System for entrenching submerged elongated structures

• US 4,330,225 A
• Filed: 09/24/1979
• Issued: 05/18/1982
• Est. Priority Date: 09/24/1979
• Status: Expired due to Term

• Current Assignee: Santa Fe International Corp.
• Original Assignee: Santa Fe International Corp.
• Inventors: Glasgow, James S.
• Primary Examiner(s): Pate, III, William F.
• Application Number:
  US06/078,184
• Time in Patent Office: 967 Days
• Field of Search: 405/157, 405/159-164
• US Class Current: 405/160
• CPC Class Codes: E02F 5/104 for burying conduits or cables in trenches under water floating substructures per se E02F9/06
• CPC Class Codes: E02F 5/107 using blowing-effect devices, e.g. jets digging devices using a blowing effect per se E02F3/9206

Abstract

A system for excavating a trench in the sea bottom and removing the excavated spoil from the trench in order to enable an elongated structure, such as a pipeline, to settle into the trench is disclosed along with a corresponding method of operating such system. The system includes a sea sled which can be towed along the bottom of the sea and a towing vessel for towing the sled along the sea bottom. The sea sled includes a frame to be disposed over the area in which the trench is to be formed, a fluid jet excavator mounted on the frame and located adjacent to the rear end of the sea sled and a spoil removal mechanism mounted on the frame and located aft of the fluid jet excavator. Both the center of buoyancy and the center of gravity of the sled are located in front of the fluid jet excavator. The towing vessel is coupled to the sled through a tow line connecting mechanism and is coupled in such manner that the effects of any heaving and/or pitching movements of the vessel upon the fluid jet excavator sled are minimized. The spoil removal machanism is appropriately constructed so that the spoil that is removed from the trench is deposited at a sufficient distance from the sides of the sled so as to minimize the possibility of the spoil immediately slipping back into the trench.

Claims

1. Apparatus for use in excavating a trench in a sea bottom and for removing spoil formed during the excavation, said apparatus comprising:

a base structure capable of being towed along a sea bottom, said base structure including;

two pontoons, each having a sufficiently high mass concentrated at its front end so that the center of gravity of said base structure is approximately at the longitudinal center of said base structure when said base structure is out of water;

a frame for disposition over the area in which the trench is to be formed;

tow line connecting means arranged at the forward section of said base structure;

fluid jet excavating means mounted on said frame and located adjacent to the rear end of said base structure;

spoil removal means mounted on said frame and located aft of said fluid jet excavating means;

said spoil removal means including at least one eductor for sucking up spoil excavated by said fluid jet excavating means and a discharge tube coupled to an output end of said eductor, said discharge tube extending past the side of said base structure by approximately a distance L where ##EQU3## where w equals the average width of the trench to be excavated and d equals the depth of the trench to be excavated; and

said base structure having its center of buoyancy and center of gravity located so that said base structure rides along substantially undisturbed soil on the sea bottom during the excavating operation; and

a towing vessel capable of moving along the water surface and having towing means coupled to said tow line connecting means so that said towing vessel can tow said base structure along the sea bottom.

3. Trenching apparatus according to claim 1 wherein said discharge tube is sufficiently long so as to deposit the spoil created by the operation of said fluid jet excavating means and sucked up by said eductor at a sufficient distance to the side of said base structure so that the spoil is unlikely to immediately reenter the trench being formed before.

4. Trenching apparatus according to claim 1 wherein said fluid jet excavating means includes at least one jet manifold tube having a plurality of output nozzles extending therefrom for directing water towards the surface to be excavated.

5. Trenching apparatus according to claim 4 wherein said discharge tube has its output end sufficiently spaced to the side of said base structure so that said output end lies over the center of the intended spoil pile.

6. A trenching apparatus according to claim 1 or 2 wherein said fluid jet excavating means includes at least one jet manifold tube having a plurality of output nozzles extending therefrom for directing water towards the surface to be excavated.

7. Trenching apparatus according to claim 6 wherein said jet manifold tube can be pivoted so that its upper end leans towards the front of said base structure and the surface being excavated.

8. Trenching apparatus according to claim 6 wherein said nozzles are concentrated near the lower portion of said jet manifold tube.

9. Trenching apparatus according to claim 6 wherein during utilization of said base structure for excavation, the majority of said nozzles face towards the direction of movement of said base structure and some of said nozzles face in a transverse direction towards a plane along the longitudinal axis of said base structure.

10. Trenching apparatus according to claim 6 wherein said jet manifold tube is an L-shape tube having a vertical portion and a transverse portion extending from the lower end of said vertical portion and said nozzles extend along the same side of both said vertical and transverse portions of said jet manifold tube in such a direction that said nozzles face in the direction of movement of said base structure during the excavating operation;

and further comprising means for causing said transverse portion of said jet manifold tube to extend along the longitudinl axis of said base structure both before and after the excavation operation and to extend towards a plane along the longitudinal axis of said base structure during the excavating operation.

11. Trenching apparatus according to claim 6 further comprising flow detection means for detecting the flow through said discharge tube and density detection means for detecting the density of the output spoil flowing through said discharge tube.

12. Trenching apparatus according to claim 1 further comprising flow detection means for detecting the flow through sid discharge tube and density detection means for detecting the density of the output spoil flowing through said discharge tube.

16. Trenching apparatus according to claim 1 wherein each of said pontoons is constructed so as to receive an increasing quantity of ballast when said base structure is lowered through the water to the sea bottom.

17. Trenching apparatus according to claim 16 wherein each of said pontoons expels the ballast received when said base structure is raised from the sea bottom to the water surface.

2. Apparatus for use in excavating a trench in a sea bottom and for removing spoil formed during the excavation, said apparatus comprising:

a base structure capable of being towed along a sea bottom, said base structure including;

a frame for disposition over the area in which the trench is to be formed;

tow line connecting means arranged at the forward section of said base structure;

fluid jet excavating means mounted on said frame and located adjacent to the rear end of said base structure;

spoil removal means mounted on said frame and located aft of said fluid jet excavating means; and

said base structure having its center of buoyancy and center of gravity located so that said base structure rides along substantially undisturbed soil on the sea bottom during the excavating operation;

a towing vessel capable of moving along the water surface and having towing means coupled to said tow line connecting means so that said towing vessel can tow said base structure along the sea bottom;

said frame includes an assembly pivotable about an axis lying adjacent to a plane along the front end of said base structure and said pivotable assembly extending rearwardly along said base structure; and

said fluid jet excavating means and said spoil removal means are attached to said pivotable assembly so taht said fluid jet excavating means and said spoil removal means are vertically adjustable with respect to said base structure.

13. Trenching apparatus according to claim 2 wherein:

said spoil removal means includes at least one eductor for removing spoil excavated by said fluid jet excavating means and a discharge tube coupled to an output of said eductor for depositing the excavated spoil at a distance spaced from the trench that is excavated;

said fluid jet excavating means including at least one jet manifold tube having a plurality of output nozzles extending therefrom for directing water towards the surface to be excavated; and

said frame including an upstanding support mounted between said pontoons and said pivotable assembly is connected to a top portion of said upstanding support and is pivotable about said upstanding support.

14. Trenching apparatus according to claim 13 further comprising flow detection means for detecting the flow through said discharge tube and density detection means for detecting the density of the output spoil flowing through said discharge tube.

15. Trenching apparatus according to claims 13 or 14 wherein:

said fluid jet excavating means includes two jet manifold tubes;

said spoil removal means including two eductors and two corresponding discharge tubes, each of said discharge tubes being coupled to an output of said corresponding eductor;

each of said jet tubes and eductors being mounted on said frame so as to be capable of being arranged on opposite sides of an elongated member that is to be entrenched in the trench to be formed.

18. An apparatus for entrenching a submerged elongated structure such as a pipeline in the soil along the bottom of the sea, said apparatus comprising:

a sled structure capable of being towed along the sea bottom so as to straddle the elongated structure to be entrenched for excavating a trench under such elongated structure so that such elongated structure can eventually settle into the trench, said sled structure including;

a frame;

fluid jet excavating means mounted on said frame and located adjacent to the rear end of said sled structure;

spoil removal means mounted on said frame and located aft of said fluid jet excavating means, said spoil removal means including a discharge tube for discharging soil excavated from the trench at a location spaced to the side of said frame of said sled structure, said discharge tube extending a minimum distance dependent upon the volume of soil to be removed from the trench; and

said sled structure having its center of bouyancy and center of gravity located so that said sled structure rides along substantially undisturbed soil on the sea bottom during the excavating operation; and

a toward vessel capable of moving along the water surface and having towing means coupled to said sled structure so that said towing vessel can tow said sled structure along the sea bottom.

19. Trenching apparatus according to claim 18 wherein said towing means is coupled to said sled structure so as to minimize any effects upon the vertical positioning with respect to the trench of said fluid jet excavating means and said spoil removal means due to any heaving and pitching movements of said vessel.

20. Trenching apparatus according to claims 18 or 19 wherein:

the front end of said frame lies approximately adjacent to a plane along the front end of said sled structure;

said frame has a towline connecting means at its front end; and

said towing means includes a tow cable connected between said vessel and said towline connecting means.

21. Trenching apparatus according to claim 18 further comprising position determining means for determining the relative angular and vertical positioning of said sled structure to the elongated structure being entrenched as said sled structure moves along the elongated structure and warning means for generating a warning signal if said sled structure comes into contact with the elongated structure.

22. Trenching apparatus according to claims 18 or 19 wherein said fluid jet excavating means includes two jet manifold tubes;

said spoil removal means includes two eductors and two corresponding discharge tubes, each of said discharge tubes being coupled to an output of said corresponding eductors; and

each of said jet tubes and eductors and mounted on said frame so as to be capable of being arranged on opposite sides of the elongated structure that is to be entrenched in the trench to be formed.

23. Trenching apparatus according to claim 22 further comprising position determining means for determining the relative vertical positioning of said eductors to the elongated structure, said position determining means including warning means for generating a warning signal if said sled structure comes into contact with the elongated structure.

24. Trenching apparatus according to claim 18 wherein said sled structure includes two pontoons, each of said pontoons having a sufficiently high mass concentrated at its front end so that the center of gravity of said sled structure is approximately at the center of the longitudinal axis of said sled structure when said sled structure is out of the water.

25. Trenching apparatus according to claim 24 wherein:

said frame includes an assembly pivotable about an axis lying adjacent to a plane along the front end of said sled structure and said pivotable assembly extending rearwardly along said sled structure; and

said fluid jet excavating means and said spoil removal means are attached to said pivotable assembly so that said fluid jet excavating means and said spoil removal means are vertically adjustable with respect to said sled structure.

26. Trenching apparatus according to claims 18 or 25 wherein said spoil removal means includes two eductors for sucking up spoil excavated by said fluid jet excavating means and two discharge tubes each being coupled to a respective one of said eductors, said discharge tubes extending past the sides of said sled structure by approximately a distance L where ##EQU4## where w equals the average width of the trench to be excavated and d equals the depth of the trench to be excavated.

27. Trenching apparatus according to claim 26 wherein said discharge tube is sufficiently long so as to deposit the spoil created by the operation of said fluid jet excavating means and sucked up by said eductor at a sufficient distance to the side of said sled structure so that the spoil is unlikely to immediately reenter the trench being formed.

28. Trenching apparatus according to claims 19, 24 or 27 wherein said fluid jet excavating means includes at least one jet manifold tube having a plurality of output nozzles extending therefrom for directing water towards the surface to be excavated and said jet tube can be pivoted so that its upper end leans towards the front of said sled structure and the surface being excavated.

29. Trenching apparatus according to claim 28 wherein said nozzles are concentrated near the lower portion of said jet tube.

30. Trenching apparatus according to claims 18 or 26 further comprising flow detection means for detecting the flow through said spoil removal means and density detection means for detecting the density of the output spoil flowing through said spoil removal means.

31. An apparatus for entrenching a submerged, elongated structure such as a pipeline in the soil along the bottom of the sea, said apparatus comprising:

a sled structure capable of being towed along the sea bottom so as to straddle the elongated structure to be entrenched for excavating a trench along such elongated structure so that such elongated structure can eventally settle into the trench, said sled structure including;

a frame;

fluid jet excavating means mounted on said frame;

spoil removal means mounted on said frame and located aft of said fluid jet excavating means, said spoil removal means including two eductors for sucking up spoil excavated by said fluid jet excavating means and two discharge tubes each being coupled to a respective one of said eductors, said discharge tubes extending past the sides of said sled structure by approximately a distance L where ##EQU5## where w equals the average width of a trench to be excavated and d equa the depth of a trench to be excavated; and

,

a towing vessel capable of moving along the water surface and having towing means coupled to said sled structure so that said towing vessel can tow said sled structure along the bottom.

32. Trenching apparatus according to claim 31 wherein said frame includes an assembly pivotable about an axis lying adjacent to a plane along the front end of said sled structure and said pivotable assembly extending rearwardly along said sled structure;

and said fluid jet excavating means and said spoil removal means are attached to said pivotable assembly so that said fluid jet excavating means and said spoil removal means are vertically adjustable with respect to said sled structure.

33. Trenching apparatus according to claims 31 or 32 wherein said fluid jet excavating means is located adjacent to the rear end of said sled structure and said towing means is coupled to an upper front portion of said sled structure.

34. Trenching apparatus according to claim 31 wherein said fluid jet excavating means includes two jet tubes and each of said jet tubes and eductors are mounted on said frame so as to be capable of being arranged on opposite sides of the elongated structure that is to be entrenched in the trench to be formed.

35. Trenching apparatus according to claim 34 wherein said jet tubes and said eductors are capable of being extended in length in order to increase the depth of the trench formed during the excavation operation of said sled structure.

36. Trenching apparatus according to claim 31 wherein said sled structure includes two pontoons, each of said pontoons having a sufficiently high mass concentrated at its front ends so that the center of gravity of said sled structure is approximately at the center of the longitudinal axis of said sled structure when said sled structure is out of the water.

37. Trenching apparatus according to claim 36 wherein said fluid jet excavating means is mounted on said frame at a location adjacent to the rear end of said sled structure.

38. Trenching apparatus according to claim 31 further comprising guide rollers mounted on said frame for guiding said sled structure along the elongated structure and position determining means for determining the relative vertical positioning of said eductors to the elongated structure, said position determining means including a plurality of load cells housed within cylindrical casings and said cylindrical casings serving as the rotational axles for said guide rollers.

39. Trenching apparatus according to claims 33, 36 or 38 wherein said sled structure includes two pontoons, each of said pontoons having a sufficiently high mass concentrated at its front end so that the center of gravity of said sled structure is approximately at the center of the longitudinal axis of said sled structure when said sled structure is out of the water and said pontoons are constructed so as to automatically receive an increasing quantity of ballast when said sled structure is lowered through the water to the sea bottom and to automatically expel the ballast received when said sled structure is raised from the sea bottom to the water surface.

40. Trenching apparatus according to claim 39 wherein said fluid jet excavating means is located adjacent to the rear end of said sled structure.

41. Trenching apparatus according to claim 40 wherein said frame includes an assembly pivotable about an axis lying adjacent to a plane along a front end of said sled structure and said pivotable assembly extending rearwardly along said sled structure and said fluid jet excavating means and said spoil removal means are attached to said pivotable assembly so that said fluid jet excavating means and said spoil removal means are vertically adjustable with respect to said sled structure.

42. A sea sled for entrenching a submerged elongated structure such as a pipeline in the soil along the bottom of the sea, said sea sled being capable of being towed along the sea bottom so as to straddle the elongated structure to be entrenched for excavating a trench under such elongated structure so that such elongated structure can eventually settle into the trench, said sled structure comprising:

a frame;

fluid jet excavating means mounted on said frame and located adjacent to the rear end of said sled structure;

spoil removal means mounted on said frame and located aft of said fluid jet excavating means, said spoil removal means causing soil excavated from the trench to be displaced to a location spaced to the side of said frame by a minimum distance dependent upon the volume of soil to be removed from the trench;

said spoil removal means including at least one eductor for sucking up spoil excavated by said fluid jet excavating means and a discharge tube coupled to an output end of said eductor, said discharge tube extending past the side of said base structure by approximately a distance L where ##EQU6## where w equals the average width of the trench to be excavated and d equals the depth of the trench to be excavated; and

said sled structure having its center of buoyancy and center of gravity located in front of said fluid jet excavating means so that said sled structure rides on substantially undisturbed soil on the sea bottom during the excavating operation.

43. A sea sled according to claim 42 further comprising towline connecting means for coupling a towing vessel to an upper front portion of said sea sled.

44. A sea sled according to claim 43 wherein the front end of said frame lies approximately adjacent to a plane along the front end of said sled structure and said frame has said towline connecting means at its front end.

45. A sea sled according to claims 42 or 44 further comprising position determining means for determining the relative angular and vertical positioning of said sled structure to the elongated structure being entrenched as said sled structure moves along the elongated structure and said position determining means including warning means for generating a warning signal if said sled structure comes into contact with the elongated structure.

46. A sea sled according to claim 42 wherein said fluid jet excavating means includes two jet tubes;

said spoil removal means includes two eductors and two corresponding discharge tubes, each of said discharge tubes being coupled to an output of said corresponding eductor; and

each of said jet tubes and eductors being mounted on said frame so as to be capable of being arranged on opposite sides of the elongated structure that is to be entrenched in the trench to be formed.

47. A sea sled according to claim 42 wherein said sled structure includes two pontoons, each of said pontoons having a sufficiently high mass concentrated at its front end so that the center of gravity of said sled structure is approximately in juxtaposition with the center of the longitudinal axis of said sled structure when said sled structure is out of the water.

48. A sea sled according to claim 47 wherein:

said frame includes an assembly pivotable about an axis lying adjacent to a plane along the front end of said sled structure and said pivotable assembly extending rearwardly along said sled structure; and

said fluid jet excavating means and said spoil removal means are attached to said pivotable assembly so that said fluid jet excavating means and said spoil removal means are vertically adjustable with respect to said sled structure.

49. A sea sled according to claim 42 or 48 wherein said spoil removal means includes two eductors for sucking up spoil excavated by said fluid jet excavating means and two discharge tubes each being coupled to a respective one of said eductors, said discharge tubes extending past the sides of said sled structure by approximately a distance L where ##EQU7## where w equals the average width of the trench to be excavated and d equals the depth of the trench to be excavated.

50. A sea sled according to claim 49 wherein said discharge tube is sufficiently long so as to deposit the spoil created by the operation of said fluid jet excavating means and sucked up by said eductor at a sufficient distance to the side of said sled structure so that the spoil is unlikely to immediately reenter the trench being formed.

51. A sea sled according to claim 42, 47 or 50 wherein said fluid jet excavating means includes at least one jet tube having a plurality of output nozzles extending therefrom for directing water towards the soil surface to be excavated and said jet tube can be pivoted so that its upper end leans towards the front end of said sled structure and the surface being excavated.

52. A sea sled according to claim 42 or 49 further comprising flow detection means for detecting the flow through said spoil removal means and density detection means for detecting the density of the output spoil flowing through said spoil removal means.

53. Sea sled for entrenching a submerged, elongated structure such as a pipeline in the soil along the bottom of the sea, said sled being capable of being towed along the sea bottom so as to straddle the elongated structure to be entrenched for excavating a trench along such elongated structure so that such elongated structure can eventually settle into the trench, said sled comprising:

a pontoon structure;

a frame mounted on said pontoon structure;

fluid jet excavating means mounted on said frame; and

spoil removal means mounted on said frame and located aft of said fluid jet excavating means, said spoil removal means including two eductors for sucking up spoil excavated by said fluid jet excavating means and two discharge tubes each being coupled to a respective one of said eductors, said discharge tubes extending past the sides of said pontoon structure by approximately a distance L where ##EQU8## where w equals the average width of the trench to be excavated and d the depth of a trench to be excavated.

54. A sled according to claim 53 wherein said frame includes an assembly pivotable about an axis lying adjacent to a plane along the front end of said sled and said pivotable assembly extending rearwardly along said sled;

and said fluid jet excavating means and said spoil removal means are attached to said pivotable assembly so that said fluid jet excavating means and said spoil removal means are vertically adjustable with respect to said pontoon structure.

55. A sled according to claim 53 or 54 wherein said fluid jet excavating means is located adjacent to the rear end of said pontoon structure and said sled further comprising towline coupling means for coupling said sled from an upper front portion thereof to a towing vessel.

56. A sled according to claim 53 wherein said fluid jet excavating means includes two jet tubes and each of said jet tubes and eductors are mounted on said frame so as to be capable of being arranged on opposite sides of the elongated structure that is to be entrenched in the trench to be formed.

57. A sled according to claim 56 wherein said jet tubes and said eductors are capable of being extended in length in order to increase the depth of the trench formed during the excavation operation of said sled.

58. A sled according to claim 53 wherein said pontoon structure includes two pontoons, each of said pontoons having a sufficiently high mass concentrated at its front ends so that the center of gravity of said sled is approximately at the center of the longitudinal axis of said sled when said sled is out of the water.

59. A sled according to claim 58 wherein said fluid jet excavating means is mounted on said frame at a location adjacent to the rear end of said sled.

60. A sled according to claim 53 further comprising guide rollers mounted on said frame for guiding said sled along the elongated structure and position determining means for determining the relative vertical positioning of said eductors to the elongated structure, said position determining means being mounted in said guide rollers and including a plurality of load cells housed within cylindrical casings and said cylindrical casings serve as the rotational axles for said guide rollers.

61. A sled according to claim 53 wherein said pontoon structure includes two pontoons, each of said pontoons having a sufficiently high mass concentrated at its front end so that the center of gravity of said sled is approximately at the center of the longitudinal axis of said sled when said sled is out of the water and said pontoons are constructed so as to automatically receive an increasing quantity of ballast when said sled is lowered through the water to the sea bottom and to automatically expel the ballast received when said sled is raised from the sea bottom to the water surface.

62. A sled according to claim 61 wherein said fluid jet excavating means is located adjacent to the rear end of said sled.

63. A sled according to claim 62 wherein said frame includes an assembly pivotable about an axis lying adjacent to a plane along a front end of said sled and said pivotable assembly extending rearwardly along said sled and said fluid jet excavating means and said spoil removal means are attached to said pivotable assembly so that said fluid jet excavating means and said spoil removal means are vertically adjustable with respect to said sled.

64. A sea sled for entrenching a submerged elongated structure such as a pipeline in the soil along the bottom of the sea, said sea sled being capable of being towed along the sea bottom so as to straddle the elongated structure to be entrenched for excavating a trench under such elongated structure so that such elongated structure can eventually settle into the trench, said sled comprising:

a frame;

fluid jet excavating means mounted on said frame;

spoil removal means mounted on said frame and located aft of said fluid jet excavating means, said spoil removal means including a discharge tube for discharging soil excavated from the trench at a location spaced to the side of said frame, said discharge tube extending a minimum distance dependent upon the volume of soil to be removed from the trench; and

said sled structure having its center of buoyancy and center of gravity located in front of said fluid jet excavating means so that said sled structure will ride on substantially undisturbed soil on the sea bottom during an excavating operation and having its center of gravity located approximately at its longitudinal center so that said sled structure will be balanced when said sled structure is suspended out of the water.

65. A sea sled according to claim 64 further comprising means for increasing the weight in the forward portion of said sled structure as said sled structure is lowered into the water to the sea bottom.

66. A sea sled according to claim 65 further comprising towline connecting means for coupling said sea sled from an upper portion thereof to a towing vessel.

67. A sea sled according to claim 66 wherein the front end of said frame lies approximately adjacent to a plane along the front end of said sled structure and said frame has said towline connecting means at its front end.

68. A sea sled according to claim 65 further comprising position determining means for determining the relative angular and vertical positioning of said sled structure to the elongated structure being entrenched as said sled structure moves along the elongated structure.

69. A sea sled according to claim 64 wherein said sled structure includes two pontoons, each of said pontoons having a sufficiently high mass concentrated at its front end so that the center of gravity of said sled structure is approximately at the center of the longitudinal axis of said sled structure when said sled structure is out of the water.

70. A sea sled according to claim 69 wherein:

said frame includes an assembly pivotable about an axis lying adjacent to a plane along the front end of said sled structure and said pivotable assembly extending rearwardly along said sled structure; and

said fluid jet excavating means and said spoil removal means are attached to said pivotable assembly so that said fluid jet excavating means and said spoil removal means are vertically adjustable with respect to said sled structure.

71. A sea sled according to claim 64 or 70 wherein said spoil removal means includes two eductors for sucking up spoil excavated by said fluid jet excavating means and two discharge tubes each being coupled to a respective one of said eductors, said discharge tubes extending past the sides of said sled structure by approximately a distance L where ##EQU9## where w equals the average width of the trench to be excavated and d equals the depth of the trench to be excavated.

72. A method for entrenching a pipeline in the soil along the bottom of the sea utilizing a trenching system including a sea sled having a pontoon structure, a frame mounted on the pontoon structure, a fluid jet excavating mechanism mounted on the frame at a location adjacent to the rear end of the sled and a spoil removal mechanism mounted on the frame at a location aft of the fluid jet excavating mechanism and a towing vessel capable of moving along the water surface having a towline connection mechanism for coupling the vessel to the sled and towing the sled along the sea bottom, the entrenching method comprising the steps of:

lowering the sled into the water to the sea bottom;

arranging the sled on the sea bottom in a position so as to straddle the pipeline to be entrenched;

excavating a trench under the pipeline so that the pipeline can eventually settle into the trench;

removing the excavated spoil from the trench and transporting such spoil through the spoil removal means by a distance L where ##EQU10## where w equals the average width of the trench and d equals the depth of the trench, before the spoil leaves the spoil removal mechanism; and

towing the sled along the sea bottom.

73. A method according to claim 72 further comprising the step of increasing the ballast in the forward end of the sled as the sled is lowered through the water to the sea bottom.

74. A method according to claim 73 further comprising decreasing the ballast in the sled as the sled is lifted from the sea bottom back to the surface of the water.

75. A method according to claim 72, 73 or 74 further comprising the step of depositing the spoil removed from the trench at a sufficient distance away from the sides of the trench so that the spoil is unlikely to immediately reenter the trench being formed.

76. A method according to claim 72, 73 or 74 further comprising the step of determining the relative angular and vertical positioning of the sled with respect to the pipeline being entrenched as the sled moves along the pipeline and controlling the navigation of the towing vessel in response to information indicative of such determination regarding the positioning of the sled.