Development of a super large resonance free vibration impact pile hammer

Abstract: With the rapid development of wind power technology, offshore wind power has become a focus of attention for the development of renewable energy in the world. In recent years, China's offshore wind power industry has developed rapidly, with increasing installed capacity of offshore wind power. Offshore wind power piles have shown characteristics of ultra long, super large, overweight, and high-precision construction. At present, the main equipment for offshore wind power pile construction are impact hammers and vibration pile hammers. The impact hammer is a traditional offshore wind power pile driving equipment, and its main problems include low striking efficiency, heat dissipation, construction safety, and quality control. The principle of pile driving with a vibrating pile hammer is to "liquefy" the soil and reduce resistance. As long as the necessary amplitude, excitation force, weight and impulse, and power are met, the pile can be sunk. The main problems currently faced are start-up difficulties, resonance hazards, manufacturing difficulties, and transportation difficulties. To this end, Shanghai Zhenzhong Company has developed a super large resonance free vibration impact pile hammer for the construction of offshore wind turbine large pile foundations.

The necessity of developing ultra large vibration impact pile hammers

1.1 Characteristics of offshore wind power piles

In recent years, China's offshore wind power industry has developed rapidly, with increasing installed capacity of offshore wind power. Offshore wind power piles have shown characteristics of being ultra long, super large, overweight, and high-precision. At present, the heaviest offshore wind power pile has reached over 1500 tons, and the construction verticality requirement is within 0.3%.

1.2 Analysis of the main equipment and characteristics currently used for offshore wind power pile construction

1.2.1 Impact Hammer

The impact hammer is the first equipment used in the construction of offshore wind power pile foundations, including diesel hammers and hydraulic hammers. Due to unresolved heat dissipation issues, the development of large diesel hammers has been constrained. In recent years, large hydraulic hammers have become the main equipment for offshore wind power pile foundation construction. Traditionally, it is believed that the impact hammer has a high efficiency of over 30%, but with the increasing size of wind power piles, the impact efficiency of hydraulic hammers is lower than the traditional perception of 30%.

The impact efficiency η refers to the ratio of the energy E obtained by the pile after being hit by the impact hammer to the energy E0 obtained during the impact hammer,

approach （1）

The impact efficiency formula derived by the Japan Architectural Society based on the momentum theorem

（2）

In the formula, m is the mass of the impact hammer core, and M is the mass of the pile.

It can be seen that in order to achieve a 30% striking efficiency, the mass of the pile cannot exceed 3 times the mass of the hammer core, and the ideal state is 1.5-2.5 times. At present, the quality of offshore wind power piles is constantly increasing, and the quality of hydraulic impact hammers is also constantly increasing, but the quality of piles far exceeds three times that of impact hammers. Assuming the core mass of a large impact hammer is 100 tons and the weight of a large pile is 900 tons, the impact efficiency of the impact hammer is only 10%.

During construction, we can complete pile driving by continuously increasing the impact energy. However, in the process of continuously increasing the impact energy, due to the low impact efficiency and large energy dissipation, the heat dissipation problem is quite serious, which not only affects the quality and service life of the pile, but also reduces the service life of the impact hammer itself, resulting in frequent failures.

1.2.2 Vibration pile hammer

In recent years, vibratory pile hammers have also become the main force in offshore wind power pile construction. The principle of vibration generated by a vibrating pile hammer is that the eccentric bodies symmetrically arranged on two axes (or even multiple axes) of the vibrating pile hammer rotate in opposite directions under the drive of a synchronous gear. The centrifugal forces generated by the eccentric bodies on each pair of two axes are combined, and the centrifugal forces in the horizontal direction cancel each other out, and the centrifugal forces in the vertical direction are superimposed on each other, forming an excitation force that varies according to a sine function law (as shown in Figure 1 and Figure 2).

Fig.1 Diagram of pile hammer

Fig.2 Diagram of sine of exciting force

When the vibrating pile hammer is connected to the pile for pile sinking, the excitation force causes the pile to vibrate at a frequency consistent with the excitation frequency. The frictional resistance of the soil on the side of the pile and the resistance at the end of the pile will rapidly decrease during pile vibration. In the case where the total gravity of the vibrating pile hammer and the pile is greater than the soil resistance at the end of the pile, the pile begins to sink.

It should be noted that:

(1) The pile sinks under the action of gravity, and the vibration under the excitation force only reduces the resistance of the soil to the pile (including lateral friction resistance and end face resistance).

(2) There is no strict distinction between vibration and impact, there is impact within vibration and vibration within impact. The up and down vibration of the vibrating pile hammer under the action of excitation force is equivalent to driving the piston of the impact hammer core to reciprocate up and down in the oil cylinder (as shown in Figure 3).

Fig. 3 vibratory pile hammer is equivalent to impact hammer diagram

And when the frequency of the vibrating pile hammer is high, the vibration characteristics are significantly reflected; When the frequency of the vibrating pile hammer is low, its vibration characteristics weaken while its impact characteristics enhance. Large vibrating pile hammers, with large eccentricity and low rotational speed, not only have vibration characteristics, but also have particularly obvious impact characteristics. Therefore, large vibrating pile hammers with low frequency and large eccentricity are also called vibrating impact hammers. The principle of pile sinking with a vibrating impact hammer is different from that of a hammer sinking a pile through impact force. It uses up and down vibration to "liquefy" the soil, reduce resistance, and complete pile sinking construction under the action of gravity.

The vibration pile hammer for pile sinking must meet the following conditions:

(1) The actual amplitude A is greater than the critical amplitude A0.

The actual amplitude is （3）

In the formula, K - eccentric moment of the vibrating pile hammer (kgm), m - mass involved in vibration (kg).

critical amplitude(mm)。 （4）

In the formula, N - is the standard penetration number of soil.

(2) The excitation force F is greater than the dynamic lateral resistance f of the pile.

The excitation force is （5）

In the formula, k - eccentric moment of the vibrating pile hammer (kgm), - vibration angular velocity (s-1).

(3) The total weight of the pile and hammer G>the dynamic end resistance f of the pile.

(4) The actual total power consumption P is less than 1.5 times the rated power P of the vibration pile hammer motor.

At present, large vibration impact pile hammers also face difficulties in starting, resonance hazards, manufacturing, and transportation during manufacturing and use.

1. Development of ultra large resonance free vibration pile hammer

Shanghai Zhenzhong Company is committed to the development of ultra large vibration impact pile hammers, and its main technologies include resonance free technology for large vibration impact pile hammers and linkage technology for large resonance free impact pile hammers.

1.1 Non resonance technology for large vibration impact pile hammers

2.1.1 Resonance and Hazards

The vibration frequency of a vibrating pile hammer is the rotational frequency of the eccentric shaft. Changing the rotational frequency of the eccentric shaft correspondingly changes the vibration frequency of the vibrating pile hammer, but any object or equipment has its own natural frequency. When the vibration frequency of the vibrating pile hammer is close to or equal to the natural frequency of the vibrating pile hammer, pile and other vibration systems, it will cause strong vibration of the vibrating pile hammer, which is called "resonance". When resonance occurs, the amplitude of vibration often reaches more than 15 times the normal amplitude. The occurrence of resonance not only causes damage to equipment, but also affects construction safety, which is a hazard.

Due to the vibration of the pile hammer and the system composed of piles, their natural frequencies are often very low, usually a few hertz, far below the normal vibration frequency of the vibration pile hammer. Generally, vibrating pile hammers go through the "resonance" frequency region during the start-up or shutdown process, so there is a "resonance" phenomenon during both start-up and shutdown processes.

It should be noted that the "resonance" of the vibrating pile hammer refers to the "resonance" of the equipment machinery, which is a hazard. Some people understand that the resonance between piles and soil is wrong. However, under certain soil conditions, there does exist an optimal vibration pile driving frequency and an optimal vibration pile pulling frequency. Using this optimal frequency vibration for pile sinking or extraction not only results in fast sinking and extraction, low power consumption, but also reduces surface vibration sensation. This is different from the characteristics of strong vibrations when resonance occurs. So, the vibration at this optimal frequency can be called "resonance". Note that the "resonant" frequency varies depending on the soil type.

2.1.2 Resonance free technology and its significance

Introduction to resonance free technology: The core of resonance free technology for large vibration impact pile hammers is the infinitely adjustable eccentric moment technology. Under normal non working conditions, the eccentric block on the rotating shaft of the vibrating pile hammer is in a vertical downward position under the action of gravity, and the maximum eccentric moment occurs in this state; At startup, the hydraulic adjustment mechanism is used to rotate the movable eccentric body on the eccentric shaft 180 ° relative to the fixed eccentric body. In this state, the eccentric torque is zero, achieving stepless continuous control of the eccentric torque from "maximum to zero"; On the contrary, the hydraulic adjustment mechanism can also achieve seamless continuous control from 0 to maximum. Figure 4 shows a schematic diagram of the eccentric moment adjustment technology for resonance free vibration pile hammers.

The resonance free vibration pile hammer achieves zero eccentricity start and zero eccentricity stop through the stepless adjustment technology of eccentricity, avoiding the environmental problems of strong vibration and noise caused by "resonance" and the safety hazards caused by equipment damage; Simultaneously achieving zero eccentric moment starting, i.e. no-load starting, solves the problem of requiring a large capacity power supply for ordinary electric vibration pile hammers to start with eccentric moment or load, and has a significant energy-saving effect; The amplitude can be infinitely adjusted from "0-maximum" or "max-0", which can adapt to soil changes and has good pile sinking efficiency and effect. Shanghai Zhenzhong Company has three types of large resonance free vibration impact pile hammers for offshore wind power pile construction: EP800, EP1100, and EP1600. The main technical parameters are shown in Table 1.

Table1. Main technical parameters of large vibratory impact hammer

The vibration and impact characteristics of the resonance free vibration impact pile hammer are particularly reflected in the parameter of vibration impulse in Table 1. The maximum vibration impulse of the EP1600 resonance free vibration impact hammer is 88000kg/m/s, which is equivalent to the impact impulse of a free fall from a height of 1m when the weight of the hammer, fixture, and pile is 20 tons; It is also equivalent to the impact impulse of a free fall from a height of 1cm when the weight of the hammer, fixture, and pile is 200 tons.

Using a large resonance free vibration impact pile hammer to vibrate and sink the pile, due to the total weight of the hammer pile being several hundred tons, the vibration amplitude is only a few millimeters, truly achieving light hammering with a heavy hammer; Furthermore, due to the absence of collision between the hammer and the pile, the hammer and pile as a whole directly vibrate and impact the soil, causing the particle structure of the soil to be destroyed, the soil to liquefy, and the soil resistance to decrease, resulting in the sinking of the pile under the action of gravity. So, using vibration to drive piles often has the advantages of high efficiency, good results, and good pile quality.

2.2 Large scale resonance free vibration impact pile hammer linkage technology

Offshore wind power foundation piles are showing: (1) super large. The pile diameter exceeds 2m, even reaching 5m or 8m or more. (2) Super deepening. The bearing layer of pile foundation often exceeds 50m deep, even up to 100m deep. (3) Overweight. Due to the wall thickness of these super large and ultra deep piles reaching 40-80mm, the weight of the entire pile often exceeds several hundred tons, and even reaches over 1000 tons. In order to use a vibrating pile hammer with sufficient amplitude and excitation force to sink super large piles, a very large vibrating pile hammer is required, with a power of several megawatts or more and an external size of several meters or tens of meters. It is imaginable that it would be difficult to manufacture such a large vibrating pile hammer, whether from the factory's processing capacity or from supporting procurement such as specialized motors and bearings. However, even if such a super large vibrating pile hammer can be manufactured in the factory, it cannot be transported to the construction site due to its high height, width, and weight. To solve these contradictions and difficulties, vibration pile hammer multi hammer linkage technology is usually used for construction.

Multi hammer linkage refers to the use of linkage technology to make multiple vibrating pile hammers of the same specification and model into a super large vibration system, which requires high synchronization of multiple vibrating pile hammers during vibration. In order to avoid the occurrence of "resonance" during the start-up and shutdown of ultra large vibration systems, which can cause significant damage to construction equipment, especially large cranes (crane ships), and pose hidden dangers to construction safety. Shanghai Zhenzhong Company has adopted multiple large resonance free vibration pile hammers with adjustable eccentric moments of the same specifications and models for linkage. The linkage technology of multiple large resonance free vibration pile hammers with adjustable eccentric moments mainly includes rotational synchronization technology and eccentric moment (amplitude) adjustment synchronization technology. Rotational synchronization is to connect the eccentric shafts of each hammer through a coupling, ensuring that the vibration phases of each hammer are the same during operation (as shown in Figure 5). The synchronization of eccentric moment (amplitude) adjustment is ensured by sensors installed in each hammer adjustment mechanism, using sensing technology to ensure the synchronization of each adjustment mechanism when adjusting the eccentric moment action (as shown in Figure 6).

In the construction of offshore wind power pile foundations, due to spatial limitations, the linkage of multiple hammers is arranged in parallel directions.

Here are several cases of large-scale resonance free vibration impact pile hammer construction for offshore wind power piles:

Case 1: The auxiliary pile diameter of the Rudong Zhongguang Nuclear Wind Power Project is 2000mm, the pile length is 50m, and the pile weight is 55t. The pile is buried 35m into the soil, with a soil silt layer at the pile end and a standard penetration number N>35. The vibration pile hammer EP800 has an average pile driving time of 20 minutes and an average pile pulling time of 30 minutes.

Case 2: The diameter of the rock socketed pile in the Longyuan Wind Power Project on Nanri Island is 1000mm, the pile length is 30m, and the pile weight is 50t. The pile is buried 20m into the soil, and the pile end is weathered rock layer, with a standard penetration number N>80. The vibration pile hammer EP800 has an average pile driving time of 40 minutes and an average pile pulling time of 20 minutes.

Case 3: The auxiliary pile diameter of the Nanpeng Island wind power project in Guangdong is 2000mm, the pile length is 65m, and the pile weight is 95t. The pile is 28m deep into the soil, with a fine sand layer at the pile end and a standard penetration number N>40. The vibration pile hammer EP1100 has an average pile driving time of 20 minutes and an average pile pulling time of 30 minutes.

The pile is buried 30m into the soil, with a soil layer of silt at the pile end and a standard penetration count N>30. The vibration pile hammer EP1100 has an average pile driving time of 25 minutes and an average pile pulling time of 35 minutes.

Case 5: The pile diameter of the Rudong Electric Power Boosting Station project in Jiangsu is 2000mm, the pile length is 67m, and the pile weight is 90t. The pile is buried 35m into the soil, and the soil at the pile end is a layer of silt, with a standard penetration number N>50. The EP1300L double hammer linked vibration pile hammer has an average pile driving time of 15 minutes and an average pile pulling time of 20 minutes.

Case 6: The diameter of the pile in the Rudongzhong Hydropower Project is 1700mm, the length of the pile is 50m, and the weight of the pile is 49t. The pile is 42m deep into the soil, and the soil at the pile end is a layer of silt, with a standard penetration number N>45. Double hammer linked vibration pile hammer EP1300L, with an average pile driving time of 30 minutes.

Based on the above cases, large resonance free vibration impact pile hammers and ultra large resonance free vibration impact pile hammers formed by multi hammer linkage have extremely strong power, excitation force, and impact impulse, and are compact in structure and reliable in use. They are particularly suitable for pile foundation construction in large-scale projects such as offshore wind power generation, and have the characteristics of high quality, environmental protection, high efficiency, safety, and economy. They are important options for pile foundation construction in large-scale projects such as offshore wind power generation.

reference:

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[3]. He Qinghua Engineering Machinery Manual Pile Machinery [M]. Beijing: Tsinghua University Press, 2018

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Cao Rongxia: First author (contact person)

Unit: Shanghai Zhenzhong Construction Machinery Technology Co., Ltd

Research direction: Research, manufacturing, and application of pile driving machinery

Detailed mailing address: No. 2518 Jiangshan Road, Nanhui New Town, Pudong New Area, Shanghai

Phone number: 021-58286177

Mobile phone: 13806852460

Email: caorx@zz-p.com