In the 1960s and 1970s, the Chinese government and the Petroleum and Natural Gas Corporation of China began exploring for oil and gas in Bohai Bay. Until 1995, little oil and gas were discovered in this area. As seismic technology and computer data analysis methods were improved with international cooperation, more oil and gas was discovered in Bohai Bay [30, 31]. The Xinhua Net recently announced the discovery of the Nanpu oilfield, which has reserves of 1 billion tons (China National Petroleum AG, 2016). This discovery is significant for the economic development of the Caofeidian Port Area and the Bohai region and is affecting international oil prices. The stratigraphic division and determination of the engineering properties of submarine soil layers are important in the early-stage evaluation of an oilfield and petroleum geology research. However, the engineering properties of the submarine soil layers in this area have rarely been reported in the literature [32, 33]. Therefore, investigating the engineering properties of the submarine soil layers is important for Chinese oil exploration.
The objectives of this study are (1) to summarize the utilization of the Wison-APB borehole wireline CPTU system; (2) to measure the tip resistance, sleeve friction, and pore pressure of the submarine soil layers in the Bohai Sea using the Wison-APB borehole wireline CPTU system; (3) to describe the borehole soil samples and conduct a series of soil mechanical tests in the laboratory; (4) to classify the submarine soils; and (5) to evaluate the engineering properties of the submarine soils, such as the undrained shear strength, sensitivity, and overconsolidation ratio.
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The objective of this study is to perform a geotechnical engineering investigation on an oilfield during its second phase of development to determine the physical and mechanical properties of the subsea soils for future offshore platform designs and installations. A Wison-APB downhole wireline CPT system was used (Figure 2). The Wison-APB CPT system includes the following components: CPTU probe (Figure 3), cable pushing system, hydraulic winch with an umbilical cable, remote control system and acquisition unit, depth encoder, and a guiding wheel.
Based on the CPT test results for pile foundation designs, Eslami and Fellenius [39] proposed a soil classification method that uses the relationship between the effective tip resistance and side friction with the following equations:where is the effective tip resistance.
In offshore geotechnical engineering, due to the saturated state of the submarine soil, excessive pore pressure is generated when the probe penetrates into the soil, which significantly affects the test results. The piezocone penetration test can measure the excessive pore pressure and its dissipation. The improvement in accuracy compared to the original measurements is remarkable. In this paper, the cone tip resistance (qc), sleeve friction (fs), and pore pressure (u2) were acquired by a Wison-APB wireline CPT system and are presented in Figure 5.
The methods of estimating the ultimate axial pile capacity were introduced previously. The final determination of the ultimate pile capacity depends on the mechanical properties of the soil and the type and design of the pile. In offshore engineering, the effects of wave surge and wind loading should be taken into account. In addition, for an offshore platform, a safety coefficient must be proposed based on the environmental loading conditions. The project presented in this paper has several advantages because it is focused on the preliminary investigation for offshore platforms, whereas the methods of Eslami and Fellenius [39] and Cai et al. [24] require information about the pile design and installation. Therefore, the method proposed by Almeida et al. [1] is most practical for calculating the unit end-bearing resistance (qp) and pile unit skin friction (fp) using the net-corrected cone resistance qnet. The pile unit skin friction (fp) was computed using values of Nkt of 15 and 20 based on the results in Section 3.2. The results are shown in Figure 13.
The Wison-APB borehole wireline CPT system and piezocone penetration test while drilling were used to perform preliminary geotechnical investigations at an oilfield site. The CPT data were analyzed, and the physical and mechanical properties of the submarine soils were determined by comparisons with laboratory tests. The following conclusions can be drawn.(1)There is a large difference between the results derived from the CPT tests (Su) and the laboratory experimental results, which may be due to the compact silt and hard clay interbeds in some layers. The cone tip resistance and pore pressure measured from the downhole CPT data should be used after being corrected.(2)The method proposed by Eslami and Fellenius [39] for soil classification was found to provide the best results.(3)The laboratory test results of the undrained shear strength, clay sensitivity, and OCR match the CPT test results best when the parameters Nkt, Ns, and k have values of 15, 6, and 0.3, respectively.(4)In offshore engineering, the final determination of the ultimate pile capacity depends on the mechanical properties of the soil and the type and design of the pile. Furthermore, the effects of wave surge and wind loading should be taken into account. Moreover, due to the unique characteristics of offshore platforms, a safety coefficient must be proposed based on static load tests and other tests and the environmental conditions. The piezocone penetration test or CPTU method is cost and time-saving for calculating the unit end-bearing resistance (qp) and pile unit skin friction (fp), which can be used to evaluate the ultimate pile capacity.(5)The test results and proposed empirical parameters for evaluating the geotechnical properties at the studied offshore platform site are intended to provide a solid basis for future platform engineering designs.
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