GB2163198A - Propulsion method and apparatus for advancing a marine pipeline excavation shield - Google Patents

Abstract

A method for burying offshore pipelines utilizes a segmented excavation shield 1 for forming a trench in the seafloor, the shield being propelled by sequentially advancing shield segments 17, 18, 19, so that the shield advances by constantly having one or more segments moving forward while the non-moving segments transmit soil reaction to the moving segment or segments. <IMAGE>

Description

SPECIFICATION Propulsion method and apparatus for advancing a marine pipeline excavation shield The invention relates to a propulsion method and apparatus for advancing a marine pipeline excavation shield.

When large diameter, relatively stiff pipes are lowered into an underwater trench, the usual elastic curvature is such that an unacceptably long span develops, and the corresponding open section of the trench may be too long to be practical unless stable slopes have been cut. For example, if a 3.5 m deep trench were in a sandy soil, the slope of the trench sidewalls might be on the order of one in three, and the amount of soil to be excavated could be five times that of a vertical sidewall ditch. Accordingly, it is not only desirable to reduce the volume of soil to be excavated, but further to prevent infilling of the soil until the pipe has reached the ditch, both to reduce power requirements and save time in construction. It is known to solve this problem by using an underwater trencher which is combined with a pipe laying guide.

With the known device a trench is formed while simultaneously an elongated shield is positioned in the trench to prevent the sidewalls of the trench from collapsing. Then, a pipeline is deflected into the trench so that the pipeline enters the trench at one end of the shield and approaches the bottom of the trench before exiting at the other end of the shield. To reduce the length of shield required the pipeline is subjected to bending strain during the pipelaying operation, but nonetheless the shield is a large piece of equipment, typically over 30 m in length. If a shield of this size is pulled (e.g. with cables) to overcome soil resistance (friction and cutting head pressure), the draw force required is typically in the order of 106N.Accordingly, it is desirable to provide means for reducing or obviating these extreme requirements of pulling force necessary for advancing the shield.

The primary purpose of the present invention is to provide a method and apparatus for the propulsion of a marine pipeline excavation shield, which greatly reduces the power requirements normally necessary to move the apparatus and which thereby saves construction time. Thus, the present invention provides a method for advancing an elongated segmented excavation shield in a soil trench comprising sequentially advancing shield segments by moving fewer segments forward at any one time than the non-moving segments can transmit soil reaction to the moving segments. Preferably, a surface is utilized on the shield segments which is resistant to rearward movement.

The present invention also pertains to an elongated excavation shield comprising a series of individual movable, connected shield segments, and means for sequentially advancing the shield segments in a soil trench by moving fewer segments at one time than the non-moving segments can transmit soil reaction to the moving segments.

Other purposes, distinctions over the art, advantages and features of the invention will be apparent to one skilled in the art upon review of the following.

Figure 1 shows an elevation view and partial section of a trench shield and excavating module of the invention.

Figures 2 and 3 show plan and end view of the trench shield in the excavating module of Fig. 1.

Figure 4 depicts one embodiment of the sequential advancement of the trench shield, wherein the various steps of advancement are illustrated in a vertical sequence.

The present invention pertains to an apparatus and method for the simultaneous trenching and installation of submarine pipelines and more particularly, to a propulsion method for moving the apparatus. The apparatus includes, in a single unit, an excavating portion, an attached ditch sidewall retention shield, and a pipeline installation guide. Individual segments comprising the shield are connected by moving means, more particularly described hereinafter, which furnishes propulsion to the shield. The excavator creates a trench of slightly larger cross section than required, the shield prevents the trench from closing due to collapse of the sidewalls, and the pipe guide causes the pipeline to be deflected into that portion of the trench protected by the shield.

The pipeline preferably is fabricated on the surface of the sea or on floating ice and is supported and guided to enter the forward portion of the shield by a stinger or other means, when required. The pipeline installation guide and excavator used may be of any suitable type known in the art and require no detailed description.

The excavation shield must readily move forward through the soil during the construction process. For practical pipe sizes and burial depths, geometrical constrains result in a very large piece of equipment, typically over 30 m in length. If such a system is pulled to overcome soil resistance, the draw force required is in the order of 106N. The propulsion method for the shield, as disclosed herein, eliminates the need to pull the device. The method may be described as an "inch worm method of advancement and may be implemented as follows. Preferably, the shield segments are sequentially advanced as shown in the embodiment of Fig. 4. Thus, the shield advances by constantly having one or more segments moving forward while the non-moving segments transmit soil reaction to the moving segments.The motion described preferably is achieved by placing double acting hydraulic rams between the segments. It is necessary that the reaction available from the stationary segments exceed the force required to advance the cutting head or overcome the static and dynamic soil friction encountered by the moving segments. This is assured by having a large number of segments.

Advantages of the "inch worm" method are, (1) there is no limiting size; the method can be scaled up as required to accomodate pipe and trench geometry; (2) there is a minimum of moving parts; and (3) a reverse control sequence permits the unit to back up.

Other improvements that add to the efficient utilization of the device include, (1) a special surface (e.g. a "fish scale" surface) on the segment to improve an advance/engage friction ratio so that larger fractions of segments can be moving at a given time, and (2) the installation of variable length segment behind the cutting mechanism which is operated in such a way that the cutting head moves forward at a constant speed while the trailing segments are advancing cyclically.

Having thus generally described the apparatus and method of the invention, as well as its advantages over the art, the following is a detailed description thereof given in accordance with specific reference to the drawings.

Figs. 1, 2 and 3 show respectively, elevation, plan and front views of the invention. A trench shield 1 having pipeline guide rollers 2 and 3, has at the forward end thereof, excavating module 4. Excavating module 4 has cutters 5 which may be lengthened if required.

Cutters 5 may be tilted or advanced or retracted into the work space by hydraulic pistons internal to the unit (not shown). The cutters 5 may also be raised or lowered relative to the module 4 as a means of adjusting the elevation of the trench bottom. Mould boards 6 and 7 at either side of the excavating module 4 serve to push soil away from the trench shield 1. Sled 8 maintains the trench shield at an appropriate level in the soil. Pipeline 9 passes over module 4, between rollers 2 and 3 and exits at the lower rear of shield 1.

Fig. 3 is a front view of the excavating module 4 and shows motors 10 and 11 which may be hydraulic or electric powered and which powers the cutters 5 and 1 2 (e.g.

augers) which are tiltable to form a trapozoidal cross section, for example, about 1 6 degrees apart, to clear the suspended pipeline 9 in the event cutters 5 and 1 2 are to be replaced while the module 4 remains in the trench.

As shown in Fig. 1 and 2 hydraulic rams 1 3, 14 and 15, or other prime mover means such as a symmetrically threaded shaft, provide motivation or propulsion for the device.

These rams may be actuated by hydraulic fluid (not shown) or by electric means, e.g. a motor (not shown). In either event, the rams operate to alternate or sequentially push the shield segments apart and then pull them back together. Thus, ram 1 3 has segments 1 6 and 1 7 pushed apart while rams 14 and 15 have segments 17 and 18, and 18 and 19, respectively, pulled together. This operation may be done sequentially as shown in Fig. 4, wherein the third line Ill of segments 20 shows how the first segment 23 is advanced, and then each subsequent line of segments shows advancing an additional segment until the sequence reaches line VIII which is a complete cycle. Line IX is the same as line Ill and the sequence then starts over.

Manifestly, while this type of sequence is a preferred sequence, the operation can be conducted by advancing various combinations of segments together.

In yet another embodiment not shown in Fig. 4, which would be the same as having the advance segment 23 (or cutter section as shown in Fig. 1) always spaced ahead of the trailing segment, the advance segment (or cutter section) can be made to move forward at a constant rate rather than in a cyclical mode by using a special prime mover means.

Claims (7)

1. A method for advancing an elongated, segmented excavation shield in a soil trench comprising: sequentially advancing shield segments by moving fewer segments forward at any one time than the non-moving segments can transmit soil reaction to the moving segments.

2. The method of claim 1 including utilizing a surface on the shield segments which is resistant to rearward movement.

3. The method of claim 1 including advancing the forward end of the shield at constant speed while advancing trailing segments cyclically.

4. An elongated excavation shield comprising: a series of individually movable, connected shield segments; and means for sequentially advancing the shield segments in a soil trench by moving fewer segments at any one time than the nonmoving segments can transmit soil reaction to the moving segments.

5. The excavation shield of claim 4 including a surface on the shield segments which is resistant to rearward movement.

6. A method according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

7. An elongated excavation shield according to claim 4 substantially as hereinbefore described with reference to the accompanying drawings.