducing the cumulative pore water pressure (S4) calculated in S4 into formula (1) and introducing the equipment settlement amount (S4) into the calculation formula (S4) of the average vertical effective stress in formula (1), thereby transforming formula (1) into:

[0019] (5).

[0020] As a preferred solution, step S1 specifically comprises the following steps:

[0021] S1.1: segmenting the in-situ obtained pore water pressure data and removing the linearly varying trend thereof to obtain the pore water pressure fluctuation value (S1.1) caused by waves; the method for removing the linearly varying trend is to subtract the average value (S1.1) of the pore water pressure data in the segmented period from the original pore water pressure data (S1.1);

[0022] S1.2: performing power spectral density estimation on the pore water pressure fluctuation value (S1.1) obtained in S1.1, and the estimation of the power spectral density is based on the Welch function of Matlab;

[0023] S1.3: taking the power spectral density of the pore water pressure fluctuation value (S1.2) obtained in S1.2 as the ordinate and taking the corresponding frequency as the abscissa, and plotting the power spectrum graph of the pore water pressure fluctuation value (S1.2) in the double logarithmic coordinates;

[0024] S1.4: determining the range (S1.4) of the wave-induced pressure characteristic frequency (S1.4) according to step S1.3, ​​​​​​​​​​​​​​​​

[0025] S1.5: The upper boundary of the wave pressure characteristic frequency determined according to S1.4 ( ), the original pore water pressure data ( )by Low-pass filtering is performed to cut off the frequency, thereby removing the original pore water pressure data. The high-frequency noise signal in the pore water was filtered out to obtain the pore water pressure data. ).

[0026] Furthermore, the period of the segment in step S1.1 is 10 minutes.

[0027] Furthermore, step S2 specifically includes the following steps:

[0028] S2.1: Reference pore water pressure data after filtering out noise signals from S1.5 ( ) is used for power spectral density estimation to determine the characteristic frequency of the tides ( );

[0029] S2.2: The tidal characteristic frequency determined in S2.1 ( The reference pore water pressure data with S1.5 as the cutoff frequency and noise signal filtered out is used. Low-pass filtering is applied to the target object to remove wave and tidal signals in order to obtain the absolute water pressure change on the seabed measured by the equipment. ;

[0030] S2.3: The average absolute water pressure after settling ( ) and the average absolute water pressure before settling ( The difference is calculated to obtain the equipment settlement () ):

[0031] ………(2)

[0032] In the formula Let g be the density of seawater and g be the acceleration due to gravity.

[0033] Furthermore, the cutoff frequency of the low-pass filter in step S2.2 is less than 2 to 4 tidal characteristic frequencies ( ).

[0034] Further, step S3 specifically includes the following steps: filtering the reference pore water pressure data after removing noise signals via S1.5 (…). It contains pressure signals caused by waves. ) and the pressure signal caused by tides ( Located at different depths inside the seabed ( , , ..., At point ), the pore water pressure data after noise signal removal via S1.5 ( It contains pressure signals caused by waves. ), pressure signals caused by tides ( ) and cumulative pore pressure signal The wave pressure characteristic frequency determined according to S1.4 ( )scope( ) and the tidal characteristic frequency determined by S2.1 ( The pressure signal caused by the tide is filtered by low-pass filtering. ) and the wave pressure signal caused by waves in the pore water pressure data inside the seabed through high-pass filtering. Extract it.

[0035] By employing the above technical solutions, this invention has the following beneficial effects compared to existing technologies:

[0036] (1) This invention provides a simple and efficient method for preprocessing in-situ pore water pressure data. It solves the problem that in-situ pore water pressure data is affected by many factors such as equipment settlement, tides, and waves, making it difficult to calculate key elements such as wave-induced cumulative pore water pressure used to determine wave-induced cumulative liquefaction of the seabed.

[0037] (2) Based on the characteristic frequency characteristics of tidal and wave dynamics in shallow sea environments, and according to the mechanism of wave-induced cumulative liquefaction of the seabed, this invention patent proposes an accurate method for calculating cumulative pore water pressure by combining spectral analysis methods.

[0038] (3) This invention patent takes into account the impact of equipment settling on the accuracy of traditional seabed wave-induced cumulative liquefaction determination. By utilizing the differences in the characteristic frequencies of pressure signals in marine environments such as tides and waves, the amount of equipment settling is accurately calculated, and the amount of equipment settling is incorporated into the discrimination formula for seabed wave-induced cumulative liquefaction, thus developing a formula for discriminating seabed wave-induced cumulative liquefaction.

[0039] Additional aspects and advantages of the invention will become apparent in the following description or may be learned by practice of the invention. Attached Figure Description

[0040] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0041] Figure 1 Schematic diagram of an array-type pore water pressure sensor probe device;

[0042] Figure 2 A schematic diagram of the power spectral density of pore water pressure fluctuations caused by waves;

[0043] Figure 3 A schematic diagram of the power spectral density of water pressure caused by tides;

[0044] Figure 4 This is a schematic diagram showing the changes in absolute water pressure on the seabed as measured by a pore water pressure sensor for reference. Detailed Implementation

[0045] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0046] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0047] The following is combined Figures 1 to 3 The method for determining wave-induced cumulative liquefaction of the seabed considering equipment subsidence, according to an embodiment of the present invention, will be described in detail.

[0048] like Figure 1 As shown, this invention patent is based on an array-type pore water pressure sensor probe device, which is currently widely used in in-situ pore water pressure monitoring. Figure 1 As shown. This type of device generally includes a reference pore water pressure sensor for measuring fluctuations in seabed surface water pressure and pore water pressure sensors buried below the seabed surface. The pore water pressure sensors are generally evenly distributed along the seabed depth direction. Based on the general characteristics of this type of device, this invention proposes a method for determining wave-induced cumulative liquefaction of the seabed considering equipment subsidence, characterized by the following steps:

[0049] S1: Preprocess the in-situ acquired pore water pressure data to obtain the characteristic frequency range of waves in the raw data and filter out high-frequency noise signals; specifically including the following steps:

[0050] S1.1: The in-situ acquired pore water pressure data is segmented and its linear variation trend is removed to obtain the pore water pressure pulsation value caused by waves. The optimal period for segmentation is generally 10 minutes. The method for removing linear trends is to use the original pore water pressure data (…). Subtract the average value of the pore water pressure data within the segmented period ( );

[0051] S1.2: The pore water pressure fluctuation value obtained in S1.1 ( Power spectral density estimation is performed based on the Welch function in Matlab.

[0052] S1.3: The pore water pressure fluctuation value obtained from S1.2 ( Plot the pore water pressure fluctuations on a logarithmic scale, with the power spectral density as the ordinate and the corresponding frequency as the abscissa. The power spectrum of ) such as Figure 2 As shown;

[0053] S1.4: Based on the power spectrum from step S1.3 ( Figure 2 Determine the characteristic frequency of wave pressure caused by in-situ waves. )scope( ), This represents the lower boundary of the characteristic frequency of wave pressure. This represents the upper boundary of the characteristic frequency of wave pressure.

[0054] S1.5: The upper boundary of the wave pressure characteristic frequency determined according to S1.4 ( ), the original pore water pressure data ( )by Low-pass filtering is performed to cut off the frequency, thereby removing the original pore water pressure data. The high-frequency noise signal in the pore water was filtered out to obtain the pore water pressure data. ).

[0055] S2: Calculate the sedimentation of the array-type pore water pressure sensor probe device ( The average absolute water pressure after settling ( ) and the average absolute water pressure before settling ( The difference is calculated to obtain the equipment settlement () ):

[0056] ………(2)

[0057] In the formula, Let g be the density of seawater, and g be the acceleration due to gravity.

[0058] Determine whether wave-induced cumulative liquefaction has occurred on the seabed; since the determination of wave-induced cumulative liquefaction is generally based on the effective stress principle, formula (1); when the array-type pore water pressure sensor probe device settles, the formula (1) This will increase; if the initial position of the pore water pressure sensor at the time of deployment is used to determine whether wave-induced liquefaction has occurred on the seabed, it may lead to misjudgment. Therefore, the settlement of the probe equipment should be calculated ( This is a prerequisite for accurately determining whether wave-induced liquefaction has occurred on the seabed. Its expression is as follows:

[0059] (1)

[0060] in, The vertical distance from the seabed surface is The cumulative pore water pressure at the location; The average vertical effective stress is the sediment overlying the seabed at the location of the pore water pressure sensor. The buoyant density of the sediment; The vertical distance from the seabed surface to the location of the pore water pressure sensor on the seabed. This is the ratio of the horizontal effective force to the vertical effective force of the sediment.

[0061] Specifically, it includes the following steps:

[0062] S2.1: Reference pore water pressure data after filtering out noise signals from S1.5 ( ) is used for power spectral density estimation to determine the characteristic frequency of the tides ( ),like Figure 3 As shown;

[0063] S2.2: The tidal characteristic frequency determined in S2.1 ( The reference pore water pressure data with S1.5 as the cutoff frequency and noise signal filtered out is used. Low-pass filtering is applied to the target object to remove wave and tidal signals in order to obtain the absolute water pressure change on the seabed measured by the equipment. ,like Figure 4 As shown; the cutoff frequency of a low-pass filter is typically less than 2 to 4 tidal characteristic frequencies ( This allows for better elimination of water pressure changes caused by waves and tides.

[0064] S2.3: The average absolute water pressure after settling ( ) and the average absolute water pressure before settling ( The difference is calculated to obtain the equipment settlement () ):

[0065] ………(2)

[0066] In the formula Let g be the density of seawater and g be the acceleration due to gravity.

[0067] S3: Calculate cumulative pore water pressure; reference pore water pressure data after noise signal filtering by S1.5 ( It contains pressure signals caused by waves. ) and the pressure signal caused by tides ( Located at different depths inside the seabed ( , , ..., At point ), the pore water pressure data after noise signal removal via S1.5 ( It contains pressure signals caused by waves. ), pressure signals caused by tides ( ) and cumulative pore pressure signal The wave pressure characteristic frequency determined according to S1.4 ( )scope( ) and the tidal characteristic frequency determined by S2.1 ( The pressure signal caused by the tide is filtered by low-pass filtering. ) and the wave pressure signal caused by waves in the pore water pressure data inside the seabed through high-pass filtering. Extracted. Located at different depths inside the seabed ( , , ..., The cumulative pore water pressure at () ):

[0068] = (3)

[0069] In the formula, To filter out noise signals from pore water pressure data, To extract wave pressure signals caused by waves from seabed pore water pressure data using high-pass filtering, To filter the pressure signal caused by tides using a low-pass filter;

[0070] S4: Correct the cumulative pore pressure calculation result; when the equipment settles, the cumulative pore water pressure calculated according to formula (3) is... The data will include pressure changes caused by changes in the position of the equipment settlement sensor. These pressure changes due to equipment settlement cannot be removed using low-pass or high-pass filtering methods; therefore, they must be combined with the equipment settlement calculated in S2.3. The cumulative pore water pressure calculated for S3 ( Corrections are made to obtain the corrected cumulative pore water pressure. ):

[0071] = (4);

[0072] S5: Substitute the cumulative pore water pressure calculated in S4 into formula (1), and add the equipment settlement calculated in S2.3 ( Introducing the formula for calculating the average vertical effective stress in formula (1) Therefore, formula (1) can be transformed into:

[0073] (5).

[0074] In the description of the application, the term "a plurality of" refers to two or more, unless otherwise expressly specified. The terms "upper", "lower", and the like, indicate the orientation or positional relationship as shown in the drawings, which are for purposes of description only and are not intended to indicate or imply that a device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the application. The terms "connection", "mounting", "fixing", and the like, should be interpreted broadly, for example, "connection" can be fixed connection, can also be detachable connection, or integral connection; can be directly connected, or indirectly connected through intermediate media. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0075] In the description of the present application, the terms "one embodiment", "some embodiments", "a specific embodiment", and the like, mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0076] The above is only the preferred embodiment of the present application, and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for determining seabed wave-induced liquefaction considering equipment settlement, comprising reference pore water pressure sensors for measuring fluctuation of water pressure on the seabed surface and pore water pressure sensors embedded below the seabed surface, the pore water pressure sensors being distributed at equal intervals in the depth direction of the seabed, characterized in that ,Specifically comprising the following steps: S1: preprocessing the in-situ acquired pore water pressure data to obtain the characteristic frequency range of the waves in the original data and filter out high-frequency noise signals; S2: Calculate the settlement of the arrayed pore water pressure sensor probe device (S2) ), and the difference between the average absolute water pressure after settlement (P2 ) and the average absolute water pressure before settlement (P1 ) is obtained. The settlement of the device (S2 ): （2） wherein is the density of seawater and g is the acceleration due to gravity. Determine whether the seabed has wave-induced cumulative liquefaction; The expression is as follows: （1） wherein, is the cumulative pore water pressure at a vertical distance of from the seabed surface; is the average vertical effective stress of the overlying sediment at the seabed location where the pore water pressure sensor is located; is the buoyant unit weight of the sediment; is the vertical distance from the seabed surface of the seabed location where the pore water pressure sensor is located; is the ratio of the horizontal effective stress to the vertical effective stress of the sediment. S3: Calculate cumulative pore water pressure; cumulative pore water pressure ( , ,… ) at different depths within the seabed ( ): = （3） wherein Pw, is the pore water pressure data filtered for noise signals, Pw, is the pore water pressure data filtered for wave-induced pressure signals by high-pass filtering the pore water pressure data inside the seabed, Pw, is the pore water pressure data filtered for tidal-induced pressure signals by low-pass filtering the pore water pressure data inside the seabed, S4: Correcting the cumulative pore pressure calculation result; combining the equipment settlement amount (S3) to correct the cumulative pore water pressure (S3) calculated in S3 to obtain the corrected cumulative pore water pressure (S4): :​​ = （4）； S5: The cumulative pore water pressure calculated in S4 is brought into equation (1), and the settlement of the equipment (S) is brought into the calculation formula of the average vertical effective stress (σv) in equation (1), whereby equation (1) can be transformed into: )​ （5）。 2. The method of claim 1, wherein the method further comprises: determining a wave-induced liquefaction potential of the seabed based on the wave-induced pore pressure and the effective stress. ,The step S1 specifically comprises the following steps: S1.1: segment the in-situ acquired pore water pressure data and remove the linearly varying trend to obtain the pore water pressure fluctuation values caused by waves ( ); the method for removing the linearly varying trend is the original pore water pressure data ( ) minus the average value of the pore water pressure data in the segmented period ( ); S1.2: The pore water pressure fluctuation value obtained in S1.1 ( Power spectral density estimation is performed based on the Welch function in Matlab. S1.3: The pore water pressure fluctuation value obtained from S1.2 ( Plot the pore water pressure fluctuations on a logarithmic scale, with the power spectral density as the ordinate and the corresponding frequency as the abscissa. The power spectrum of ) S1.4: determining the wave pressure characteristic frequency of the waves caused by the in situ waves (f ) range (f ), is a lower boundary of the wave pressure characteristic frequency, is an upper boundary of the wave pressure characteristic frequency; S1.5: the upper boundary of the wave pressure characteristic frequency determined according to S1.4 , the original pore water pressure data (Pw) is low-pass filtered with a cutoff frequency of , so as to remove the high-frequency noise signal in the original pore water pressure data (Pw) and obtain the pore water pressure data (Pw) filtered with the noise signal. ​​​ 3. The method of claim 2, wherein the method further comprises: determining a wave-induced pore pressure at the seabed surface; and determining a wave-induced pore pressure at the seabed surface using the wave-induced pore pressure at the seabed surface and the wave-induced pore pressure at the seabed surface. The segmented period in the step S1.1 is 10 minutes.

4. The method of claim 2, wherein the method further comprises , The step S2 specifically comprises the following steps: S2.1: Filter the reference pore water pressure data of S1.5 for noise signals ) for power spectral density estimation to determine the characteristic frequency of the tides ); S2.2: low-pass filtering the reference pore water pressure data (Pw) filtered in S1.5 with the tidal characteristic frequency (fT) determined in S2.1 as cut-off frequency, to remove wave and tidal signals and to obtain the absolute sea bed pressure changes measured by the device ;​​ S2.3: The average absolute water pressure after settling ( ) and the average absolute water pressure before settling ( The difference is calculated to obtain the equipment settlement () ): （2） In the formula is the density of seawater and g is the acceleration due to gravity.

5. The method for determining wave-induced cumulative liquefaction of the seabed considering equipment subsidence according to claim 4, characterized in that... , the cut-off frequency of the low-pass filtering in step S2.2 is less than 2 to 4 characteristic frequencies of the tide (f = 1 / T) ).

6. The method of claim 4, wherein the method further comprises: , the step S3 specifically comprises the following steps: filtering out the wave-induced pressure signals ( ) and the tide-induced pressure signals ( ) in the reference pore water pressure data ( ) of the noise signals through S1.5; , , …, ) in the pore water pressure data ( ) of the noise signals through S1.5, which contains the wave-induced pressure signals ( ), the tide-induced pressure signals ( ) and the cumulative pore pressure signals ; The wave-induced wave pressure signature frequency (f ) range (f ) and the tidal characteristic frequency (f ) determined according to S1.4, the tide-induced pressure signal (p ) is extracted by low-pass filtering and the wave-induced wave pressure signal (p ) in the seabed internal pore water pressure data is extracted by high-pass filtering.

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