Specifications for offshore platform engineering geology investigation
Some standard content:
GB 17503--1998
This specification is specially formulated to unify the technical requirements for engineering geological surveys of offshore platform sites in my country and ensure the overall quality of survey work. The preparation of this specification refers to the relevant industry specifications of the Ministry of Geology and Mineral Resources, China National Offshore Oil Corporation, Ministry of Construction, Ministry of Water Resources and Electric Power, Ministry of Transport, State Oceanic Administration, State Seismological Bureau, etc., the general requirements for platform classification and foundation surveys of different fixed platforms formulated by the Norwegian Classification Society (DNV), and the recommended practices for planning, design and construction of offshore fixed platforms formulated by the American Petroleum Institute (API).
Appendix A and Appendix B of this standard are the appendices of the standard; this standard is proposed by the State Oceanic Administration and is responsible for interpretation; this standard is under the jurisdiction of the National Marine Standards and Metrology Center; the drafting unit of this standard is the Second Institute of Oceanography of the State Oceanic Administration; the main drafters of this standard are Ye Yincan, Li Quanxing, Pan Guofu, Chen Xitu, Li Qitong, Gu Wu, and Chen Xiaoling. 844
1 Scope
National Standard of the People's Republic of China
Specifications for offshore platform engineering geology investigation This specification specifies the content, methods and technical requirements for engineering geological investigation of offshore platform sites. GB 17503-1998
This specification is applicable to the engineering geological investigation of offshore fixed platform sites. It can also be used as a reference for the engineering geological investigation of other offshore fixed structure sites. bzxz.net
2 Cited standards
The provisions contained in the following standards constitute the provisions of this standard through reference in this standard. When this standard is published, the versions shown are valid. All standards will be revised, and parties using this standard should explore the possibility of using the latest versions of the following standards. GBJ123—1988 Standard for geotechnical test methods GB12327-1990 Specification for hydrographic survey GB/T13909—1992 Specification for marine survey Marine geological and geophysical survey GB17501-1998 Specification for marine engineering topographic survey DLJ204—1981, SLJ2—1981 Rock test procedures for water conservancy and hydropower projects JTJ224-1987 Technical specifications for port engineering Geological survey SY/T10009—1996 Recommended practice for planning, design and construction of fixed offshore platforms - Load and resistance factor design method (idtAPI 2A-LRFD:1993) 3 Definitions This standard adopts the following definitions 3.1 Fixed offshore platforms on piles refers to offshore platforms supported by piles driven into the seabed. According to the number and type of piles, they can be divided into three types: pile group type, leg column type and jacket type.
3.2 Fixed offshore gravity platforms refer to offshore platforms supported by a base (sunken pad) composed of several large reinforced concrete or steel cylinders. Generally, they are composed of a base, leg columns, steel decks and assembly modules on the deck. 3.3 Fixed offshore jack-up platforms refer to offshore platforms that stand on the seabed supported by their pile legs. The platform is raised and lowered by its own hydraulic lifting system to achieve its placement or evacuation.
4 General provisions
4.1 Purpose and tasks of survey
Approved by the State Administration of Quality and Technical Supervision on October 12, 1998 and implemented on April 1, 1999
GB 17503--1998
4.1.1 The purpose of survey is to provide basic data for platform foundation design, installation and prevention and control measures for adverse geological phenomena. 4.1.2 The task of the survey is to find out the distribution of rock and soil layers and their physical and mechanical properties within the influence range of the platform structure; to find out the adverse geological phenomena that affect the stability of the foundation.
4.2 Main contents of the survey
a) The water depth and seabed topography of the platform site; b) Seabed geomorphic features, special geomorphic phenomena and natural or man-made seabed obstacles; c) The division of engineering geological units and their causes, ages, physical and mechanical properties, thickness, burial depth and spatial distribution; d) Disaster geological factors such as landslides, collapses, weak interlayers, shallow gas, buried ancient river valleys, sand waves, active faults, and earthquake activities. 4.3 Survey procedures
It is divided into technical design, offshore survey, sample analysis and testing, data quality control, data collation and preparation of results reports, results acceptance, and data archiving.
4.4 Survey methods
Adopt engineering geology-geophysical comprehensive survey methods, based on high-resolution geophysical detection and engineering geological drilling. The main methods are as follows:
a) Depth measurement;
b) Side scan sonar detection;
c) Surface layer detection;
d) High-resolution multi-channel digital seismic survey; e) Magnetic detection;
f) Bottom survey;
g) Engineering geological drilling;
h) Engineering geological test.
4.5 Scope and workload of survey
4.5.1 The engineering geological survey of offshore platforms is carried out within a certain range of the platform location and its surroundings (i.e., the platform site). According to the importance of the project and the accumulation of previous site data, the work area is generally determined to be 0.5km×0.5km～1km×1km. 4.5.2 The workload of the survey depends on the complexity of the engineering geological conditions of the platform site, the existing survey data and the results of previous work. Generally, the following provisions are made:
a) Geophysical survey lines are arranged in a grid. The survey line spacing in the marginal area of the site is 100m, and the spacing in the central area of the site is increased to 50m. The drilling well site must have a survey line passing through, and the positioning point spacing is 50m; b) There are no less than 10 stations for bottom sampling;
c) At least 1 engineering geological sampling hole and 1 in-situ test hole should be arranged on the site, or the engineering geological sampling hole can also be used as an in-situ test hole. 4.6 Basic requirements for survey
4.6.1 The geophysical exploration operation ship can adapt to the sea conditions of level 5 and below, and can maintain a speed of 4kn~6kn. 4.6.2 When several survey methods are operated simultaneously, the positioning time and station number must be unified. If the survey line is interrupted for some reason or the same survey line is operated in batches, the same method must be used for supplementary surveys, and more than 3 positioning points must be overlapped. 4.6.3 Strictly implement the shift report system, and record at any time when encountering special geological phenomena, interference signals, and abnormal navigation. 4.6.4 The geophysical record paper roll shall indicate the operation area, date, paper roll number, survey line number, operation unit, and operator. 4.6.5 The original data shall be 100% self-checked by the operator and the head of the business department (technical person in charge), 50% shall be randomly checked by the competent department, and 10% shall be randomly checked by the task-assigning unit. The data can only be used when the qualified rate reaches more than 95%. Data that has not been accepted or failed the acceptance shall not be used. 4.6.6 Onboard and indoor geotechnical tests and their data analysis and calculation shall be carried out in accordance with GBJ123-88. 4.6.7 The compilation and drawing of the results map must have the map name, scale, geographic coordinates, legend, map number, responsibility table and necessary instructions. The responsibility table includes the mapping unit, map compiler, drafter, technical person in charge, data source, compilation and publication date, etc. 846
5 Navigation and positioning
5.1 Basic technical requirements
5.1.1 Work content
GB 17503--1998
a) Land plane and elevation control measurement, establish control network, and determine the location of navigation and positioning shore station; b) Navigation and positioning of marine survey vessels:
c) Positioning data collation.
5.1.2 Positioning accuracy
a) The plane and elevation control measurement accuracy shall be implemented in accordance with 4.4.1 and 4.4.2 of GB17501-1998 respectively. b) The error in marine positioning shall not be greater than 1.0mm on the map. 5.1.3 Coordinate system and projection
The plane coordinate system adopts the national coordinate system, and the WGS-84 geodetic coordinate system or independent coordinate system can also be used according to the requirements of the task entrusting party. Gauss-Kruger projection is used. 5.2 Plane and elevation control survey
Plane and elevation control survey establishes a navigation and positioning control network for marine surveys, which serves as the starting point for survey area positioning, and also determines the location of the shore station of the navigation and positioning system (including the GPS reference station). It is implemented in accordance with Chapters 6 and 7 of GB17501-1998. 5.3 Navigation and positioning of survey vessels
5.3.1 Navigation and positioning requirements
5.3.1.1 Navigation and positioning requirements in underway geophysical exploration a) 1 km before the survey line, the ship speed is reduced or stopped, and the survey towing cable is released. 0.5km, the ship enters the extension line of the survey line; b) In actual work, the ship maintains a speed of 4kn~6kn, and the maximum deviation between the track and the designed survey line is less than 20% of the survey line spacing; c) When encountering ships and underwater obstacles, the survey ship should slow down and detour to avoid them. After the towing cable is bypassed, it will slowly accelerate and navigate the ship to the original designed survey line to continue working;
d) The spacing between the positioning points on the map is not more than 1cm; e) The first and last point numbers of the survey line are recorded in the duty record. During the work, fill in the data every 10 points, and check the point number with the relevant investigators every 20 points. Record signal interference, interruption and handling opinions at any time; f) The aiming center or antenna center of the survey ship positioning instrument should coincide with the geophysical detection positioning center as much as possible. For mapping with a scale greater than or equal to 1:10000, the maximum horizontal distance between the two shall not exceed 2m, otherwise an eccentricity correction should be made and calculated to the positioning center of each geophysical detection. The geophysical detection and positioning time are kept synchronized. 5.3.1.2 Requirements for navigation and positioning in fixed-point surveys: a) When the survey instrument reaches the seabed, record the positioning data. When the actual survey station deviates from the designed station by more than 1 cm on the map, it needs to be redone;
b) To ensure safety, the navigation personnel must adjust the ship's navigation operated by the external operating instrument to the upwind side. 5.3.2 Navigation and positioning methods
The navigation and positioning methods for survey vessels mainly include microwave ranging positioning, GPS positioning, and hydroacoustic positioning. 5.3.2.1 Microwave ranging positioning shall be implemented in accordance with 8.2 of GB17501-1998. 5.3.2.2 GPS positioning uses differential positioning method, which shall be implemented in accordance with 8.3 of GB 17501-1998. 5.3.2.3 The hydroacoustic positioning system includes long baseline, short baseline and ultra-short baseline hydroacoustic positioning systems, which are implemented in accordance with 6.6.7.2, 6.6.7.3 and 6.6.7.4 of GB12327--90 respectively. When the ultra-short baseline hydroacoustic positioning system is used for underwater geophysical transducer positioning, the underwater sound beacon is installed in the transducer, and the three-dimensional positioning of the transducer is performed in combination with the positioning information of the survey ship. 5.4 Positioning data collation
5.4.1 Land plane control survey data collation Plane control survey data collation shall be implemented in accordance with 6.5 of GB17501-1998. 847
5.4.2 Data collation for height control survey
GB 17503—1998
Leveling survey shall be carried out in accordance with 7.2 of GB17501-1998, photoelectric ranging trigonometric height measurement shall be carried out in accordance with 7.3 of GB17501--1998, and cross-sea height measurement shall be carried out in accordance with 7.4 of GB17501-1998. 5.4.3 Data collation for survey vessel positioning
5.4.3.1 Field data collation and inspection shall be carried out in accordance with 9.4.2 of GB17501-1998. 5.4.3.2 According to the positioning data, the navigation detection track map and the fixed-point survey operation location map shall be compiled, which can be compiled together or separately; computer-assisted mapping can also be used, or it can be manually recorded in the working drawing plate for drawing; when computer-assisted mapping is used, it is required to be implemented in accordance with 9.8.2 of GB17501-1998, and when it is manually drawn, it is required to be implemented in accordance with 9.5.2 of GB17501-1998. 6 Engineering geophysical exploration
Engineering geophysical exploration includes water depth measurement, side-scan sonar detection, and stratigraphic surface detection to identify the seabed topography, seabed surface conditions, seabed obstacles, shallow seabed geological features and adverse geological phenomena. High-resolution multi-channel digital seismic surveys and magnetic detection can be carried out as needed.
6.1 Mapping scale and mapping division
6.1.1 Mapping scale
The mapping scale of engineering geophysical exploration must be determined according to actual needs and the complexity of the shallow geological landforms of the seabed. Generally, it is measured at a scale of 1:5000, and the scale can also be selected according to the requirements of the task entrusting party. 6.1.2 Mapping division
Each platform site is mapped separately. When the sites are very close to each other, multiple sites can be combined into a map. 6.1.3 Map size
The standard map size is: 50cm×70cm; 70cm×100cm; 80cm×110cm. 6.2 Depth measurement
6.2.1 Technical requirements
6.2.1.1 The accuracy of depth measurement is measured by the difference between the depth measurement values at the intersection of the main survey line and the detection line. The root mean square error should not be greater than 0.20m when the water depth is less than 20m, and should not be greater than 1% of the actual water depth when the water depth is greater than or equal to 20m. 6.2.1.2 The layout of survey lines shall be carried out in accordance with 4.5.2 of this specification. When using a multi-beam sounding system for full coverage depth measurement, a reasonable survey line spacing should be selected based on the water depth and instrument performance to ensure that there is no less than 10% overlap between adjacent survey lines. If there is a lack of sound velocity or hydrological data in the survey area, an appropriate number of sound velocity or hydrological observation points should be arranged to obtain sound velocity profile data for sound velocity correction of the sounding data. 6.2.1.3 The datum of the water depth map is the "1985 National Height Datum", and other datums may also be used according to the requirements of the survey task entrusting party. 6.2.2 Implementation of water depth measurement
The implementation of water depth measurement shall be carried out in accordance with 9.2.6, 9.2.7 and 9.2.8 of GB17501-1998. 6.2.3 Data collation
6.2.3.1 Depth measurement shall be carried out in accordance with 9.5.4 of GB17501-1998. 6.2.3.2 Depth correction shall be carried out in accordance with 9.5.5 of GB17501-1998. 6.2.3.3 The result map of water depth measurement shall include water depth map and water depth profile map, which shall be drawn by computer according to data files, or drawn manually according to water depth data reports.
The depth chart should be based on the track chart. The interval of the isobath value is generally 0.5m, 1m or 2m, depending on the complexity of the terrain. One isobath should be added and annotated for every 5 isobaths. The legend should indicate the sounding instrument model and water depth reference plane. The horizontal scale of the depth profile is the same as that of the depth chart, and the vertical scale is generally 1:100 or 1:200, depending on the size of the terrain fluctuation.
6.3 Side-scan sonar detection
6.3.1 Technical requirements
6.3.1.1 Select a reasonable sonar scanning range according to the survey line spacing. 100% coverage is required in the survey area, and adjacent survey lines must have 20%~848
GB 17503—1998
30% repeated coverage; 100% repeated coverage is required near the proposed platform location; when the water depth is too shallow, the repeated coverage rate can be appropriately reduced. 6.3.1.2 The operating frequency of the side-scan sonar is 50~500kHz, the horizontal beam angle is less than or equal to 1°, the pulse length is less than or equal to 0.2ms, and the maximum single-side effective distance is greater than or equal to 200m; it has functions such as water removal, speed correction, and tilt distance correction. 6.3.1.3 Supplementary surveys should be conducted for the following situations: missed survey line segments, coverage not meeting requirements due to heading deviation, and poor quality of recorded maps that cannot be correctly interpreted.
6.3.2 Implementation of marine surveys
6.3.2.1 Before the survey begins, representative sea areas should be selected in or near the survey area to debug instruments and equipment and determine the best instrument working parameters.
6.3.2.2 After the sonar towfish enters the water, the survey vessel shall not stop or reverse, and shall keep the heading stable as much as possible, and shall not use a large rudder angle to correct the heading; the small rudder angle and large turn method shall be used for changing the survey line. 6.3.2.3 The height of the sonar towfish from the seabed should be 10% to 20% of the scanning range. 6.3.2.4 When the sonar recorder records the maps after water removal, speed correction and tilt distance correction, the uncorrected original data shall be recorded and stored in electronic media.
6.3.2.5 Use appropriate positioning equipment to automatically locate the sonar towfish, or use manual calculation to correct the position of the sonar towfish. 6.3.2.6 Carry out preliminary interpretation of the sonar map records on site, and add additional survey lines in different directions around the suspicious targets as needed for further detection.
6.3.3 Interpretation of detection data
Combined with the results of seabed sampling, core drilling and shallow stratum profile detection, interpret the seabed surface conditions: identify and eliminate interference signals and noise on the sonar map records and echo signals that have no engineering significance; identify the types of seabed sediments, determine the distribution range of various types of sediments and exposed bedrock on the seabed; analyze the seabed micro-topography; identify and locate seabed obstacles. For the identified seabed surface features and seabed obstacles, the speed, tilt distance and towfish position correction should be carried out to determine their true position, distribution range, size and shape, and plot them on the track chart. For vertically undulating seafloor features, their approximate height and depth should be determined based on the length of the acoustic shadow on the sonar recording map. The irregularities of the seafloor that are clearly undulating should be added to the water depth map. 6.3.4 Results Map
Based on the side-scan sonar detection data, the seafloor status map is compiled. The map uses the track map as the base map and includes geological data and coastlines of seafloor sampling, drilling cores, surrounding land and major land features. According to the requirements of the task entrusting party, the sonar mosaic map of the survey area is completed based on the sonar detection data. Computer automatic digital mosaic or manual mosaic methods can be used. 6.4 Stratigraphic profile detection
Stratigraphic profile detection includes shallow stratigraphic profile detection, middle stratigraphic profile detection and deep stratigraphic profile detection. Three types of stratigraphic profile detection can be carried out simultaneously according to needs, or shallow and middle stratigraphic profile detection or shallow and deep stratigraphic profile detection can be carried out. 6.4.1 Technical requirements
6.4.1.1 Shallow stratigraphic profile detection obtains the stratigraphic distribution characteristics and adverse geological phenomena within a depth of 30m below the seafloor, and the stratigraphic resolution is not worse than 30cm.
6.4.1.2 Medium stratigraphic detection obtains stratigraphic distribution characteristics and adverse geological phenomena within a depth of 100m below the seabed, with a stratigraphic resolution of no less than 1m.
6.4.1.3 Deep stratigraphic profile detection obtains stratigraphic structure, geological structure, shallow gas and other adverse geological phenomena within a depth of several hundred meters (typically 200-600m) below the seabed, with a stratigraphic resolution of 3-6m. 6.4.1.4 The recorded surface image is clear, without strong noise interference and image blur, blank, discontinuity and other phenomena. 6.4.2 Implementation of offshore detection
6.4.2.1 Shallow stratigraphic profile detection uses a shallow penetration stratigraphic profiler, and the transducer is immersed in water at a depth of no less than 0.5m. 6.4.2.2 Medium stratigraphic profile detection uses a medium penetration stratigraphic profiler, and its source and hydrophone array must be towed a certain distance outside the stern vortex area and float at a certain depth.
6.4.2.3 For deep formation profile detection, a deep penetration formation profiler shall be used, and the towing method and technical requirements of its lightning source and hydrophone array shall be the same as those of the medium penetration formation profiler.
GB 17503-1998
6.4.2.4 Before the start of marine detection, a test shall be conducted in the survey area to obtain the best formation penetration depth and resolution so as to establish the detection operation parameters, while reducing noise and interference to the minimum. 6.4.2.5 The requirements for the navigation of survey vessels in marine detection are the same as those for side-scan sonar detection. 6.4.2.6 When the water depth changes, the scanning time and time delay of the recorder shall be adjusted in time. When detecting medium and deep formation profiles, the gun spacing shall be adjusted in time according to the water depth.
6.4.2.7 The recorded profile image shall be complete, with the missed or missing part in the middle not exceeding 50m, and the cumulative missed section less than 2% of the total length of the survey line. 6.4.2.8 Preliminary analysis and recording of profile images, if any suspicious target is found, additional survey lines should be set up to determine its nature and define its scope. 6.4.3 Stratigraphic profile data interpretation
The interpretation is carried out using the replicated stratigraphic profile record data, mainly including the following contents: a) Identify various interference signals on the section record; b) Carry out seismic (acoustic) sequence division of the profile and compare it with the geological drilling layer data of the survey area; Analyze the spatial morphology of each sequence and the contact relationship between each sequence, and determine the geological characteristics and engineering properties of each sequence; The layers at the intersection of the main survey line and the detection line must be closed; When multiple stratigraphic layers are detected in the same area, the sequence division and label names should be unified; c) Carry out seismic phase analysis based on seismic (acoustic) parameters such as reflection structure, amplitude, continuity, frequency, seismic phase unit shape and plane combination recorded in the profile, and infer the sedimentary environment, sedimentary phase, sediment type and its engineering properties; d) Identify the following adverse geological phenomena: shallow gas, ancient river valley, landslide, collapse, fault, mud dome, salt dome, submarine soft interlayer, erosion groove, active sand wave, etc. Determine their nature, size, shape and distribution range. 6.4.4 Results Maps
Compile stratigraphic section maps and shallow geological feature maps based on stratigraphic profile detection data and geological data of the survey area. 6.4.4.1 Stratigraphic profile maps are compiled by selecting survey lines as needed. The plane scale is generally the same as other interpretation maps. The vertical scale is determined according to the profile depth to be reflected, and the vertical and horizontal scales should be reasonable. The map includes important features such as the water depth at the profile line position, stratigraphic interfaces, lithology of each layer, shallow gas distribution interface, faults, etc., and marks the main landforms passed, the locations of seabed sampling and drilling cores, and the corresponding seabed sedimentary columnar diagrams, layered descriptions of cores and test results. When multiple stratigraphic profiles are detected simultaneously, stratigraphic profiles can be interpreted and compiled separately, or compiled into one stratigraphic profile.
6.4.4.2 The shallow geological characteristic map mainly includes the following contents: a) thickness contour lines or top surface burial depth contour lines of important strata; b) important topography, landforms and shallow geological phenomena; c) distribution of major adverse geological phenomena and their causes, forms, properties, scale, etc. The shallow gas area should mark the top burial depth of the gas-bearing strata and the possible distribution range;
d) main landforms, seabed sampling stations, drilling positions, descriptions of geological sampling results and sediment test results in the survey area. When the content of the shallow geological characteristic map is relatively small, it can be compiled together with the seabed status map. 6.4.4.3 The time-depth conversion of the stratigraphic profile detection data interpretation map is carried out based on the sound velocity logging data or other sound velocity data in the sea area within and near the survey area. When there is no actual sound velocity data, the assumed sound velocity of 1500m/s can be used for time-depth conversion, but it should be indicated on the map. 6.5 High-resolution multi-channel digital seismic survey
High-resolution multi-channel digital seismic survey is used to survey the strata, structures, landslides, high-pressure air pockets and other hazardous geological factors within 1000m below the seabed.
6.5.1 Technical requirements
6.5.1.1 The number of channels shall not be less than 24, the channel spacing shall not exceed 25m, and the data sampling interval shall not exceed 1ms. 6.5.1.2 The number of abnormal working channels shall not exceed 1/24 of the total channels; the empty shot rate of the entire survey line shall be less than 6%; the timing line of the monitoring record shall be clear, the track shall be uniform, and the breakpoints of the airgun synchronization signal and the excitation signal (TB) shall be clear. 6.5.1.3 The survey line layout shall be consistent with other geophysical survey lines as much as possible according to actual needs, especially through drilling well locations. 6.5.2 Implementation of marine survey
GB 17503-1998
6.5.2.1 Before the start of marine survey, comprehensive tests of instruments and equipment should be carried out in the survey area or nearby sea areas to determine the best instrument working parameters. 6.5.2.2 The horizontal selective addition (common depth point) method is generally used for ground survey. The number of coverage additions and the length of arrangement are determined according to actual needs. 6.5.2.3 The navigation requirements of the survey ship during marine survey are the same as those of stratigraphic profile detection. At the same time, the driving personnel should frequently monitor the towed capsule source and cable. When a ship is found to pass through the water surface of the cable, preparations should be made in advance to sink the cable. 6.5.2.4 Playback and inspection of monitoring records shall be carried out in accordance with 41.4 of GB/T13909--92. 6.5.3 Data Processing
The processing flow is basically the same as that of conventional seismic survey data processing, and mainly includes the following steps: preprocessing, spherical divergence correction, velocity analysis, normal time difference correction and removal, selection, deconvolution, time-varying filtering, averaging, migration imaging, etc. The processing parameters are determined based on the experiment. Select key sections and time periods as needed, and perform special processing and calculations such as "bright spot" technology and AVO (reflection amplitude and offset) technology to determine whether there are high-pressure gas pockets and analyze the strength of the gas. 6.5.4 Seismic data interpretation
Seismic data interpretation mainly includes the following contents: a) Combine regional geology, drilling in the survey area and other geophysical data to divide the seismic sequence and determine the corresponding relationship with the geological strata; b) Perform seismic phase analysis to infer the sedimentary environment, lithology and lithofacies and engineering geological conditions; c) Identify landslides, faults (layers) and high-pressure gas pockets and other disaster geological factors; d) Analyze velocity data, extract root mean square velocity, average velocity, and layer velocity for time-depth conversion and further interpretation. 6.5.5 Result drawings
The main result drawings are comprehensive interpretation profiles and geological feature maps. The profiles are drawn according to the typical survey lines selected by the belt; the geological feature maps mainly reflect the important topography and geological structures and the existing disaster geological factors . If necessary, layered structural maps (isot. maps or isobath maps) of typical layers or isopach maps and other maps of typical layers can be compiled. 6.6 Marine magnetic detection
6.6.1 Technical requirements
6.6.1.1 The mean square error of magnetic detection shall not exceed 2nT. The mean square error calculation shall be carried out in accordance with 36.2.5 of GB/T13909-92. 6.6.1.2 The layout of the survey line shall be based on actual needs and should be consistent with other geophysical survey lines as much as possible; Additional survey lines should be added to areas where the survey data indicates that the abnormal areas may be caused by magnetic objects. Additional survey lines should be added to areas where historical data indicate the presence of magnetic objects (pipelines, cables, wellheads, sunken ships, mines, bombs, etc.).
6.6.2 Implementation of marine surveys
6.6.2.1 When the water depth is greater than 100m, the sensor should be deep-towed. 6.6.2.2 Requirements for the navigation of survey vessels in marine magnetic surveys are the same as side-scan sonar surveys. 6.6.2.3 Before the start of the survey, a ship magnetic orientation influence test should be conducted in the survey area in accordance with 34.2 of GB/T13909-92. 6.6.3 Interpretation of survey data
6.6.3.1 Identify the seabed magnetic objects based on magnetic anomalies, and calculate and determine the nature, location, size, shape and occurrence of these objects. The interpretation should be combined with the results of side-scan sonar and stratigraphic profile detection. 6.6.4 Results map
The detected seabed magnetic objects should be marked on the seabed surface map, and some of the more important parts should be separately mapped and explained as needed. 7 Bottom survey
7.1 Bottom sampling method
Bottom sampling has two types: columnar sampling and surface sampling. Columnar sampling of bottom sediments is carried out using equipment such as gravity samplers, gravity piston samplers and vibration piston samplers. Surface sampling is carried out using clam samplers and box samplers. 7.2 Technical requirements for bottom sampling
7.2.1 Sampling location
The bottom sampling stations can be arranged at equal intervals according to the grid, or at unequal intervals according to the changes in the bottom. The spacing between sampling stations is generally not more than 851
- 300 mm.
7.2.2 Sampling depth
GB 17503 -1998
The length of the columnar sampling should be greater than 2m on the muddy seabed and greater than 0.3m on the sandy seabed. The surface sample should be no less than 1000g. 7.2.3 Untouched soil sample
The geotechnical test requires the use of untouched soil samples. When taking untouched soil samples with a columnar sampling tube, a liner must be built in to reduce the disturbance of the sample. The sample diameter must be greater than 72mm.
7.3 Inventory of Sediment Samples
7.3.1 On-site Inventory
Record the location, water depth, and sample length of the sampling station. Describe and stratify the samples and take photos if necessary. The description should include: a) color;
b) odor;
c) particle size;
d) consistency;
e) structure;
f) minerals;
g) biological content.
7.3.2 On-site sample processing
a) For columnar samples used for indoor geotechnical analysis, mark the station number and the up and down direction, fasten with high-pressure adhesive tape, seal tightly with wax, place upright, wrap with soft stuffing, pack in boxes, and keep warm;
b) For columnar samples used for geological analysis, fasten with high-pressure adhesive tape, seal with wax, mark the station number, sample length, and upper and lower ends; c) For surface disturbance samples, use bottles or plastic bags and mark the station number. 7.3.3 Indoor cataloging
a) Use a sample cutter to cut the columnar sample tube or push the sample out of the tube, observe the lithology, describe, stratify, and take photos. The description content shall be implemented in accordance with 7.3.1 of this chapter.
b) According to the lithology stratification, according to the sampling requirements of geotechnical, geological, chemical, biological and other projects, select representative samples for each layer. c) After receiving the sample, the laboratory shall immediately arrange for testing and analysis. The storage time of geotechnical test samples shall generally not exceed 1 month. d) The content of the test report shall include the test method, the instrument used, the test results, the test accuracy, the data interpretation and conclusion. 8 Engineering geological drilling
8.1 Hole layout
Engineering geological drilling holes are determined based on the interpretation of geophysical data and are generally selected in the center of the platform site. 8.2 Drilling technical requirements
8.2.1 Hole position
The distance between the actual drilling hole position and the designed hole position must be less than 20m, otherwise the measurement should be re-positioned. The spacing between the engineering geological sampling hole and the in-situ test hole should be less than 10m.
8.2.2 Hole depth
Different types of platform sites have different requirements for the drilling hole depth. Generally, the hole depth is required to be the foundation burial depth of the platform plus the width of the influence zone below the foundation.
a) The hole depth required for a pile-type fixed platform is the length of the pile in the soil plus the width of the pile foundation influence zone (generally considered as 10 times the pile diameter); b) The hole depth required for a gravity-type fixed platform is greater than the maximum width of the platform base. The soil layer at this depth should include any possible critical shear surface and all soil layers affected by foundation settlement; c) The hole depth required for a self-elevating fixed platform must be greater than the depth that the pile leg may penetrate plus the width of the influence zone of about 10 times the pile diameter;
GB 17503—1998
d) If bedrock is encountered, drill to 3m~5m into the fresh bedrock to complete the hole. 8.2.3 Hole diameter
The core diameter shall not be less than 69mm.
8.2.4 Water depth measurement at the hole position
Before drilling, water depth measurement shall be carried out first, and correction shall be made using an echo sounder and drill pipe readings. After obtaining the first core sample, water depth shall be measured again for further correction.
8.2.5 Requirements for sampling of undisturbed soil
For clay soil, cores shall be collected by hydraulic pressure with a thin-walled corer, and for sandy soil, cores shall be collected by hammering. 8.2.6 Core sampling rate
For clay soil, not less than 80%; for bedrock, not less than 70% (using a diamond drill bit); for sandy soil, weathered fractured zone and pebble layer, not less than 50% (using biological glue sampling).
8.2.7 Interval without cores
For clay soil, the interval without cores shall not exceed 1 m, and for other soils, the interval shall not exceed 2 m. 8.2.8 Interval of sampling of undisturbed samples
For soil layers with a thickness of less than 2 m, one core shall be collected; for layers with a thickness of more than 3 m, the maximum sampling interval shall not exceed 3 m. 8.2.9 Drilling correction
Drilling correction must be carried out when the footage is 50 m. The hole depth error is required to be less than 3%, the hole inclination is less than 1° when it is 50 m, and less than 2° when it is 100 m. 8.3 Borehole cataloging
8.3.1 Cataloging contents
a) Lithology description;
b) Color photography;
c) Sampling records;
d) Field test records;
e) Borehole structure;
f) Construction status.
8.3.2 On-site sample processing
a) The samples obtained by drilling shall be stored in a special core box in order and in a timely manner. Each core shall be separated by a core card. The core card shall be painted with paint to indicate the start and end depth of drilling. The missing parts of the core shall be marked and filled with fillers;b) All samples shall be packaged with cling paper and tin foil;c) The original soil samples shall be sealed with wax, marked with depth, up and down, and numbered, and then placed vertically in the box, and heat preservation shall be required. 8.4 Drilling results and completion report
8.4.1 Documents to be submitted after drilling completion
a) Drilling completion report;
b) Compilation book;
c) Sample distribution and sample delivery list;
d) Drilling engineering geological comprehensive column chart;
e) Field test chart;
f) Core transfer and storage table.
8.4.2 Main contents of drilling completion report
a) Drilling purpose and task;
b) Borehole coordinates, elevation, water depth;
c) Construction time;
d) Drilling and coring methods;
e) Abnormal conditions during drilling;
f) Drilling quality acceptance.
9 Engineering geological tests
GB 17503—1998
Engineering geological tests include in-situ tests, shipboard geotechnical tests and indoor geotechnical tests. This chapter only lists two types of in-situ tests: static penetration test and cross-plate shear test.
9.1 Static penetration test
9.1.1 Technical requirements
a) The offshore static penetration instrument is generally a double-bridge probe with a cone angle of 60°, a cone head cross-sectional area of 10cm2, and a friction tube side area of 200cm2; b) The penetration speed should be uniform, 20mm/s; c) The straightness error of the probe rod axis is less than 0.1%. When conducting deep static penetration, in order to avoid rod breakage accidents, the deflection angle of the penetration hole should be measured. When the deflection angle exceeds 3°, the penetration should be stopped; d) The nonlinear error, repeatability error, hysteresis error, temperature drift, and zeroing error of the probe should not be greater than 1%Fs during indoor calibration. The insulation resistance of the probe should not be less than 500MQ after 2 hours under 500kPa water pressure. During field tests, the insulation resistance should not be less than 20Me) There should be sufficient reaction force and pulling force to ensure the verticality of the probe rod; f) The accuracy of penetration depth recording is less than 5cm for every 20m. 9.1.2 Field operations
a) After the drilling ship is in place, a static penetration test is first performed; b) Each time the penetration is 3m, continuous penetration, and a continuous and complete cone head resistance curve, side wall friction curve, pore water pressure curve and friction ratio curve are obtained:;
c) At the beginning of the penetration, the probe is penetrated for a short distance, and the initial reading is recorded after the probe temperature is consistent with the ground temperature. At the end of the test, recalibrate and the error between the two calibrations should be less than 1%. Otherwise, the test results will be discarded. d) The drilling vessel must be equipped with a wave compensator. 9.1.3. Summarization of test results
a) Correction of original data includes: depth correction, zero correction, and original record curve correction. Depth correction: When the recorded depth is inconsistent with the measured depth, the depth error should be corrected linearly according to the actual depth. Zero drift correction: Generally, the test value is corrected by linear interpolation according to the depth interval of zero check. Correction of original record curve: The trumpet or peak that appears in the penetration pause interval curve is smoothly connected. b) Drawing of penetration curve: relative penetration resistance depth (P,-h) curve, cone head resistance depth (f.-h) curve, side wall friction depth (Rr-h) curve, friction resistance relative depth (F,-h) curve: c) Dividing soil layer boundaries: According to the static penetration curve, the soil is mechanically stratified, or the engineering geological stratification is carried out by referring to the drilling stratification combined with the shape and numerical value of the static penetration curve; d) Calculation of soil layer penetration resistance: The arithmetic average method is used, or the penetration resistance of each layer of the borehole is calculated by the area method according to the penetration curve. 9.1.4 Application of test results
The results of static penetration testing are mainly used in the following aspects: a) Classify soil layers using cone resistance and friction ratio; b) Determine the bearing capacity and deformation modulus of foundation soil using applicable empirical formulas; c) Determine the relative density and internal friction angle of sand based on cone resistance and specific penetration resistance; d) Estimate the bearing capacity of a single pile based on the measured curve of static penetration testing. 9.2 Cross-plate shear test
9.2.1 Test conditions
The in-situ cross-plate shear test is performed under the condition that the soil body basically maintains the in-situ stress. It is suitable for homogeneous saturated soft clay with a sensitivity S less than or equal to 10 and a consolidation coefficient C less than or equal to 100 m2/y. For uneven soil layers, especially soft clay with thin layers of fine sand or silty 854
GB 17503—1998
soil, the test will have a large error and must be used with caution. 9.2.2 Technical requirements
a) The size of the cross plate is usually rectangular, with a height-to-diameter ratio (H/D) of 2, as determined in Table 1; Table 1 In-situ cross plate size
Drilling hole outer diameter
Blade thickness
Cross plate drill rod diameter
b) During the test, the depth of the cross plate inserted below the bottom of the hole is greater than 5 times the borehole diameter to ensure that the cross plate can be tested without disturbing the soil;
c) All clay layers with a thickness greater than 1m are subject to cross plate shear tests, and thick clay layers are tested every 2m; d) The torsional shear rate should generally be controlled at 6°/min to 12°/min. When the torque reaches a peak or stable value, continue to measure for 1 minute to confirm the peak or stable torque;
e) After the maximum torque is measured, the cross plate is rotated continuously for more than 6 circles, and the peak or stable torque of the reshaped soil is measured after 1 minute of reshaping process.
9.2.3 Test data collation
a) Draw the cross-plate undrained shear strength depth (Cu-h) curve; b) Draw the sensitivity depth (S.-h) curve; c) Draw the undrained shear strength-torsion angle (Cu-α) curve of each test point. 9.2.4 Application of test results
a) Evaluate the on-site undrained shear strength. The cross-plate undrained shear strength value measured in situ is generally too high and needs to be corrected by shear failure time, soil anisotropy, soil overconsolidation ratio, etc. b) The bearing capacity of soft soil foundation is determined by the following empirical formula: ft 2(Cu)riela + h
Where: fk-
Standard value of foundation bearing capacity, kPa;
—Natural floating density of soil, kN/m;
h-foundation burial depth, m;
(Cu)Fiela on-site undrained shear strength, kPa. c) The ultimate bearing capacity of a single pile is the sum of the pile side friction and the pile end bearing capacity, where the pile side friction is calculated as follows: f ac.
Where: a-reduction factor,
Cu——undrained shear strength of soil, kPa. The pile end bearing capacity is calculated as follows:
9 = 9Cu
The ultimate bearing capacity of a single pile can be calculated based on f and. 9.3 Geotechnical tests on board
Geotechnical tests are carried out on the collected soil samples on the drilling ship. The main test items are: a) pocket cross plate shear test,
b) pocket penetrometer test;
c) water content;
d) natural density;
(2)
(3)
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