# Field Test on Buoyancy Variation of a Subsea Bottom-Supported Foundation Model

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Field Tests

#### 2.1. Test Model

#### 2.2. Test Principles

#### 2.3. Test Plan

#### 2.4. Tests Results

_{2}) was obtained from the top pore pressure transducer. The test results in Figure 5 are fluctuating because of the influence of tides. The state of the tide at the beginning stage of each test can be obtained according to u

_{2}. At the beginning of Case a, the tide was rising, and at the beginning of Cases b, c, d, e, the tide was falling. The initial phase of the tide had an influence on the readings of the transducers (as shown by the difference between Case a and Case b) but had no effect on the variation of the buoyancy, since both methods of calculating buoyancy are based on subtraction. As the sitting time increased, the result of $\overline{{\mathit{u}}_{1}}$ showed a decreasing trend, which is shown in Figure 5e (the dotted red line is the trend line for $\overline{{\mathit{u}}_{1}}$). On the basis of the tests results, the buoyancy variation of the model was determined during a long sitting time. Therefore, further discussion on pore pressure difference, effective stress, and buoyancy is based on the sitting time of 5 days.

## 3. Numerical Simulation

#### 3.1. Soil Property

#### 3.2. Model and Mesh

#### 3.3. Comparison between Numerical Analysis and Field Test

## 4. Conclusions

- (a)
- The model foundation was subjected to buoyancy during the entire sitting time because of the connectivity of the pore water in the seabed to the outside seawater.
- (b)
- At the initial stage of the sitting time, the buoyancy of the model could reach twice the theoretical value. As the sitting time increased, the buoyancy gradually decreased and eventually stabilized near the theoretical value. The fluctuation of buoyancy was due to the difference of the pore pressure response speed between the top and the bottom surfaces when the water level changed. The pore pressure response of the bottom surface had a phase lag relative to that of the upper surface, since the lower half of the model was buried in the lowly permeable seabed.
- (c)
- The soil–water coupled numerical analysis demonstrated that the buoyancy acting on the model was closely related to the pore water pressure at the bottom of the model. The buoyancy reached twice the theoretical value at the beginning as a consequence of the significant excess pore pressure at the bottom. With the dissipation of the excess pore pressure at the surface of the seabed, the buoyancy decreased. The deep excess pore pressure had little effect on the buoyancy acting on the model foundation.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 4.**Photographs taken during the test: (

**a**) numbering of the transducers and calibration of their initial values; (

**b**) dynamometer used in the test; (

**c**) lowering of the model to start the test; (

**d**) test site.

**Figure 5.**Field test results of total stress and pore water pressure during the sitting time of (

**a**) 3 h, (

**b**) 6 h, (

**c**) 22 h, (

**d**) 2.5 days, and (

**e**) 5 days.

**Figure 6.**Variation of the pore pressure difference and effective stress during the sitting time of 5 days.

**Figure 7.**Variation of buoyancy (based on two calculation methods) during the sitting time of 5 days.

**Figure 9.**Distribution of the excess pore pressure after sitting for (

**a**) 0.5 day, (

**b**) 1 day, (

**c**) 2.5 days, and (

**d**) 5 days.

Test Case | a | b | c | d | e |
---|---|---|---|---|---|

Sitting time | 3 h | 6 h | 22 h | 2.5 days | 5 days |

Sampling frequency | every 1 min | every 1 min | every 10 min | every 10 min | every 10 min |

Parameter | Symbol | Value |
---|---|---|

Slope of normally consolidated line in e − ln p’ space | λ | 0.2 |

Slope of swelling and recompression line in e − ln p’ space | κ | 0.04 |

Slope of critical state line in p’ − q space | M | 1.2 |

Poisson’s ratio | ν | 0.3 |

Void ratio at p’ = 1 kPa on critical line | e_{cs} | 1.28 |

Permeability | k (m/s) | 10^{−9} |

Saturated bulk density of soil, | γ (kN/m^{3}) | 18 |

Coefficient of earth pressure | K_{0} | 0.6 |

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**MDPI and ACS Style**

Fang, T.; Liu, G.; Ye, G.; Pan, S.; Shi, H.; Zhang, L.
Field Test on Buoyancy Variation of a Subsea Bottom-Supported Foundation Model. *J. Mar. Sci. Eng.* **2019**, *7*, 143.
https://doi.org/10.3390/jmse7050143

**AMA Style**

Fang T, Liu G, Ye G, Pan S, Shi H, Zhang L.
Field Test on Buoyancy Variation of a Subsea Bottom-Supported Foundation Model. *Journal of Marine Science and Engineering*. 2019; 7(5):143.
https://doi.org/10.3390/jmse7050143

**Chicago/Turabian Style**

Fang, Tianyi, Guojun Liu, Guanlin Ye, Shang Pan, Haibin Shi, and Lulu Zhang.
2019. "Field Test on Buoyancy Variation of a Subsea Bottom-Supported Foundation Model" *Journal of Marine Science and Engineering* 7, no. 5: 143.
https://doi.org/10.3390/jmse7050143