GB2187256A - Apparatus for towing a pipe string at a controlled depth through a body of water - Google Patents

Abstract

Subsea pipelay by bottom towing frequently is not feasible in areas of difficult undersea terrain or due to the location of other pipelines. The present invention overcomes this problem by flying the pipe string underwater with the use of wings 19,20 which provide sufficient lift to maintain the pipestring off bottom as it is towed. The wings 19,20 may be mounted on the pipe string by detachable cross-ties 21,22. Launch skids 16,17 and buoys 28,29 may be provided. The wings 19,20 may be adjustably mounted and may be formed of foamed polyurethane, glass-reinforced polyester or epoxy. <IMAGE>

Description

SPECIFICATION Apparatus for towing a pipestring at a controlled depth through a body of water Offshore submerged pipelines are installed usually either by dragging the pipeline along the seabottom, pulling the pipeline while maintaining it in buoyant condition, or deploying the pipeline from a conventional lay barge. Pipelaying procedures involving moving contact between the pipeline and either the seabed or handling equipment may be damaging to the pipeline, particularly if the pipeline is coated with a material sensitive to abrasion, or in cases where existing pipelines need to be crossed and contact with them needs to be avoided.

Several procedures are currently available for pulling submerged pipelines through the water while providing means for supporting the pipeline during the pull so that the pipeline does not touch the seabed and avoids abrasive contact with the seabed and other obstructions or existing pipelines.

It is, of course, possible to lift the pipe totally off the bottom, without towing it, but the required tension in the tow cables would be very large.

It is known from US patent 4,474,507, to transport a pipe by the so-called "controlled depth tow method" wherein the pipe itself (with or without additional buoyancy tanks attached) is kept at a predetermined height above the seabed by attaching chains at certain intervals. The pipe ends are attached with tow cables to a leading and trailing tug. If the assembly is not moving, the pipe will float at some height above the seafloor. The weight of the chain hanging from the pipe, but not resting on the seabed, is equal in magnitude to the positive buoyancy of the pipe itself, and the remainder of the chain on the seafloor provides resistance against hydrodynamic forces from lateral currents. When the tow starts, the chains will start to deviate from their vertical position and the pipe will start to lift off from the bottom.The reason for this phenomenon is that the hydrodynamic drag force on the chains splits into two components: an upwards force which reduces the total system submerged weight and a horizontal force. Thus, the pipeline configuration will take the shape of a catenary. When the tow speed increases, the inclination of the chains, measured from the vertical, increases also, and thus provides a greater upwards force. To some extent this upwards force would also be present without the use of chains, due to the hydrodynamic drag on the pipeline itself.

The chains are not an optimum-means to provide this upwards force, because they also create a drag force along the pipe axis, and they have a large weight to start with.

It is a principle object of the present invention to provide an apparatus which is able to support a pipestring at a controlled depth in a body of water with only little additional drag.

In accordance with the invention this object is accomplished by an apparatus comprising a series of gripping means being clamped to the pipestring at regular intervals along the length thereof; and wing means connected to each of said gripping means for lifting said pipestring out of contact with the seabed during the towing operation.

The invention will now be explained in more detail with reference to the accompanying drawings, in which: Figure 1 is a view of the invention wherein a pipestring is being towed between two tugs using fixed wings attached to the pipestring.

Figure 2 shows a one section wing.

Figures 3 and 4 depict a wing inclination adjustment.

Figures 5 and 6 show wing rotation.

Figure 7 is a plan view of the invention wherein a pipeline bundle is pulled fnto the water from an onshore location.

Figure 8 is an exploded perspective on a larger scale of the gripping and wing means for supporting the pipe bundle shown in Figure 7 and for permitting it to be pulled into the water and flown above the seabed.

Figure 9 is a fragmentary detail of the detachable means for attaching the wing assembly to the pipe gripping assembly shown in Figure 8 whereby the former, after the pipe bundle has been pulled to a final location, may be detached by remote control from the surface and retrieved.

Principles of airfoil theory known in the art are applicable to determine the magnitude of the lift force as a function of tow speed, wing geometry, and wing inclination relative to the direction of the fluid flow, even though operation is in relatively low Reynolds numbers.

Airfoils have seen extensive use for trimming purposes, for example, on submarines and on seismic floats.

A key parameter is the angle of the wing to the direction of the fluid flow (which is for simplicity assumed to be parallel to the longitudinal axis of the pipe). The pipe configuration will change from an essentially straight position to a catenary during the course of increasing tow speed from zero to the desired level. Each point along the pipeline, therefore, undergoes a change in inclination. Having the wings 1 rigidly attached along the axis of pipe 2 towed between tugs 3 and 4 will not be efficient (Figure 1), because the trailing end of the pipe experiences a downward directed force 5 on the wings while the forward end of pipe 2 experiences an upward force 6. In some cases, it is desirable to move the pipe back in the direction it came from, using the trailing tug 4 as the leading tug instead.It is therefore preferred to have some means to adjust the inclination of the wing.

The wings can comprise one or two sections (Figures 2 and 7). if the wing 7 comprises one section, it can pivot around a hinge 13 (Figure 3) with tails 8 (Figure 2) providing lateral strength. Preferably, the wings 7 are not left on the pipe 2 after installation of the pipe because (1) they may have a negative effect on hydrodynamic stability of the pipe, and (2) using wings may not be economically viable if they are not re-used. Detachment of the wings 7 may be achieved remotely or with use of a remotely operated vehicle (not shown).

A long pipe string which is towed without buoys or chains, will roll around its longitudinal axis. Wing design is such that this roll is avoided. This is achieved by constructing the wing assembly such that, if the assembly is positively buoyant, its centre of gravity is above the centre of gravity of the pipe string, and if the wing assembly is negatively bouyant, its centre of gravity is below the centre of gravity of the pipe string. This is also important during start-up of the tow procedure, when the pipe is close to the seabed: the wings are not allowed to dig into the seabed or hook behind an obstruction.

It is likely, as above stated, that the largest forces on the wing 7 will be due to wave impact forces during pipe launch from shore (aside from accidental loads). Vertical forces on the wing 7 (downwards and upwards) can be orders of magnitude higher than those encountered during tow. Other forces are hydrodynamic forces during tow, impact forces with the seabed, hydrostatic forces, and handling forces during installation on the pipe and launch from the beach.

Based on the foregoing requirements, the wing components are: (1) the wing itself, (2) wing inclination adjustment mechanism, (3) wing rotation mechanism, (4) wing attachment to pipe, (5) wing release mechanism, and (6) pipe slides.

It is probably more advantageous to have relatively small wings at close intervals, say one every joint or two joints, than larger wings, further apart. The smaller the wing area, the easier the handling becomes, Chances of damage or inadvertently hitting an obstruction decrease also. in addition, "mass production" may become attractive, especially if the wings are made from mouldable materials (high density foams, plastics). The wings preferably are light and they preferably have a solid cross section, to withstand the hydrostatic pressure. The low density of foams results in positively buoyant wings, which increases the roll stability in the configurations shown. Wings preferably are placed symmetrically and comprise one or two sections (Figures 2 and 8), depending on primarily economic considerations.

The wings 7 are preferably made using syntactic foam, a high-density polyurethane, fibre reinforced polyester or epoxy. Manufacturing costs for the wing are low, especially if the cost of moulds, etc. are spread out over several hundred units.

As above mentioned, it is preferred to have the capability to adjust the inclination of the wings 7. This inclination may be set prior to launch of the pipe 2, but this may not result in the desired lift force distribution during varying conditions under tow. It is unlikely that the inclination of the wings will be adjusted while towing (in which case the required torque to rotate the wing 7 would be high), so a simple, low torque mechanism is employed (Figures 3 and 4). This mechanism comprises screw bolt 9 extending through threaded holder 10 and activating rotating arm 11 which moves along slide 12 to swivel wing 7 about hinge 13. Torque to activate bolt 9 can be provided by a pre-loaded spring, a remotely operated vehicle equipped with an hydraulic arm, or by a small hydraulic cylinder (energy provided by an accumulator) which is activated by the remotely operated vehicle.Alternatively, inclinometers can be placed at regular intervals, with a system of transponders placed along the line, to automatically actuate the wing inclination mechanism. The rotating arm 11 and the hinge 13 may include hinge bolts (not shown) which will fail at a predetermined shear force. This will ensure that, if the wing accidentally gets caught behind an obstruction, the wing will shear off without damage to the pipe.

The wing assembly may experience the largest forces when being pulled through the surf zone. To minimize the exposure to onrolling waves passing from surf zone 14 to beach 15, the wings 7 may be horizontally rotated 90 degrees (Figure 5). A simple spring loaded system (not shown) may be activated by divers or a remotely operated vehicle, to rotate the wings 7 back into their proper position for tow (Figure 6).

It is suitable in accordance with the invention to attach the wings 7 to the pipe 2 or pipe bundle in several ways. Main considerations are given to ease of assembly, compatibility with movements between individual pipes, avoidance of damage to the pipeline, and ease of release. If launch forces can remain low, then clamps 38 do not have to transfer significant forces. The weight of the clamp preferably is kept low, and various materials other than steel, such as moulded clamps made of a hard plastic, may be more economical. An emergency release preferably is incorporated, which releases the wing if it gets stuck, or hangs behind an obstruction.

Various different release mechanisms are suitable for use with the invention. It can be a chain reaction type trigger mechanism, with a trigger rope or cutting cable along the pipe, or the clamps may be released individually. The mechanism shown in Figures 8 and 9 is a typical example of the latter.

The purpose of slides 16 and 17 as shown in Figures 7-9 is to prevent contact of the pipe with the beach 15 when the pipe is being launched, or when the tow speed is still too low for "lift-off". For the slides 16 and 17 to be successful, it is necessary to have a fairly close wing spacing (to avoid midspan contact between pipe and seabed) and to have a hard, and fairly smooth seabed surface, for example, a sandy soil. In soft mud, more commonly found in deeper waters, the slides sink into the sea bottom and restarting a tow requires high tow forces. Some contact of coating and seabed over short distances may not be detrimental, while continuous contact over a long tow would be, so slides are not always necessary.

In Figure 7, pipe bundle 18 is shown being pulled from the shore 15 into the surf zone 14 by a suitable towing means, while sliding on spaced pairs of slides 16 and 17 on the shore. Once the pipe bundle 18 is in the water, and the tow speed has increased sufficiently, wing 19 and 20 lift the pipe bundle 18 off the seabottom and cause it to fly in the water, whereupon the pipe bundle may be pulled to a desired location before depositing it on bottom. Thus, the pipe bundle 18 is kept out of contact with the seabed and may be moved eventually to a final location where the wing assemblies may be detached and retrieved. The ends of the pipes comprising pipe bundle 18 are sealed by closures (not shown) and are attached to or are an integrated part of a pull assembly (not shown) permitting attachement of tow cable 37.

The construction of the wing assembly is made clear from Figure 8. At intervals of about 12 to 36 m centres along the pipestrings, combination wing and clamping assemblies are provided for gripping the pipe bundle 18 and supporting it both above the land while being pulled into the sea and above the seabottom when the pipe bundle is under water, so that the pipe bundle will not touch the ground on the shore or the seabed during the placement. Details of one combination clamping and wing assembly appear in Figures 8 and 9. Each member of a pair of cross ties 21 and 22 is suitably grooved to receive and clamp between the ties the pipe bundle 18.

This clamping assembly may be used with one, two or many pipestrings or pipe bundles which can be arranged not only side-by-side but also above each other, either aligned or staggered, but preferably parallel. The bottom tie 21 is notched on the top side to receive the pipe bundle 18 and the top tie 22 is notched on the bottom side to nestle the pipe bundle 18 therebetween. The upper tie is then bolted or otherwise fastened to the lower tie by means not shown and also to the sleds or skids 16 and 17, thereby forming a structure which firmly binds the pipe bundle to the clamping assembly and also the sleds or skids as well. The ties may also be attached by releasable means to permit burying the pipelines. Attachable to the tie 22 is an axle 23 and wings 19 and 20 which form parts of the wing assembly. Preferably, the wings 19 and 20 are rotatably attached to the clamping assembly.Active means (not shown) may be utilized to provide rotation of the wings.

Flanges 24 and 25 extend from the front and back faces of the axle 23, respectively, and have apertures which match with the apertures shown in lugs 26 and 27. Buoys 28 and 29 are attached via lines 30 and 31 to eyebolts 32 on one face of the axle 23 and another eyebolt (not shown) on the opposite face of axle 23 to a shear pin 33 at the end of eyebolt 32 and also at the end of line 30 (not shown) which are insertable into the matching holes in flanges 24 and 25 and lugs 26 and 27. Manifestly, a single buoy could be employed instead of the double buoys 28 and 29.

As more clearly shown in Figure 9, the line 31 passes through a ring 34 which is, in turn, attached to a ring 35 which is affixed to flange 36. Line 31 then attaches to the eyebolt 32 which passes through the holes in the flanges 24 and at the opposite end thereof, is attached to a shear pin 33. When it is desired to release the eyebolt from flanges 24 and 25, and lugs 26 and 27, respectively located between the flanges, so that the wings 19 and 20 may be retrieved at the surface, a suitable force is exerted upon buoys 28 and 29 and lines 30 and 31 to break the shear pin 33 and the similar shear pin on the opposite side of axle 23 whereupon the wings 19 and 20 may be retrieved at the surface. This is, of course, after the pipe bundle 18 has been towed to a desired on-bottom location.

When the force is exerted to break the shear pins and release the wing assemblies from the gripping assembly, the eyebolt 32 preferably does not pass through the ring 34 but catches thereat and permits retrieval of the wing assembly by means of the lines 30 and 31.

The foregoing description of the invention is merely intended to be explanatory thereof.

Various changes in the details of the described method and apparatus of the invention may be made within the scope of the appended claims without departing from the spirit of the invention.

Claims (8)

1. An apparatus for towing a pipestring at a controlled depth through a body of water comprising: a series of gripping means being clamped to the pipestring at regular intervals along the length thereof; and wing means connected to each of said gripping means for lifting said pipestring out of contact with the seabed during the towing operations.

2. The apparatus of claim 1 wherein the wing means comprise pairs of foils rotatably attached to an axle detachably secured to said gripping means.

3. The apparatus of claim 1 including remotely operated means for detaching and retrieving said gripping means and wing means after the pipeline has been pulled to a predetermined location.

4. The apparatus of claim 1 wherein means for supporting the pipeline above the seabed or shore is attached to the gripping means.

5. The apparatus of claim 4 wherein the support means are sleds or skids.

6. The apparatus of claim 1 wherein the wings are horizontally rotatable to align with the pipeline.

7. The apparatus of claim 1 wherein the wings are inclinable relative to a horizontal axis.

8. An apparatus as claimed in claim 1, substantially as described with reference to the accompanying drawings.