4.2.2. Foundation-Supported Structures

As listed in Table 1, many marine facilities experienced strong earthquakes and were damaged to a certain extent. Among various foundation-supported structures (offshore drilling platforms, oil platforms, wind turbines, cross-sea bridges, etc.), offshore wind turbines are gradually becoming the focus of attention. This is mainly because of the trend of developing clean and eco-friendly energies. Wind energy as a representative has aroused great research interest. Europe is a pioneer of offshore wind turbine (OWT) construction [64]. A 2019 report on European offshore wind turbines showed that OWTs are moving towards the deeper sea (Figure 4). As OWTs are deployed in deeper water, the OWT foundations are being modified, as shown in Figure 5.

have been proposed (Table 2).

have been proposed (Table 2).

on [2,26,68–71]).

on [2,26,68–71]).

from [67]).

Meanwhile, the wind turbine is a slender structure, which has a larger length/width ratio compared to other marine structures, so it is very sensitive to lateral loads [65]. Under the combined effect of winds, waves, and possible seismic loads, the structure–soil interaction will become quite complicated. The soils around the foundations of wind turbines may be greatly disturbed and have a great potential of liquefaction. Thus, in this section, we take the wind turbine as a typical marine structure and highlight research on new foundation structures of liquefaction resistance as applied to it. Meanwhile, the wind turbine is a slender structure, which has a larger length/width ratio compared to other marine structures, so it is very sensitive to lateral loads [65]. Under the combined effect of winds, waves, and possible seismic loads, the structure–soil interaction will become quite complicated. The soils around the foundations of wind turbines may be greatly disturbed and have a great potential of liquefaction. Thus, in this section, we take the wind turbine as a typical marine structure and highlight research on new foundation structures of liquefaction resistance as applied to it. compared to other marine structures, so it is very sensitive to lateral loads [65]. Under the combined effect of winds, waves, and possible seismic loads, the structure–soil interaction will become quite complicated. The soils around the foundations of wind turbines may be greatly disturbed and have a great potential of liquefaction. Thus, in this section, we take the wind turbine as a typical marine structure and highlight research on new foundation structures of liquefaction resistance as applied to it.

*J. Mar. Sci. Eng.* **2020**, *8*, x FOR PEER REVIEW 8 of 16

Meanwhile, the wind turbine is a slender structure, which has a larger length/width ratio

**Figure 4.** Average distance to shore of offshore wind turbine (OWTs) in Europe (modified from [66]). **Figure 4.** Average distance to shore of offshore wind turbine (OWTs) in Europe (modified from [66]). **Figure 4.** Average distance to shore of offshore wind turbine (OWTs) in Europe (modified from [66]).

**Figure 5.** Major foundation types used in OWT design: (**a**) gravity foundation; (**b**) mono-pile; (**c**) jacket **Figure 5.** Major foundation types used in OWT design: (**a**) gravity foundation; (**b**) mono-pile; (**c**) jacket foundation piles; (**d**) suction bucket (mono-pod); (**e**) floating wind turbine with anchors (modified **Figure 5.** Major foundation types used in OWT design: (**a**) gravity foundation; (**b**) mono-pile; (**c**) jacket foundation piles; (**d**) suction bucket (mono-pod); (**e**) floating wind turbine with anchors (modified from [67]).

foundation piles; (**d**) suction bucket (mono-pod); (**e**) floating wind turbine with anchors (modified from [67]). At present, there are five main types of foundation structure for OWTs: gravity, monopile, jacket, suction bucket and floating foundation. Many researchers have studied the damage-mitigation performance of the above foundation forms, mainly by numerical methods and dynamic centrifuge experiments. To meet the various marine engineering challenges in the future and improve the At present, there are five main types of foundation structure for OWTs: gravity, monopile, jacket, suction bucket and floating foundation. Many researchers have studied the damage-mitigation performance of the above foundation forms, mainly by numerical methods and dynamic centrifuge experiments. To meet the various marine engineering challenges in the future and improve the liquefaction-resistant performance of the foundations, many innovative structure improvements At present, there are five main types of foundation structure for OWTs: gravity, monopile, jacket, suction bucket and floating foundation. Many researchers have studied the damage-mitigation performance of the above foundation forms, mainly by numerical methods and dynamic centrifuge experiments. To meet the various marine engineering challenges in the future and improve the liquefaction-resistant performance of the foundations, many innovative structure improvements have been proposed (Table 2).

liquefaction-resistant performance of the foundations, many innovative structure improvements

**Table 2.** Main OWT foundation types and novel liquefaction-resistant structure improvements (based

**Table 2.** Main OWT foundation types and novel liquefaction-resistant structure improvements (based


**Table 2.** Main OWT foundation types and novel liquefaction-resistant structure improvements (based on [2,26,68–71]).

The monopile foundation is the main type of OWT foundation currently in use. A new adaptation is the multi-pile foundation (to some extent, the jacket foundation can also be classified as a multi-pile foundation). Hao et al. carried out dynamic centrifugal model tests on the tripod foundation and found that it has better resistance to liquefaction than the common monopile one [75]. Wang et al. proposed a new hybrid monopile foundation, also based on centrifugal tests, and found that the mixed foundation has smaller lateral displacement and enhanced liquefaction resistance than ordinary monopile foundations [73,74]. General views of these two alternatives are illustrated in Figure 6. In fact, the concept of a hybrid monopile can be used to strengthen existing structures. *J. Mar. Sci. Eng.* **2020**, *8*, x FOR PEER REVIEW 10 of 16

**Figure 6.** New structures of monopile foundation to increase liquefaction resistance: (**a**). hybrid monopile; (**b**). tripod foundation (based on [74,75]). **Figure 6.** New structures of monopile foundation to increase liquefaction resistance: (**a**). hybrid monopile; (**b**). tripod foundation (based on [74,75]).

research stage of model testing and numerical calculations; these include suction buckets with honeycomb compartments, a modified suction caisson with an external skirt, an umbrella suction anchor foundation, and so on (Figure 7). Experiments by Wang et al. showed that the honeycombcompartment bucket can reduce soil settlement by about 50% according to experimental data [81]. Li et al. found that the external skirt provides the modified suction caisson with a higher lateral capacity [79,80]. Liu et al. studied a new umbrella suction anchor foundation with anchor branches that closely fit the seafloor; this system improves the anti-overturning ability of the master cylinder and the antiscouring ability of the surrounding seabed soil [64,68]. Compared with conventional foundations, these new structures show good liquefaction-resistance performance, and have broad application

**Figure 7.** New structures of suction bucket foundations with increased liquefaction resistance: (**a**) modified suction buckets with honeycomb compartment; (**b**) modified suction caisson with external

skirt; (**c**) umbrella suction anchor foundation (based on [64,79,81]).

prospects in practical marine engineering.

As shown in Table 2, the suction bucket foundation is a hot topic currently. Many modifications

New anti-liquefaction jacket foundations have not been proposed in published articles to our knowledge. However, Ju et al. used the finite element method to analyze the seismic response of NREL 5-MW jacket-type OWT under combined loads (earthquakes, waves and winds), and found that the first-mode tuned mass dampers are necessary, which can effectively reduce the vibration induced by combined loads when liquefaction occurred [82]. **Figure 6.** New structures of monopile foundation to increase liquefaction resistance: (**a**). hybrid monopile; (**b**). tripod foundation (based on [74,75]). As shown in Table 2, the suction bucket foundation is a hot topic currently. Many modifications

*J. Mar. Sci. Eng.* **2020**, *8*, x FOR PEER REVIEW 10 of 16

As shown in Table 2, the suction bucket foundation is a hot topic currently. Many modifications of the suction bucket foundation have been proposed and implemented, such as the large-scale prestressed concrete bucket foundation, with certain success in real engineering by mitigating liquefaction damage (details in Section 4.2.3). Many other new liquefaction resistant structures are still in the research stage of model testing and numerical calculations; these include suction buckets with honeycomb compartments, a modified suction caisson with an external skirt, an umbrella suction anchor foundation, and so on (Figure 7). Experiments by Wang et al. showed that the honeycomb-compartment bucket can reduce soil settlement by about 50% according to experimental data [81]. Li et al. found that the external skirt provides the modified suction caisson with a higher lateral capacity [79,80]. Liu et al. studied a new umbrella suction anchor foundation with anchor branches that closely fit the seafloor; this system improves the anti-overturning ability of the master cylinder and the anti-scouring ability of the surrounding seabed soil [64,68]. Compared with conventional foundations, these new structures show good liquefaction-resistance performance, and have broad application prospects in practical marine engineering. of the suction bucket foundation have been proposed and implemented, such as the large-scale prestressed concrete bucket foundation, with certain success in real engineering by mitigating liquefaction damage (details in 4.2.3). Many other new liquefaction resistant structures are still in the research stage of model testing and numerical calculations; these include suction buckets with honeycomb compartments, a modified suction caisson with an external skirt, an umbrella suction anchor foundation, and so on (Figure 7). Experiments by Wang et al. showed that the honeycombcompartment bucket can reduce soil settlement by about 50% according to experimental data [81]. Li et al. found that the external skirt provides the modified suction caisson with a higher lateral capacity [79,80]. Liu et al. studied a new umbrella suction anchor foundation with anchor branches that closely fit the seafloor; this system improves the anti-overturning ability of the master cylinder and the antiscouring ability of the surrounding seabed soil [64,68]. Compared with conventional foundations, these new structures show good liquefaction-resistance performance, and have broad application prospects in practical marine engineering.

**Figure 7.** New structures of suction bucket foundations with increased liquefaction resistance: (**a**) modified suction buckets with honeycomb compartment; (**b**) modified suction caisson with external **Figure 7.** New structures of suction bucket foundations with increased liquefaction resistance: (**a**) modified suction buckets with honeycomb compartment; (**b**) modified suction caisson with external skirt; (**c**) umbrella suction anchor foundation (based on [64,79,81]).

skirt; (**c**) umbrella suction anchor foundation (based on [64,79,81]).

With the development of marine engineering, gravity foundations have been gradually phased out because they can only be used in shallow waters and cannot meet future demands. In contrast, floating foundations are suitable for deep-sea environments. It is foreseeable that research on floating foundations will increase in the coming years, and new liquefaction-resistant structures of floating foundations may be developed and applied in the field, which will become the next research hotspot.
