water intake riser bundle

TECHNICAL FIELD

The present invention relates to a floating structure, in particular a floating structure comprising a liquid natural gas plant, arranged in a body of water, comprising a floating body and a water intake riser bundle being suspended from the floating body hanging downwardly into the body of water, the water intake riser bundle comprising a structural riser and at least one water intake riser. 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.

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.

WO2012066039 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 risers that generally extend side by side along a length direction. The term riser is used in this text to denote a substantially vertical element, such as a pipe or a conduit. Risers are typically tubular shaped. The distal end 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 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 one or more spacers. In this example, each spacer comprises eight guiding sleeves, through which the other eight tubular risers are guided to keep the tubular risers bundled, although spaced apart. The spacers may be provided at predetermined locations along the structural riser. The structural riser may, or may not, convey water to the surface (i.e., may or may not serve as a water intake riser) .

With the present design it is cumbersome to establish a transmission path for signals, fluids other than the cooling water or energy (e.g. electric/pneumatic/hydraulic) between the off-shore structure and the lower or distal end of the water riser intake bundle. Also, with the present design it is difficult to reach the distal end of the structural riser for maintenance or other purposes .

SHORT DESCRIPTION

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

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 being suspended from the floating body 102 hanging downwardly into the body of water 101, the water intake riser bundle 106 comprising a structural riser 106B and at least one water intake riser 106A, 106C, wherein a distal end of the structural riser 106B is located at a depth greater than the distal ends of the at least one water intake riser 106A, 106C.

The distal end of the structural riser may in particular be located at a depth greater than the depth of distal ends of each water intake risers.

The structural riser 106B has a length which is greater than the length of the respective water intake risers 106A, 106C.

The portion of the structural riser extending beyond the distal end of the at least one water intake riser is referred to as the protruding portion.

The structural riser and the water intake risers each comprise a proximal end, via which they are suspended from the floating body and a distal end, located away from the floating structure. The distal ends of the risers are the ends of the risers which are, when suspended from the floating body, furthest away from the floating body and closest to the seabed.

The proximal ends may be connected to the floating structure by 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.

The water intake risers comprise a water inlet section, preferably located in a distal portion of the water intake riser. The water intake sections may be provided with water intake openings distributed along the water-intake sections. The water intake openings may be distributed along the length and the circumference of the water intake sections. The water inlet section may be formed as a perforated sleeve. An example thereof is described in WO2012066039.

It is noted that the water intake risers may be provided in a staggered orientation as for instance described in WO2012066039, having distal ends at different depths. In such a case, the distal end of the structural riser is at a depth greater than the distal end of the water intake riser having its distal end at the greatest depth. The protruding portion of the structural riser is the portion extending beyond the distal end of the water intake riser having its distal end at the greatest depth.

The above described floating structure allows divers or ROVs (remotely operated underwater vehicles) to be able to access the distal end of the structural riser, for instance to connect transmission lines or cables, e.g. flying leads, between the distal end/protruding portion of the structural riser and one or more of the water intake risers . Such transmission lines or cables may be used to transmit signals (control signals/sensor data), fluids other than the cooling water or energy (e.g. electric/pneumatic/hydraulic) between the distal portion of structural riser and the water intake risers .

Such a construction also allows easy access to the distal end/protruding portion of the structural riser for

maintenance .

The construction also allows sensor equipment or parts thereof, such as a memory unit comprising sensor data, to be positioned, replaced and/or removed to/from the distal end or protruding portion of the structural riser.

The structural riser and the water intake risers are preferably made of steel, e.g. carbon-steel.

According to an embodiment the water intake riser bundle 106 comprises at least one transmission line between the structural riser and at least one of the water intake risers.

Such a transmission line may be formed by a (flexible) cable or a conduit, for instance in the form of a flying lead and may be used 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).

According to an embodiment the structural riser 106B comprises transmission lines running between the proximal end and the distal end or protruding portion of the structural riser .

These transmission lines, which may also be referred to as umbilicals, may be connected to transmission line

connectors positioned at the distal end of the structural riser or along the protruding portion of the structural riser, and/or may be connected to equipment positioned at the distal end or protruding portion of the structural riser, such as a measurement unit .

The structural riser 106B may comprise transmission lines which run through the inside of the structural riser between the proximal end and the distal end or protruding portion. At the proximal end, the transmission lines may be connected to further equipment on the floating body. The transmission lines 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) . The term

transmission line comprises transmission cables and

transmission conduits.

According to an embodiment the structural riser comprises one or more transmission line connectors.

Such transmission line connectors may be positioned at the distal end of the structural riser or along the

protruding portion of the structural riser. This allows easy connection of transmission lines between the structural riser and the water intake riser (s) by a diver or ROV.

In case one or more transmission lines are to be provided between the structural riser and the water intake riser (s), transmission line connectors may also be provided on the water intake risers. This way a transmission path can be established between the structural riser and the water intake riser ( s ) .

So, according to an embodiment, the structural riser comprises one or more transmission lines which are connected to transmission line connectors provided at the distal end or protruding portion of the structural riser and transmission lines are further provided to connect the transmission line connectors provided at the distal end or protruding portion of the structural riser to the water intake risers. According to an embodiment the structural riser comprises a measurement unit positioned at the distal end or protruding portion of the structural riser.

The measurement unit may comprise measurement equipment for measuring movement of the structural riser, an energy source for energizing the measurement equipment and a memory unit for storing measurement data.

The measurement unit may be connected to transmission lines running through the structural riser to the floating body to energize the measurement unit and/or to establish a data connection between the measurement unit and the floating body .

Now the distal end or protruding portion of the

structural riser is easily accessible, replacing the energy source of the measurement unit is made easier. Also, a memory unit present in the measurement unit may be replaced in an easy manner by using a ROV or diver.

According to an embodiment the structural riser comprises one or more spacers 110 arranged to keep the at least one water intake riser at a predetermined position with respect to the structural riser.

The distal end of the structural riser is located at a greater depth than the lowest spacer.

As described above, the structural riser may serve as a support for one or more spacers. The spacers may be formed as guiding sleeves, through which the water intake risers are guided to keep the structural riser and the water intake risers bundled, but avoiding collisions. The spacers may be provided at predetermined locations along the structural riser. The structural riser may, or may not, convey water to the surface (i.e., may or may not serve as a water intake riser) . According to an embodiment the structural riser comprises a riser element extending between the proximal end and the distal end which is formed as a single piece.

The riser element, in particular being a tubular riser element, may be formed as a single piece, which includes being formed by a plurality of (tubular) riser segments being connected in a non-releasable manner, in particular being connected by welding. Although it is understood that parts connected by welding can be separated, for this text direct welding is considered to be a non-releasable connection.

This has the advantage that the structural riser, or at least the tubular riser element thereof, can be assembled on ^¬ shore with the transmission lines running between the proximal end and the distal end of the structural riser already in place. This makes complicated off-shore assembling of the structural riser no longer necessary, as the riser element can be manufactured on-shore. The term on-shore relates to activities performed on land or in the harbour.

The tubular riser element may have a length of more than 100 meter, preferably more than 150 meter, more than 200 meter, more than 250 meter or even more than 300 meter.

Additional parts may be connected to the riser element, such as spacers 110 formed as guiding sleeves for other risers and a coupling for coupling the risers to the floating structure providing some freedom of movement for the water intake riser bundle when being suspended from the floating structure as described above.

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.

According to an embodiment the floating structure comprises a LNG (liquid natural gas) plant. Such a floating structure is usually referred to as a floating LNG plant. The LNG plant may be 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. In other words, the LNG plant may cool and liquefy natural gas to form LNG , and/or heat and gasify LNG .

In case of a liquefaction plant, the water riser bundle is suspended from the floating structure to take in water at depth and convey the water upward to the LNG plant for cooling purposes. In case of a gasification plant, the water riser bundle is suspended from the floating structure to take in water at depth and convey the water upward to the LNG plant for heating purposes.

In both cases the water may be input to heat exchangers provided on the floating structure as part of the LNG plant to add or remove heat to/from a process performed on the off ^¬ shore structure. Heated or cooled water from the outlet of the heat exchangers may be discharged back into the ocean at the surface, or alternatively conveyed back to depth with a discharge system.

According to another aspect there is provided a method of installing a floating structure 100, the method comprising a) providing a floating structure 100 in a body of water (101), the floating structure 100 comprising a

floating body 102,

b) moving the floating structure 102 to its production location,

c) suspending a water intake riser bundle 106 from the floating body 102, the water intake riser bundle hanging downwardly into the body of water 101, the water intake riser bundle 106 comprising a structural riser 106B and at least one water intake riser 106A, 106C, wherein a distal end of the structural riser 106B is located at a depth greater than the distal ends of the at least one water intake riser 106A, 106C,

d) deploying a remotely operated vehicle into the body of water 101 and operating the remotely operated vehicle to mechanically interact with the distal end of the structural riser 106B.

The mechanical interaction may involve connecting or disconnecting transmission lines. The mechanical interaction may comprise removing, installing or replacing a device to the distal end of the structural riser, such as measurement equipment, an energy source, e.g. a battery, or a memory unit .

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 structural riser may comprise transmission lines running between the proximal end and the distal end of the structural riser. The distal end or protruding portion of the structural riser may comprise transmission line connectors. The water intake risers may also comprise transmission line connectors .

According to an embodiment d) comprises providing at least one transmission line between the structural riser and at least one of the water intake risers.

Such a transmission line may be formed by a cable or a conduit, for instance in the form of a flying lead and may be used 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) .

According to an embodiment the water intake riser bundle comprises first water intake risers have distal ends at a first depth and second water intake risers have distal ends at a second depth, the first depth being deeper than the second depth and step d) comprises

dl) providing transmission lines between the structural riser and the first water intake risers and

d2) providing transmission lines between respective first water intake risers and respective second water intake risers .

In this embodiment, the distal end of the structural riser is located at a depth greater than the first depth.

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 drawings in which:

Fig. la-lc schematically illustrate a floating structure arranged in a body of water comprising a water intake riser bundle according to embodiments of the present invention. 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-B. By means of such spacers 110A-B, the risers 106A-C are physically associated or connected together. In one embodiment, enough spacers 110A-B 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.

The water intake riser bundle 106 hangs freely downwardly into the body of water 101.

The one or more risers 106A-C are configured to take in cold water at depth via water intake sections 108 provided with water intake openings, and to convey the cold water upward to the floating body 102. The water intake openings may be provided as a plurality of through holes through the side wall of the one or more water intake risers.

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. The hardware needed for adding/removing heat is not shown. 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 structural riser 106B supports the spacers 110A-B.

The water intake risers and the structural riser each comprise a proximal end, closest to the floating body 102 and a distal end 107 A-C . The distal ends 107A-C are considered to be the part furthest removed from the floating body 102.

The structural riser 106B comprises a protruding portion 110, being the portion extending beyond the (deepest) distal ends 107A, C of the water intake risers 106A, C.

The distal ends of the water intake risers 106A, C may be arranged in a staggered configuration such as proposed in US 2013/0239480.

Although the examples shown in the figures show 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 106 may comprise nine risers arranged in a three-by-three rectangular array or circular pattern, according to one particular embodiment. The array has eight water intake 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.

Fig. lb shows an alternative embodiment further

comprising transmission lines 120 inside the structural riser 106B, transmission line connectors 121 positioned at the distal end 107B of the structural riser 106B connected to the transmission lines 120 inside the structural riser 106B, further transmission line connectors 123 positioned on the water intake risers 106A, C and further transmission lines connecting the respective transmission line connectors 121 on the structural riser 106B to the further transmission line connectors 123 on the water intake risers 106A, C.

This embodiment may for instance be used to supply anti- fouling agents to the water intake risers 106A, C to prevent clogging of the water intake riser

Fig. lc shows an alternative embodiment wherein the structural riser 106B comprises a measurement unit 130 positioned at the protruding portion of the structural riser 106B. A transmission line 131 is provided running through the structural riser 106B to the floating body, e.g. for

energizing the measurement unit 130 or transmitting

measurement data.

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.