GB2153318A

GB2153318A - A method of towing a pipeline structure in a body of water and a structure for use therein

Info

Publication number: GB2153318A
Application number: GB08400636A
Authority: GB
Inventors: Jan Veldwijk; Erik Jan Rooduyn
Original assignee: SMIT INTERNATIONAL MARINE SERV
Current assignee: SMIT INTERNATIONAL MARINE SERV
Priority date: 1984-01-11
Filing date: 1984-01-11
Publication date: 1985-08-21
Also published as: GB2153318B; GB8400636D0; CA1249182A; NO841440L

Abstract

A pipeline structure (1) is towed suspended between two tugs (4, 6), the structure having weights (2) attached at points along its length. In the static condition the combustion of pipeline structure and weights has a positive submerged weight, but the structure and/or weights have surfaces that generate hydrodynamic lift forces when the structure is towed along. The arrangement of the surfaces and the speed of tow are such that the submerged weight of the combination is reduced by at least 40% as compared with the static condition. <IMAGE>

Description

SPECIFICATION A method of towing a pipeline structure in a body of water and a structure for use therein This invention relates to a method of towing a pipeline structure suspended between two tugs at a controlled depth in a body of water, e.g. to bring this structure from a point of assembly to a zone where it has to be laid on the sea bottom, and to a structure for use in this method.

Such a method is e.g. known from USP 4.363.566. Therein a pipeline structure is provided with weighting means, consisting of chain lengths. The pipeline structure itself is given a slight buoyancy and together with the weighting means the total structure is given a submerged weight to such an extent that, as soon as part of the chain lengths rest on the bottom, the pipeline structure is kept floating at a short distance above the bottom, in which condition it is towed by a tug to the site to be reached, there being a trailing tug connected to the trailing end of the structure as usual. If obstacles on the sea bottom, such as ship wrecks or reefs are encountered, the restraining force exerted by the trailing tug will be increased to lift the structure off the bottom to pass the obstacle.It is also possible to exert this restraining force continually during towing to maintain the structure at a controlled distance above the bottom during towing.

The present invention aims at improving such known methods. It was found that a much better control of the pipeline structure during towing as to deflections is possible, also allowing considerably longer pipeline structures to be towed in one operation without considerable increase of towing and restraining forces. This is based on the insight that weighting means such as chains together with the pipeline structure itself, if moving at sufficient speed with respect to the surrounding water, are subject not only to drag forces but also to lifting forces which may be used as a major contribution to decrease deflections and allow an efficient and well-controlled towing at the required depth without high towing and hold back forces.

In view thereof, a method as given in the preamble is according to this invention characterized in that the pipeline structure is given a weight and volume so as to have nett buoyancy, that weighting means are connected to the pipeline structure in a number of points along its length so that the combination of pipeline and weighting means has a submerged weight, said combination having protruding parts with surfaces, which during towing are subjected to lift forces caused by the water flowing along them, the towing speed with respect to the ambient water being at least during part of the tow path such as to give the said combination a submerged weight being decreased by at least 40% of and with respect to the submerged weight of the combination in the stationary position of the pipeline structure.

The protruding parts subjected to the lift forces may be combined with the weighting means entirely or in part.The weighting means may be chains as in said known method, being flexible by having links easily pivotable with respect to each other, and such chains cause such lift forces when the structure is towed at sufficient speed through the water, causing the chains to take up inclined positions trailing with respect to their vertical suspended positions in stationary condition of the structure.

Instead thereof or together therewith there may be other protruding parts giving such lift forces such as hydrofoils or inclined vanes connected rigidly or pivotably to the pipeline structure itself and/or to the weighting means.

Preferably the said towing speed is chosen so as to give the said combination a vertical deflection between its ends of 30-70 m.

If the towing speed becomes too high, the combination may rise to the surface by the fact that the said lift forces become high enough to reduce the submerged weight to zero. In many cases. this might be less desirable under adverse weather conditions in view of the deflecting forces of waves thereon. In view thereof, it is often preferred to keep said towing speed at a level, which is at least 2% below the speed at which the submerged weight of the combination is zero.

Further preferred features and details about the realisation of this method will be given in the following description of the annexed drawings. Therein: Fig. 1 is a diagrammatic elevational view of a pipeline structure with leading and trailing tugs during towing through a body of water; Fig. 2 is a graph showing the vertical load (submerged weight) of the pipeline structure with weighting means versus tow speed.

In Fig. 1 there is a pipeline structure 1, of which the buoyancy may be controlled, e.g.

by filling or emptying parts thereof with a suitable liquid or gas such as nitrogen. Such pipeline structures usually are complex, i.e.

they consist of more than one pipe one within the other and often also one to the side of the other. The normal pipeline structure has a surrounding carrier pipe, in which there are one or more pipes e.g. for guiding oil or gas, if desired entirely or in part also to be used as TFL-lines (through-flow-line pipes for guiding tools etc.) and one or more pipes for other purposes, cables, control lines etc. and (part of) such pipes may have a surrounding heat insulation layer, sleeve pipes around such layers and suitably coatings.There may also be a carrier pipe with external pipes and lines connected thereto or a combination of both possibilities. All this is known as such in many different embodiments so that it is not shown in detail.At least part of the pipes and of the spaces between the pipes may in known manner be closed at the end of the structure, be connected to means to fill them with gas such as nitrogen, whether compressed or not, and/or liquid such as oil or sea water. Together with the possibility to apply floats or weighting means, to apply closed or closable bulkheads in (part of) the structure to fill or empty separate compartments with (different) fluids along the length of the structure, and with the possibility to vary wall thicknesses and materials chosen, all this allows to choose and to vary the buoyancy of the structure in the water.

At preferably regular intervals, chain lengths 2 are connected to this structure to be suspended therefrom. The chains may be normal link chains with or without studs. They may be connected to the pipeline structure by having their top link engaging a lug welded to said structure or a strap or bracket surrounding the (outer) pipe of the structure. Instead thereof the top link may be connected to the pipeline structure by a short steel wire or cable. Instead of chain lengths only consisting of the usual links one immediately engaging the other there may be thin steel wires in each chain length, connecting two adjacent links in one or more points of the chain length to allow easy adjustment of the length of each chain length.In this case the chains with their connections to the pipeline structure form the only weighting means, but there may be additional weighting means such as heavy straps, weighting blocks or the like rigidly connected to the pipeline structure. The chain lengths 2 in this case form the only protruding parts subjected to lift forces but, as stated, there may be additional such means such as protruding vanes, hydrofoils or the like connected to the structure.

In usual manner the pipeline structure may have tow heads 3 at its ends. A leading tug 4 will be connected by tow line 5 to the leading tow head 3 of the structure and a trailing tug 6 will be connected by tow line 7 to the trailing tow head 3. The tow lines 5 and 7, the tow heads 3 and the tugs 4 and 6 may be of known and usual design, the tow lines 5 and 7 may in part consist of NylonR and may consist of or be provided with the usual parts, such as shackles, hawsers, pennants, bridles and floaters.

In Fig. 1 the sea bottom is shown at 8.

When this pipeline structure is moved through the water, the chains 2 will take up inclined positions as shown by lines 2' for the two leftmost chainlengths. This will give lift forces relieving part of the weight of these chains by the water flowing along them. This effect is used to advantage by the present invention, which will now be described in more detail with reference to Fig. 2.

This gives the tow speed 9 e.g. in m/s of the pipeline structure with respect to the water along the abscissa and the resultant vertical load on the pipeline structure with chains in the water along the ordinate, e.g. In N/m length of the structure. In calculating said vertical load the tow force and the holdback force exerted thereon by the tow lines 5 and 7 are assumed to be horizontal at the towheads 3.

It will be seen that the vertical load on the entire structure at zero speed is at a certain maximum (negative) value (maximum submerged weight) and at increasing speed there is at first not much decrease of this vertical load. At higher speeds, this load decreases gradually until it reaches zero value, at which speed the structure will just be floating, so that it will reach the surface of the water.

According to the invention, the tow speed is chosen so as to be at least at the value, at which the submerged weight is decreased by at least 40% of and with respect to the submerged weight in stationary condition of the structure. indicated by the horizontal dashed line 10. At these 60% remaining submerged weight the tow speed is represented by the vertical dashed line 11, which speed is indicated as v min. For short combinations this 40% is a good practical limit. In particular for long combinations with a high submerged weight it is preferred to tow at higher speeds in view of maximum deflections to be kept sufficiently low by the lift action of the protruding parts such as the chains.

Preferably, the tow speed is chosen so as to decrease the vertical load (submerged weight) so much that the vertical deflection between the ends of the pipeline combination is between 30 and 70 m. This will for a'particular case e.g. be so at dashed line 12, at which the speed Vp is represented by vertical line 13.

To be on the safe side in view of water currents, deviations in temperature and salinity and thus specific gravity of the water and normal tolerances in the structure as to weights and dimensions, the chosen tow speed will in many cases have an upper limit below the speed at zero vertical load, and this upper limit is preferably at 98% of the speed at zero vertical load (2% below the speed at zero vertical load), as indicated by vertical dashed line 14.

In the known method as described above, the hold back force when lifting the structure from the bottom so as to pass obstacles is relatively high. When applying the invention the hold back force can be much lower and is preferably between 100 and 300 kN even for very long pipe structures.

The following example, for which values of speeds and vertical loads have been given in Fig. 2, will explain a possible embodiment of the method of the invention in more detail.

A pipeline consisting of an outer carrier pipe and a number of oil lines and control lines therein has an outer diameter of the carrier pipe of 876 mm (including coating) and a length of 5000 m. The total weight in air of the pipeline structure without chains is 5800 N/m and its displacement expressed in weight of sea water of a given normal temperature and salinity is 6100 N/m, so that there is a nett buoyancy of 300 N/m.

The chains 2 are connected to the pipeline structure in lengths of about 4,1 5 m, at mutual distances of 12 m. The chains 2 consist of usual anchor chain with links having a thickness of 78,5 mm. The submerged weight of each chain length of 13 links is 4200 N. This gives per unit length of the structure an additional submerged weight of 350 N/m. The total submerged weight of the combined structure with the chains will thus be 350-300 = 50 N/m in stationary condition.

When towing this structure with chains the chain lengths will, in their inclined positions (as 2' in Fig. 1) be subjected both to drag forces and to lift forces and these can and have been determined rather easily by experiments. It will be clear that at zero speed of the structure in the water both drag and lift will be zero and that at increasing speed both the drag force and the lift force will increase.

The lift force will be low at lower speeds and rise considerably at higher speeds, which is reflected by Fig. 2. In this example, the structure will float at a speed of 2,48 m/sec, the towing speed should be at least 1,73 m/sec for 40% decrease in vertical load (line 11 in fig. 2) and is preferably 2,25 m/sec for a 85% decrease in vertical load, corresponding to a maximum deflection of the bundle between its ends of approximately 60 m. For keeping the structure sufficiently submerged, the speed should be below 2,35 m/sec (98% of 2,48m/sec).

As compared with the known method as indicated in the preamble of this specification and as stated before the tow and hold back forces may be considerably lower when applying the invention for the same length of structure, or the structure may be much longer for the same tow force. In said known method, the hold back force will be relatively high with respect to the towing force. In applying the invention it is preferred that the hold back force is a set value which results normally in a value between 10% and 40% of the towing force. In the given example, the towing force at the speed chosen according to the invention (of 2,25 m/sec) is about 1200 kN and the hold back force is 150 kN. One of the grounds for a high hold back force in the known method is the necessity to limit deflections in the structure by means of tension forces at its ends.When applying the invention, the deflections appear to be so low that the hold back force need not be high to counteract such deflections. A lower hold back force also means a lower towing force and this is thus an important advantage of applying this invention.

The tow heads 3 may also be of known and usual design, having means for adjusting their buoyancy, skids to rest on and slide over the sea bottom when starting towing in shallow water and when sinking the structure at the required site, means to connect the pipes of the structure to underwater connections, e.g.

for oil or gas at oil wells, submarine storage means etc. The values given above of course relate to the structure together with such tow heads in their normal operation.

The structure may be assembled on shore entirely or in part on shore and in part in shallow water, where adjustments of the chain weights may be performed. When towing begins, the towing and hold back forces may either immediately be set at the values which in equilibrium conditions will give the structure the chosen speed or the towing and hold back forces may be increased gradually or in steps with, as soon as the hold back force reaches the required value, a further increase of the towing force only.If towing begins in shallow water, where (part of) the chains rest on the bottom to keep the structure floating in submerged condition by part of the weight of the chains being taken up by the sea bottom, the forces may be adapted thereto e.g. by being at the beginning of towing equal and at a level higher than the final hold back force at speed towing according to the invention, to lift the structure with the chains entirely from the bottom as soon as possible before any considerable towing speed is reached, after which the hold back force is decreased to the required value.

In general, it is preferred that the submerged weight of the combination in stationary condition of the entire structure with chains as both weighting means and lift means is between 8 and 50% of the nett buoyancy of the structure without chains and between 7 and 40% of the submerged weight provided by the chains. These values are preferred in view of practical design considerations, tolerances and lift requirements for the chains.

Claims

1. A method of towing a pipeline structure suspended between two tugs at a controlled depth in a body of water, characterized in that the pipeline structure is given a weight and volume so as to have nett buoyancy, that weighting means are connected to the pipeline structure in a number of points along its length so that the combination of pipeline and weighting means has a submerged weight, said combination having protruding parts with surfaces, which during towing are subjected to lift forces caused by the water flowing along them, the towing speed with respect to the ambient water being at least during part of the tow path such as to give the said combination a submerqed weight being decreased by at least 40% of and with respect to the submerged weiqht of the combination in the stationary position of the pipeline structure.

2. A method accordinq to claim 1, characterized in that the weighting means themselves are such protruding parts subjected to the lift forces.

3. A method according to claim 1 or claim 2, characterized in that the said protruding parts comprise hydrofoils or vanes on the structure.

4. A method according to claim 1, 2 or 3 characterized in that said speed is chosen so as to give the said combination a vertical deflection between its ends of 30-70 m.

5. A method according to claim 1 or claim 2 or claim 3, characterized in that said speed is at least 2% below the speed at which the submerqed weight of ffle combination is zero.

6. A method according to any of the preceding claims, in which the hold back force during towing at said speed is preset independently of the allowable deflection at a value between 100 and 300 kN.

7. A method according to any of the precedinq claims with chains as both weighting means and lift means, characterized in that the submerged weight of the combination in stationary condition of the entire structure with chains is between 8 and 50% of the nett buoyancy of the structure without chains and between 7 and 40% of the submerged weight provided by the chains.

8. A pipeline structure with chains as both weighting means and lift means for use in the method according to claim 7, characterized in that the pipeline structure without such chains has buoyancy, the combination of said structure and said chains has a submerged weight at zero towing speed, in which the submerged weight of the combination in stationary condition of the entire structure with chains is between 8 and 50% of the nett buoyancy of the structure without chains and between 7 and 40% of the submerged weight provided by the chains.

9. A method of towing a pipeline structure substantially as described herein with reference to the accompanying drawings.

10. A pipeline structure constructed and arranged substantially as described herein with reference to the accompanying drawings.