GB2221276A

GB2221276A - Towing and installing prefabricated submarine pipelines

Info

Publication number: GB2221276A
Application number: GB8914972A
Authority: GB
Inventors: Karel Karal
Original assignee: Norwegian Contractors AS
Current assignee: Norwegian Contractors AS
Priority date: 1988-07-01
Filing date: 1989-06-29
Publication date: 1990-01-31
Anticipated expiration: 2009-06-29
Also published as: GB8914972D0; NO882947D0; NO882947L; NO170739C; GB2221276B; NO170739B

Abstract

Long prefabricated pipelines (1) are towed for installation in offshore environments and are designed for transporting oil and/or gas. The pipelines are along their length equipped with a plurality of linked or flexible, relatively lightweight weights (13), which weights during the towing movement generate a lifting force. The pipelines are further equipped with at least one through-going ballast pipe (3) for ballasting purposes. <IMAGE>

Description

TOWING AND INSTALLING PREFABRICATED SUBMARINE PIPELINES The present invention relates to methods and means for towing and installing prefabricated submarine pipelines of substantial lengths. More particularly the invention relates to long pipelines which are towed from a production site on land for installation on the sea bed or bottom in offshore environments.

The expression "pipelines" includes single pipes or pipe bundles, either kept together with clamping means attached to the outside of one or more support and protection pipes, or assembled inside a circumscribing guarding pipe.

In connection with oil- and gas production from subsea wells, a need exists for pipelines which are placed on the sea bottom to establish connections for instance between oil and gas wells and other installations on the sea bottom or to the shore. In consideration of the costs it is found to be most attractive to produce such pipelines in a single full length or in as few sectional lengths as possible on a shore site in order to reduce the required number of coupling connections on the offshore installation sites. It is thus desirable to prefabricate such pipelines in lengths which can exceed 5 to 15 kilometers in certain cases.

Prefabrication of long pipelines for oil- and gas production designed for installation on the sea bottom is known. The lines are joined together in a suitable length ashore, and are thereafter pulled out along the sea bottom and towed in its full length to the installation site.

The towing can be carried out in various ways.

It is known to provide the pipeline, which is given a certain positive buoyancy, with a number of chain lengths each of which has one end attached at spaced positions along the pipeline. The chains have such a length and weight that they, together with the pipeline and with part of the chain length resting on the sea bottom, result in a certain definite equilibrium situation together with the pipeline at a certain distance above the bottom.

Part of the chains will then, during the towing, slide along the sea bottom, and the towing can take place with a pipeline at a suitable safe distance from the sea bottom. This method is called "Off Bottom Tow".

Such a method requires, however, a more or less ideal sea bottom surface, and necessitates in addition a detailed pre-examination of the nature of the sea bottom along the route. Furthermore, the required pulling force in order to accomplish such towing will be large, and in connection with pipeline lengths of 5 kilometers or more and with heavy chain parts resting on the sea bottom the required pulling force can be exceedingly large, with the result that this method is practically impossible to accomplish.

The crossing of other already installed pipelines may also imply complicating factors.

Another known method of towing a pipeline in submerged but still in a clear position off the bottom, is the Controlled Depth Tow method (CDT). In this case the pipeline is also equipped with a number of suspended chain lengths, and by controlling the chain weights and adjusting the buoyancy of floats fastened to the pipeline, the towing can be carried out without contact with the sea bottom, while at the same time as the pipeline is being kept submerged at a suitable distance below the sea surface in order to avoid the risk of damage due to waves, ocean traffic or the like.As auxiliary means, in order to maintain the desired distance from the sea bottom and also from the sea surface, is further proposed to utilize chain lengths and pipelines equipped with protruding or staked elements which together with the chains contribute to increase available lifting force depending upon towing velocity and when the towing is initiated from sea bottom position. The applicability of this method is limited by the length of the pipeline and the total submerged weight, since the required sea depth and/or tension in the pipeline will increase proportionally with an increase in one of the beforementioned parameters. Further, this method would involve practical problems in that the weights have to be adjusted in submerged position within rather small tolerances.

Further, the towing of the pipelines must be initiated from an underwater position clear of the bottom without possibilities for ballasting.

Known methods have the common disadvantage that the weights in the form of chains or the like which are utilized will be so substanial that the pipeline must be equipped with possibilities for an additional positive buoyancy which can compensate these weights. The dimensioning must further be based upon the largest depth along the towing route. The drag resistance will be large and the towing velocity will necessarily be low, and will further be limited to a small maximum value in order to prevent the pipeline from being forced up to the sea surface.

A consequence of this situation is that, also in connection with this method, the necessary towing force will be substantial and will also limit the maximum pipe length which it is possible to tow by means of available and economically feasible towing operation both in connection with initial lift-up from the sea bottom and during normal towing. In the worst case, difficult sea bottom conditions may exclude utilization of such method.

The invention provides a method of towing and installing on the sea bottom long, prefabricated pipelines of the kind where the pipelines are equipped with mutually spaced, suspended flexible weight means and the pipeline system includes at least one through-going ballast line for adjustment of the buoyancy of the pipeline, wherein the pipeline for the towing operation is ballasted to a relatively small submerged weight or small positive buoyancy and the weight means are designed, configured and suspended on the pipeline such that the towing movement through the water generates a lifting force with sufficient magnitude such that the pipeline floats up to the sea surface during the towing operation, but will sink down as soon as the towing velocity is reduced to or below a certain value.

In accordance with the present invention the shortcomings with known methods are overcome by carrying out the towing operation along the water surface in this manner. In connection with the launching and the subsequent towing in shielded areas the pipeline floats at the water surface, (Surface Tow), and the remainder of the towing stretch in open sea to the installation site takes place with the pipeline still floating on the surface but with possibilities for controlled submersion of the pipeline in order to protect it. This method is usually called: "Controlled Surface Tow" (CST).

This is accomplished by reducing the towing velocity by ballasting the pipeline or in connection with the same in dependence upon the actual situation which requires special precautions. A reduction of the towing velocity results in an immediate submersion of the entire pipeline. Such submersion is for instance carried out in order to avoid collision with a surface vessel, floating objects or the like.

Ballasting of the pipeline causes it to sink in the shape of an S-wave which propagates along the pipeline in pace with the increasing length which is ballasted. This is particularly advantageous if it is necessary to temporarily position the pipeline on a selected site on the sea bottom due to severe weather or wave conditions such that further towing has to be discontinued or postponed.

The pipeline together with small weights represents a small submerged weight during the towing, and the lifting forces which are created by the weights will keep the pipeline in its surface position.

It is thus possible to accomplish economically justifiable towing pipelines having substantial lengths, such as 5 to 10 kilometers and longer.

The invention is especially suitable in situations including difficult sea bottom, both where the pipeline is to be launched after prefabrication ashore, and along the towing route. The present invention further permits relatively large variations in the weight and volume of the pipeline without necessecitating individual adjustments subsequent to launching.

A further attractive feature of the present invention is that the towing velocity is not critical. The lower limit (CST) is the lowest velocity necessary in order to keep the pipeline in surface position. This velocity is selected upon the basis of actual conditions during towing and not the basis of the parameters of the pipeline. As to maximum velocity no practical limits exist.

If submersion is initiated by ballasting the pipeline, the towing velocity will have little importance, and if submersion is initiated by reducing the towing velocity alone, the towing velocity must be reduced to a value below the velocity which is necessary in order to keep the pipeline in the surface position.

In accordance with the present invention it is thus possible to control the vertical movements of the pipeline with rather small weights which, during the towing, maintain the pipeline in the desired position along the sea surface, due to the lifting forces which are created by said relatively small weights.

The invention is based upon acknowledgement of the fact that the forces and loads which a pipeline is exposed to due to the weather and wind when being towed in surface position, contrary to what is taught in the profession, are almost negligible under normal weather conditions, that is all conditions permitting safe towing, regardless of which towing method is used. The generally accepted limit is in this connection a sea surface situation with a defined wave height of 2,5 meters.Subject to such conditions, typical pipelines will be subjected to maximum 15-30% of permitted strength capacity and can thus warrant towing of even very long pipelines positioned on the sea surface, and simultaneously obtaining sufficient safety in situations of emergency, when the pipeline in a simple and quick fashion can be submerged to a position below the sea surface or all the way down to the sea bottom for protection.

The weights which are utilized in accordance with the invention can preferably be produced from concrete elements which are cast around an 8mm steel wire and positioned spaced from each other along the steel wire. The total weight of the concrete is selected to obtain an optimum effect. The weights can further for instance be given a plastic covering in order to avoid being damaged or fractured in the corrosion preventing layer on the pipeline. The expression "weights" as used in this connection is actually somewhat misleading, since the principal contribution of the weights actually consists in the hydrodynamic lift they create.

In the accompanying drawings: Figure 1 shows a pipeline furnished with, the necessary equipment for surface tow or controlled surface tow (ST/CST); Figure 2 is a diagram illustrating the depth as a function of the load caused by waves which a pipeline is subjected to; Figure 3a, b, c, d and e show the various steps in the towing operation for a pipeline equipped with a coupling unit; Figures 4a, b, c and d show different weights Figure 4c being a section on Figure 4b, and Figures 5a and b show the effect of the weights in comparison with the prior art.

In Figure 1 a pipeline 1 is shown in floating position on the sea surface 2. The pipeline or pipeline unit as shown comprises a number of separate pipes and the like (not shown) encompassed by a shielding or protection pipe. The pipeline also comprises an internal pipe 3 for ballasting purposes. The pipeline unit is towed by a first towing vessel 4, a second towing vessel 5 guiding the aft end of the towing unit. The towing arrangement forms no part of the invention and is therefore not shown.

On the second towing vessel 5 is located a seawater pump 6 and an air compressor 7 which are connected to the ballast pipe 3 in the pipeline by means of a hose including quick couplings 8 and 9.

The other end of the ballast part leads to the first towing vessel 4 by means of a hose including quick couplings 10 for venting or bleeding of the ballast pipe.

During the first part of the towing operation of the pipeline in shielded water it is sufficient that the pipeline has sufficient buoyancy in order to remain floating on the sea surface at zero towing velocity, since in this phase no problems exist in connection with waves, and it is possible to possess full control as regards crossing traffic or the like, such that the towing operation can be defined as simple surface towing (ST). When the towing offshore is initiated, towing is changed to controlled surface towing (CST) subsequent to mounting weights 13.When the offshore towing is initiated, the ballast pipe 5 is supplied with the ballast water until the unit attains a small submerged weight, and the lifting forces which are added due to the weights 13 will force the pipeline unit to the surface during the towing with a veloc. t which should lie above a selected minimum va If obstructions or difficulties should arise during the offshore towing, the towing is discontinued, having the effect that the pipeline immediately will begin to sink. If necessary, further ballast can be supplied into the ballast pipeline 3. When the danger is over and the way is clear, the towing is resumed, and the pipeline will ascend or rise towards the sea surface. If necessary, some of the ballast water will be blown out of the ballast pipe by means of the compressor 7.

Figure 2 is a graphic presentation related to depth of the maximum induced tension (a ) caused by waves in the most severely exposed part of the pipeline, expressed as a percentage of the float tension (of) ) in steel utilized in the steel pipe (a X 100) Of The surprising fact is that the exposures at the surface are relatively small and they will reach a maximum at a depth of about 7-8 meters, whereafter the loads will reduce with increasing depth. At a depth of about 15 meters the exposures will be the same as at the surface, and will then reduce gradually with increasing depth.

Figures 3a, b, c, d and e show various steps during the towing of a pipeline which is connected to a relatively large coupling body element which finalizes the pipeline at the one end. A coupling body 11 includes a ready made coupling for quick and safe coupling to for instance a platform or an oil well. The coupling body 11 is provided with a buoyant body 12 which is pivotally connect to the coupling body.

Figure 3a shows the situation during towing in shielded waters. The pipeline 1 is being towed by a first towing vessel 4. The pipeline is equipped with weights and has an accummulated net buoyancy. The weights 13 have during this step no function. If desired, the mounting of the weights can be postponed to just before the offshore towing is initiated.

Figure 3b shows the situation, subsequent to the offshore towing being initiated. A substantial part of the pipeline 1 is still on the surface aided by the lifting forces rendered by the weights 13 during the towing, even if the pipeline and the coupling body are given a small submerged weight by being ballasted. The coupling body 11 is however, in a submerged position, since the floating body 12 will point downwards due to the towing. The coupling body 11 is being kept in a submerged position during the towing in order to reduce the loads on the transition means between the coupling body and the pipeline.

Figure 3c shows the situation close to the installation site. The towing velocity has now been reduced to a value below the selected minimum value, and the pipeline leaves the surface and sinks towards the sea bottom 14.

In Figure 3d the second towing vessel 5 has picked up the leading cable 18 to the well 15 to which the coupling body 11 is to be connected, and the pipeline 1 including the coupling body is being pulled into position. During this step, the coupling body 11 is given a small buoyancy which facilitates the entry into the well 15.

Figure 3e shows the situation wherein the coupling between the coupling body 11 and the well 15 has been accomplished.

Figure 4a, b, c and d show in detail how the weights can be designed.

Figure 4a shows a weight 13 attached to a pipeline 1 by means of a cable 16. The mass is primarily a number of separate single weights 17, which are concrete elements cast around the cable 16. The concrete elements 17 have a selected mass/specific weight which is adapted to the requirement for minimum towing velocity when the pipeline is positioned on the sea surface.

Figures 4b and c show separate weights which are given a surface which will offer increased water resistance during towing, and with corresponding increased lifting force.

The weights can also, as shown in Figure 4d, be realized as spearate single weights which are kept in place in a plastics casing. It is of great importance that the weights have large flexilibity.

It is further desirable that the weights have an external configuration such that the corrosion preventing covering on the pipeline is not damaged when the pipeline is brought to a resting position on the sea bottom, and that they are not made of a metal which may accelerate corrosion.

Figures 5a and 5b illustrate the effects of the weights of Figures 4a to d in comparison with the effects of conventional, heavy weights which are used in connection with the prior technique.

Figure 5a illustrates the relationship between required minimum weight in order to keep the pipeline at the sea surface at controlled depth tow (CDT) during towing compared with necessary weight and controlled surface tow (CST).

At a towing velocity of l,Sm/sec. the required minimum weights at CDT will be six times the weight necessary at CST, in order to obtain the same effect. At a towing velocity of 0,5 m/sec., the required minimum weight at CDT will be 10 to 15 times the weight which is required at CST.

Figure 5b graphically shows the part of the drag resistance which is caused by the weights at various towing velocities in connection with CST and CDT, respectively. The curves are composed with the underlying criteria that the pipelines both with CDT and CST, respectively, will come up to the sea surface at a towing velocity of 1,5m/sec. The graphic presentation includes a curve illustrating commercially available towing forces, illustrating that available towing velocity at CDT will radically limit actual slowing velocities.

The curve shows clearly that to start the towing of a pipeline at CDT (which must be started from the bottom) will require a substantial towing force and will to a large degree limit the maximum pipeline length which it can be used for.

The graphic representation shows that, at a towing velocity of l,5m/sec. in CDT, all the available towing force is already consumed by the weights alone. In connection with CST the resistance which is caused by the weights will be rather small, and they will reach a maximum at a towing velocity of 1,5 m/sec., and will thereafter decrease with increasing towing velocity, and will constantly be available.

Claims

1. A method of towing and installing on the sea bottom long, prefabricated submarine pipelines of the kind where the pipelines for towing operations are equipped with mutually spaced, suspended flexible weight means and the pipeline system includes at least one through-going ballast line for adjustment of the buoyancy of the system, wherein the pipeline for the towing operation is ballasted to a relatively small submerged weight or small positive buoyancy, and the weight means are designed, configured and suspended on the pipeline such that the towing movement thereof through the water generates a lifting force with sufficient magnitude such that the pipeline floats up to the sea surface during the towing operation, but will sink down as soon as the towing velocity is reduced to or below a certain value.

2. A method as claimed in claim 1, wherein the pipeline during the towing, in order to obtain a quick submersion and sinking of the pipeline, is ballasted with water simultaneously as the towing velocity is reduced.

3. A method as claimed in claim 1 or 2, in which the pipeline is prefabricated at a shore site and is delivered successively out to sea during the production and kept floating by means of its own buoyancy, initial towing operation of the pipeline in shielded waters being carried out with the pipeline floating on the sea surface, the weight means being mounted on the pipeline while it is floating in the sea and then, when offshore towing is initiated, decreasing the buoyancy of the pipeline by ballasting the pipeline and then continuing the towing towards the installation site, the necessary buoyancy or upwardly directed forces being provided by the weight means.

4. Means for carrying out the method as claimed in any preceding claim, wherein each of the weight means constitutes relatively short lengths of linked or flexible, elongate elements.

5. Means as claimed in claim 4, wherein the weight means constitute lengths of chain or wire encompassed in a plastic casing with the aim to protect the weights and provide the weight members with suitable outside dimensions in order to provide the desired upward lifting force during the towing.

6. Means as claimed in claim 4 or 5, wherein the weight means are constituted by concrete elements cast or imbedded around lengths of wire or chain.

7. Means as claimed in any of claims 4 to 6, wherein the weight means are provided with protruded elements in order to increase available lifting force in dependence upon the towing velocity.

8. Method and means substantially as herein described with reference to and as shown in the accompanying drawings.