Elsevier

Ocean Engineering

Volume 234, 15 August 2021, 108770
Ocean Engineering

Effect of sitting time on the breakout force of mat foundation on soft marine clay seabed

https://doi.org/10.1016/j.oceaneng.2021.108770Get rights and content

Highlights

  • Field tests were conducted to study the breakout force of mat foundation.

  • Suction generated by negative pore pressure accounted for most of the breakout force.

  • Breakout force has a linear relationship with the consolidation degree of seabed soil.

  • Uplift process of mat foundation changes from drained to undrained as uplift rate increases.

Abstract

Jack-up offshore platform mat foundation has lower requirements on the bearing capacity on clay seabed than spudcan foundation, which is more suitable for the soft clay seabed. It is of great significance to reveal the mechanism of breakout force of mat foundation to ensure the safety and stability of offshore platform. In this study, field tests with different consolidation times were conducted to investigate the breakout force of mat foundation on soft marine clay seabed. Field results show that the suction generated by negative pore pressure in the seabed soil accounted for most of the breakout force, generally up to 90%. Then, the effects of consolidation time, preloading and uplift rate on the effective stress and breakout force were numerically studied. Numerical results show that the breakout force has a linear relationship with the consolidation degree of seabed soil. Under the same degree of consolidation, the breakout force per unit area increases with the increase of preloading. The uplift process changes from drained to undrained as the uplift rate increases.

Introduction

Jack-up platforms are extensive used as exploratory drilling platforms and offshore and wind farm service platforms. When the platform needs to move from one location to another, the process of foundation lifting is necessary. Generally, the penetration depth of a traditional spudcan is very deep on soft clay seabed, even up to 30 m deep. So the process of extract the embedded spudcan is time-consuming (Na, 2005). Mat foundations are more suitable on soft clay seabed due to its good terrain adaptability, shallow penetration depth and short foundation lifting time (Yuan et al., 2019). During the operation of the platform, only the lower part of the mat foundation is sunk into the seabed, so the uplifting process does not need to overcome the weight of the overlying soil (Fang et al., 2019). However, the breakout force acting on the mat foundation during the uplift process is a critical issue, which is related to the stability and safety of the platform (Li et al., 2015).

The breakout phenomenon of uplift an object from the seabed surface was explained by Sawicki et al. (2003). Generally, the uplift force of an object is equal to the submerged weight when the object is in water. However, the force needed to extract an object from the seabed surface is much larger than the submerged weight of the object itself. In order to raise the object from the seabed surface, the uplift force should be applied which has to overcome both the submerged weight and some additional force Rs, which is the resultant of some stresses generated at the contact between the object and seabed surface. Therefore, the breakout force should be larger than Rs.

Researches on the breakout process of an object lying on seabed surface is relatively scarce. Vesic et al. (1971) pointed out that the breakout force could defined as the difference in pore pressure between the two sides of the embedded structure. For mat foundation, the breakout force also includes the friction force between soil and the side wall of the mat foundation, and the adhesion force between soil and mat foundation bottom surface. Foda (1982) proposed an analytical model to describe the negative pressures developing underneath a body as it detaches from the seabed. Mei et al. (1985) proposed an alternative theory to study the breakout process of object lying on a porous but rigid seabed. Zhou et al. (2008) numerically investigated the breakout process of a disk lying on the porous, elastic saturated seabed surface. Chang et al. (2015) analytically reinvestigates the lift-up problem for a rigid porous bed that was studied by Mei et al. with a more general model.

Existing research on breakout force has focused on marine structures such as plate anchors, mudmats, and suction foundations (Acosta-Martinez et al., 2012; Gourvenec et al., 2009; Lehane et al., 2008). In laboratory model tests, indirect method was usually used to measure the breakout force of plate anchor. The hollow bolt was vented to eliminate the breakout force, and the results were compared with those without ventilation (Bemben et al., 1975). The filter paper was placed at the bottom of the anchor to make the results breakout force more accurate (Singh and Ramaswamy, 2008). Chen et al. (2012) conducted a series of centrifuge tests to investigate the uplift resistance of plain mudmats resting on lightly over-consolidated clay. The suction developing of the mudmat was instrumented with pore pressure transducers. The centrifuge tests results shown that the uplift resistance increases as pull-out velocity increases. Li et al. (2014) investigated the uplift resistance of rectangular mudmats resting on lightly over-consolidated kaolin clay through centrifuge model tests. The effects of perforation on the uplift resistance and suction generation at the foundation were discussed, and the results shown that perforations could only alter the magnitude of suction force. The breakout mechanism of the foundation was discussed through the pore pressure variations under the foundation (Li and Cassidy, 2018).

Besides, Du et al. (2017) conducted a series of uplift tests on a suction foundation by using a beam centrifuge, in which the uplift displacement, suction force, and loading rate were monitored. Centrifuge tests results shown that the pull-out resistance increases as the uplift loading rate increases. The shape effects on the undrained uplift capacity of skirted suction foundations in clay were investigated through centrifuge model tests (Fan et al., 2013).

To date, studies on the breakout force of large mat foundations are limited. Besides, there are few researches on the effect of sitting time, which also represents the consolidation time, on the breakout force of mat foundation lying on soft marine clay seabed. Therefore, it is essential to understand the mechanism of breakout force of mat foundation on soft marine clay seabed. In this research, a serious of field tests were carried out to investigate the mechanism of the breakout force of mat foundation. The pore pressure at the top and bottom of mat foundation were measured by pore pressure transducers (PPT). The breakout force was measured by a dynamometer. The effects of mat foundation sitting time on the uplift force and pore pressure were investigated. Then numerical analysis was carried out to investigate the effects of consolidation time, external load and uplift rate on the breakout force.

Section snippets

Field test

In order to measure the breakout force and investigate the relationship between the negative pore pressure at the bottom of mat foundation and the breakout force, a serious of field tests were conducted at East China Sea, as shown by the red dot in Fig. 1. The average water depth near the wharf is about 5 m. The physical properties of different soil layers near the field test is shown in Table 1.

Numerical model and mesh

Due to the large crane was operated by professional technicians in the field tests, the lifting rate and continuity were difficult to control, as shown in Case B. Meanwhile, due to the long test time, only five sets of field tests were conducted. Therefore, in order to fully understand the effects of consolidation time, external load and uplift rate on the breakout force, a further numerical analysis was carried out.

Mechanism of negative pore pressure

During the entire operation of the mat foundation, the seabed soil undergoes the processes of loading, consolidation and unloading. The mechanism of negative pore pressure during the unloading process can be explained by triaxial test. Therefore, numerical triaxial tests under isotropic consolidation and K0 consolidation are carried out by using the element model. The shearing process of the test is undrained. The element model is subjected to pressures in both axial and radial directions.

Conclusions

In this research, field tests were carried out to investigate mechanism of the breakout force of mat foundation with different sitting time. A model box installed with pore pressure transducers was designed to simulate the mat foundation. The effects of consolidation time, preloading, and uplift rate on the breakout force were discussed numerically. Based on the field tests and numerical results, the following conclusions are drawn:

  • (1)

    The suction force caused by the negative pore pressure accounts

CRediT authorship contribution statement

Qi Zhang: Writing – original draft, Visualization, Investigation. Tianyi Fang: Investigation, Writing – review & editing. Guanlin Ye: Conceptualization, Supervision. Guojun Liu: Resources, Project administration. Rong Wang: Resources, Investigation. Yinghui Tian: Methodology, Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The study is supported by National Natural Science Foundation of China (Grant Nos. 42072317), Shanghai Sailing Program (Grant No. 20YF1418500) and CCCC Research & Development Project (2018-ZJKJ-01).

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