A method for determining the bottom deck elevation of an offshore platform

Abstract

This invention discloses a method for determining the bottom deck elevation of an offshore platform, mainly comprising the following steps: collecting and processing relevant data such as wave and tide levels in the engineering area; selecting appropriate samples; considering the joint probability distribution of storm surge and wave-related marine hydrological environmental elements; generating an environmental envelope; and then obtaining the joint return period design parameters of relevant marine hydrological environmental elements from the environmental envelope, which are then used to calculate and determine the bottom deck elevation of the offshore platform. This invention reasonably considers the characteristic laws of important marine hydrological environmental elements in setting the bottom deck elevation of an offshore platform, and uses the idea of ​​statistical modeling of the joint distribution of marine hydrological environmental elements to construct environmental envelopes under different return periods. It can conveniently select the joint return period design parameters of marine hydrological environmental elements and obtain appropriate combinations of environmental parameter return values. The scheme is simple and easy to implement, with clear and reasonable engineering significance, and can effectively guide the design of actual engineering structures.

Description

Technical Field

[0001] This invention relates to the field of marine hydrological analysis and hydraulic engineering technology, specifically to a method for determining the bottom deck elevation of an offshore platform. Background Technology

[0002] Determining the lowest deck elevation is a crucial step in the structural design and evaluation of certain offshore platforms. On one hand, the accurate calculation of the lowest deck elevation directly impacts the platform's structural safety: if the elevation is set too low, in extremely harsh marine environments, waves may reach or exceed the platform's lowest deck. These waves will directly impact the platform's upper deck and superstructure, generating load effects that hinder normal platform operations and could even damage the lowest deck, superstructure, and equipment. More importantly, after a wave overtopping event, the wave directly impacts the deck, generating immense loads that drastically increase the load on the platform structure, potentially leading to platform collapse. This wave-in-deck (WID) load effect under extreme storm conditions is a significant cause of failure in stationary offshore platforms. On the other hand, a reasonable determination of the lowest deck elevation also relates to the project's economic viability: an overly conservative elevation setting can lead to redundancy in the platform's structural design, increasing project investment. Determining the bottom deck elevation of an offshore platform requires statistical analysis of marine hydrological factors. In hydraulic and marine engineering, the concept of "Still Water Level (SWL)" is defined. The still water level does not consider water surface changes caused by waves, and wave crest height is relative to the still water level. Based on this, the concept of "Extreme Water Level (EWL)" is introduced. The extreme water level (EWL) is crucial for determining the platform's bottom deck elevation; when some uncertainties are ignored, it can be considered as the starting point for the platform's bottom deck elevation. The determination of the extreme water level and the bottom deck elevation should comprehensively consider wave crest height, storm surge, and astronomical tides.

[0003] However, current technical methods for determining bottom deck elevation have several problems. First, the design specifications for offshore platforms are not clearly defined, causing difficulties in practical engineering design. Some design specifications do not provide detailed calculation processes or specific steps for determining the bottom deck elevation of offshore platforms, only offering vague guidance; some design specifications have very unclear definitions of the relevant parameters in the calculation of bottom deck elevation; in addition, the traditional setting methods used in engineering practice and the technical methods presented in some studies are mostly crude and unreasonable. First, most traditional methods conduct separate statistical studies on wave crest height, storm surge, and astronomical tide level, calculating their respective extreme values ​​of design parameters separately. Such traditional methods do not consider the probability of various marine hydrological environmental factors occurring together, which has obvious defects. Second, many methods based on joint probability analysis treat astronomical tides as a random phenomenon, which is also unreasonable, because astronomical tides are phenomena with clear patterns, exhibiting periodic changes. Due to the periodicity and determinism of astronomical tides, directly treating astronomical tides as random variables is inaccurate and needs to be corrected. Third, traditional two-dimensional or multi-dimensional joint probabilistic analysis of marine hydrological environmental elements still suffers from problems such as complex and difficult solutions and unclear engineering implications. Design parameter combinations for a specific return period can result in numerous solutions, making it crucial to rationally determine the joint return value to address the problem of uncertain solutions. Multi-dimensional joint probability models are complex, and return period analysis based on purely mathematical joint transcendental probabilities lacks clear engineering implications, thus failing to reflect practical engineering significance and guide engineering design. Therefore, an effective method for conveniently obtaining the joint return value combination of marine environmental elements is urgently needed for engineering design.

[0004] In summary, the bottom deck elevation of offshore platforms has a significant impact on the safety and economy of engineering projects. Accurate calculation of the bottom deck elevation requires statistical analysis of marine hydrological environmental factors, referencing relevant engineering specifications. However, existing calculation methods often have flaws or limitations, which can cause difficulties in practical engineering design and potentially lead to deviations in the calculated bottom deck elevation. Therefore, a reasonable method for calculating and determining the bottom deck elevation of offshore platforms is needed to facilitate the design and evaluation of marine engineering structures. Summary of the Invention

[0005] (1) Technical problems to be solved

[0006] The calculation and determination of the bottom deck elevation of offshore platforms requires comprehensive consideration of marine hydrological environmental factors such as astronomical tides, storm surges, and waves. However, the requirements for determining the bottom deck elevation in various engineering specifications are not clear, and some concepts remain ambiguous. Furthermore, the technical methods proposed in current reference specifications do not reasonably consider the characteristic patterns of relevant marine hydrological environmental elements, cannot accurately reflect the probability of various environmental factors occurring together, and are difficult to derive appropriate combinations of marine hydrological environmental parameter recurrence values. This hinders convenient and effective guidance for engineering design and evaluation, leading to inaccuracies in the setting of the bottom deck elevation of offshore platforms, affecting the safety of the engineering structure and the economic efficiency of the project investment. Therefore, a simpler, more practical, and reasonable method for calculating the bottom deck elevation of offshore platforms is needed.

[0007] (2) Technical solution

[0008] The objective of this invention can be achieved through the following technical solutions:

[0009] A method for determining the bottom deck elevation of an offshore platform includes the following steps:

[0010] Step 1: Collect and obtain relevant data on marine hydrological environmental elements in the project area; collect long-term wave and tide data for the project area, including wave height, storm surge, and astronomical tide level. Wave, astronomical tide, and storm surge data can be: ① Measured data from the project area. ② When measured data is lacking or marine hydrological observation is not feasible, data obtained from numerical simulations of the area can be used. In practical engineering, wave observations and numerical simulations usually provide characteristic wave height values ​​such as significant wave height and maximum wave height, but generally not wave crest height. Therefore, when specific wave crest height data is unavailable, data on significant or maximum wave heights in the project area can be collected as a substitute, and then the wave crest height can be estimated using a theoretical model.

[0011] Step 2: Preliminary processing of the data from Step 1, selecting data samples; different sample selection methods are adopted according to the different data obtained in Step 1 and the actual situation; probabilistic analysis and fitting of marine hydrological environmental elements requires the selection of specific data samples. Depending on the data samples, sample selection methods can be divided into full value method, block extreme value sampling method, process extreme value sampling method, and overthreshold sampling method, etc. Specifically: ① When there is complete, high-quality tide level and wave height time history data with few missing data, the full value method can be used to include all valid storm surge and wave height time history data in the statistical analysis. ② When extreme sea states in the engineering area are caused by a single weather process, the process extreme value sampling method is also applicable. For example, extreme sea states in southern my country are mostly controlled by tropical cyclone weather processes, so tropical cyclone process extreme value data sample sequences can be used. ③ Sampling can also be performed using the block extreme value method (such as the annual extreme value method) or the overthreshold method, etc.

[0012] Step 3: Construct a joint probability distribution model of wave height and storm surge in the engineering area; Based on the data obtained in Steps 1 and 2, perform joint probability analysis on the wave height and storm surge data samples to establish a joint statistical distribution function for wave height and storm surge; This invention assumes that astronomical tides are phenomena with clear patterns, and only performs joint probability analysis on wave height and storm surge to establish a joint statistical distribution function for wave height and storm surge. There are many models for joint statistical modeling of marine hydrological environmental parameters. Here, the hierarchical conditional model, i.e., the conditional modeling method (CMA), is used as an example for detailed introduction. The joint probability model of wave height and storm surge is constructed according to the hierarchical conditional model, i.e., the conditional modeling method (CMA). It consists of the marginal probability distribution of wave height and the conditional probability distribution of storm surge based on wave height (the conditional distribution of storm surge under wave height):

[0013] ① Fitting the marginal distribution of wave height. Marginal statistical analysis of the long-term distribution of wave height can be performed using the Weibull distribution family. Generally, a three-parameter Weibull distribution is used to fit the wave height distribution. Based on the data samples, relevant parameters are estimated, thus yielding the probability density function (PDF) and marginal distribution function (CDF) of the wave height's marginal distribution.

[0014] ② Analyze the conditional distribution of storm surge based on wave height. A two-parameter Weibull distribution can be used to describe the conditional distribution of storm surge under wave height. The parameters in the two-parameter Weibull distribution are related to wave height and are functions of wave height, requiring fitting with specific wave height and storm surge data from the engineering area. Once the two parameters of the two-parameter Weibull distribution are obtained, the conditional distribution density function of storm surge based on wave height (the conditional distribution density function of storm surge under wave height) can be determined, and the corresponding distribution function can be calculated by integrating the density function.

[0015] ③ Obtain the joint probability distribution of wave height and storm surge. The joint probability density function of wave height and storm surge is the product of the edge probability density function of wave height and the probability density function of storm surge based on wave height conditions.

[0016] Step 4: Based on the joint probability distribution model obtained in Step 3, and using the environmental envelope construction definition method, generate wave height-storm surge environmental envelopes for different return periods. Based on the joint probability distribution of wave height and storm surge obtained in the above steps, apply methods such as the inverse first-order reliability method (IFORM) to generate wave height-storm surge environmental envelopes (HS Environmental Contours) for different return periods. There are many methods for defining and generating environmental envelopes, with IFORM being the most commonly used. Points on the wave height-storm surge environmental envelope for a certain return period can be considered as the joint extreme value combination of wave height and storm surge for that return period.

[0017] Step 5: Select the wave height and storm surge values ​​corresponding to specific points on the wave height-storm surge environmental envelope under the return period from Step 4 as the joint design parameter values ​​for determining the bottom deck elevation. The wave height-storm surge environmental envelope is a set of extreme design condition points for the joint return period of wave height-storm surge. Selecting the wave height and storm surge values ​​corresponding to specific points on the environmental envelope under the return period as joint design parameters is used to determine the bottom deck elevation of the offshore platform in the engineering area. The advantage of generating the environmental envelope is that the return value of the combination of marine hydrological environmental elements most unfavorable to the structural design can be obtained by searching and selecting specific points on the environmental envelope. Since this method determines the bottom deck elevation by adding the extreme values ​​of the design parameters of wave crest height and storm surge, the selection principle for specific points on the environmental envelope is to select the points that maximize the sum of storm surge and wave crest height. If the effective wave height or maximum wave height data sequence is used when calculating the joint design parameters of wave height and storm surge under the return period, the extreme values ​​of wave peak height under the corresponding return period need to be obtained through the conversion relationship between wave peak height and characteristic wave height. Depending on the type of wave height, the selection of specific operating points on the environmental envelope can be categorized as follows:

[0018] ① When using peak height H crest When generating the wave height-storm surge environmental envelope from the data sequence, the sum of the wave peak height and the storm surge height on the environmental envelope is directly selected (H). crest The point with the largest (+S) is taken as the final specific point, and the wave crest height and storm surge value of this specific point are used as the joint design parameter values ​​for determining the bottom deck elevation.

[0019] ②When using the maximum wave height H max When generating the wave height-storm flood envelope from the data sequence, the extreme peak height H crest According to formula Hcrest =0.635H max To deduce this, the environmental envelope line (0.635H) should be selected. max The point with the largest sum of +S is taken as the final specific point.

[0020] ③ When using effective wave height H s When generating the wave height-storm flood envelope from the data sequence, the extreme peak height H crest According to formula H crest =0.635H max =1.27H s To extrapolate (without considering wave breakage), the environmental envelope line (1.27H) should be selected. s The point with the largest sum of +S is taken as the final specific point.

[0021] After selecting specific points from the environmental envelope, the wave height and storm surge values ​​corresponding to those points become the design parameters for determining the bottom deck elevation. If the significant wave height or maximum wave height data sequence was used in the initial calculation, the extreme values ​​of the wave crest height design parameters under the corresponding return period need to be determined using the conversion relationship between extreme wave crest height and characteristic wave height (e.g., H). crest =0.635H max H crest =1.27H s To find the solution.

[0022] Step 6: Obtain the astronomical tide height design parameter value; select the maximum astronomical tide height above the mean sea level within a 19-year cycle from the astronomical tide data of the engineering area as the astronomical tide height design parameter value for determining the bottom deck elevation of the offshore platform; in this invention, astronomical tides are considered to exhibit a periodic variation, therefore, astronomical tide height is not treated as a random variable and combined with wave height and storm surge for joint probability analysis. Considering the characteristic of astronomical tides having a long cycle of 18.61 years and the service life of the offshore platform, the maximum astronomical tide height above the mean sea level within a 19-year cycle from the astronomical tide data of the engineering area is selected as the extreme value of the design parameter for determining the bottom deck elevation of the offshore platform.

[0023] Step 7: Calculate and determine the bottom deck elevation of the offshore platform; using mean sea level as the reference surface, the bottom deck elevation of the offshore platform is the sum of the extreme values ​​of the design parameters for wave crest height, storm surge, and astronomical tide height under the return period, plus the height of the air gap of 1.5m. This invention's method calculates and determines the bottom deck elevation of the offshore platform according to relevant API specifications and commonly used concepts in the domestic engineering field. Without considering some uncertainties, using mean sea level (MSL) as the reference surface, the bottom deck elevation of the offshore platform is the sum of the extreme values ​​of the design parameters for wave crest height, storm surge, and astronomical tide height under the return period, plus the height of the air gap of 1.5m. The extreme value of the design parameter H for the wave crest height under a 100-year return period is used.crest,100 Extreme values ​​of storm surge design parameters S 100 Taking the astronomical tide height design parameter t as an example, the formula for calculating the bottom deck elevation h of an offshore platform is h = H crest,100 +S 100 +t+1.5.

[0024] (3) The beneficial effects of the present invention:

[0025] Compared with existing technologies, the beneficial effects of this invention are as follows: The technical method for determining the bottom deck elevation of an offshore platform based on an environmental envelope proposed in this invention is convenient and easy to implement, with clear and reasonable engineering significance, and can effectively guide engineering design. First, it is not limited by the original data sample sequence and is applicable to various data sample sequences such as long-term time history full values, annual extreme values, process extreme values, and threshold values. Second, the constructed environmental envelope can conveniently provide joint return period design parameters for marine hydrological environmental elements, making it easy to apply in actual engineering structural design and also facilitating subsequent structural response and reliability assessment. Attached Figure Description

[0026] Figure 1 This is a diagram illustrating the range of elevation of the bottom deck of the offshore platform in this invention;

[0027] Figure 2 This is a flowchart illustrating the method for calculating and determining the bottom deck elevation of an offshore platform based on environmental envelope in this invention;

[0028] Figure 3 This is a probability fitting diagram of the wave height edge distribution in Embodiment 1 of the present invention;

[0029] Figure 4 This is a parameter estimation diagram of storm surge based on wave height condition distribution in Embodiment 1 of the present invention;

[0030] Figure 5 This is an envelope diagram of wave height-storm flooding environment under different return periods in Embodiment 1 of the present invention;

[0031] Figure 6 This is a diagram illustrating the point (final specific point) on the once-in-a-century wave height-storm surge environmental envelope in Embodiment 1 of the present invention where the sum of storm surge and wave crest height reaches its maximum. Detailed Implementation

[0032] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0033] Example 1:

[0034] This embodiment provides a method for determining the bottom deck elevation of an offshore platform, and uses marine hydrological environmental data from a certain sea area in the East China Sea as a basis for a case study calculation to determine the bottom deck elevation of an offshore platform in that area. The technical solution of this invention is further explained by combining marine hydrological analysis and bottom deck elevation design calculations for that sea area in the East China Sea.

[0035] 1. Collect and obtain marine hydrological environmental data for a certain sea area in the East China Sea; obtain wave and tide data for tropical cyclone processes in a certain sea area in the East China Sea over many years, covering the required wave height, storm surge, astronomical tide and other data, among which the wave height is the data of the significant wave height.

[0036] 2. Preliminary data processing and data sample selection. The data obtained are wave and storm surge data of tropical cyclone processes. Since most extreme sea states in the East China Sea are controlled by tropical cyclones (typhoons), the extreme value sampling method of tropical cyclone processes is adopted, and the maximum significant wave height and corresponding storm surge of each tropical cyclone process are used as the data sample sequence.

[0037] 3. Construct a joint probability distribution model of wave height and storm surge in this sea area. Based on the data samples of extreme wave height and corresponding storm surge during tropical cyclone processes, a joint probability model of wave height and storm surge in this region is constructed using a hierarchical conditional model, i.e., conditional modeling method. The joint distribution model of wave height and storm surge consists of the marginal probability distribution of wave height and the conditional probability distribution of storm surge based on wave height (conditional distribution of storm surge under wave height).

[0038] In this embodiment, a three-parameter Weibull distribution is used to fit the marginal distribution of wave height. The probability fitting diagram of the marginal distribution of wave height data samples is shown below. Figure 3 As shown, a two-parameter Weibull distribution is used to describe the conditional probability distribution of storm surge based on wave height. The parameters in the two-parameter Weibull distribution are related to wave height and are functions of wave height; they need to be obtained by fitting specific wave height and storm surge data for the region. The parameter estimation diagram of the storm surge conditional distribution based on wave height in this embodiment is shown below. Figure 4 As shown.

[0039] 4. Based on the joint distribution model of wave height and storm surge, the environmental envelope of wave height-storm surge under different return periods is generated using the environmental envelope construction definition method. The inverse first-order reliability method is used to construct and generate wave height-storm surge environmental envelopes for 100-year and 1000-year return periods. In this embodiment, the wave height-storm surge environmental envelopes for 100-year and 1000-year return periods are as follows: Figure 5 As shown.

[0040] 5. Select the corresponding wave height and storm surge values ​​from the wave height-storm surge environmental envelope at the return period as design parameter values. Select the wave height and storm surge values ​​corresponding to specific points on the environmental envelope at the return period as joint design parameters for determining the bottom deck elevation. This embodiment uses the effective wave height H. s The data sequence generates a wave height-storm surge environmental envelope. Therefore, a value is selected on the environmental envelope such that the sum of the storm surge and the wave crest height is 1.27H. s The point where +S) reaches its maximum is taken as the final specific point. On the once-in-a-century environmental envelope, the sum of storm surge and wave crest height (1.27H) is considered the maximum. s +S) reaches its maximum point, which is the final specific point, such as... Figure 6 As shown. Calculations and analysis yielded the following results: the combined design parameters for the effective wave height and storm surge during a 100-year event are 6.6481m and 0.9262m, respectively; therefore, the combined design parameters for the wave crest height and storm surge during a 100-year event are 8.4431m and 0.9262m. The combined design parameters for the effective wave height and storm surge during a 1000-year event are 7.5742m and 1.1393m, respectively; therefore, the combined design parameters for the wave crest height and storm surge during a 1000-year event are 9.6192m and 1.1393m.

[0041] 6. Obtain the astronomical tide height design parameter value. The tide height in the tide table is astronomical tide data. Therefore, the maximum astronomical tide height above the mean sea level within the 19-year cycle of the tide table for the engineering area is selected as the design value for determining the bottom deck elevation of the offshore platform. In this embodiment, the maximum astronomical tide height above the mean sea level is 2.4m, so the astronomical tide design parameter value for determining the bottom deck elevation of the offshore platform is 2.4m.

[0042] 7. Calculate and determine the bottom deck elevation of the offshore platform. Without considering any uncertainties, the mean sea level is used as the reference surface. The bottom deck elevation of the offshore platform is the sum of the extreme values ​​of the design parameters for wave crest height, storm surge, and astronomical tide height under the return period, plus an air gap height of 1.5m. The calculation results of the bottom deck elevation of the offshore platform in this embodiment are shown in Table 1.

[0043]

[0044] Table 1 Calculation results of bottom deck elevation of offshore platform

[0045] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. While the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. The preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. These embodiments have been selected and specifically described in this specification to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The scope of protection of this invention should be determined by the appended claims.

Claims

1. A method of determining the elevation of a moonpool deck of an offshore platform, characterized by, The method comprises the following steps: Step 1: collecting data related to marine hydrological environment elements in the engineering area; obtaining long-term wave and tidal level data, which covers the required wave height, storm surge and astronomical tide related data; Step 2: preliminarily processing the data in step 1 and selecting data samples; according to the different data obtained in step 1 and the actual situation, different sample selection methods are adopted; Step 3: constructing a joint probability distribution model of wave height and storm surge in the engineering area; according to the data obtained in steps 1 and 2, the joint probability of wave height and storm surge data samples is analyzed, and a joint statistical distribution function of wave height and storm surge is established; Step 4: based on the joint probability distribution model obtained in step 3, and using the environmental envelope line construction definition method, the wave height-storm surge environmental envelope line under different return periods is generated; Step 5: selecting the wave height and storm surge values corresponding to the specific points on the wave height-storm surge environmental envelope line under the return period in step 4 as the joint design parameter values for determining the bottom deck elevation of the offshore platform; Step 6: obtaining the design parameter value of astronomical tide height; selecting the maximum astronomical tide height above the average sea level in the 19-year period in the engineering area as the design parameter value of astronomical tide height for determining the bottom deck elevation of the offshore platform; Step 7: calculating and determining the bottom deck elevation of the offshore platform; taking the average sea level as the reference surface, the bottom deck elevation of the offshore platform is the sum of the design parameter extreme values of wave height, storm surge and astronomical tide height under the return period plus the height of the air gap of 1.5m; The specific point on the wave height-storm surge environmental envelope line under the return period in step 5 refers to the point that makes the sum of storm surge and wave height maximum.

2. A method of determining the elevation of a jacket leg of an offshore platform according to claim 1, wherein: The data in step 1 is measured data or numerical simulation data in the engineering area.

3. The method of determining the elevation of a jacket leg of an offshore platform according to claim 1, wherein: The sample selection method in step 2 includes one of the whole value method, the block extreme value sampling method, the process extreme value sampling method and the threshold sampling method.

4. The method of determining the elevation of a jacket leg of an offshore platform of claim 1, wherein: The joint probability distribution model established in step 3 is constructed according to the data samples of wave height and storm surge, and using the joint distribution statistical modeling method.

5. The method of determining the elevation of a jacket leg of an offshore platform of claim 1, wherein: The astronomical tide in steps 1-7 is a periodic regularity phenomenon.

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