GB2267945A - Installation of submerged pipelines - Google Patents

Abstract

A pipeline or pipelines for installation on the sea bottom are prepared ashore in a number of long prefabricated sections, each for instance having a length of from 3 to 10 km. The pipe sections are moved out into the sea for temporary storage in shielded waters, either in floating position or in a submerged position on the sea bottom, whereafter the pipe sections are moved to the laying site and located in position to be joined. Joining takes place at the surface by equipment on an auxiliary vessel, the preceding joined sections being laid continuously down on the sea bottom and the end part of the section closest to the vessel being raised to the surface in order to be joined to the next pipe section, whereafter said interconnected pipe section is submerged in controlled manner down to the sea bottom by means of suitable ballasting equipment. The pipeline may be combined with one or more ballasting pipelines having adjustable buoyancy, or may be provided with a non-permanent, buoyancy providing, flexible support structure. <IMAGE>

Description

"Method and Means for Production, Temporary Storage, Towing and Installation of Long Pipelines along the Sea Bottom".

The present invention relates to methods and means for production, temporary storage with subsequent towing and installation of long and very long pipelines of medium and great size or diameter, along a sea bottom. The invention comprises also the task of pulling pipelines up to platforms or subsea installations, installations along the sea bottom presenting a strongly varying topography, frequently occurring at the shore line, and possibly also including shore landing of such pipelines.

The invention is particularly attractive when two or more pipelines shall be installed side by side. A particular advantage with the invention is that pipelines installed in accordance with the invention may be removed when use or active operation has ceased. Removal of the pipelines is particularly simple to carry out when permanent ballasting pipes are being used.

Installation of long pipelines on the sea bottom is priorily known. Many such long pipelines have been installed, particularly in connection with exploration of hydrocarbons.

The invention comprises also small diameter pipelines, for instance well piping, injection- or service pipelines for supplying chemical products (methanol, glycol) and the installation of control cables. All such pipelines can be installed simultaneously in replacement of traditional pipe bundles.

For smaller pipe dimensions, it is known to use prefabricated long pipelines, winded up on large drums. The drums with the coiled pipeline are positioned on special laying vessels, from where the pipeline is feeded out in the sea.

The use of drums offers several advantages in that for instance the laying time in open sea can be rather short with correspondingly reduced operational intermissions and delays caused by bad weather. The drum method limits, however, the size of the diameter of the pipeline, and is thus not suitable for the large pipeline diameters which in many cases are required. This method further reduces the possibility to select special cover materials which are frequently required for pipelines.

For large size pipeline diameters, therefore, special pipe laying vessels are frequently used, carrying standard pipelines of about 12 m length, which are welded together continuously or intermittently on the laying vessel, and thereafter being dispensed out in the sea where the readymade part of the pipeline is laid down on the sea bottom directly.

The above method is well known. The laying operation will, however, due to the many working steps which are necessary to carry out on the laying vessel in open sea, be rather time consuming. The method is therefore expensive due to large investments in equipment. The laying method requires also that the pipeline presents a relatively smooth surface, so that it can slide out in the sea along a stinger from the laying vessel down to the bottom without the surface of the pipeline being damaged. The method is thus only attractive for laying of one large diameter pipe at the time. If two or more pipes are to be positioned along the same route or bed, the pipelines must be laid and positioned separately and such that they are not disturbing each other. In other words, the pipelines will not be positioned as an assembly, and will therefore occupy a larger sea bottom area, which area must be pre-inspected and clarified. Furthermore, the respective pipelines will, from a hydrodynamic view, operate or act separately, and must consequently be separately dimensioned with regard to those hydrodynamic forces to which they will be exposed. The same consideration must be taken regarding collisions from bottom trawl boards: two pipes positioned at a distance from each other, will be subjected to two hits or blows during the passage of the trawl, while two pipes positioned side by side will only be exposed to one hit at each passage. Finally, a later inspection could be comprehensive, if the two pipes in question cannot be inspected simultaneously as an assembly.

A further factor which makes this laying method expensive, particularly if it is a question about large diameter pipelines and large depths, is that it will require the pipelines to be subjected to large tensional forces in order to avoid breakage or cracking during a laying operation in accordance with the S-method, i.e. powerful equipment in the shape of anchors, winches, tensioning machines etc., - equipment which frequently is not standard. Large tensional forces will further offer only limited possibilities for bending of the installed and laid pipe, both in the horizontal and the vertical plane, a fact which limits the possibility to wind or "wriggle" the pipes along areas with difficult or "crooked" topography along an optimal route. Known methods for crossing sea bottom areas having a strongly varying topography are usually based either on levelling the sea bottom by removing tops and filling crevices along a relatively straight route or alignment and thereby a not optimalized alignment, or by assembling the pipelines by specially prepared sections having the correct bends for the pipeline to follow the sea bottom.

The first method may offer good results, but may in many cases be unsuitable because of the high costs involved. The second method will offer several technical difficulties and is not compatible with known laying methods. The J-method does not require large tensional forces, but is on the other hand rather time consuming as long as traditional welding methods are being applied.

It is also known to prefabricate ashore relatively long lengths of a pipeline, hereinafter designated "pipeline sections", which are to be installed on a sea bottom. The method requires considerable areas at a suitable site along the shore. As a rule, such pipeline sections are not independently buoyant, and neither are they equipped to be joined together in open seas in order to form continuous lengths.

The laying of long pipelines formed by prefabricated long pipeline sections, is with the technology available today a difficult and in any case a very expensive method.

A common feature of known methods is that most of the work has to be carried out in open seas, the methods are cost consuming and the work will extend over a long period of time in open seas. Different technologies are used in connection with the various parts of the pipeline, such as the pulling in to the platform, the crossing of difficult terrain, and the task of extending the pipeline to a shore station. Thereby, the enterprise -is divided into several contracts and one will lack a unitary optimalizing supervision of the enterprise.

Pipelines with large diameter are further, in accordance with known methods, located separately and are not designed to cooperate when they are subjected to loads from currents and waves, and neither are they designed for removal after use.

A known method for pulling a pipeline towards a platform is based upon initially installing a transition piece on the sea bottom, which thereafter is pulled into the platform. The pipeline is thereafter connected to the transition piece, usually by means of hyberbaric welding. If the job in question concerns two or more pipes which shall be connected to the platform, each of the pipes requires individual installation, a fact which requires sufficient space around the transition pieces in order to effect the joints to the pipelines.

The method is expensive, particularly if hyperbaric welding is to be used. Further, one can meet problems if expansion loops are to be built in to compensate for thermical expansion contractions of the pipeline.

Known methods for extending subsea pipelines up to a shore position, are: a) through a tunnel from a suitable location below the sea, b) positioning the pipeline along a ditch provided in the shore vicinity, or c) laying the pipe or pipes in concrete culverts or conduits.

All of these three methods are costly and time consuming.

Thus, concrete culverts require for instance large and heavy structures in order to obtain the necessary stability against wave and current loads.

The main object of the present invention is to provide a method and equipment for carrying out a complete installation of large diameter pipelines, small diameter pipelines and cables and the like on the sea bottom, including attachments or connections to the platform, the crossing of strongly varying sea bottom contour and bringing ashore the pipelines without implying those limitations which inherently are connected to the conventional technique in the field, simultaneously as the solution is attractive from an economical point of view.

The present invention relates to equipment and a method for prefabrication, temporary storage with subsequent towing and installation on a sea bottom of one or more, very long (also with large diameter) pipelines, simultaneously and along the same path, and also including a possibility for removal of the pipelines after the use period if suitable ballasting pipes are being used.

The pipelines are laid down on the sea bottom in conformity with the topography of the same, and with a possibility for pulling the pipeline in to the bottom construction, to the platform or for bringing the pipeline ashore. None of these working operations will require divers. The pipelines are being prefabricated ashore from standard 12 m pipe lengths, which are launched into the sea continuously and in the shape of larger lengths and - if not connected to permanent ballast piping - on to a supporting structure and in lengths of 3,5 - 10 km. If two or more pipelines with or without cables and including possible ballast pipes shall be located along the same path or route, they will be prefabricated simultaneously and launched as a unit. The pipeline section, or the bundle of pipeline sections, if two or more pipelines are to be put together and be launched as a unit, are towed away and stored in shielded waters, either floating or lowered down to the sea bottom until they are to be transported to and installed in offshore position. Welding toget- her of these long pipe sections may be carried out in offshore position at the installation site on a specially equipped vessel. Lowering down to the sea bottom takes place along a long, flexible and ballastable stinger or by means of permanent ballasting piping, if such is being used.

In other words, the invention can be used in connection with pipelines without special ballasting means as well as for pipelines including such ballasting means, usually in the shape of one or more ballasting pipes, possibly including other auxilliary equipment.

The method to be used in connection with installation of long, continuous pipelines along a path on the sea bottom, usually extending from a shore bay to an offshore platform, is thus of the kind whereby the pipeline or the pipeline unit is prefabricated ashore at a shore base and thereafter is transported out in the sea in floating or submerged position, and the method in accordance with the invention is generally characterized by the following steps: the pipeline or pipelines for the total laying distance are being made ashore in a precalculated number of long sections, each for instance having a length of from 3 to 10 km, and where each section consists of a number of welded together pipe elements each of which having a length of for instance 12 m, the pipe sections are transported/pulled out, one after the other, into the sea and are thereafter moved for temporary storage in shielded waters, preferably side by side, either in floating or submerged position on the sea bottom, whereafter the pipe sections are being transported/towed, preferably one after the other, out to the installation site along a preselected pipeline path on the sea bottom, and are thereafter correctly positioned in order to be joined to the aft end of the preceding section, the joining operation taking place in water surface position by means of equipment mounted on an auxilliary vessel, while the preceding, already joined pipe sections are being laid down continuously down on the sea bottom, while the end part of the pipe section which at any time is located close to the auxilliary vessel is being lifted up to water surface position at the vessel in order to be joined to the next section, whereafter the said pipe section is being submerged in controlled manner down to the sea bottom by means of suitable ballasting equipment.

The necessary tension of the system (pipes between sea bottom and surface supported either by the stinger or the ballasting pipe), joining vessel and the floating part of the pipeline section on the bow side of the vessel, positioning power (to counteract side current) and discharge power are not unduly great and are provided by means of a tug (a further tug is kept at hand as a reserve to take over in case of failure of the first one). The discharge power may be increased by using separate tugs, tension machine on the joining vessel, or by providing rotation force to the roller of the supporting structures. This, or tension machine, is not necessary when permanent ballasting pipes are used.

Prefabrication of the long pipe sections is carried out in suitable fashion along the shore. The space requirement for the assembling plant is relatively modest, since subsequent to said standard 12 m pipe elements being welded together, the ready made part of said pipe section is pulled out into the sea on a supporting structure, or without a such structure if sufficient buoyancy is provided by means of permanent ballasting piping. Some existing plants offer the possibility to weld together standard 12 m piping in lengths up to several hundred meters, which lengths thereafter are welded together and pulled out into the sea. Pipe sections having a length of 3,5 - 10 km are being towed for storage in shielded waters, either on the sea bottom subsequent to the supporting structure being removed for repeated use, or floating at the sea surface by means of the beforementioned permanent ballasting pipes. The method makes it possible to produce very long pipe sections before the laying operation, so that all or about all necessary pipe sections are ready for installation when the actual laying operation is to be initiated, a fact contributing to a drastic reduction of the total laying time. The supporting structure may also be used in connection with the towing operation to the laying site if separate ballasting piping is not being used.

On the laying site, the pipe sections are welded together on the vessel equipped therefore. The foregoing section is already laid down in correct position on the sea bottom, and only the rear end of same is hoisted up to the surface in order to be welded to the next section. The weight of this end is supported by ballast piping, or by a long, flexible stinger-like support means. The curvature of the pipe and thereby also the prevailing bending stresses are continuously controlled by means of tension supplied from a towing vessel connected to the free end of said section, which is floating at the sea surface, subsequent to the stinger or the ballast piping having been supplied with a correct quantity of ballast water, a condition which is simple to monitor by measuring the internal air pressure at the upper end of the stinger or the ballast piping.

The separate pipe sections are being prefabricated in complete fashion by means of auxilliary equipment and equipment depending upon the sea bottom conditions and other requirements which are assumed to be necessary for operational safety and correct functioning in installed position.

In most cases, it is desirable to provide the pipe lines with expansion loops close to the receiving location on the subsea or bottom installation or at the platform. This involves that the pipeline unit which is to be connected to such receiving station is being prefabricated complete with attachment piping, expansion loop or loops and with a total length sufficient for the free end of this pipeline section to be temporarily maintained at the sea surface in order to be coupled to the next pipeline section. Such a pipeline unit, hereinafter designated the pulling module, may present special problems when it is to be towed to the installation site. In order to facilitate and secure the towing operation in cases where the laying operation concerns two or more parallel pipelines each having an expansion loop, said two expansion loops may, by means of a suitable yoke and winches, be pulled towards each other, and thereby improve the situation during the towing operation. This end of the pulling module will, due to such tensioning, attain a larger draught, and thereby reduce the dynamic response versus waves and currents, and prepare the end part of the pulling module for further submersion during the work with coupling of this section to the installation at or close to the sea bottom. Pivoting of the pipe or pipes including expansion loops is made possible by the design of the pulling module itself.

If parts of the pipeline are to be laid in an area with largely varying sea bottom contour, and/or in areas with large loads from waves and currents (hydrodynamic forces, rolling stones or the like), the pipeline sections in question are provided with a permanent supporting and protecting external construction as a rigifying element. Such a construction will not be required if the pipeline section is provided with permanent ballasting piping, which also can be dimensioned to meet the prevailing loads in consequence of a varying sea bottom contour. By passing or crossing larger recesses or the like in the sea bottom, the pipeline unit can be supported by such a supporting construction or by means of said permanent ballast piping, which may offer a positive buoyancy and be anchored by means of cables or the like to the sea bottom.

Such a permanent supporting construction may be designed for installation of further piping subsequent to the installation of a first pipeline unit. A supporting construction or the permanent ballast piping may be provided with bulkheads or the like, which offers a possibility for selective ballasting, i.e. parts which are in contact with the sea bottom may be filled with water or even with heavier material such as concrete, while the parts which are crossing recesses in the sea bottom may remain wholly or partly filled with air. In some such cases, it will be necessary to increase the buoyancy of such parts in a free span which is anchored to the sea bottom.

Thereby, one can obtain sufficient rigidity in the system and thus enable the pipeline unit to sustain all prevailing hydrodynamic forces without risking undesired movements of the pipeline.

The pipeline section which is to be coupled to the installation ashore, is provided with a supporting construction based on the same principle as is being used in connection with a varying bottom contour, where the pipelines are positioned side by side and may slide coaxially in sliding supports, independent of each other and relative to the supporting construction. This construction is hereinafter designated "shore landing module". The possibility of the piping being able to slide coaxially is necessary in order that the final interconnection of the piping, which must be carried out in the vicinity of such shore landing module, can be carried out in the sea surface in the same fashion as the other pipe joining operations. At the transfer of the already interconnected part of the pipeline from the sea surface down to the sea bottom, the surplus length of the piping, which is created by the pipeline on either side of the coupling site forming an S-curve down to the sea bottom, is "absorbed" by pulling the pipeline towards the shore into the shore landing module. The shore landing module can also be prepared for possible supplementing pipelines which may be desirable to lay later. Sections of such supplementing pipelines are located and pulled into the shore landing module prior to the laying of the other pipeline units, which already have reached this location, whereafter the free end is brought up to the sea surface (by deballasting) and is being pulled to the laying vessel in order to be coupled to the other parts of the pipeline. Well ashore, the end of the pipeline is being pulled in to an extent such that the surplus length for the laying operation may be removed.

The invention and further features in connection therewith shall in the following be further described by means of some practical embodiments with reference to the accompanying drawings, wherein: Fig. la and Tb show schematically an arrangement of prefabrication on a shore site with continuous transport of pipeline sections in position on a supporting construction, and including a vertical sectional view showing how the pipeline sections are being loaded on to the supporting construction, Fig. 2a and 2b show schematically an arrangement of the storage site and the way of removing the pipeline sections from a supporting construction, Fig. 3a-c illustrate a possible coupling solution for a transport pipe with a permanent ballast pipe, Fig. 4a-c show three (two large and one small diameter) transport pipes provided with permanent ballast piping and being positioned down onto the sea bottom, Fig. 5 shows three pipelines positioned side by side without permanent ballast piping, Fig. 6a-c show a supporting construction for transporting a pipe section consisting of one single pipe, Fig. 7a-b show the same as Fig. 6a-b, but presenting a double pipe section, Fig. 8a-b show a laying operation by means of a stinger and a supporting construction in a typical state of the operations (welding on a platform), Fig. 9a-b show a laying operation by means of a permanent ballast pipe subsequent to the welding operation having been carried out, Fig. 10a-b show a suitable vessel provided with a platform for carrying out the interconnection of the pipeline sections, Fig. 11 shows the principle used in connection with a pulling module for two large diameter pipes, including expansion loops designed for being connected to a platform, Fig. 12 shows an arrangement of a part of a pulling module, including expansion loops, Fig. 13a-c show auxilliary equipment and arrangements for towing of expansion loops into a pulling module, Fig. 14a-b show a towing operation of a pulling module, designed to be pulled to a platform and wherein the pulling module is positioned temporarily on the sea bottom, due to an emergency situation, Fig. 15a-f illustrate the installation of a pulling module, Fig. 16a-d show supporting constructions for three transport pipes designed for an uneven bottom contour, including shore bringing of the same, Fig. 17 shows an example of a design of the route for the pipeline, in order to bring the shore bringing module to the shore (vertical view), Fig. 18a-f show the main operations in connection with a pipe laying project consisting of fabrication, transport and installation of the pulling module including expansion loops, laying the pipeline across a sea area and shore bringing to a shore based terminal, by means of the shore bringing module.

Fig. la illustrates a preferred method for prefabrication of the pipeline sections. The standard 12 m pipe elements or prewelded 24 m elements, or even longer units 1 are welded together in a welding hall 2 located close to the shore. As the welding together of the pipe elements proceeds, the readymade part of the section is intermittently or continuously pulled or pushed out in the sea, while supported on a suitable support 3. If more than one pipeline shall be laid out at the same time, both pipelines are welded together simultaneously and provided with interconnecting straps or the like, and other necessary implements. Details in this connection shall be described later. The readymade part of the pipeline sections are preferably pulled out by means of a winch 7, located on a vessel 6 which is made stationary by means of anchorings 8. If the pipeline sections do not include special pipelines for ballasting purposes, the pipeline sections are pulled out on a ballastable support construction 4 which is moored by means of anchors 5.

When each of the pipeline sections are made ready for installation, it is transported in floating state to a suitable temporary storage site, for instance in a shielded bay (Fig. 2), where the pipeline sections one after the other are being stored, either in floating state or submerged down to the sea bottom.

Fig. lb illustrates in a vertical section how such a pipeline section 9 is pulled out from the welding hall 2 along a suitable support 3 out into the sea by means of a winch 7 on a vessel 6, and being supported on a supporting construction 4.

Fig. 2a and b illustrate how a pipeline section 9 is being pulled by the supporting construction 4 by means of a winch arrangement 10, positioned either ashore or on a barge 11, moored with anchors 12, simultaneously as a vessel 13 drags the supporting construction away. This version of the method is preferably utilized if the pipeline unit is not equipped with ballasting piping. It should be mentioned that the separate lengths are positioned side by side 14 on to a prepared sea bottom bed 15, so that they, when hoisted up, slip away from the sea bottom without causing a risk for undesired accumulation of ground masses around and on top of the pipe units.

The pipeline sections, which may have a length of for instance 3,5 - 10 km, may be joined together by one or more pipe lengths which are to be used for transport of hydrocarbons for ballasting purposes, or for control or monitoring purposes. In other words, a pipeline section as herein described, shall designate a complete unit including necessary equipment for connecting or joining of such pipe lengths together, and also taking into account the prevailing sea bottom conditions and the various functions which the pipeline units shall cover.

Typical joining methods for pipeline sections which are to be installed on an even sea bottom, are shown in Fig. 3, wherein Fig. 3a, b and c illustrate a solution for a transport pipeline 16 which is connected to a superimposed ballast pipe 17, connected together with spacers 18 and straps 19 (shown with stitched lines in Fig. 3a). The transport pipeline, which in this embodiment is made without weight mantle, is given a corrosion resistant layer. This layer is also resis tant against blows, for instance from fishing gear. The transport pipeline rests in this embodiment on fundaments 20, which by their weight offer stability and provide a well defined fundament, and also lifts the pipeline somewhat up from the sea bottom. This reduces the effects from currentand wave loads, and thereby the requirement to submerged weight (the size of the ballast pipe). The spacers 18 and the fundament 20 are such configurated that they do not represent any obstruction for fishing gear.

Fig. 4a-c illustrate two transport pipelines 16 which are bundled togehter with a ballast pipe 17, with spacers 18 made for instance of concrete, including straps 19 and provided with a layer 21 of elastic material between the pipe end and the concrete in order to reduce shear loads in parallel direction with the longitudinal axis, and thereby reducing bending stresses, for instance in connection with positioning and installation in a direction at an angle relative to currents.

In order to improve the stability, the underside of the concrete blocks are provided with pins 23 in order to provide firm contact with the sea bottom. None of the shown embodiments demand use of special weight mantles. A lining 22 provides safe passage of trawl gears and the like. On the underside of the ballast pipe 17 is mounted in protected position a control cable or pipe 24, of small diameter, by means of special straps 25 which can be mounted at a different spacing from each other than the straps 19.

Fig. 5 shows two transport pipelines 16 arranged together in accordance with the present invention by means of a supporting construction which is removed subject to use in connection with transport and installation. In replacement of spacers (giving weight and fundament, and keeping the pipes together), the transport pipelines are here provided with a weight mantle 25. The demand on this weight mantle is however considerably reduced relative to what would be the case if the pipelines had been installed in accordance with conventional methods, i.e. in relatively large distance from each other. By installation in accordance with the invention, the pipelines may be placed adjacent to each other and will so to speak cooperate with each other from a hydrodynamic point of view.

The pipe 24, which in this case is intended for use for instance in connection with transport of chemicals to an offshore production plant, will be positioned well protected against currents and contact for instance with trawl gear. With one or more permanent ballast pipes, the weight mantles may be replaced by lighter ballast weights, for instance as combined spacer means between the pipes and the "fundament". These fundaments improve the side stability of the pipeline and will lift the pipe up from the sea bottom and thereby reduce hydrodynamic forces and the need for submerged weight.

Fig. 6a, b and c show a supporting c not to be continuous or throughgoing, and can for instance consist of lengths which follows those units which include the supporting construction. The frames 27 including roller and sliding bearings 28 are made as streamlined as possible, and the bearings should be so large that the rolling resistance is kept at an acceptable level. Some of the roller bearings may, if desired, be equipped with a power drive. Suitable spacers 18 will facilitate a quick mounting job, the necessary flexibility and low bending rigidity, and distribution of the various point loads. A bottom plate 29 renders support against the sea bottom, for instance when a towing operation is initiated subsequent to storage on the sea bottom. Axial forces are sustained by tension legs 30. The interconnection of the buoyant piping 26, the ballast piping 17 and the frame 27 can be provided by means of straplike fasteners 31.

Fig. 7 illustrates how a supporting construction can be adapted for transport and installation of a pipeline section, consisting of two pipelines. It shall be understood that the shown supporting easily can be modified for supporting more that two pipelines, and pipelines having other dimensions.

The supporting construction is assembled with elements having suitable length, for instance 250 m to a unit of for instance 3 - 5 km. The elements are mechanically coupled together in a releasable fashion in order to sustain the prevailing forces therebetween, and the ballast piping is being coupled together to one continuously extending chamber.

In emergency situations where it is difficult to carry out loading or towing with available towing power, the mechanical coupling may be released (with a slack limiting means, for instance a chain), in order to concentrate the loading power to that section which creates too large resistance.

An assembled supporting construction is finalized at the ends with unitary equipped end pieces. These means include the following special parts: - fastenings for towing cables - valves for ballasting - couplings to the coupling platform - supplementary buoyancy which is utilized in connection with the picking up of pipeline sections from the sea bottom (only at one end) - means for monitoring the end of the pipeline section during the hoisting operation - a removable means for brushing and jet cleansing of the pipe surface during the hoisting operation from the sea bottom at the storage site, or by removal of one or more pipelines after use of the pipeline - means for holding the pipeline or lines during transport.

Interconnection or coupling and laying of a pipeline WITHOUT permanent ballast piping shown in Fis. 8a and b.

The method is comparable with the known S-method, but the pipeline 32 is entirely (or almost) supported by a "stinger" 33 which is substantially identical with the supporting construction shown in Fig. 6 and 7, except for the length, which is shorter, about 3 to 5 times the water depth. Correct dimensioning relative to the buoyancy and the submerged weight results in that the sum of the forces on the stinger and the pipeline in the vertical plane is small. In dependence upon the dimensions of the ballast pipe, the sum of the forces from the sea bottom to about half of the water depth will be downwardly directed (submerged weight), and further at the water surface be directed upwards (net buoyancy). These forces will be small, implying that also the demands to tension in order to maintain prevailing bending stresses within given limits will be small, and may usually be taken care of by an oceangoing towing vessel 34. Anchoring is not necessary, and will otherwise, paying regard to the large potential laying velocity, constitute a limiting factor.

The interconnection of the pipeline 32 supported by the stinger 33 and with the pipeline section 9 supported by the supporting construction 4 is carried out on an especially designed platform 35 on a vessel 36 (Fig. 8) in accordance with in per se known methods. The requirements to the vessel 36 will be: - small movements caused by wind and currents - own machinery to compensate for possible currents, so that these are not transferred to the pipelines.

The loading is controlled by a thug 34.

During the first stage of the loading, the resistance from the unloading of the pipeline section 9 from the supporting construction 4 and the pipeline 32, now coupled to the section, from the stinger 33 will be greater that the propulsion forces due to the gravity forces on the pipe 32. It will therefore be necessary with an increased power from the tug 34, possibly also from a special tension means on vessel 36, not shown, and/or separate propulsion means on the supporting construction 4 in order to pull the stinger 33 and the support construction 4 and the vessel 36 in direction away from the pipelines 32 and 9. The loading resistance will decrease proportionally with increasing length of pipeline which has left the support construction 4 and later the stinger 33 and has been lowered down to the sea bottom. Prior to this situation, the pipeline section 9 must be held back and the loading must be controlled either by means of a tension means, or by means of cables, coupled to one or more winches placed on vessel 36. An auxilliary tug vessel 37 is coupled to the construction 4, and this vessel will in case of failure in the main tug vessel 34 automatically take charge of control and monitoring of the support construction 4 and the stinger 33.

The laying of parallel pipes without permanent ballast piping positioned close to each other, is advantageous for the following reasons: 1) to reduce hydrodynamic forces and thereby reduce the requirements to the weight mantle, 2) to bisect the number of hits or knocks caused by the passing of fishing gear crossing the pipelines, 3) to create protection of pipelines and cables which may be placed between the main pipelines.

Intercouplinq and laying of pipelines WITH permanent ballast sining.

Joining of pipeline sections provided with permanent ballast piping is carried out similarly to the methods used in connection with pipelines sections without permanent ballast piping on a platform positioned on a vessel as shown in Fig.

8. The pipeline units are floating without help from special supporting constructions, ref. Fig. 9, which illustrates the situation subsequent to the pipeline sections 38 and 39 having been coupled together and the laying operation has been initiated.

When the joining of the pipeline sections is finalized, these sections are deloaded from the joining platform 35 to floating condition. The platform 35 is thereby liberated and can be moved to the free end of the pipeline section 39. At the same time, the ballasting of the part of the pipeline section 38 which is ready for laying is initiated, resulting in that it assumes an S-form which gradually propagates along the pipeline. The ballasting is finalized at the end of the pipeline section 39, which still is floating at the sea surface. The end of the pipeline is attached to the platform 35.

A new pipeline unit 39 is coupled up to the platform 35, whereafter a further joining operation can be initiated.

Fig. 10 illustrates one of a series of possible solutions for an intercoupling platform and a vessel for supporting the same. A mini semi submersible vessel 36 supports a platform 35. The platform is such designed that the pipeline section 38 and 39, which shall be coupled together, are arranged in horisontal position along the sea surface (i.e. without being subjected to any bending stresses). The ends of the pipelines are being fixed on the platform without relative movements, and in correct position for carrying out the joining operation. During the entire joining operation, all forces and loads from either side of the pipeline section, with or without supporting constructions, are being transferred to the platform. During the work, the workingsarea 40 on the platform is emptied for water. The working area is designed as a dry dock, wherein the end walls including through passages for the pipe ends, are being equipped with sealing means (not shown). Subsequent to the coupling operations, the dock is filled anew with water, the fixation of the pipeline is released and removed, and the pipeline (constituting an intercoupling of pipeline 38 and 39) may float free at the sea surface.

The platform or the dock 35 is displaced laterally and the floating pipeline is thereafter removed from the platform.

As mentioned before, it is presupposed that possible expansion loops shall form part of a prefabricated pulling module. Such a prefabricated pulling module is schematically shown in an example for two pipelines shown in Fig. 11, and consists of three sections: 1): the pulling- and spacing section 41 which guides the pipelines 44 to correct position and direction in order to be pulled into the sealing- and pulling pipes 45 mounted in the platform 46, 2): a section with expansion loops 42, including the buoyancy- and ballast piping 47, and 3): a coupling section 43 which can be ballasted so that the end 48 floats at the sea surface and the remainder of the section forming a slack S-form between the sea bottom and the sea surface when it shall be coupled to the subsequent (regular) pipeline sections which are described and shown in Figs.

8 and 9. Section 43 is principally equal to the unit shown in Fig. 6 and 7, and is not removed subsequent to pulling of the piping 44 into the platform 46, since it remains a part of the installation. Section 41 possesses the same characteristic features as section 43, but forms in addition one in the vertical plane tilted ramp which is guiding the pipeline 44 in correct level relative to the pulling pipes 45. If the pipes 45 are not parallel, or have a different distance than the pipes 44 on the prefabricated unit, the section 41 is such designed that the lines 44 attain the desired direction in order to carry out the pulling. The sections 41 and 43 permit the pipes 44 to slide along and rotate about the longitudinal axis. A rotation of the pipeline is necessary prior to launching from ashore in order to subject the expansion loops 49 to a desirable angle during towing and subsequent to the lowering operation in order to return the loops to a horisontal position. This design will be described in detail later.

A sliding of the pipes 44 in the sections 41 and 43 is necessary during the pulling operation into pipes 45 and during the later operation, when the pipes 44 are being subjected to expansion and contraction due to temperature- and pressure variations. The end of the pulling module is equipped with a special end piece 50 which guides this end of the expansion module into the complementary pieces 51 on the platform 46 and thereby, one obtains an accurate positioning.

The pulling module (permanent): - guides and supports the pipelines into the platform, - may include expansion loops, - the expansion loop or loops including the pipe sections can pivot about the longitudinal axis to be positioned for transport/towing, - the module is being towed at the sea surface, but can be submerged down to the sea bottom for temporary protec tion, for instance against waves.

Fig. 12 illustrates several important details in connection with the section with the expansion loop (supporting construction on adjacent section, wherein the pulling- and distance section 41, including the coupling section 43 are not shown). The loop section 42 consists of the following main parts: pipe fitting parts 52, supports 53 with pipe brackets 54 on the one side, and slide bearings 55 on the other side, slide fundaments 57, elements 58 transferring tension between the pulling section 41 and the coupling section 43 during transport and installation of the entire prefabricated pipeline units, buoyancy and ballasting piping 47. The pipelines 48 including the expansion loops 49 may be pulled into the platform individually. The support construction 4 for sections 41 and 43, including fundaments 56 and 57, are immovable during the pulling of pipe 48. When the pipe 48 is pulled into the platform, the pipe fitting part 52 slides into the support construction 4 for section 41 and the loop 49 with supports 53 and the buoyant piping 47 slides along the sea bottom, supported by the sliding fundament 57 towards the platform. A corresponding length (some few meters) of the pipe 48 will slide out of the support construction 4 in section 43. When the production of hydrocarbons is initiated, the pipes 48 will be heated up, and the loop 42 will enter into function. The pipe in loop 49 will be deformed and slide horisontally in slide bearings 54. At production breaks, the pipes will cool off, and the loop will slide back to original position. The expansion loops are difficult to transport in floating horisontal position due to wave induced movements and forces. Towing of one separate pipe including expansion loop may be carried out with the loop in vertical position. Fig.

13a, b and c illustrate a preferred method, where one single pulling module having two expansion loops can be towed to an installation side in an advantageous fashion.

The expansion loops 49, which initially or before transportation has been initiated, float in a more or less horizontal fashion to either side, are pulled up towards each other prior to towing by means of a frame 59 and wires 60 which are tensioned and winded up by means of winches 61 until the two loops attain a V-shape.

As mentioned previously, the pipelines in the pulling section 41 and the coupling section 43 are supported such that a rotary movement along the longitudinal axis will not be hindered. Weight and buoyancy of the loop is such dimensioned that when the loops are in transport position, the tension in the wire 60 will be greater than expected wave forces on the loop 49, so that the loops, the wires and the frame behave in the wave as a solid or rigid body.

Fig. 14 illustrates a prefabricated pulling module 62 with expansion loops 49 in two phases. In Fig. 14a, the pulling module is shown under towing to the installation site by means of two towing vessels or tugs. The tug 34 does the towing while the tug 63 assists when passing narrow waters and later during transfer of the pulling end 48 to the sea bottom.

In Fig. 14b, the pulling module is shown in position on the sea bottom. Such a situation is not in accordance with a regular procedure, but may incidentally occur due to unexpected bad weather which may lead to the towing having to be discontinued. The pulling module 62 will rest safely on the sea bottom, while the tugs 34 and 63 stand by in a waiting position with the towing wires 64 and auxilliary wire 65 either released or not released.

Fig. 15 illustrates the separate steps during manoeuvering of the prefabricated pulling module 62 with expansion loops 47 to a platform 46, and the separate operations which are carried out in connection with the subsequent pulling of the pipeline into the platform. The separate steps illustrated in the figures are: a: The towing unit arrives at the installation site, the auxilliary wire 65 is coupled to pulling wires 66 and kept at the sea surface by means of a buoy 67.

b: The tug 63 couples the line 68 to the inlet on the loop 47. The line is utilized in later steps for ballasting of the buoyant piping in the loop and for operating winches during laying/rotation of the loops at the sea bottom.

c: The tug 34 tightens up the towing wire 64 and the pulling wire 66. The end 48 of the pipeline unit 62 is being pulled down into the water and will pose or reduce the net buoyancy of the pulling module 62. A winch on plat form 46 is actuated to pull in the wire 66.

d: The end 48 of the pulling module 62 is pulled in until the platform comes to rest against a special biasing or contact member 51.

e: Ballasting of the pipeline unit 62 has started. The pipeline unit 62 forms a slack S-curve which propagates with controlled velocity to the tug 34 until the entire ballast pipe is being filled with water and the free end 69 of the unit 62 is lowered down to the sea bottom by means of a winch on the tug 34 which controls the towing wire 64. The Figure illustrates the S-curve in several typical positions.

f: The entire pipeline unit 62 is positioned along the cor rect route along the sea bottom, the loop is ballasted by tug 63, whereafter the loop by means of winches is moved down to the sea bottom, the frame with the winches is removed and finally, the pipelines are cleared for indi vidual pulling in to the platform by means of two wires which connects the respective ends on the pipelines to winches inside the platform. These wires are coupled to the pipelines at a suitable moment, either prior to lowe ring of the unit 62, or by means of a remotely controlled submarine vessel subsequent to the unit 62 having been positioned down on the sea bottom.

In those cases where the pipeline units shall pass especially broken-up or hilly terrain on the sea bottom, or in areas with particularly large wave actions, currents or moving material occurring along the shores, the prefabricated pipeline units should be supported and protected by a support construction as shown in Fig. 16, which is built on the same principles as the support construction shown in Fig. 6.

Further to providing transport and installation of the pipeline unit, such a permanent construction will support and protect the pipelines during the entire lifetime of the pipeline by liberating the pipelines relative to bending stresses due to varying bottom topography, from tension and strain concentrations due to local bumps in the sea bottom like rocks, it will shield the pipeline from hydrodynamic forces induced by waves and currents, and further protect the pipeline from other mechanical stresses. In that the ballast piping is being filled with air, and assuming the desired net buoyancy for a particular part of the pipeline unit, the pipeline unit can thereby be kept at a desirable level above the sea bottom, at which level it can be kept in place by means of cables or the like attached to bottom anchors. The support construction can also be used in combination with other known means, such as supports of various kinds and also by removing large "bumps" in the sea bottom contour.

A construction for crossing uneven sea bottom contour (permanent) being adapted for selective ballasting in accordance with varying sea bottom support conditions, consists of: 1) sections in contact with the sea bottom: ballasting with water, or heavier materials such as concrete, 2) free spans over a certain length remain in the water in pertaining parts of the buoyant piping. At longer spans, where the pipeline is kept in correct position by means of cables/tension stays, the buoyancy may be improved by improving the diameter on the buoyant piping and by use of several buoyant pipelines.

The construction may be used in connection with other known solutions in connection with free spans, like levelling of the sea bottom, special supports etc.

Constructions used for bringing the pipeline ashore will in principle be equal to the constructions for crossing uneven sea bottom contours. The following tasks come in addition: - preparation for positioning of future pipes, - the end of the pipeline must be equipped for ballasting/ deballasting.

The support construction for transport is preferably composed of sections which together form the desired total length, and in order to solve problems in connection with the deloading or installation if the frictional resistance during deloading exceeds the available pulling force. The end section is especially designed for loading or hoisting up prefabricated pipe sections stored on the sea bottom, and is arranged with separate buoyancy means, controls and means for cleansing the pipe surface.

Fig. 16a and b show the support construction or that part of the same which is adapted to rest on an uneven sea bottom.

The unit in this example show a shore bringing module comprising three large transport pipelines 70, a smaller service pipe 24 and two buoyancy- and ballasting pipelines 71. Pipe 71 rests on a support 72 against an upper part 73 and a lower frame 74. The ballast piping is filled with air during the transport, and is being filled with water during the installa tion, or, if a large weight is desirable, with heavier masses, for instance concrete. In places where net buoyancy is desirable, these pipes will remain filled with air.

The stingerlike support construction which is attached to the vessel for laying of the pipeline without separate buoyancy, is in principle equal to the described support construction and is equipped for: - formation of a symmetrical S-curve (submerged weight of the lowermost section equals the net buoyancy of the uppermost section and the section at the sea surface, - contact with the sea bottom and sliding along the same.

If the installation conditions make it appropriate to work with only a small bending stiffness, the joint between ballast pipes 71 can be made somewhat flexible by means of a rubber list 75 which does not transfer shear forces, but permits bending. On the top side of the construction are placed protective mats 76, protecting the pipelines 70 and 24 against falling objects, large stones which are rolling due to heavy wave actions or the like. Lower frame 74 is equipped with fundaments 77, which can adapt themselves to an uneven basis by having flexible fundamentation, for instance in the shape of links 78. The underside can also be prepared for means which will improve the support conditions (incidental free span) by for instance using jackable support, sand bags, bags which are being injected in situ with masses which will rigify or the like (not shown, known technique). The transport pipe 70 rests on roller/slider bearings 28 which permit axial movements during prefabrication, interconnection with standard pipeline sections and during possible movements caused by expansion or contractions.

Fig. 16c and d illustrate a method to be used when it is desirable that the shore bringing module/support construction shall be kept at a distance above the sea bottom, for instance for crossing a cleft or crevice in the sea bottom. The joining of the transport piping and ballast piping will be largely the same, but in this example, the ballast piping is kept filled with air all the time, with an air pressure equal to or greater than the prevailing hydrostatic pressure, in order to reduce any risk for leakages. The construction is preferably made rigid in that the ballast piping on either side co-operate in that the rubber list 75 is omitted, and bracings 79 are provided. The unit is kept in correct height above the sea bottom by means of anchorings 80 and 81. Correct positioning of the unit takes place by a longitudinal adjustment in connection with the placing of the anchors on the sea bottom (not shown). Net buoyancy of the unit is being balanced during transport and installation by means of temporary weights 82, for instance a chain formed by concrete blocks, which are removed subsequent to the construction being installed and the anchorings 80 being tightened. When the weights 82 have been removed, the anchorings 80 and 81 will attain the desired tensioning, which tensioning can be adjusted if needed.

Fig. 17 shows the shore bringing module 83 placed along a difficult sea bottom in connection with the shore bringing along a rather uneven sea bottom 84 (the heights are about ten times the lengths in this Fig.). The construction 83 extends along a length 85. The conditions outside the construction downwards are such that pipelines 70 may rest freely on the sea bottom. On the opposite end, the conditions are such that the pipeline 75 may either rest unprotected or will be above the sea surface and the "splashing zone". In longitudinal direction, the shore bringing module 83 consists of three characteristic sections 86, 87 and 88 in regard design of the various cross sections, which are adapted to the different tasks these sections will meet. Section 86 is a "standard" section with support and protection as the principal task.

Section 87 may "absorb" the surplus length of the pipeline 70 and pipelines lifted out from section 88 to the sea surface in order to be coupled to adj -ent pipeline sections. Section 88 is such designed that the pipelines can be lifted out of the section for intercoupling purposes. With regard to buoyancy and weight in operation, the construction 83 consists of sections 89, 90, 91, 92 and 93. Sections 89 and 91 have relatively large submerged weight in order to secure stability of these sections when subjected to waves and currents. Sections 90 and 92 have a sufficiently large net buoyancy to keep the tension in the vertical anchoring cables 80 greater than the greatest expected downwardly pointing wave load on the construction 83. This will be required in order to avoid vertical movement of the construction 83 and slack in the anchorings 80 anchored to the sea bottom 84 or to anchor blocks 94. Lateral loads are absorbed either by the stiffness in the construction 83 which is dimensioned for or in combination with lateral anchorings including stays/wires (not shown). Section 93 extends above the sea bottom where - due to water depth and bottom contour - both weights and the basis are more advantageous, and the construction 83 may rest with its own weight (the support pipelines being filled with water) without other precautions.

Fig. 18 illustrates the most important steps in connection with the laying of one or more pipelines from a platform 46 to a shore plant 95 where the same technology (based on prefabrication of pipe sections ashore) and operations (based on towing and ballasting) is used in order to create a unification of the entire project including transport and pulling of pipelines with expansions loops into a platform, laying of pipelines from a platform to a shore location, and shore bringing of pipelines to a shore plant also crossing a submerged shore site, possibly also with the passing of areas with difficult sea bottom. The typical steps will be: a: towing of the pulling module 62 including expansion loops 47, b: submersion of the pulling module 62 by means of wire pulling from platform 46, c: intercoupling of prefabricated sections 38 and 39 which will be carried out on vessel 36 where a part of the pipelines 32 have been installed on the sea bottom, simultaneously as the shore bringing module including pipeline sections 83 at the shore bringing are under installation, d: the shore bringing module 83 has been installed and the laying of pipeline 32 proceeds, e: the ends of the pipeline sections 97 installed in the shore bringing module 83 are being deballasted up to the sea surface, are guided on the intercoupling platform on vessel 36 and are subjected to welding with the pipeline sections 38, f: the intercoupling is being finalized, pipeline sections 38 and 97 are being pulled downwards against a buoyancy force by means of a winch 98 and wires 99, in which final step, in order to reduce the tension in wire 99, the sections 38 and 97 are slowly ballast controlled directed from the vessel 36 through pipelines 100 until the entire section 38 and 97 are being laid down on the sea bottom and in to the support construction 83 and the ballast piping in section 97 is being filled with water.

Claims (19)

1. Method for installation of long, continuous pipelines along a selected course on a sea bottom, usually extending from a shore base to an offshore platform, of the kind where the pipeline or the pipeline unit is prefabricated ashore at a land base and thereafter transported out into the sea in floating or submerged state, comprising the following features: the pipeline or pipelines with sufficient length for the total laying distance are being made ashore in the shape of a precalculated number of long sections, each for instance having a length of from 3 to 10 km, each of which sections consists of a plurality of welded together pipe elements each of which having a length of for instance 12 m, the pipe sections are being transported/pulled out one after one into the sea and are thereafter moved for temporary storage in shielded waters, preferably side by side, either in floating position or in a submerged position on the sea bottom, the pipe sections are thereafter transported/towed, preferably one after one, out to the laying position along the preselected pipe course, and are thereafter located in correct position to be welded to the outer end of the preceding section, the joining operation taking place in water surface position by means of equipment provided on an auxilliary vessel, while the preceding, already joined pipe sections are being laid continuously down on the sea bottom, so that the end part of the section which at any time is close to the auxilliary vessel is being lifted up to the water surface at the vessel in order to be joined to the next section, whereafter the actual pipe section is being submerged in controlled manner down to the sea bottom by means of suitable ballasting equipment.

2. Method in accordance with claim 1, wherein the pipeline or pipeline unit is being joined together permanently with one or more ballasting lines having adjustable buoyancy, which lines are gradually ballasted subsequent to being joined at the auxilliary vessel such that the pipeline or pipeline unit is heavier than water and is submerged down on to the sea bottom simultaneously as the vessel is moving along the selected laying path on the sea bottom.

3. Method in accordance with claim 1, wherein a pipeline or pipelines are used without built-on or built-in buoyancy elements, the pipelines being submerged from the vessel to the sea bottom by means of a not permanent, buoyant, flexible support structure which is attached at the delivery side of the vessel and is pulled away and removed from the pipeline or pipelines as these are laid down on the sea bottom from the support structure.

4. Method in accordance with claim 1, wherein the pipe sections, after having been produced and made ready at the land base, are pulled out in the sea one by one on a floating, supporting, elongate structure placed in the sea in a stable manner and whose length substantially corresponds to the length of the pipe section, said support structure being provided with slide/roller bearings to receive the pipe section, whereafter the support structure with the pipe section is moved to the temporary storage location where the pipe section is put down, whereafter the support structure is moved back to its prior position to receive a new pipe section, etc.

5. Method in accordance with claim 4, wherein two or more pipe sections are placed on the support structure before moving it to the storage location.

6. Method in accordance with claim 5, wherein a support structure is used for transport of one single or several pipe sections simultaneously to the vessel at the laying position, where the support structure is moored to the vessel, whereafter the pipe section is pulled in by a part of the support structure for joining to the end of the preceding pipe section or sections which are placed in joining position at the vessel.

7. Method in accordance with claim 6, where the pipe section or sections comprise a socalled pulling module intended to be joined to the platform, wherein the pulling module (Fig. 11) initially is coupled or joined to the platform, whereafter the other end of this module is floated or pulled up to the sea surface, where it is joined to the next pipe section, etc., until the pipeline reaches the landing place where the last pipe section in the row is joined to a pipe section into a socalled landing module which extends ashore.

8. Method in accordance with claim 3, wherein said not permanent supporting construction for the pipeline is equipped with one or more ballast pipelines with sectionwise adjustable buoyancy, adapted in such a way that a certain, small net buoyancy is obtained at empty ballast pipe, and a small submerged weight with water filled ballast piping including attached pipeline section.

9. Method in accordance with claim 1, wherein is utilized pipeline sections which in a per se known fashion are provided with one or more permanent ballast pipes, and in that said pipeline sections subsequent to fabrication at a shore base are pulled out into the sea and are moved in floating state to said temporary storage site and which by ballasting are submerged down to a selected storage bed on the sea bottom.

10. Method in accordance with claim 1 with use of pipeline sections provided with permanent ballast piping, wherein said pipeline sections are floated out on own buoyancy to the installation site, where one end of the preceding pipe section is loaded up on a working platform, the one end of the floating, not interconnected pipe section is positioned in working position on said working platform and is joined to the end of the preceding pipe section, whereafter the vessel including the working platform is displaced forwardly to the free end of the said joined pipeline section in order to be utilized for joining the said free end to the end of the next pipeline section, etc., simultaneously as the preceding pipeline section is submerged down to a position on the sea bottom subject to synchronized ballasting.

11. Means to be used with the method in accordance with claim 1, to join pipeline sections, said means comprising one on the vessel installed supporting platform including a ramp provided with a troughshaped, longitudinal dock having openings in both ends for reception/discharge of pipeline sections, and means for sealing off the openings in the dock towards the pipelines, and equipment for alternative dispensing and filling of water in and out of the dock in connection with the joining of introduced end parts of pipelines from respective ends of the dock, respectively for removal of joined and readymade pipelines from the dock.

12. Means in accordance with claim 11, where said platform with the dock may be displaced laterally in the horisontal plane relative to the vessel, and possibly also in the vertical plane.

13. Means in accordance with claim 11, wherein one end of a ballastable stingerlike support construction is permanently attached to the outlet of the dock on the vessel, in order to support and provide dispensing of a pipeline or pipelines without separate buoyancy, which are joined together in said dock, which support construction with pipeline or lines to be laid down on the sea bottom, normally extends down in an Scurve towards the sea bottom during the laying operation, simultaneously as the vessel moves forward in the direction of the laying course, which support construction at the lower end is provided with suitable means such that the construction may slide towards the sea bottom.

14. Support construction to be used with the method as claimed in claim 3 for transport of pipe sections which are not provided with permanent ballast piping or the like, wherein said support construction is divided up in a number of separate lengths or elements, each for instance having a length of 250 to 300 m, and which are releasably coupled together for instance by means of links, chains or the like, so that the support construction assumes a length of for instance 3 - 5 km.

15. Means on support construction in accordance with claim 14, wherein said end element is provided with means for loading or hoisting up prefabricated pipe sections which are stored on the sea bottom, which means include equipment for ballasting control, manuoevering and mechanical gear for handling the pipe sections.

16. Means on pipe section to be used in the method as recited in claim 7, wherein said pulling module is provided with expansion means, which expansion means include a sideways outwardly directed expansion loop in each of the pipelines in the pipeline unit.

17. Means at the pulling module in accordance with claim 18, wherein said expansion loops are suspended in handling equipment so that the loops may be pivoted and angularly fixed relative to each other about the longitudinal axis of the pipeline.

18. Means on pulling module in accordance with claim 19, wherein said expansion loops may be pivoted such that they point in diagonally opposite directions and extend substantially in a common plane, and in that each of the expansion loops are provided with sliding elements designed to slide against the sea bottom during the pulling operation towards the platform, and thereby serve as a guide.

19. Means at pulling module in accordance with claims 18-20, wherein said pipe ends in the pulling module facing the platform during the mounting are passed into complementary pipe ends mounted inside the platform and with a somewhat larger dimension, such that a telescopic coupling is provided between the receiving pipe elements in the platform and the pipelines, where the clearance or space between the opposed pipe surfaces subsequent to the mounting is being sealed off, for instance by concrete.