FLOATING STRUCTURE COMPRISING A WATER INTAKE RISER BUNDLE, METHOD OF INSTALLING SUCH A FLOATING STRUCTURE, METHOD OF PRODUCING A LIQUEFIED HYDROCARBON STREAM AND METHOD PRODUCING A VAPOROUS HYDROCARBON STREAM

TECHNICAL FIELD

The present invention relates to a floating structure arranged in a body of water, comprising a floating body and a water intake riser bundle, the water intake riser bundle comprising at least one riser, the water intake riser bundle being suspended from the floating body hanging downwardly into the body of water. The invention further relates to a method of installing such a floating structure, a method of producing a liquefied hydrocarbon stream and a method of producing a vaporous hydrocarbon stream with such a floating structure .

STATE OF THE ART

A commercially important liquefied hydrocarbon is liquefied natural gas (LNG) , which is typically produced by extracting heat from a natural gas stream whereby the natural gas is cooled to reach a temperature that is below the bubble point of the LNG at atmospheric pressure. The temperature is typically about -162 °C. The removed heat is generally brought into the ambient. In case of a water-cooled LNG production process, the heat is removed by cooling water and generally released into the sea.

Before use by an end user, the LNG is typically

revaporized, which involves withdrawing heat from the ambient and adding this heat to the LNG. The heat may be taken from a stream of sea water.

US pre-grant application publication No.

2013/0239480 discloses an off-shore structure from which a water intake riser assembly is suspended into a body of water. The water riser assembly is employed to take in water from a certain depth from the body of water, and supply the water to the off-shore structure via the water intake riser assembly. The water is used to add heat to, or remove heat from, a hydrocarbon stream. Subsequently the water is disposed of.

The water intake riser assemblies can be formed as a water intake riser bundle comprising a plurality of tubular risers that generally extend side by side along a length direction. The term riser is used here to denote a

substantially vertical element, such as a pipe or a conduit. The distal part of the water intake riser assembly may hang free from the ocean floor, for instance at a depth of between around 130 to 170 meters from the surface of the body of water. It is further disclosed that the water intake riser assembly may be employed at other depths as well.

The water intake riser bundle comprises a plurality of tubular risers, of which one may serve as a structural riser, also referred to as a structural conduit. For instance, the bundle may comprise nine tubular risers arranged in a three by three array, wherein the tubular riser at the centre serves as structural riser. The structural riser may serve as a support for spacers formed as guiding sleeves, through which the other eight tubular risers are guided to keep the tubular risers together, though spaced apart. The spacers may be provided at predetermined locations along the structural riser. The structural riser at the centre may, or may not, convey water to the surface (i.e., may or may not serve as a water intake riser) .

The water intake riser assembly is built off-shore, i.e. at the intended production location of the floating

structure. The tubular risers are formed at this location by sequentially lowering and connecting together tubular riser segments (carried by the off-shore structure) from the off ^¬ shore structure using a crane. The tubular riser segments are attached to each other to form a tubular riser. First, the structural riser is built with spacers attached to it, such that the other tubular risers can be lowered through the spacers during building. The different riser segments are connected by using connectors, for instance Merlin™

connectors. The connectors are for instance formed by a male connector attached to the top end of a first riser segment and a female connector attached to the lower end of a second riser segment, where the male and female connector parts are arranged to engage each other without requiring off-shore welding. Such connectors provide a releasable connection.

Building the water intake riser bundle is a time

consuming process and therefore expensive. Also, building the structural riser is in particular time consuming when the structural riser comprises transmission lines such as umbilicals, such as cables and/or additional conduits.

There is a desire to extend the depth of the water intake riser assembly, as the lower temperature water can generally be obtained deeper in the sea. This requires longer water intake riser bundles which are thus more time-consuming to install .

SHORT DESCRIPTION

It is an object to provide a floating structure which at least partially overcomes one of the above mentioned

problems .

The term 'comprising' is used in this text to indicate that all the enlisted elements are encompassed without excluding the presence of additional non-named elements. In accordance with an aspect of the present invention, there is provided a floating structure 100 arranged in a body of water 101, comprising a floating body 102 and a water intake riser bundle 106, the water intake riser bundle 106 comprising at least one riser 106A, 106B, 106C, the water intake riser bundle 106 being suspended from the floating body 102 hanging downwardly into the body of water 101, wherein at least one of the risers 106B comprises a riser element 108 which has a length of 100 meter or more.

The floating structure may comprise a LNG plant for liquefying a vaporous hydrocarbon containing feed stream and/or for gasifying a liquefied hydrocarbon stream. In other words, the LNG plant may cool and liquefy natural gas to form LNG and/or heat and gasify LNG.

The term riser element is used in this text to refer to a single element having a length of more than 100 meter.

Preferably, the riser element 108 has a length of more than 150 meter, more than 200 meter, more than 250 meter or even more than 300 meter.

The riser element may be a single piece element, which includes being formed by a plurality of parts being connected in a non-releasable manner, in particular being connected by welding .

The riser element is manufactured on-shore, thereby saving off-shore assembling time. The term on-shore relates to activities performed on land or in the harbour.

Additional parts may be connected to the riser element, such as spacers 110 formed as guiding sleeves for other risers and a coupling providing some freedom of movement for the water intake riser bundle when being suspended from the floating structure. The coupling may comprise a swivel joint, a ball joint, a riser hanger, or other pivotable or hingeable couplings. Particular reference is made to US Patent

7,318,387 which describes a riser hanger construction involving a flexible load transfer element and a hose to convey the water.

In case a water intake riser bundle is needed with a longer length than the riser element, additional riser segments may be attached to the riser element. This may be done off-shore.

The riser element and riser segments are formed as a pipe with an internal passage, for instance for taking in water or guiding transmission lines, such as umbilicals. The riser element and riser segments comprise a wall which is of closed geometry creating the passage. The riser element and riser segments may be of a tubular shape.

As indicated above, the riser element may be a single piece element. This includes a riser element which is formed by a plurality of parts, such as riser segments, which are permanently joined, e.g. by welding. Although it is

understood that parts connected by direct welding can be separated, for this text direct welding is considered to be a non-releasable connection.

According to a further embodiment the riser element 108 is formed by a plurality of riser segments 109, wherein the plurality of riser segments 109 are welded together to form the riser element 108.

The riser segments are preferably welded directly together. So, no connectors or the like are used to form the riser element . The riser element is preferably formed on ^¬ shore, thereby saving time and money as less off-shore time and effort is needed to build and deploy the water intake riser bundle.

According to an embodiment the riser element 108 is rigid. The riser element 108 and further riser segments 109 are made of steel, e.g. carbon-steel. Although it will be understood that in use the riser element will deform to some extent under the influence of gravity, water current, wave motion, the term rigid is used here to indicate that a rigid riser element can't be bent, folded or wound around a reel during transportation.

According to an embodiment the water intake riser bundle 106 comprises a structural riser 106B, the structural riser 106B comprising the riser element 108.

The risers not being the structural riser may be referred to as water intake risers. The structural riser itself may or may not be arranged to take in water.

The structural riser is relatively complicated to assemble off-shore, as it usually comprises additional parts. By forming the structural riser using the riser element having a relatively long length, the structural riser can be put in place requiring less off-shore time. Preferably, the structural riser 106B comprises the riser element without further riser segments being connected thereto off-shore, e.g. by using connectors.

According to an embodiment the riser element 108 extends between a proximal end of the structural riser 106B and a distal end of the structural riser 106B. Thus, the structural riser does not comprise additional riser segments.

According to an embodiment the riser element 108

comprises transmission lines. As indicated above, the

structural riser 106B may comprise transmission lines which run through the inside of the structural riser and may serve to transmit signals (control signals/sensor data), fluids other than the cooling water (e.g. anti-fouling agents to prevent clogging of the water intake riser) or energy (e.g. electric/pneumatic/ hydraulic) from the floating structure to the lower end of the water intake riser bundle 106 and/or vice versa. The term transmission lines comprises

transmission cables and transmission conduits.

The transmission lines may be connected to one or more water intake risers. The distal end of the structural riser may comprise connectors arranged to connect to a further transmission line, such as a flying lead. The water intake risers may also comprise connectors arranged to connect to a further transmission line, such as a flying leads.

In such a case, forming the structural riser 106B using the riser element is in particular advantageous. The riser element can be made on-shore including the transmission lines. As a result, no complicated building operations including guiding the transmission lines through the

structural riser are needed.

According to an embodiment the structural riser 106B has a length which is at least equal to length of the other risers 106A, 106C. The other risers are water intake risers. Preferably, the length of the structural riser is greater than the length of the water intake risers.

The distal end of the structural riser is thus at the same depth or at a greater depth than the distal end of the other risers or water intake risers.

This allows divers or ROVs (remotely operated underwater vehicles) to be able to access the distal end of the

structural riser, for instance to connect the transmission lines to the water intake risers. Such a connection may be made using further transmission lines, such as flying leads.

According to an embodiment the structural riser comprises spacers 110. The spacers may be formed as guiding sleeves for the other risers are guided and serve to keep the risers spaced together without colliding. According to an embodiment the floating body 102

comprises supports 120 for supporting the riser element 108 in a horizontal position.

This provides an advantageous way of transporting the riser element from the on-shore location to the intended off ^¬ shore production location.

According to an embodiment the floating body 102

comprises a hull 121 and the supports 120 are provided on the outside of the hull 121 above the water line of the floating body 102.

The water line or International Load Line (positioned amidships), is the line to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy.

Preferably, the supports are positioned well above the water line such that the supports are positioned above the wave impact zone. The supports are for instance at a height of at least 6 or 9 or 12 metres above the water surface.

This is an advantageous position for the supports as it provides a good starting position for manoeuvring the riser element 108 to a vertical and partially submerged position. No sideward movements are required, as the riser element is already in an overboard position during transport. Also, no scarce deck space is used.

According to an embodiment the floating body 102

comprises handling devices, such as a crane 150 and one or more winches 151, for moving the riser element 108 from a horizontal storage position to a vertical position.

The horizontal storage position may be defined by the supports described above. The handling devices are positioned such that the storage position is within reach of the handling devices. The handling devices are preferably positioned along a side edge of a deck of the floating structure .

In another aspect of the present invention there is provided a method of installing a floating structure 100 comprising a floating body 102, the method comprising:

a) providing a floating body 102 with supports 120for

supporting the riser elementl08 in a horizontal position, the floating body 102 comprising handling devices for moving the riser element 108 from the supports 120 and positioning the riser element 108 in an overboard, vertical orientation,

b) providing a riser element 108 preferably having a

length of more than 100 meter,

c) positioning the riser element 108 on the supports.

The floating body may be a FLNG, i.e. a floating plant for liquefying a vaporous hydrocarbon containing feed stream.

The floating body may also be a floating plant for gasifying a liquefied hydrocarbon stream.

The handling devices may comprise a crane 150 and one or more winches 151.

According to an embodiment the floating body 102 comprises a hull 121 and the supports 120 are provided on the outside of the hull 121.

According to an embodiment the riser element 108 provided in step (b) comprises transmission lines running through the riser element. Transmission lines may be cables and/or additional conduits and may serve to transport information

(for instance control signals, sensor data), energy, fluids, chemicals and the like.

According to an embodiment the method comprises

d) moving the floating structure 102 to its production location, e) positioning the riser element 108 in an overboard, vertical orientation using the handling devices, and optionally

f) removing the supports and/or at least one of the

handling devices.

Action f) is an optional step. If performed, it may be performed during action and/or after e) .

An additional action g) may comprise using ROV's and/or divers to connect the transmission lines which run through the structural riser to the water intake risers, for instance for providing anti-fouling agent to the water intake risers to prevent clogging of the water intake riser. The connection between the transmission lines and the water intake risers may be established by further transmission lines, such as flying leads.

According to a further aspect there is provided a method of producing a liquefied hydrocarbon stream, comprising:

- feeding a vaporous hydrocarbon containing feed stream to a floating structure according to the above;

- on the floating structure, forming a liquefied hydrocarbon stream from at least a part of the vaporous hydrocarbon containing feed stream comprising at least extracting heat from at least said part of the vaporous hydrocarbon containing feed stream;

- supplying water to the floating body via the water intake riser bundle of the floating structure;

- adding at least part of the heat removed from said at least a part of the hydrocarbon containing feed stream to at least part of the water supplied via the water intake riser bundle;

- subsequently disposing of the at least part of the water . According to a further aspect there is provided a method of producing a vaporous hydrocarbon stream, comprising:

- providing a liquefied hydrocarbon stream on a floating structure according to the above;

- on the floating structure, forming a vaporous

hydrocarbon stream from at least a part of the liquefied hydrocarbon stream comprising adding heat to the said part of the liquefied hydrocarbon stream;

- supplying water to the floating body via the water intake riser bundle of the floating structure;

- drawing at least part of the heat for adding to the said part of the liquefied hydrocarbon stream from at least part of the water supplied via the water intake riser bundle;

- subsequently disposing of the at least part of the water .

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be further illustrated hereinafter by way of example only, and with reference to the non-limiting drawing in which:

Fig.'s la - lb schematically illustrate a floating structure arranged in a body of water comprising a water intake riser bundle according to two embodiments;

Fig. 2a - 2g shows a method of installing a floating structure according to an embodiment .

DETAILED DESCRIPTION

In this description same reference numbers refer to similar components. The person skilled in the art will readily understand that, while the invention is illustrated making reference to one or more a specific combinations of features and measures, many of those features and measures are functionally independent from other features and measures such that they can be equally or similarly applied

independently in other embodiments or combinations.

Figure la schematically illustrates a floating structure 100 arranged in a body of water 101. The floating structure 100 comprises a floating body 102, and a water intake riser bundle 106 suspended from the floating body 102 into the body of water 101. The floating body 102 floats on a water surface 104 of the body of water 101. The floating body 102 may be moored to a turret (not shown) in weathervaning relationship with the turret comprising rotatability about a vertical axis through the turret .

The water intake riser bundle 106 comprises at least one, preferably more than one, risers 106A - C. The risers may be formed as tubulars .

The term "bundle" is used to imply that a plurality of risers 106A-C are positioned parallel and close to each other such that the plurality of risers behave like a bundle.

Preferably, the risers are bundled together side-by-side forming a single water intake riser assembly, as described for instance in US 2013/0239480.

In the example of Figure la, for instance, the risers 106A-C are laterally connected to each other by means of at least one spacer 110A-C. By means of such spacers 110A-C, the risers 106A-C are physically associated or connected together. In one embodiment, enough spacers 110A-C may be provided to keep the risers 106A-C from striking into one another and to make the risers 106A-C behave as one unified bundle 106. However, a single riser - not bundled side-by- side with other risers- may also qualify as a "bundle" for the purpose of the present disclosure.

The water intake riser bundle 106 hangs freely downwardly into the body of water 101, and comprises a proximal end 107 and a distal end 117. The distal end 117 is considered to be the part of the water intake riser bundle 106 that is furthest removed from the floating body 102.

The one or more risers 106A-C are configured to take in cold water via water intake openings at depth, and to convey the cold water upward to the floating body 102. The cold water may be input to heat exchangers to add or remove heat to/from a process performed on the off-shore structure 100. Heated or cooled ocean water from the outlet of the heat exchangers may be discharged back into the body of water 101 at the surface, or alternatively conveyed back to depth with a discharge system. One of the risers 106B may be a

structural riser, possibly not arranged for taking in water.

The lower ends of the risers 106A-C may be arranged in a staggered configuration such as proposed in US 2013/0239480. The distal portions of the risers may further be provided with water intake openings provided as a plurality of through holes through the side wall of the one or more riser tubes 106A-C .

Although the example in Fig. la shows three risers 106A-

C, the water intake riser bundle 106 can comprise any suitable number of risers, preferably a larger number. The water intake riser bundle may comprise nine risers arranged in a three-by-three rectangular array or circular pattern, according to one particular embodiment. The array has eight risers along the periphery and one at the center. The riser at the centre may serve as a structural support riser or structural riser for the spacers through which the other eight risers guided. Such a configuration is known from US pre-grant application publication No. 2013/0239480. The structural riser may or may not serve to take in water.

As schematically indicated in Fig. la, risers 106A, 106C may be formed out of a plurality of riser segments 109 which are connected to each other using connectors 112. Riser segments 109 typically have a length of 6 - 12 meters.

Fig. la also shows that one of the risers 106B comprises a riser element 108 which has a length of 100 meter or more. The riser element 108 is formed as a single integrally formed element. Alternatively, the riser element 108 may be formed as a single element formed by fixedly connecting a plurality of riser segments 109. This means that they cannot be separated without employing destructive measures such as cutting. Fixedly connecting includes welding the riser segments 109 directly together without using connectors as described above.

This one riser element 108 has a length which is equal to the length L of the riser 106B between a proximal end 107 of the riser and a distal end 117 of the riser. The length L is typically measured between the lowest point of the riser and the top edge of the riser where the coupling to the floating structure 102 starts. The length L is schematically indicated in Fig. la.

In the embodiments shown, the riser 106B comprising the riser element 108 is a structural riser, to which spacers 110A-C are attached. The structural riser is shown as having a length which is equal to length of the water intake risers 106A, 106C. However, the structural riser may have a length greater than the length of the water intake risers. The distal end of the structural riser 106B may thus be

positioned deeper than the distal ends of the water intake risers 106A, 106C.

Fig. lb shows an alternative embodiment in which

transmission lines 131 are provided running through the structural riser 106B. These transmission lines 131 may be provided in the structural riser 106B on-shore making off ^¬ shore assembly less time consuming. The transmission lines may be connected to the water intake risers, for instance for providing anti-fouling agent to the water intake risers to prevent clogging of the water intake riser. The connection between the transmission lines and the water intake risers may be established by further transmission lines, such as flying leads 132.

The distal end of the structural riser 106B is located at the same depth or preferably at a lower depth than the distal ends of the risers 106A, 106C, making the distal end of the structural riser 106B accessible for divers and ROV's.As the riser element 108 can't be suspended from the floating body 102 hanging downwardly into the body of water 101 during construction of the floating structure and transport of the floating structure 100 from its construction location to its production location, it is stored on the floating structure 100 in a horizontal position during transport. This and the method of putting such a riser element 108 in place will be explained in more detail with reference to Fig.'s 2a - 2g.

In order to safely transport the riser element 108 and position it in the intended vertical submerged position, the floating body 102 comprises supports or sea fastening frames 120 for supporting the riser element 108 in a horizontal position .

As schematically shown in Fig. 2a, the supports 120 are positioned on the outside of the floating body 102. The floating body 102 may comprise a hull 121 and the supports 120 are provided on the outside of the hull 121 above the water line. The supports 120 may be attached to the outside of the hull 121 in a removable manner.

The floating body 102 comprises handling devices, such as a crane 150 and one or more winches 151, for lifting the riser element 108 from the supports 120 and positioning the riser element 108 in a mainly submerged, vertical position. The supports 120 are positioned such that the riser element 108 when in a supported position resting on the supports 120 is in reach of the handling devices 150, 151. The handling devices 150, 151 are preferably positioned at predetermined positions along the supported position of the riser element 108. As shown in Fig. 2a, the handling devices 150, 151 are positioned along the edge and on the deck of the floating structure 102.

The crane 150 may be a pedestal crane positioned on deck of the floating body 102. The crane is preferably positioned at or near the intended proximal end of the riser element 108 when in the supported position.

The winches 151, for instance 3, 4 or 5 winches may be positioned on the deck of the floating body 102 distributed along the riser element 108 when in the supported position.

Transporting and handling of the riser element 108 with a length of more than 150 meter is challenging. Next, a method of installing a floating structure 100 comprising such a riser element 108 is described in more detail with reference to Fig. ' s 2a - 2g.

The method comprises

a) providing a floating body (102) with supports (120) for supporting the riser element (108) in a horizontal position, the floating body (102) comprising handling devices, such as a crane (150) and one or more winches

(151), for moving the riser element (108) from the supports (120) and positioning the riser element (108) in an overboard, vertical orientation,

b) providing a riser element (108) having a length of

more than 100 meter,

c) positioning the riser element (108) on the supports. Step (b) may comprise providing spacers 110A-C (as shown) and transmission lines (not shown) running through the riser element 108. Step (b) is preferably performed on-shore. Steps (a) and (c) are preferably performed at a construction facility in a dock, a dry dock, a harbour or the like. For the purpose of this text this is also considered on-shore, as being opposite from off-shore, i.e. being remote from the shore .

The result of these steps is schematically shown in Fig. 2a. The floating body 102 comprises a hull 121 and the supports 120 are provided on the outside of the hull 121. The supports are preferably positioned above the water line and are attached to the outside of the hull 121 in a removable manner .

Next, the floating structure 100 including the riser element 108 supported by the supports 120 is moved to its production location (step (d) ) . The floating structure may be moored, for instance to a turret (not shown) .

In a next step (e) , the riser element 108 is attached to the handling means 150, 151 (shown in Fig. 2b) and optionally lifted from the supports 120.

In a next, optional step (f) , the supports 120 are removed. This is in particular advantageous in situations where the supports 120 are provided on the outside of the hull 121. By removing the supports 120, the riser element 108 can simply be lowered by the handling devices 150, 151 and no significant lifting and/or horizontal movements are required. Fig. 2c shows the floating structure after step (f) with the supports 120 removed.

Next, the riser element 108 is positioned in an

overboard vertical position, by lowering the riser element 108. This can be done by giving out rope by the handling means 150, 151. Different handling devices give out different amounts of rope to rotate the riser element 108 to a

substantial vertical orientation. This is shown in Fig.'s 2d - 2f. First the riser element 108 is lowered to a horizontal submerged position (Fig. 2d) . Optionally, all but one winch 151 and the crane 150 are disconnected and the riser element 108 is rotated to a substantial vertical position (Fig. 2e-f) . When the riser element 108 is vertical or almost vertical, the last winch 151 is also disconnected.

The further procedure is in line with known procedures and may comprise connecting an additional rope 142 which runs through a caisson 141 to pull the riser element 108 to its intended position (e.g. by keel hauling) . In case

transmission lines 131 are present inside the riser element 108, these can be connected to corresponding hardware on the floating structure 102.

As shown in Fig.'s 2a - 2g, the floating structure 100 may have a number of further riser segments 109 stored, which may be assembled and positioned in any known matter using crane 150 (not shown) .

After the winches 151 are disconnected, the winches 151 may be removed from the floating structure 100 as they are no longer needed.

The water intake riser bundle as described above may be used to supply process water to any process carried out on the floating body of the floating structure.

In one specific example, it may be used in a method of producing a liquefied hydrocarbon stream, comprising:

- feeding a vaporous hydrocarbon containing feed stream to the floating structure;

- forming a liquefied hydrocarbon stream from at least a part of the vaporous hydrocarbon containing feed stream comprising at least extracting heat from at least said part of the vaporous hydrocarbon containing feed stream; - supplying water to the floating body via the water intake riser bundle;

- adding at least part of the heat removed from said at least a part of the hydrocarbon containing feed stream to at least part of the water supplied via the water intake riser bundle;

- subsequently disposing of the at least part of the water.

A well known example of a liquefied hydrocarbon stream is a liquefied natural gas stream. A variety of suitable installations and line ups are available in the art for extracting heat from a vaporous hydrocarbon containing feed stream, particularly a natural gas stream, as well as other treatment steps such as removal of unwanted contaminants and components from the feed stream often performed in

conjunction with producing a liquefied hydrocarbon stream, and need not be further explained herein.

In another specific example, the water intake riser bundle may be used in a method of producing a vaporous hydrocarbon stream, comprising:

- providing a liquefied hydrocarbon stream to the floating structure;

- forming a vaporous hydrocarbon stream from at least a part of the liquefied hydrocarbon stream comprising adding heat to the said part of the liquefied hydrocarbon stream;

- supplying water to the floating body via the water intake riser bundle;

- drawing at least part of the heat for adding to the said part of the liquefied hydrocarbon stream from at least part of the water supplied via the water intake riser bundle; and

- subsequently disposing of the at least part of the water.

A variety of suitable installations and line ups are available in the art for regasification or vaporisation of previously liquefied hydrocarbons streams and adding heat to such a liquefied hydrocarbon stream, and need not be further explained herein.

The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims.