Marine natural gas must be transported by
offshore pipelines, and crude oil produced in shallow water can be directly loaded into tankers by production platforms. However, the crude oil produced in the deep sea, the large oil tanker docking on the production platform will threaten the platform safety, so there is a single point mooring in the sea dedicated to docking large oil tankers. In this way, there must be an oil pipeline connecting the production platforms to a single point mooring.
The marine pipeline laying operation is developed by the construction method of land pipelines crossing rivers and lakes. There are three main methods of pipe laying: pipe laying, towing, and drum boat laying. Which operation method is selected depends on the diameter of the pipe, the depth of the seawater, the sea conditions, and the distance from the shore.
① Pipelaying vessel laying
This is the most commonly used method. The operation process is that after the pipe passes the onshore prefabrication plant and the cement weight layer, it is transported to the pipe laying ship by ship, and the pipe is assembled and welded section by section. When the welded pipe section is moved forward, the pipe is managed by the stern The shelf slides into the sea. During the whole pipe laying operation, the length of the pipe section sliding down must be synchronized with the displacement of the ship, and the pipe laying ship must be in a relatively stable state. For this reason, 4 to 6 anchors are arranged at the front, rear, left and right of the pipe laying ship, and the ship can be stabilized by adjusting the tightness of the anchor rope; the ship position can be adjusted by adjusting the length of the anchor rope. When the pipe section slides from the tail of the stinger to the bottom of the sea, it is suspended in the seawater to form an S-shape that turns from an upper arch to a lower curve, so that the pipe section is subject to complex bending stress. Therefore, the ship has the ability to clamp the pipe section with sufficient tension machine, so that it cannot slide freely, and the pipe section slides in accordance with the displacement distance of the ship.
This method first appeared in the 1950s. When developing oil and gas fields in shallow waters, a river channel capable of passing shallow water vessels was artificially created, and the pipes were assembled on a pipe-laying barge assembled with floating tanks. When the barge moves backwards, the welded pipe section slides into the water. This pipe-laying barge has gradually developed into a large-scale pipe-laying ship. In 1956, the first larger pipelayer was put into service. In 1979, the semi-submersible "Castro" pipe-laying vessel successfully laid 500 mm in the 608-meter-deep sea area when constructing a gas pipeline from Algeria in Africa to Tunisia through Tunis Strait to Europe and Italy. The diameter of the pipe.
② Laying method
In recent years, the traction method has been used to lay pipelines in shallow waters, and has been widely used. This method does not require the use of pipe laying ships and trenchers, and can reduce many auxiliary vessels at a lower cost. The operation method is generally: first assemble the pipe on the coast into a 1-2 km long pipe section, and then tow the pipe section to the sea with a tugboat, tow it to a predetermined location one by one, and dock in the sea to form a whole pipeline.
In calmer waters, pipe sections can be floated and towed on or under the water; in seas with strong winds and waves, they can be towed on the ocean floor. However, in this method, care must be taken to ensure construction quality. If a failure occurs during construction, large vessels are still required to rule it out.
③Reel boat laying
The pipe is wound on a reel with a large diameter on the ship, and the pipe is unwound from the reel and sunk into the sea floor when laying. This method is mostly used for laying pipes with smaller diameters, and its advantages are fast progress and good continuity. A 400 mm diameter pipe is being studied in this way.
Reel laying was used in World War II. This method was used when the wartime oil pipeline across the English Channel was quickly laid in 1944. At that time, armored flexible pipes made of copper-tin-lead alloy were wound on the ship's reels, just like laying submarine cables. The pipes were laid on the sea floor. A total of 12 pipes with a diameter of 77 mm were laid.
