The present invention relates to a floating structure arranged in a body of water, comprising a floating body and a water intake riser bundle suspended from the floating body into the body of water. In other aspects, the present invention relates to a method of producing a liquefied hydrocarbon stream on such a floating structure and/or a method of producing a vaporous hydrocarbon stream on such a floating structure.

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 assembly of US 2013/0239480 is a bundle comprising first and second tubular conduits that generally extend side by side along a length direction. The distal part of the water intake riser assembly may hang free from the ocean floor 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 .

Since the water intake riser assembly of US

2013/0239480 may be made essentially out of steel, and as the water intake riser assembly hangs freely into the body of water suspended from the off-shore structure, it may be exposed to swaying motion as a result of drag forces imposed by the water and/or relative motion of the floating body from which the water intake riser assembly is suspended. As a result, over time the water intake riser assembly my suffer from effects of material fatigue .

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. However, it turns out that extending the water intake riser assembly from 130 m to a depth of 170 m has an adverse effect on the fatigue life expectancy of the water intake risers, which can be problematic for a number of applications in the off-shore oil-and gas industry.

In accordance with a first aspect of the present invention, there is provided floating structure arranged in a body of water, comprising a floating body and a water intake riser bundle suspended from the floating body hanging freely downwardly into the body of water, said water intake riser bundle comprising a steel riser tube attached to the floating body by a hang-off section at a proximal end of the riser tube and a water intake section at a distal end of the riser tube, whereby the water intake riser bundle extends over a length L between the hang-off section and an apex of the water intake riser bundle, wherein the length L is selected from a range of lengths of the water intake riser bundle, over which range a fatigue life indicator of the water intake riser bundle plotted over the range of lengths varies between a minimum fatigue life value and a maximum fatigue life value, in which range a reference length L0 uniquely corresponds to the fatigue life indicator having the minimum fatigue life value, wherein L is greater L0.

In another aspect of the present invention, 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 first aspect of the invention, comprising a water intake riser bundle suspended from a floating body;

- 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 .

In yet another aspect, there is provided of producing a vaporous hydrocarbon stream, comprising:

- providing a liquefied hydrocarbon stream on a floating structure according to the first aspect of the invention, comprising a water intake riser bundle suspended from a floating body;

- 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.

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

Fig. 1 schematically illustrates a floating structure arranged in a body of water comprising a water intake riser bundle;

Fig. 2 shows a plot of calculated fatigue life for a range of lengths of the water intake riser bundle;

Fig. 3 shows a graph of bending moment along the length of three water intake riser bundles of different lengths ;

Fig. 4 shows a normalized schematic representation of bending mode shapes for modes 1 to 4; Fig. 5 schematically shows a cross section of a flex joint that can be used in the water intake riser bundle.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. 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 1 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 term "bundle" is used to imply that a plurality of riser tubes (106A, 106B) may preferably be bundled together side-by-side forming a single water intake riser assembly 105, as described for instance in US

2013/0239480. In the example of Figure 1, for instance, the riser tubes 106A, 106B are laterally connected to each other by means of at least one spacer (110A; HOB, HOC) . By means of such spacers, the riser tubes are physically associated or connected together. In one embodiment, enough spacers may be provided to keep the tubular structures from striking into one another and to make the riser tubes behave as one unified bundle.

However, a single riser tube - not bundled side-by-side with other riser tubes - 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 extends over a length L between a hang-off section 112 at a proximal end

107 of the water intake riser bundle, and an apex 117 of the water intake riser bundle 106. The apex 117 is considered to be the part of the water intake riser bundle 106 that is furthest removed from the hang-off section 112.

The apex 117 does not touch the ocean floor 103.

Preferably a minimum distance of 35 m is maintained between the apex 117 and the ocean floor 103. Seen in the length direction, the water intake riser bundle has a proximal portion 107 closest to the floating body 102, followed by a connecting portion 108, followed by a distal portion 109. The distal portion of the water intake riser assembly 105 hangs free from the ocean floor 103. The one or more riser tubes (106A, 106B) are configured to take in cold water 140 via the distal portion 109 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.

The distal portion 109 may be arranged in a staggered configuration such as proposed in US 2013/0239480. The distal portion 109 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, 106B in the distal portion 109.

Fatigue life of the water intake riser bundle 106 is a measure of how long the water intake riser bundle 106 can be used in the configuration described above, before it disqualifies as a result of the effects of material fatigue. Fatigue life may be expressed by a fatigue life indicator, which may for instance correspond to the expected duration of how long the water intake riser bundle 106 can be used in the configuration described above, before it disqualifies as a result of the effects of material fatigue, expressed in years.

The fatigue life of the water intake riser bundle 106 turns out to depend on the selected length L. For very short lengths, the fatigue life expectancy can be extremely long - so long that fatigue will not be a life ^¬ time limiting phenomenon compared to other factors that influence the operational life time of the floating structure. The fatigue life is expected to decrease as a function of length L. This is confirmed by numerical analysis, which shows that with increasing length of the water intake riser bundle of US 2013/0239480 from 130 m to 170 m, the fatigue life decreases from about 165 years to between 15 and 20 years. Further details on the numerical analysis will be discussed hereinbelow.

It has now been discovered that a minimum fatigue life value is reached at a certain unique length of L0, which will be referred to in the present description as reference length. For each specific water intake riser assembly configuration, one single reference length L0 uniquely corresponds to the fatigue life indicator having the minimum fatigue life value. For all lengths smaller than L0, the fatigue life decreases with increasing length. Further increasing the length beyond L0 has now surprisingly been found to result in a relative increase of the fatigue life compared to the minimum fatigue life value found at L0. In other words, a fatigue life indicator of the water intake riser bundle 106, when plotted over a range of lengths, varies between a minimum fatigue life value and a maximum fatigue life value wherein the minimum fatigue life value occurs at a length L0 whereby L0 does not coincide with an endpoint of the range of lengths considered.

Hence, it is presently proposed to select the length L of the water intake riser bundle 106 greater than L0, and preferably greater than 1.2 times L0. Herewith the fatigue life of the water intake riser bundle is

increased relative to the minimum fatigue life value for a given riser.

The water intake riser bundle may thus be designed, according to a method wherein selecting a length L for a water intake riser bundle comprising a steel riser tube to be attached to the floating body of the floating structure by a hang-off section for free-hanging

suspended operation in a body of water, wherein:

- determining a set of values for a fatigue life

indicator for a number of water intake riser bundles each extending over a length between the hang-off section and an apex of the water intake riser bundle for a range of lengths ;

- determining a reference length L0 that uniquely corresponds to the water intake riser bundle having the lowest fatigue life value of all fatigue life values in the set, whereby said reference length L0 may not correspond to an end point in the range of lengths;

- selecting a length L that is greater than L0. If the reference length LO does correspond to an end point in the range of lengths, the method can be

supplemented by expanding the set of values by

determining fatigue life indicator values for additional lengths until the minimum value is found for a reference length LO that does not correspond to an end point in the range .

The absolute value of LO expressed in a unit of length (for instance meters) depends on mechanical properties of the riser bundle, including bending modulus and weight per unit of length. Such properties depend on material selection as well as riser geometry including outer diameter, cross-sectional shape, and wall

thickness. It can be determined in various ways, including by numerical simulation and empirically. The absolute value of the fatigue life depends on assumptions regarding intensity of motions forced on the floating structure by the environment. The qualitative behaviour of the fatigue life indicator as a function of length, however, is expected to be independent from such

properties as will be made credible hereinbelow.

With proprietary software (TIARA) the reactions of various water intake riser bundle designs to 10,000 year return period survival environmental conditions that can be expected at a potential future deployment location were studied. These conditions include wind sea

dominated loading with waves travelling from bow to stern of the floating body 102, with a significant wave height of 19.7 m and a period of 17.2 s. A current with a specified velocity profile having a maximum velocity of

1.60 m/s at the surface, and levelling off at about 0.2 m/s at 600 m and deeper, was also taken into

consideration. With Morison' s equation the loading on the risers was determined. Surge (horizontal) , heave (vertical) and pitch (rotational) motions of the floating body 102 are included in the model. Heave motion only causes a change in effective tension along the water intake riser bundle 106.

The floating body 102 was assumed to be moored to a turret in weathervaning relationship with the turret comprising rotatability about a vertical axis through the turret. The water intake riser was assumed to be connected to the floating body 102 at a draft of 17 m depth, and 117 m aft of the center of gravity of the floating body 102 and 32 m starboard of the center of gravity of the floating body 102. The total length of the floating body 102 was 488 m and the width 76 m.

The calculation was done for a water intake riser bundle consisting of a circular-cylindrical single water intake riser, with an outer diameter of 107 cm (42") and a wall thickness of 3.05 cm (1.2") . The water intake riser tube was assumed to comprise a flex joint within the hang-off section 112, whereby the remainder of the water intake riser tube below the flex joint, including the connecting portion 108 and the distal portion 109, was assumed to be formed out of carbon steel. The length of the hang-off section, and the flex joint therein, was the same for each riser bundle length. Specifically in the present examples the length of the hang-off section was about 1.6% of L0, which is small compared to L0. The flex joint was taken to have the same axial stiffness as the steel of the remainder of the water intake riser tubes, but a much lower bending stiffness.

The outcome of fatigue life calculations for free- hanging suspended water intake riser bundles of varying lengths is depicted in Figure 2. On the vertical axis is a calculated fatigue life indicator, here expressed in a number of years of non-stop suspension in an ocean, for a certain assumed accumulation of motions based on

frequencies of sea state conditions. On the horizontal axis is plotted the length of the water intake riser bundles expressed relative to LO whereby LO corresponds to the fatigue life indicator having the minimum fatigue life value.

It is remarked that a safety factor may optionally be applied in the fatigue life calculations. In the present calculations a safety factor of 10 was adopted on all calculated fatigue life valves. However, as it is only necessary to find a minimum, the safety factor does not influence the determination of the reference length LO .

As can be seen, the minimum fatigue life value for a water intake riser having length LO, in the present example, is about 7.4 years. For all lengths above LO, the fatigue life is expected to be longer than 7.4 years. However, while better than 7.4 years, it is clear that preferably the length is not chosen very close to L0 as the relative improvement becomes stronger if somewhat further removed from L0. Above 1.2XL0, for instance, the fatigue life becomes increasingly better than 7.4 years. At 1.2XL0 the expected fatigue life has increased by 5 years to 12.4 years but the marginal increase in fatigue life becomes larger when increasing above 1.2XL0.

The calculation also shows that at a length of about 1.5XL0 the fatigue life has increased to a local maximum value of about 65 years . The general trend is a slight decrease again when increasing the length beyond 1.5XL0 to about 2.25XL0, and further increasing the length from 2.25XL0 to about 3.0XL0 results in another increase of the fatigue life expectancy until another local maximum is reached. It should be noted that, at said length of 2.25XL0 the fatigue life expectancy of about 28 years is still significantly better than at a length of LO. Local maxima can be seen at repetitions of NX1.5XL0, wherein N is a natural number greater than or equal to one.

Based on the repetitive signature of the local peaks in the fatigue life indicator plotted against water intake riser length, it is concluded that the best fatigue life expectancy can be found in "bands" of lengths centered around the NX1.5XL0. The fatigue life expectancy for all lengths larger than LO is better than at LO, but choices of L within the bands (1.5XN - 0.3)XL0 < L < (1.5XN + 1.2)XL0 provide better results than outside of these bands. The fatigue life expectancy for long water intake risers is best in bands that are closer to NX1.5XL0. Preferred are lengths L between (1.5XN - 0.3)XL0 and (1.5XN + 0.7)XL0.

As stated above, it is conceived that the qualitative behavior of free-hanging steel water intake riser bundles as shown in Fig. 2 is general and independent of

mechanical properties of the riser bundle. Without intending to be bound by theory, it is presently

understood that the qualitative behavior is related to a discrete nature of bending modes. This is illustrated in Fig. 3, which shows bending moments along three of the water intake riser bundles used for the calculations of Fig. 2 when loaded with a benign sea state of 5m wave height and a period of 10s. The bending moment is plotted against the horizontal axis on a linear scale. The length along the riser bundle is plotted on the vertical axis on a linear scale. Three riser bundle lengths have been plotted: 0.7XL0 (full solid line 10); L0 (short-dashed line 20); and 1.5XL0 (long-dashed line 30) . The benign sea state has a relatively large contribution in the amount of fatigue damage. Fig. 3 shows that the maximum bending moment increases when the water intake riser bundle length increases from 0.7XL0 to

L0. However, when the water intake riser bundle length is further increased to 1.5XL0, the distribution of the bending moment along the riser changes drastically and the maximum bending moment decreases. It is believed that the response of the water intake riser bundle is governed by oscillation mode 2 for a riser with length L0, and governed by oscillation mode 3 for a riser with length 1.5XL0. The amplitude of the bending moment in the third mode (mode 3) is much smaller than that in the second mode (mode 2) . It is believed that this is the reason for the improvement of fatigue life performance for lengths above L0. At a certain length the fourth mode becomes available, which leads to another local maximum in the fatigue life performance.

The mode shapes for first to fourth modes are depicted in Figure 4 in a normalized representation. The solid line 1 corresponds to the first mode, the short- dashed line 2 corresponds to the second mode, the long- dashed line 3 corresponds to the third mode, and the dotted line 4 corresponds to the fourth mode. The amplitudes of the modes are not to scale and can by definition not be compared. In a sufficiently long riser, all these modes can be excited either individually or simultaneously. However, in a shorter riser only lower modes will be excited.

A second contributing phenomenon is that the weight of the water intake riser bundle increases with

increasing length. Therefore a larger part of the wave loading is transferred by axial tensile stress, rather than by bending stress. This would also explain the improved fatigue life performance associated with water intake riser bundle lengths larger than L0.

Thirdly, deepest part of a free-hanging water intake riser bundle is affected relatively little by wave loading. Therefore the deepest part acts as a damper, which also contributes to improvement in fatigue life performance. This is observed in Figure 3, where

(disregarding periodic variations) the general base-line fatigue life trend is increasing with length for when L/LO > 1.

In the present examples the riser tubes are built up with carbon steel pipe sections each having a length of 12 m. The length of 12 m for the pipe sections is not critical and any suitable length may be used. Lengths in a range of from 8 to 25 m are recommended, to strike a suitable balance between speed of deployment and ease of handling and transport. A suitable coating may be applied on the inside as well as the outside of the riser tubes to protect the carbon steel against corrosion. In addition to these coatings, cathodic protection in the form of anode bracelets may be provided to protect areas where the coating is damaged.

The hang-off section 112 of the water intake riser tubes in the calculations were modelled according to the structure disclosed in US Patent No. 7,318,387. In summary, such hang-off section 112 may comprise a flex joint 50 as schematically illustrated in Figure 5. The illustrated flex joint comprises an internal suspension connection 52 to carry axial load, surrounded by a flexible hose piece 54. The flexible hose piece provides a flow conduit. The internal suspension connection 52, which is here shown in the form of a chain 56, extends between an upper connector piece 8 and a lower connector piece 9, for instance by means of transverse internals 18 and 19, which leave open flow communication in

longitudinal direction. Examples of such transverse internals are the cross-shaped internals such as

disclosed in US Patent No. 7,318,387. The chain 56 is secured to the transverse internals 18 and 19

respectively via coupling interfaces 28 and 29. Each of the transverse internals may suitably be welded to the inner surfaces of the upper (8) and lower (9) connector pieces, respectively. The flexible hose piece 54 is secured to the connector pieces in a watertight way. The flex joint 50 can thus be attached to the floating body and the remaining steel parts of the water intake riser tubes via these upper (8) and lower (9) connector pieces, respectively.

Generally speaking, the length of the hang-off section is small compared to L0, suitably less than about 2% of L0, typically, for instance, between 1% and 2% of

L0.

In the examples above only the hang-off section comprises the flex joint as described. No other flex joint was provided along the water intake riser bundle 106 between the hang-off section 112 and the apex 117.

Embodiments with one or more flex joints along the water intake riser bundle 106 between the hang-off section 112 and the apex 117 as additional hinge points have been considered, but these additional flex joints were not needed because no maximum stress criterion was exceeded in any of the examples above and neither did it improve the fatigue life indicator. Such additional flex joints were found to make installation and inspection of the water intake riser bundles more difficult and any benefit of additional flex joints did not outweigh the

disadvantages thereof.

An all-rubber riser was also considered, but did not meet requirements of acceptable performance in terms of compression, tension, and minimum bending radius .

Moreover, in a bundle configuration the spacers between the rubber riser parts are expected to cause

difficulties. Embodiments having a top section (below the hang-off section) of the water intake riser tubes made from steel and a bottom section made of rubber were also considered, but not selected for similar reasons as embodiments with additional flex joints and the all- rubber riser solution.

The joint axial stiffness may suitably be selected within a stiffness range of from 0.5 times the axial stiffness of the steel riser tube and 2 times the axial stiffness of the steel riser tube, but it is contemplated that satisfactory results may be achieved outside of this stiffness range as well. The joint bending stiffness is smaller than that of the bending stiffness of the steel riser tube, preferably smaller than 0.1 times the bending stiffness of the steel riser tube.

Any number of or all of the riser tubes may be provided with vortex induced vibration suppression devices. Examples of such devices include sleeve-like devices such as helical strakes, shrouds, fairings, and substantially cylindrical sleeves. Helical strakes are preferred. Examples are described in various public disclosures, including for instance US application publication No. 2012/0006053. 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.

It is remarked that the proprietary software product

TIARA has employed in the past for calculating of fatigue stresses in other riser systems, for instance in a Joint Industry project on reliability analysis of a top- tensioned TLP riser (BSEE Technology Assessment Program, project number 275) . Reference is made to the Final

Report dated 20 July 2001, published by the US Bureau of Safety and Environmental Enforcement.

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 .