GB1348487A - Method of and apparatus for burying sub-sea pipelines - Google Patents

Abstract

1348487 Jet excavating NETHERLANDS OFFSHORE CO (NEDERLANDSE MAATSCHAPPIJ VOOR WERKEN BUITENGAATS) BV 18 May 1971 [19 May 1970] 24286/70 Heading E1F [Also in Division F2P] A method of burying pipelines in the bottom of the sea includes the steps of laying a pipeline on the seabed, lowering onto the pipeline a plurality of trenching devices which are flexibly connected one behind the other to form a train so that the trenching devices straddle the pipeline, eroding the seabed material beneath the pipeline by jets of water from said trenching devices so that said pipeline sinks into said eroded seabed material under the combined weight of itself and the trenching devices, and simultaneously advancing the train of trenching devices in one direction along the pipeline such that successive trenching devices successively increase the depth of erosion into the seabed whereby the pipeline progressively sinks in an 'S' curve to the desired depth in the sea bed in one pass of the trenching devices. Each trenching device comprises a pair of parallel box section girders 31, 32 (Fig. 6) rigidly interconnected by braces 51 and having rollers 37-40 to engage the pipeline. Jets 33-36 expel water to erode the seabed. The ends of the girders divide into two channels 39, 41; 43, 45 (Fig. 7), the corresponding channels of adjacent trenching devices being connected by flexible sleeves 58, 59. Mechanical engagement is by linkage 65 which allows limited rotation about horizontal or vertical axes through the coupling. The extent of pivoting is limited by stops 61-64, and load cells 54, 55 placed therebetween allow the shape of the train (and hence the pipeline) to be determined. The centre of gravity of each tool preferably lies below the point of suspension so that the train is self righting. The train is headed by a cutting and sensing device (Fig. 10) which comprises a pair of fins 70 straddling the pipe 75 and supported on roller 72. Water jets on the fins cut into the seabed and may destroy small obstructions. Jets 73 clear surface debris from the pipe and the train is towed by a constant tension winch attached to cable 74. The presence of large obstacles may be indicated by load cells on the device of Fig. 10 or by increased load on the cable 74. The trailing tool may include vibration means for consolidating material over the pipe. Operation. The cutting tool 11 (Fig. 10) is lowered first and is supplied with water to entrench the fins, the remaining tools being lowered in succession and being operated until the whole train is straddling the pipeline. Air may be expelled from the tools via remotely controlled release valves to assist sinking of the devices. Guide wires 24 attached to base plates 25 may assist in the lowering operation, and lines 22 may be slackened. The train is towed while water is supplied to the jets, the depth of cut being controlled by the pressure of the supply. The inclination of the train to the vertical plane through the pipeline may be adjusted by supplying a different water pressure to each side of the device. The displaced seabed material may be held in suspension by further jets. Re-floating the device may be assisted by filling the box girders with air.