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Construction technology of offshore deepwater steel pipe piles

September 24, 2024

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1. Production of steel pipe piles and steel casing

The steel pipes used for steel pipe piles and the steel casing used for the underwater part of boreholes are both rolled on site. Generally, steel plates with a thickness of 10-14mm are selected, rolled into small sections, and then welded into large sections. Each section of steel pipe is welded with inner and outer rings, and the width of the weld seam is not less than 2cm.

2. Floating box assembly

A floating box is the foundation of a floating crane, consisting of several small steel boxes. The small steel box has a rectangular shape with rounded corners at the bottom and a rectangular shape at the top. The steel plate of the box is 3mm thick and has a steel partition inside. The top is welded with angle steel and steel plate with bolt holes and locking holes. The small steel boxes are connected to each other through bolts and locking pins, and anchor bolt holes are reserved at the top to connect and fix anchor machines or other equipment that needs to be fixed.

Use a car crane to lift the small steel boxes into the water one by one on the shore, and assemble them into a large floating box by connecting them with bolts and locking pins.

3. Floating crane assembly

The floating crane is a lifting device for water operation, which is composed of a floating box and a CWQ20 dismountable mast crane. From a distance, the main body of the floating crane is a tripod. The crane structure is composed of boom, column, slant support, rotary table base and cab. The foundation of the turntable base is basically a regular triangle, and three winches are located in the center of the tail of the floating crane.

 

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4. Set up an underwater platform

(1) Floating crane anchoring; Firstly, use a floating crane to anchor the anchor at a distance of 60-100m from the design pile position, and use a float as a marker.

(2) Guiding ship fixation: When positioning the guiding ship, a motorized boat is used to push the guiding ship to the designed pile position and anchor it. Then, four winches (commonly known as anchor machines) on the guiding ship are used to position the guiding ship under measurement command, and the telescopic anchor machine is used to accurately release the pile position of each steel pipe pile on the guiding ship according to its layout position, and the positioning frame is installed in sequence.

(3) Under the steel pipe pile: After the guiding ship is positioned, the motorized boat will transport the welded steel pipe pile to the pier position by transport ship and dock the floating crane.

Lift the steel pipe pile, mark the length on the steel pipe, insert it from the positioning frame, and slowly sink it by its own weight. After confirming the length mark on the steel pipe and entering the riverbed, check the verticality and make correction. Lift the electric vibration hammer, place it on the top of the steel pipe and clamp it on the steel plate. Start the vibration hammer to vibrate the steel pipe pile until the steel pipe rebounds, then it can be considered to have entered the weathered rock and the vibration sinking can be stopped. Observe the verticality at all times during the driving process.

(4) The construction platform has been completed: the steel pipe piles have been driven and the platform has been built according to the platform design.

5. Burial steel casing

Accurately determine the pile position on the platform and place the guide frame. A section of the casing that enters the riverbed is symmetrically welded with a clamp plate on the outer side of the top. It is lifted by a floating crane with a shoulder pole beam. The casing passes through the guide frame and slowly sinks by its own weight. The clamp plate is clamped on the guide frame. The next section of the casing is lifted using the same method and welded to the previous section. After the casing is long enough, it will sink due to its own weight. If it no longer sinks, it will be welded and replaced at the top of the casing, and a vibration hammer will be used to vibrate and sink. When the casing rebounds significantly, it will continue to sink for 5 minutes before stopping sinking.

6. Construction of drilled piles

After the casing is buried, the drilling rig is lifted into place for drilling construction. Connect the casing to the mud pit using a mud tank and place it on the platform. The mud pit is a steel box made of steel plates and welded onto a platform.

7. Clear hole

To ensure successful infusion, the gas lift reverse circulation method is used to replace all the mud in the hole with clean water. The main equipment for air lift reverse circulation includes one 9m ³ air compressor, one 20cm slurry steel pipe, one 3cm air injection hose, and two mud pumps. Open an inclined opening upwards 40cm from the bottom of the steel pipe and connect it to an air hose. When cleaning the hole, lower the slurry steel pipe to 40cm from the bottom of the hole, and use two water pumps to continuously send clean water into the hole. Start the air compressor and use the principle of reverse circulation to spray water from the upper opening of the slag steel pipe. During the construction process, it is necessary to ensure that the water head inside the hole is 1.5-2.0m above the river water level to reduce the external pressure on the casing wall. The cleaning of the borehole should be carried out carefully, and the thickness of sediment at the bottom of the borehole should not exceed 5cm. Before infusion (after the installation of the catheter), check the sedimentation inside the hole. If it exceeds the design requirements, perform a second cleaning of the hole using the same method to ensure that the sedimentation thickness is less than the specified value.

8. Concrete pouring

The concrete used for drilling piles is mixed in a centralized manner at the mixing plant and transported by concrete tankers to the temporary dock. Set up a chute at the temporary dock, and the concrete slides from the chute into the hopper on the transport ship. The transport ship then drags the hopper to the pier and lifts it with a floating crane for pouring. The conduit is generally buried at a depth of 4-5 meters to ensure the compactness of the concrete. It is necessary to ensure that each transportation time does not exceed 40 minutes and to ensure the slump of the concrete.

9. Platform dismantling

The pile foundation construction is completed, and the platform is dismantled from top to bottom. The pipe pile shall be pulled out after the removal of transverse and longitudinal beams and slant support. The floating crane lifting vibration hammer directly clamps the pipe wall, starts the vibration hammer, and slowly lifts the hook while vibrating to remove the pipe pile. Divers went into the water to cut off the pipe piles connected to the concrete and bedrock.

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