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25 To accelerate pipe fusion during the installation process, it is a common practice to fuse every other two lengths of pipe together two or three days before the outfall installation. This reduces by half the number of joints to be fused during the installation process. 12 FLOATING, SUBMERGING AND PLACING THE OUTFALL The HDPE submarine outfall is designed to be buoyant with the anchor collars attached and the pipe filled with air and to be strongly negatively buoyant when it is subsequently filled with water. Air is contained in the pipe by means of a sealed and plug or plate securely attached to the end of the pipe. The plate or plug is fitted with an air release/inlet valve and then it is attached to the terminal end of the pipe. By gradually releasing air from this valve and allowing water to enter the outfall at the shore end. The rate of descent is controlled through the release of air at the terminal end and the controlled entrance of water at the shoreline. It is important that the pipe be sunk from the shore progressing to the terminal end so as to preclude entrapment of air in a high spot. A number of small boats are needed to tow the pipeline out from shore as it is fused together and the anchor collars are attached and to tow it into place for submergence. A general rule of thumb for the number of boats required is one boat for every 100 meters of pipe. Fewer or more may be applicable depending on water conditions. Several boats are stationed at intervals along side the pipe on the up current side of the route pulling the pipe into place. During the sinking, the boat at the terminal of the pipe operates the air release valve. The boat at the terminal end of the pipe does not have to be in place until the final length is sent into place. Three placement boats are usually required in the vicinity of the section being sunk. It is usually easier to allow the pipe to flex with the current gradually pulling it into place along the route as it is sunk. Divers on the pipeline communicate with both the three or four placement boats, which are towing the pipe into place, and the boat on the terminal end releasing the air. The divers advise the placement boats to move the pipe to right or left, or hold, so as to keep the pipe on alignment as it reaches the bottom and also advise the air release operator when to release the air to obtain descent of the pipe. Air is released in a series of short bursts with sufficient time between bursts to receive communication from the divers. It is better to release the air too slowly than too quickly.PDF Image | SMALL DIAMETER (HDPE) SUBMARINE OUTFALLS
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