WO2015040374A1

WO2015040374A1 - Improved pipe laying methods

Info

Publication number: WO2015040374A1
Application number: PCT/GB2014/052803
Authority: WO
Inventors: Alex Mackinnon
Original assignee: Mackinnon Marine Limited
Priority date: 2013-09-18
Filing date: 2014-09-16
Publication date: 2015-03-26
Also published as: GB2518376A; GB201316599D0; GB2518376B

Abstract

Improvements to reel laying methods that do away with the need for onshore spoolbases by manufacturing pipe stalks using an s-lay vessel (3) and temporarily storing the pipe stalks on the seabed (55) in relatively shallow water at a near-shore location (55) close to the offshore installation site. Several pipe stalks can be laid in parallel as completed full length pipelines (1). A reel-lay vessel (3) thereafter retrieves and spools pipe stalk directly from the seabed (55) onto reel in a single operation without the delays associated with performing tie-in welds. The inventive method means that the entire pipeline (1) may be pressurised during spooling to preserve the integrity of lined or clad pipes, and to combat ovalisation.

Description

Improved Pipe Laying Methods

The present invention relates to the field of offshore oil and gas production and specifically the laying of pipe to construct subsea infrastructure. More specifically, the present invention concerns improvements to reel laying methods that do away with the need for onshore spoolbases.

Background to the Invention

The transportation of oil and gas from offshore locations to production facilities, from production facilities to onshore facilities, or indeed between production facilities, relies heavily on subsea infrastructure which is typically constructed by laying a network of pipelines. Pipeline installation methods involve the use of pipe laying vessels which are capable of laying pipe directly on the sea bed; generally using "S-lay" or "J-lay" installation techniques. "S" and "J" refer to the shape of the pipeline between the vessel and the seabed resulting from the direction in which pipe is fed out from the stern (or moonpool) of the pipe laying vessel as it travels along the lay route (substantially horizontal or vertical, respectively).

In either operation, a pipeline can be constructed length-by-length (sometimes referred to as "stove-piping") in which case adjoining sections of pipe are welded on board as the pipe is paid out, or by reel lay in which case a continuous length of pipe is deployed from a reel around which the pipe has been wound. The reel may be disposed horizontally or vertically on a reel barge; although if oriented horizontally the installation method is usually restricted to S-lay techniques.

Reel laying has a number of advantages over stove-piping; stove-piping is typically slow as operations need to be periodically interrupted in order to weld new lengths of pipe to the pipeline which limits fabrication speeds to one or two miles per day (approx. 1.6 to 3.2 kilometres), whereas reel lay operations are capable of laying one or two kilometres per hour. Furthermore, the welds or "field joints" need to be coated - a step that may add 15 or 20 minutes per weld. Nonetheless, as described below, stove-piping remains competitive despite comparatively slow lay rates because of inherent drawbacks in conventional reel lay operations.

Conventional reel lay operations begin by constructing one or more continuous lengths of pipe, onshore, by welding together a large number of individual pipe sections to form a continuous pipe length or "pipe stalk". Once a predetermined stalk length has been reached, the pipe stalk is stored or spooled onto a reel on a pipe laying vessel. A site at which these continuous pipe lengths are created for spooling onto a pipe laying vessel is called a "spoolbase". Spoolbases may be found in the UK, Norway, United States, Brazil and Angola.

Due to the space requirements and geographical restrictions inherent with spoolbases, there are very few sites which can accommodate them (which is reflected in the low number of permanent spoolbases). As a result, transit distances from spoolbase to installation site can be vast; for example, a pipe lay operation in Australasia may require repeated transits to the nearest spoolbase in, say, Angola. Although "portable" spoolbases have been proposed in which the facility can be dismantled, containerised and shipped to any suitable location, there are very few suitable locations (again evidenced by the low number of permanent spoolbases).

Furthermore, spoolbases are very expensive to set up and to run; and are often underutilised because of the variable nature of projects in which they are involved. Although significant stalk lengths can be produced, geographical limitations restrict the total length of an individual stalk and as such it is rare that sufficient lengths of stalk can be produced to allow efficient spooling operations - even on sites where piers or slipways can extend the workable area into the sea. While the largest spoolbases are capable of prefabricating pipe stalks up to 3km in length, space restrictions often mean that storage space is limited and much pipe handling is required to shuttle the completed stalks towards the spooling line. In any case, adjoining pipe stalks need to be welded together as they are spooled onto the reel using tie-in welds, which interrupts the spooling on operation - typically for an hour at a time.

Tidal variations at the spoolbase port, local surface currents and vessel surge also affect the tie-in weld quality. A small surge movement of the vessel could disrupt the welders performing the tie in weld as the weld may tear during the root and hot pass step.

Even though a typical reel might hold between 15-25 km of pipe (dependent on pipe diameter) and while the newest pipe laying vessels may comprise a number of fixed carousels or removable reels, when this pipe is laid the vessel needs to be reloaded before operations can recommence. This involves a transit back to the spoolbase and, as noted above, transit distances can be vast.

Attempts to overcome the issues above have focussed on modifying spoolbases in order to manufacture and/or store longer stalk lengths, thus minimising the number of tie-in welds required and hence the length of time that the pipe laying vessel has to be in dock between subsequent operations.

International Publication Number WO 97/41054 describes a storage system for storing continuous lengths of pipe prior to spooling onto a reel of a pipe laying vessel. The system comprises a loop of rail track with bogies arranged around the track to receive and retain multiple turns of the pipe. By facilitating longer lengths of pipe stalk, it is proposed that delays associated with creating tie-in welds between subsequent pipe stalk lengths can be avoided. However, the proposed arrangement requires significant investment in equipment and in land to create the storage loop. The Applicant has also identified that significant engineering design will be required to solve inherent stored energy safety issues with such an arrangement.

Other disadvantages of this arrangement are described in International Publication Number WO 2008/078983, which proposed an alternative arrangement employing supports with guide rollers which define the outer radius of a temporary storage loop. Instead of employing bogies and rails, the supports and guide rollers are arranged such that the looped pipe stalk is contained and cannot break away in a radial direction. A stalk length can be wound under itself in a spiral fashion, and subsequently retrieved from the loop as and when required. However, while this arrangement is simpler than that described in WO 97/41054, it still requires a significant amount of land to create the storage loop. The Applicant recognises that in this case there is a lack of operational flexibility in terms of which pipes can be spooled, making the logistics of spooling in a preferred sequence extremely difficult.

Accordingly, it is an object of at least one aspect of the present invention to provide an improved pipe-laying method that removes the dependence of known/prior reel-lay operations on fixed spoolbases. It is another object of at least one aspect of the present invention to provide an improved pipe-laying method that provides an alternative to portable spoolbases. It is an object of embodiments of these and other aspects of the invention to remove the dependence on spoolbases in reel-lay operations altogether. Particular embodiments are intended to provide ways of maximising the utilisation of a fleet of pipe laying vessels.

Specific embodiments of the invention obviate and/or mitigate one or more disadvantages of known/prior arrangements. Further aims and objects of the invention will become apparent from reading the following description. Summary of the Invention

According to a first aspect of the invention, there is provided a pipe laying method comprising;

temporarily laying a continuous length of pipe on the seabed; and

subsequently spooling the continuous length of pipe onto a reel of a pipe laying vessel.

Most preferably, the method comprises constructing the continuous length of pipe by welding together a plurality of individual pipe sections. Most preferably, the continuous length of pipe is constructed and temporarily laid on the seabed by a pipe construction vessel.

The above invention provides a number of significant benefits over conventional reel lay operation as described in the background to the invention and as summarised in the pre- characterising portion of the clause above. The step of temporarily laying the continuous length of pipe or "pipe stalk" on the seabed prior to spooling onto a reel on a pipe laying vessel does away with the land and space requirements associated with spoolbases, in which pipe stalks need to be stored and managed onshore. Laying on the seabed provides access to significantly greater useable surface areas, with the additional benefit that the reeled pipe laying vessel does not need to dock onshore. Furthermore, an area of seabed can be selected that is proximal to the installation site to reduce transit times associated with journeys to and from onshore spoolbases. Further advantages will be discussed further in the detailed description which follows.

Note that while the invention finds particular utility in relation to rigid pipe, it shall be understood to encompass all manner of pipe and pipe coating systems. Furthermore, there shall be no limitation to pipe-in-pipe, clad or lined pipe, although operations involving each are foreseen.

Most preferably, the continuous length of pipe is temporarily laid on a first area of the seabed for subsequent installation on a second area of the seabed following spooling onto the reel. Advantageously, the first area is selected based on a proximity to the second area. Preferably, the continuous length of pipe is constructed and temporarily laid on the seabed using an pipe laying vessel, preferably an s-lay vessel. Preferably, the pipe laying vessel is adapted or configured to simultaneously construct and temporarily lay a plurality of continuous lengths of pipe on the seabed. Optionally, the plurality of continuous lengths of pipe comprise two or more different diameters. Optionally, the plurality of continuous lengths of pipe comprise two or more different lengths.

Most preferably, the continuous length of pipe or plurality of continuous lengths of pipe are laid in relatively shallow water. Optionally, the water depth is in the region of 20 m to 40 m.

Optionally, the method further comprises the pipe construction vessel receiving additional pipe sections from one or more support vessels. These support vessels may comprise supply barges.

Optionally, the method further comprises surveying the seabed prior to constructing and temporarily laying the continuous pipe length or continuous pipe lengths.

Preferably, the continuous pipe length or lengths are laid in a spiral arrangement.

Alternatively, the continuous pipe length or lengths are laid in a linear arrangement.

Further alternatively, the continuous pipe length or lengths are laid in any suitable geometric arrangement.

Preferably, the method further comprises connecting the continuous pipe length or lengths to corresponding anchors or clump weights on the seabed to facilitate pipe lay initiation and/or to act as a positional restraint. The method may optionally comprise laying the anchors on the seabed prior to constructing the continuous pipe length or lengths.

Preferably, the pipe laying vessel is a reel-lay vessel. Preferably, the reel-lay vessel comprises a dynamic positioning system.

Optionally, the method comprises sequentially spooling a plurality of continuous lengths of pipe onto the reel. Optionally, the reel-lay vessel comprises a plurality of reels and the method comprises sequentially or simultaneously spooling a plurality of continuous lengths of pipe onto the plurality of reels. Optionally, the method comprises flooding the continuous length of pipe to assist in locating and retaining it on the seabed.

Optionally, the method comprises pressurising the continuous length of pipe prior to or during spooling onto the reel of the pipe laying vessel. This step is advantageously applied in the case where the pipe is lined or clad, but applies equally to internally uncoated or unlined pipe. Optionally, the pipe is pressurised using treated seawater or technical water.

Optionally, pressurising the continuous length of pipe comprises enclosing an internal volume, preferably using end caps or by inserting two or more pipeline pigs, or a combination of these methods. Optionally, the method comprises controlling the pressure in the internal volume during spooling. For example, a constant internal pressure can be maintained by progressively allowing fluid to exit the pipe during spooling, or by utilising a pressure compensation system to counteract positive and negative pressure variations within the pipeline.

Preferably, the method further comprises transiting the reel-lay vessel to an installation site and installing the continuous length of pipe.

In a preferred embodiment of the first aspect of the invention, the method comprises simultaneously constructing and temporarily laying several continuous lengths of rigid pipe on the seabed in one or more loops using an s-lay vessel, retrieving and spooling one or more of the continuous lengths of rigid pipe onto one or more reels on a reel-lay vessel, and subsequently laying the one or more continuous lengths of reeled rigid pipe at an installation site using the reel-lay vessel.

The s-lay vessel may comprise a barge modified with one or more of a pipeline assembly, handling, welding and deployment spread.

Optionally, the method comprises spooling the continuous length of pipe onto reels of a plurality of pipe laying vessels. Optionally, the continuous length of pipe is constructed and temporarily laid on the seabed by a plurality of pipe construction vessels. Optionally, the method further comprises laying additional lengths of pipe or infrastructure using conventional installation methods. The present invention is compatible with existing installation methods which can be used to install other pipe sections or infrastructure. For example, a piggy-back line such as a flexible flowline, control umbilical or DEH heating cable. These can also be pre-spooled onto an auxiliary reel in a conventional manner. The auxiliary reel may be permanently installed on the pipe-lay vessel, or pre-spooled offline and transported to the pipe-lay vessel seperately.

According to a second aspect of the invention there is provided a method of constructing pipe for spooling onto a reel of a pipe laying vessel comprising;

constructing a continuous length of pipe by welding together a plurality of individual pipe sections;

wherein the continuous length of pipe is constructed and temporarily laid on the seabed by a pipe construction vessel prior to spooling onto the reel of the pipe laying vessel.

Preferably, the pipe construction vessel is an s-lay vessel.

Preferably, the method comprises constructing a plurality of continuous lengths of pipe. Optionally, the plurality of continuous lengths of pipe are constructed by a plurality of pipe construction vessels.

Embodiments of the second aspect of the invention may comprise features corresponding to the preferred or optional features of the first aspect of the invention or vice versa.

According to a third aspect of the invention, there is provided a pipe laying method comprising;

temporarily laying the continuous length of pipe on the seabed; and

Optionally, the method comprises pressurising the continuous length of pipe prior to and/or during spooling onto the reel of the pipe laying vessel. The method may further comprise the method of transiting the pipe laying vessel to an installation location and installing the continuous length of pipe.

Preferably, the method comprises temporarily storing a plurality of continuous lengths of pipe on the seabed. Optionally, the method comprises spooling two or more continuous lengths of pipe onto a plurality of reels of one or more pipe laying vessels.

Optionally, the method comprises storing a plurality of continuous lengths of pipe on the seabed. Optionally, the method comprises selecting one of the continuous lengths of pipe to spool onto the reel of the pipe laying vessel dependent on field development strategy. The selection of preferred pipe stalks enables a service provider to maximise installation efficiency.

Advantageously, the continuous length of pipe is constructed using a pipe construction vessel.

Embodiments of the third aspect of the invention may comprise features corresponding to the preferred or optional features of the first or second aspects of the invention or vice versa.

According to a fourth aspect of the invention there is provided a pipe construction vessel adapted or configured to construct and temporarily store one or more continuous lengths of pipe on the seabed in accordance with the method of the preceding aspects.

Preferably, the pipe construction vessel is an s-lay vessel. Alternatively, the pipe construction vessel is a j-lay vessel.

Preferably, the pipe construction vessel comprises a plurality of firing lines to

simultaneously construct and temporarily store a corresponding plurality of continuous lengths of pipe on the seabed.

Embodiments of the fourth aspect of the invention may comprise features corresponding to the preferred or optional features of the preceding aspects of the invention or vice versa. According to a fifth aspect of the invention there is provided a reel-lay vessel adapted or configured to retrieve and spool one or more continuous lengths of pipe onto a reel of the reel-lay vessel in accordance with the method of the first or the third aspect.

Optionally, the reel-lay vessel is adapted or configured to retrieve and spool one or more continuous lengths of pipe onto a plurality of reels.

Optionally, the reel-lay vessel comprises cleaning apparatus to clean the continuous length of pipe before or as it is spooled onto the reel. The cleaning apparatus may comprises wipers, jetters and/or scrubbers for example.

Optionally, the reel-lay vessel comprises a pressurisation system adapted, arranged or configured to pressurise the continuous length of pipe during spooling and/or pipelay operations.

Optionally, the reel-lay vessel further comprises a pressure compensation system to counteract positive and negative pressure variations within the continuous length of pipe during spooling and/or pipelay operations.

Embodiments of the fifth aspect of the invention may comprise features corresponding to the preferred or optional features of the preceding aspects of the invention or vice versa.

According to a sixth aspect of the invention there is provided a storage area comprising an area of seabed selected for its suitability to receive and temporarily store one or more continuous lengths of pipe constructed and laid by a pipe construction vessel, and to accommodate a reel-lay vessel for subsequent retrieval and spooling onto a reel on the reel-lay vessel of one or more of said continuous lengths of pipe.

Optionally, the temporary storage area comprises one or more initiation anchors moored to the seabed and connectable to one or more corresponding continuous lengths of pipe to provide tension when constructing and laying the one or more continuous lengths of pipe and/or when retrieving and spooling the one or more continuous lengths of pipe. According to a seventh aspect of the invention, there is provided a continuous length of pipe constructed in accordance with the first or the second aspect of the invention. Optionally, the continuous length of pipe is temporarily stored on the seabed.

According to another aspect of the invention there is provided a pipe laying method, a method of constructing pipe for spooling onto a reel-lay vessel, an s-lay vessel, reel-lay vessel or other pipe-lay vessel, or a storage area for temporary storage of pipe for spooling onto a reel-lay vessel, substantially as herein described with reference to the appended drawings.

Brief Description of the Drawings

Aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the following drawings (like reference numerals referring to like features) in which:

Figure 1 is a schematic representation of an s-lay vessel according to an aspect of the invention temporarily laying continuous lengths of pipeline on the seabed in accordance with another aspect of the invention;

Figure 2 is a schematic representation showing further detail of the s-lay vessel illustrated in Figure 1 ;

Figure 3 is a schematic representation of a reel-lay vessel according to an aspect of the present invention retrieving a continuous length of pipeline from the seabed according to another aspect of the present invention; and

Figure 4 is a schematic representation of a geographical region particularly suitable to provide a temporary storage area according to an aspect of the present invention.

Detailed Description of Preferred Embodiments

An embodiment of the present invention will now be described with reference to the attached drawings and which overcomes the problems with the prior art discussed in the background to the invention. An improved pipe laying method begins by temporarily laying one or more continuous lengths of pipe on the seabed in a loop as indicated by reference numeral 1 in Figure 1. This operation is performed by a pipe-laying vessel 3 which (as shown in, and described with reference to, Figure 2) is configured to simultaneously lay several (in this example, five) lengths of pipe 5 on the seabed. This vessel 3 is an s-lay vessel which constructs and lays pipe using the "stove-piping" method described in the background to the invention. Individual pipe sections 7 are stored on and below deck and manipulated using pipe handling equipment including deck cranes 9 in order to provide an efficient lay rate.

The vessel 3 includes specially designed stinger 1 1 which is designed and arranged to support and guide the lengths of pipe 5 from the multiple firing lines. It will of course be understood that a single pipe may be laid instead of several, just as it will be understood that many more than five (as illustrated) can be laid simultaneously. Deck arrangements can be optimised for said multiple firing lines by providing corresponding numbers or pluralities of welding, inspection and field joint coating stations.

However, by laying several lengths of pipe 5 simultaneously, pipes of different diameters and stalk lengths can be constructed in parallel. Additional pipe sections 7 can be received from barges, again using deck cranes 9 to assist in handling the pipe sections 7. As shown in Figure 1 , the distance d from shore is typically small (for example, between one and a few kilometres). The expected corresponding inshore sheltered conditions will particularly suit such pipe replenishment operations and permit 24 hour operations unaffected by weather or vessel movement. An ideal location also minimises transit times for the reel-lay vessel (see below) and is situated on a flat and relatively featureless seabed, preferably sandy in nature with moderate to low currents. The depth of the water in which the loop is constructed (in the region indicated by reference numeral 55) will be typically in the region of 20m to 40m.

While these depths and distances from shore are typical of the inventive pipe laying method and are preferable from an operational and logistical viewpoint, the pipe can be laid anywhere convenient with installation parameters adjusted as required to suit the location and water depth. In other words, the invention is not limited to shallow water or near-shore operation.

Ideally, the area will be surveyed to identify any obstacles and the like, to assess currents etc., and to determine the best lay path or route. In this embodiment, a spiral loop of pipes resembling a running track has been selected and is subsequently being laid by s-lay vessel 3 which is moving in the direction indicated by arrow /. The loop has a length L of approximately 4km and a bend radius R at either end of 1 km. Note that the loop starts at initiation anchor 6 to which the pipe or pipes are connected at the outset so that the pipe construction vessel can lay away without dragging the pipe along the seabed. The anchor 6 may further act as a positional restraint, for example as a hold back anchor to keep the end of the pipe in position or to counteract environmental effects such as currents, seabed topography and mobility influencing pipe position along its length.

This exemplary 4km by 2km loop is anticipated to suit high speed recovery by a reel lay vessel (such as indicated by reference numeral 13 in Figure 3), and the large radius results in low strains and provides stability on the seabed. Such large radii also do not require turning bollards to assist in pipe positioning. One "lap" of the spiral loop 1 provides 10km of pipe; several laps can be laid in order to provide single continuous lengths of pipe sufficient to fill a reel in a single run - without the need to create tie in welds. It is anticipated that three or four laps will fill a reel, dependent of course on reel capacity, pipe diameter and/or pipe weight. No current spoolbases are capable of constructing such stalk lengths.

Once one or more loops of pipe have been laid as described above, the s-lay vessel 3 leaves the temporary storage area at which time it can be deployed on other projects (which may of course include temporarily laying further lengths of pipe). Reel-lay vessel 13 can then begin to retrieve individual pipes such as pipe 5a shown being spooled onto reel 15 of the reel-lay vessel 13 in Figure 3. Not only does the reel-lay vessel not have to dock at an onshore location with the attendant difficulties and risks (as outlined in the background to the invention), but an additional benefit over spoolbases is that any one of the pipe lengths 5 can be retrieved. For example, several different diameters of pipe can be constructed and the appropriate diameter pipe for a particular installation operation can then be selected from the available pipe lengths. In a spoolbase operation, pipe stalks not only need to be joined during spooling by creating tie-in welds (which the present invention negates) but the pipe stalks are only available to the reel-lay vessel in a fixed order. This selection can be made based on a strategy for developing a particular field.

During recovery and spooling, the reel-lay vessel 13 may employ dynamic position systems to control position and orientation as it reverses along the spiral loop path recovering the pipeline. This operation can be carried out at full spooling speeds of, say, 1 km per hour or more without interruption. The vessel position can be constantly adjusted to synchronise with the spooling operation. Furthermore, more than one pipeline may be sequentially spooled onto the reel and each pipeline can be of any length that can be accommodated within the reel capacity. It will also be realised that a reel-lay vessel may comprise several reels, and pipe lengths can be spooled onto any or each of these reels sequentially or, indeed, in parallel. Spooling operations are only limited by the limits of the reel and/or of the tensioner 17, efficiency of reel packing and the synchronicity of the vessel position along the lay path. Wipers, jetters, scrubbers or equivalent apparatus may be employed to clean the pipelines as they are pulled onto the reel-lay vessel.

Once the reel (or reels) has been filled, the reel-lay vessel can transit to the installation site where the pipe or pipes can then be installed using conventional reel-lay methods.

Advantageously, during laying of the pipelines, they may be flooded to provide greater on- bottom stability during laying and also while being temporarily stored on the seabed.

Furthermore, during recovery of the pipelines, they may remain flooded or indeed be pressurised, using treated seawater for example. In the case of clad or lined pipes, pressurisation can prevent wrinkling, damage and/or separation of the lining material from the internal wall of the pipe because of the internal stresses created by bending the pipe. Pressurisation may also be employed to reduce deformation (ovality) as subsequent turns of the pipelines are spooled onto the reel of the reel-lay vessel. As a result, it may be possible to spool and lay pipe sections with lower wall thicknesses because of the reduced likelihood of deformation, with obvious cost benefit.

To date there have been numerous proposals for pressurising pipe stalks during spooling at spoolbases; however none of have been successfully implemented yet. Such attempts have involved the use of internal pigging to negotiate the frequent tie-in welds since a significant loss of pressure cannot be permitted at any time during spooling. The present invention provides another significant advantage because pressurising a complete pipe length, which can be achieved because of the laying and temporary storage step, is much simpler, and presents lower risk as pressurisation may be achieved in fewer steps.

Pressurisation of the pipe length in this manner can be carried out completely subsea which has resulting safety benefits over prior art attempts as well, and personnel exposure is minimised until spooling starts and the pipe is recovered to deck. Furthermore, standard (and therefore well understood) subsea pigging operations or even relatively simple end caps - or a combination of the two - can be used to enclose the pressurised internal volumes, and pressurised fluid can be retained for the duration of spooling and controlled as required. For example, relatively small amounts of fluid may be permitted to exit the pipe as spooling progresses to maintain a constant internal pressure, or additional fluid may be injected to maintain pressure during laying or to account for temperature fluctuations and/or potential leakage during spooling, transit and laying. To this end, a pressure compensation system may be employed to maintain pressurisation.

The present invention therefore allows the concept of pressurising pipe stalk lengths during spooling to be realised in a practical and useful way.

Note that the spiral loop configuration illustrated in Figure 1 is only one of a range of shapes and paths that the s-lay vessel may lay the one or more continuous lengths of pipe. Indeed, in the simplest realisation of the present invention, the pipes are stored in substantially linear orientations which may be parallel to a shoreline, perpendicular to the shoreline, or any suitable orientation therebetween. Figure 4 illustrates a potential installation site proximal to the Roncador Oil Field which is located 125km off the shore of Brazil. The water depth in this region is in the region of 1.8 to 2.0km; however, only 50km inshore the water depth is significantly shallower, and presents an ideal location for the temporary storage of pipelines for subsequent retrieval by reel-lay vessels servicing installation operations on the Roncador Oil Field. In this area, tens of kilometres of continuous pipe lengths could be laid and temporarily stored in accordance with aspects of the invention. Several pipes could be laid simultaneously and linearly, resulting in efficient pipeline construction and reducing complexity in the retrieval operation. Furthermore, the proximity to shore means that pipe stock can be replenished on board the s-lay vessel with relative ease. It will be realised that the present invention provides a truly "portable" spoolbase, as pipe stalks can be laid in any offshore location which can be accessed by a pipe laying vessel, even in deep water if necessary, in which case j-lay vessels may even be employed. Access to new geographical locations which, to date, have been deemed too expensive due to the large capital investment required for a spoolbase, has now become feasible.

Scheduling advantages can be attributed to two primary sources - spooling duration and transit duration - and at least one secondary source - vessel activity.

Spooling duration is a function of spooling speed and tie-in weld duration (in conventional spoolbase systems). In the example of a 16 km pipeline, if it is assumed that 1 1 tie-in welds and field joints are required, an estimated total of approximately 20 hours is required which, for two trips, corresponds to approximately 40 hours spooling time alone.

According to the present invention, this time can be eliminated saving over a day-and-a- half on a notional 17 day reel lay schedule (approximately 10%). This equates to a cost saving on the order of $400k (based on a conservative estimated daily rate of $250k).

Transit duration is a direct function of transit distance and vessel speed. Increasing speed is very expensive so shortening the transit distance is far more attractive a proposition. By way of example, the Block 15 field (located approximately 145 km off the coast of Angola) is currently served by a spoolbase located in Angola some 260 km away. The spoolbase distance can be shortened to approximately 110 km (based on the nearest shoreline to the field) which at a speed of 13 knots (approx. 24 kilometres per hour) represents a time- saving of approximately 1 1 hours in each direction (to and from the field). Continuing the above example, on the notional 17 day reel lay schedule, a transit time saving of nearly 2 days can be realised (approximately 1 1 %). This equates to an additional cost saving on the order of $480k.

In the above described example, a total time saving of around three-and-a-half days over the original 17 day reel lay schedule (approximately 20%) is realised by spooling continuous lengths of pre-laid pipe in the manner described herein instead of relying on fixed spoolbases. Based on a target vessel utilisation of 250 days per annum, the shortened project duration equates to more than 50 additional pipe laying days per year with a projected revenue increase of approximately $12.5m. Vessel activities that add to the reel lay schedule involve typical logistics operations such as approaching, docking at, remaining moored at and departing from a spoolbase port. By avoiding coming into port altogether, further time savings can be realised. Furthermore, at greater transit distances the time savings are even more significant. For example, when doubling the transit distance to 520km (notional 20 day reel lay schedule c.f. 17 days) the present invention may provide for a time and corresponding cost saving of nearly 7 days. Of course, while the reel lay vessel is transiting to and from the field, a pipe construction vessel can continue laying pipe stalks to be picked up on subsequent transits back, or indeed be deployed elsewhere after laying several pipe stalks simultaneously..

In the foregoing description, the skilled person will recognise that the temporary storage area, while described as being preferably located proximal to the installation site, can be a significant distance away from the installation site in fact; the benefits of the present invention however allowing that the temporary storage area can be located in any suitable location (indeed it is foreseen that several temporary storage areas might be used for a single installation operation) meaning that it or they can be located at least closer, even if not very much closer, to the installation site than existing spoolbases. Thus, transit distances and corresponding delays in installation operations can be reduced.

Furthermore, continuous lengths of pipe may be temporarily stored for significant periods of time, for example in the event of delays to a particular project or when stockpiling pipe for an upcoming or anticipated project. Onshore spoolbases are not suitable for such longer term storage, nor would it be trivial to obtain and develop suitable areas of land and infrastructure to modify them for that purpose.

It will also be realised that the construction and temporary storage of continuous lengths of pipe may be performed by a j-lay vessel instead of an s-lay vessel if water depths permit; in fact, any suitable or equivalent method of pipe laying may be employed. Importantly, the present invention may be realised using conventional pipe laying techniques but applying them in a completely new way; thus the benefits may be realised through operational changes with minimal equipment changes required.

The invention concerns improvements to reel laying methods that do away with the need for onshore spoolbases by manufacturing pipe stalks using an s-lay vessel and temporarily storing the pipe stalks on the seabed in relatively shallow water at a near-shore location close to the offshore installation site. Several pipe stalks can be laid in parallel as completed full length pipelines. The reel-lay vessel thereafter retrieves and spools pipe stalk directly from the seabed onto reel in a single operation without the delays associated with performing tie-in welds. The inventive method means that the entire pipeline may be pressurised during spooling to preserve the integrity of lined or clad pipes, and to combat ovalisation.

Throughout the specification, unless the context demands otherwise, the terms 'comprise' or 'include', or variations such as 'comprises' or 'comprising', 'includes' or 'including' will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

The foregoing description of the invention has been presented for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims.

Claims

1. A pipe laying method comprising;

temporarily laying a continuous length of pipe on the seabed; and

2. The method according to claim 1 , comprising constructing the continuous length of pipe by welding together a plurality of individual pipe sections.

3. The method according to claim 1 or claim 2, wherein the continuous length of pipe is constructed and temporarily laid on the seabed by a pipe construction vessel.

4. The method according to any preceding claim, wherein the continuous length of pipe is temporarily laid on a first area of the seabed for subsequent installation on a second area of the seabed following spooling onto the reel.

5. The method according to claim 4, wherein the first area is selected based on a

proximity to the second area.

6. The method according to any preceding claim, wherein the continuous length of pipe is constructed and temporarily laid on the seabed using a pipe laying vessel.

7. The method according to claim 6 wherein the pipe laying vessel is an s-lay vessel.

8. The method according to claim 6 or claim 7, wherein the pipe laying vessel is

adapted or configured to simultaneously construct and temporarily lay a plurality of continuous lengths of pipe on the seabed.

9. The method according to claim 8, wherein the plurality of continuous lengths of pipe comprise two or more different diameters.

10. The method according to claim 8 or claim 9, wherein the plurality of continuous lengths of pipe comprise two or more different lengths.

1 1. The method according to any preceding claim, wherein the continuous length of pipe or plurality of continuous lengths of pipe are laid in shallow water.

12. The method according to claim 11 , wherein the water depth is in the region of 20 m to 40 m.

13. The method according to any preceding claim, further comprising receiving

additional pipe sections from one or more support vessels.

14. The method according to any preceding claim, further comprising surveying the

seabed prior to constructing and temporarily laying the continuous pipe length or continuous pipe lengths.

15. The method according to any preceding claim, wherein the continuous pipe length or lengths are laid in a spiral arrangement.

16. The method according to any of claims 1 to 14, wherein the continuous pipe length or lengths are laid in a linear arrangement.

17. The method according to any of claims 1 to 14, wherein the continuous pipe length or lengths are laid in a geometric arrangement.

18. The method according to any preceding claim, further comprising connecting the continuous pipe length or lengths to corresponding anchors or clump weights on the seabed to facilitate pipe lay initiation and/or to act as a positional restraint.

19. The method according to claim 18, comprising laying the anchors on the seabed prior to constructing the continuous pipe length or lengths.

20. The method according to any preceding claim, wherein the pipe laying vessel is a reel-lay vessel.

21. The method according to claim 20, wherein the reel-lay vessel comprises a dynamic positioning system.

22. The method according to any preceding claim, comprising sequentially spooling a plurality of continuous lengths of pipe onto the reel.

23. The method according to any preceding claim, wherein the reel-lay vessel comprises a plurality of reels and the method comprises sequentially or simultaneously spooling a plurality of continuous lengths of pipe onto the plurality of reels.

24. The method according to any preceding claim, comprising flooding the continuous length of pipe to assist in locating and retaining it on the seabed.

25. The method according to any preceding claim, comprising pressurising the

continuous length of pipe prior to or during spooling onto the reel.

26. The method according to claim 25, wherein the pipe is pressurised using treated seawater or technical water.

27. The method according to claim 25 or claim 26, wherein pressurising the continuous length of pipe comprises enclosing an internal volume using end caps and/or by inserting two or more pipeline pigs.

28. The method according to any of claims 25 to 27, comprising controlling the pressure in the internal volume during spooling.

29. The method according to claim 28, comprising maintaining a constant internal

pressure by progressively allowing fluid to exit the pipe during spooling.

30. The method according to claim 28, comprising utilising a pressure compensation system to counteract positive and negative pressure variations within the pipeline during spooling.

31. The method according to any preceding claim, wherein the method further comprises transiting the reel-lay vessel to an installation site and installing the continuous length of pipe.

32. The method according to any preceding claim, comprising simultaneously

constructing and temporarily laying several continuous lengths of rigid pipe on the seabed in one or more loops using an s-lay vessel, retrieving and spooling one or more of the continuous lengths of rigid pipe onto one or more reels on a reel-lay vessel, and subsequently laying the one or more continuous lengths of reeled rigid pipe at an installation site using the reel-lay vessel.

33. The method according to claim 32, wherein the s-lay vessel comprises a barge modified with one or more of a pipeline assembly, handling, welding and deployment spread.

34. The method according to any preceding claim, comprising spooling the continuous length of pipe onto reels of a plurality of pipe laying vessels.

35. The method according to any preceding claim, wherein the continuous length of pipe is constructed and temporarily laid on the seabed by a plurality of pipe construction vessels.

36. The method according to any preceding claim, further comprising laying additional lengths of pipe or infrastructure using conventional installation methods.

37. A method of constructing pipe for spooling onto a reel of a pipe laying vessel

comprising;

38. The method according to claim 37, wherein the pipe construction vessel is an s-lay vessel.

39. The method according to claim 37 or claim 38, comprising constructing a plurality of continuous lengths of pipe.

40. The method according to claim 39, wherein the plurality of continuous lengths of pipe are constructed by a plurality of pipe construction vessels.

41. A pipe laying method comprising;

temporarily laying the continuous length of pipe on the seabed; and subsequently spooling the continuous length of pipe onto a reel of a pipe laying vessel.

42. The method according to claim 41 , comprising pressurising the continuous length of pipe prior to and/or during spooling onto the reel of the pipe laying vessel.

43. The method according to claim 41 or 42, further comprising transiting the pipe laying vessel to an installation location and installing the continuous length of pipe.

44. The method according to any of claims 41 to 43, comprising temporarily storing a plurality of continuous lengths of pipe on the seabed.

45. The method according to any of claims 41 to 44, comprising spooling two or more continuous lengths of pipe onto a plurality of reels of one or more pipe laying vessels.

46. The method according to any of claims 41 to 45, comprising storing a plurality of continuous lengths of pipe on the seabed.

47. The method according to claim 46, comprising selecting one of the continuous

lengths of pipe to spool onto the reel of the pipe laying vessel dependent on field development strategy.

48. The method according to any of claims 41 to 47, wherein the continuous length of pipe is constructed using a pipe construction vessel.

49. A pipe construction vessel adapted or configured to construct and temporarily store one or more continuous lengths of pipe on the seabed in accordance with the method of any preceding claim.

50. A reel-lay vessel adapted or configured to retrieve and spool one or more continuous lengths of pipe onto a reel of the reel-lay vessel in accordance with the method of any preceding claim.

51. The reel-lay vessel according to claim 50, comprising cleaning apparatus to clean the continuous length of pipe before or as it is spooled onto the reel.

52. The reel-lay vessel according to claim 51 , wherein the cleaning apparatus comprises wipers, jetters and/or scrubbers.

53. The reel-lay vessel according to any of claims 50 to 52, comprising a pressurisation system adapted, arranged or configured to pressurise the continuous length of pipe during spooling and/or pipelay operations.

54. The reel-lay vessel according to any of claims 50 to 53, comprising a pressure

compensation system to counteract positive and negative pressure variations within the continuous length of pipe during spooling and/or pipelay operations.

55. A storage area comprising an area of seabed selected for its suitability to receive and temporarily store one or more continuous lengths of pipe constructed and laid by a pipe construction vessel, and to accommodate a reel-lay vessel for subsequent retrieval and spooling onto a reel on the reel-lay vessel of one or more of said continuous lengths of pipe.

56. The storage area according to claim 55, comprising one or more initiation anchors moored to the seabed and connectable to one or more corresponding continuous lengths of pipe to provide tension when constructing and laying the one or more continuous lengths of pipe and/or when retrieving and spooling the one or more continuous lengths of pipe.

57. A pipe laying method, a method of constructing pipe for spooling onto a reel-lay vessel, an s-lay vessel, reel-lay vessel or other pipe-lay vessel, or a storage area for temporary storage of pipe for spooling onto a reel-lay vessel, substantially as herein described with reference to the appended drawings.