CA2040833A1 - Method of preventing formation of hydrates in flowing hydrocarbons under subsea piping of same and a subsea plant for processing of a well to prevent hydrate formation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE.

Method of preventing formation of hydrates in flowing hydrocarbons under subsea piping of same and a subsea plant for processing of a well flow to prevent hydrate formation.

Formation of hydrates is to be prevented in subsea pipelining of hydrocarbon flows by establishing the phase diagram for said flows including a hydrate line. The hydrocarbon flow is separated under a pressure at which light hydrocarbons boil off the liquid phase and enters the gas phase to such an extent that hydrates will not be formed or substantially not be formed in the liquid phase, determined by the composition of the flow of hydrocarbons. The gas phase is processed according to known methods so that formation of hydrates is prevented during the subsequent piping. A subsea plant for processing of a well flow to prevent formation of hydrates during the subsequent piping includes a separator for separation of the well flow into liquid and gas, connection means for connection of the separator to a well flow pipeline, a pump unit including a pump with motor and a compressor unit comprising a compressor with motor, and fluidum carrying lines between the separator and the pump resp. the compressor and a pressure line from pump resp.
compressor, connected to the conveying pipelines and a choke device in the well flow pipeline for reduction of the pressure in the separator and a control device for adjustment of the choke device in accordance with the pressure desired in the separator.

Description

2~0~33 The lnvention relates to a method of preventlng formation of hydrates in flowing hydrocarbons under subsea plplng of same.

The lnvention is also related to a subsea plant for pro-cesslng of a well flow to pre~ent hydrate formatlon durlng the subsequent piplng in pipelines, lncluding a separator for separation of the well flow into liquid and gas, cormectlng means for connectlon of the separator to the well flow llne, a pump unit having a pump with motor and a compre~sor unit comprlsing a compressor wlth motor together wlth fluld conveying plpelines between the separator and the pump resp.
the compressor and a pressure plpellne from pump resp.
compressor connected to the actual plpelines.

When conveying flows of hydrocarbons in subsea pipelines, the formatlon of hydrates in the pipelines represent a problem.
The condltions for formation of hydrates (which 1~ a form of lce) ln plpellnes where hydrocarbons are conveyed, are present when the flow of hydrocarbons contalns light hydrocarbon components, particularly parafins and free water and the temperature ls below a certaln level, for example (and the pressure slmultaneously ls above a certaln level).

The condltlons for formatlon whlch are thermodynamlcally deflned, can be found quite acGurately for a given mixture of hydrocarbons contalning water by constructing a so called phase diagram for the ml~ture and by lnserting a so called hydrate line ln same. A skllled person wlll be acqualnted wlth approved computer programs able to lnsert the hydrate llnes when the composition of the hydrocarbon mixture is known. The effect of hydrates in a plpeline can reach from the harmless, i.e. that the hydrates are entralned ln the well flow to the processlng plant in whlch they are dissolved (melt) wlthout actually belng dlscovered, and to the more 6erlous sltuatlon where the hydrates will block the pipellne completely as an "lce plug" and consequently ~top the 2 20~al83~
productlon. In the latter case, the plpellne must be pressure released 80 that the hydrates will melt. Thiæ may take several days.

Normally the formation of hydrates ls counteracted by ln~ections of either methanol or glycol into the pipeline.
Thls method is, however, for economical reasons limited to pipelines for gas/condensate as a usual rule indicates that about half a liter of in~ection fluid must be added per liter free water. Thls addition will be acceptable in gas/-condensate pipelines, but not ln multiphase pipellnes which may convey several thousand m3 of water per day.

A further hydrate limiting posslbility is to insulate the pipeline but this will only have effect as long as there is a fluid flow through the pipeline. Prior to a possible stop, methanol (or glycol) must be in~ected or the pipeline ma~y have heating cables which are connected during a stop or the pressure in the pipeline must be released. These methods are 20 expensive and no well developed technology is available for this purpose.

The ob~ect of the present invention is to prevent formation of hydrates in a simple and cost effective way.

It is therefore according to the inventlon suggested a method of preventing formation of hydrates in flowing hydrocarbons under subsea piplng of same, the method being characterized in that the phase diagram for flow of hydrocar-30 bons is established with hydrate llne and the flow ofhydrocarbons is separated at a pressure under which the light hydrocarbons boil off the liquid phase and enter the gas phase to such an extent that hydrates will not be formed or substantially not be iormed in the liquid phase, as regulated 35 by the composition of the flow of hydrocarbons and in that the gas phase is processed in such a manner that the formation of hydrates is prevented during the subsequent piplng.

The invention is based on the use of phase diagrams estab-lished for the compositions ln question of the well flow andon the concept that it is primarily the light hydrocarbons which form hydrates. The llght hydrocarbonæ are removed from the well flow by separating the well flow into liquid and gas. The water will be entralned in the liquid. The gas phase will contain water vapor up to saturation. It has been found that in practice satisfactory conditions will be obtained already at 40 bar. It is important that the pressure is reduced to a level at which the conditions in the phase diagram are fulfilled to such an extent that the ~5 formation of hydrates is prevented in the liquid phase.

It is advantageous to let the processing of the gas phase include in~ection of compounds counteracting the formation of hydrates, as for example methanol and glycol.

During the subsequent piping of the well flow it is possible, with simple means, to obtain the desired pipeline pressure in the gas and liquid provided by the separation.

25 Said separation pressure may be provided by a choking of the pressure in the hydrocarbon flow.

A partlcularly advantageous method to prevent formatlon of hydrates durlng subsea piping of hydrocarbon flow ls a method 30 utillzing a subsea statlon for pumping of a well flow, the well flow being in the subsea station conveyed through a separator and further through a pump and a compressor respectlvely, the method being dlstingulshed by the feature that in order to prevent formation of hydrates, the separator 35 iS operated with a controlled pressure which results ln the boiling off of a sati6fatory portion of light hydrocarbons from the liquid phase and enters the gasious phase so that no 4 ~0~833 hydrates or substantlally no hydrates are formed ln the liquid phase in which most of the water is present, the conditions being defined by the composition of the well flow and by processing of the gas phase so that the hydrate 5 formatlng is prevented.

The formation of hydrates can, for example, be prevented by in~ection of compounds such as methanol and glycol counterac-tlng hydrates in the gas phase. ~eating is also a posslbl-o llty.

As mentloned it is in accordance with the invention alsosuggested a subsea plant for processlng of a well flow to prevent formatlon of hydrates durlng the subsequent piping, including a separator for separating of well flow into liquid and gas, connecting means for connecting of the separator to a pipeline carrying the well flow, a pump unit including a pump with motor, and a compressor unit comprising a compres-sor with motor, together with fluid conveying pipellnes 2~ between the separator and the pump resp. the compressor and a pressure pipellne from pump resp. compressor connected to pipelines for the well flow, the characterizing feature of the new plant according to the invention being choke means ln the well flow pipe line to reduce the pressure ln the 25 separator and control means for controlling the choke means in accordance with the deslred pressure in the separator.
Thls pressure will be the pressure which, accordlng to the phase diagram with hydrate curve ls sufficlently low to enable boiling of the desired quantity of light hydrocarbons 30 from the liquid phase and into the gas phase.

It is advantageous to have inJection means for lnJectlon of compounds counteracting formation of hydrates in the pressure llne for the compressor.

The invention will be explained more in detail with reference to the drawings on which:

Z~0~3;~

Figure 1 diagrammat~cally di~closes a ~ubsea st,ation wlth separator, pu~p and co~pressor, 5 Flgure 2 discloses a typlcal phase dlagram for a well flow, Figure 3 discloses dlagrammatlcally the lay-out of the statlon, and Figures 4a, b and c disclose halves of sectlons through a practlcal embodlment of a ~ubsea station.

Diagrammatically shown, subsea plant on Figure 1 is a subsea station for production of hydrocarbons. The statlon lncludes a separator 2, a pump 3 and a compressor 4. A well flow ls supplled to the separator 2 (oil/water/gas/partlcles) through a pipeline 1 from one or more well heads (not shown) on the sea bottom. From the fluld chamber in the separator a pipe-line 5 extends to the pump 3. This pipellne will consequent-20 ly convey a mixture of oll, water and particles. In the pump3, the liquid flow is energized for the conveyance and continues further through the pipeline 6. From the gas chamber of the separator a plpellne 7 leads to the compressor 4. At this polnt the gas ls energlzed for the further 25 conveyance and contlnues through the pipellne 8.

The motors for the pump 3 and the compressor 4 are referred to as M. The supply of electrlclty to the motors ls referred to by the dotted llnes 9.

On Figure 3 it is shown ln principle how the subsea station can be constructed as a compact unit. The same reference numbers as in Figure 1 are used for the corresponding components in the statlon.

As is evldent from Figure 3, the separator 2, the pump 3 and the compressor 4 (the motors are no-t shown on Figure 3) are ~O~Q~33~

assembled as a compact unlt wlth the three components arranged in the column structure shown, with the pump at the bottom, then the separator and wlth the compressor at the top. The fluid carrylng pipellnes 1, b and 8 are a6sembled 5 in a common connectlng unit 10 at the bottom of the column structure. In the gas plpeline 8 a volume flow meter 12 may be inserted. In the same way a meter 11 may be inserted in the line 6. The pipeline 1 i6 close to the separator provided with a choke 31.

In~ection means is indicated at 32.

The pump 3 is with its inlet directly connected to the liquid chamber 13 in the separator, and in a corresponding way the 15 inlet of the compressor 4 is directly connected to the gas chamber 14 of the separator.

Both units, i.e. the pump unit with the pump 3 and also the compressor unit with the compressor 4 are self draining, i.e.
20 gas will be able to bubble upp from the pump and liquid will drip down from the compressor.

It would be an advantage if the gas chamber 14 of the separator is insulated as indicated with the insulatlon 15.
25 The liquid chamber 13 of the separator can with advantage be provlded with cooling ribs 16. With these provisions a stabilization of the phases may be obtained, i.e. the liquid phase and the gas phase.

30 Figure 4 discloses a preferred embodiment of a subsea station ln the form of a compact unit lncluding a separator, a pump and a compressor and it is adapted to be located on the sea bottom. The unit shown on Flgure 4 has an advantageous size corresponding to a blow out preventer (BOP). This unit could 35 be installed using a drilling rig or a modlfied diving vessel.

~0~083~

The same reference numbers are used on Figure 4 aæ on Figure 1 and 3 concerning the componentæ which are found on these Figures. Consequently the æeparator 2 is on Flgure 4 in the form of a container having a cyllndrical upper section and a 5 conlcal bottom section and a the pump is here ln the form of a multistage centrifugal pump while the compressor 4 is constructed as a multistage rotational compressor. These components are as shown assembled to a compact unit in a column structure. At the bottom there iB provided a common o connectlon unlt or connector 10.

The compressor as well as the pump are on Figure 4 deslgned as vertically positioned centrifugal machines. The motor 17 for the compressor is at the top and the motor 18 for the pump is located under the pump in the column structure. The motors are vertically positioned elec-tric motors (asyncronous motors) with separat RPM-control.

As shown on Figure 4, the column structure is built into a 20 frame work 19 which includes guide funnels 20 i'or cooperation with guide posts in a standard module pattern in a manner known per see, for e~ample as known from blow out preventers and other kinds of equipment, which are adapted to be lovered and installed on the seabed in a desired position.

The gas chamber of the separator may be heat lnsulated as indlcated with the reference number 15. The liquid chamber of the separa-tor may have cooling means for example external cooling ribs not shown on Figure 4, but as diæclosed on 30 Figure 3 where the coollng ribs are referred to as 16.

The pump 3 and its motor 18 are located ln a common, closed pressure houslng 22. The pressure housing is assembled from a plurality of closely assembled housing sections.

The compressor 4 and its motor 17 and gearing ~2 are also arranged ln a common pressure housing 24 constructed from a 2040~

plurallty of closely assembled houslng sections by means of the flange connectlons lndlcated.

The pressure houslng 24 ls at the lower end deslgned as a supply 25 for lubricatlng oll for lubrlcation of the bearings ln the compressor unit.

The æuctlon side of the compressor 4 i6 by means of a plpe 26 dlrectly connected to the gas chamber of the separator 2.
10 The pressure side of the compressor 16 connected to the plpe llne 27, extendlng downwardly on the outslde of the column structure to the connector 10. A plpe connectlon 28 extends from the pressure slde of the compressor for return of gas of the separator 2. The pipe llne 28 extends down lnto the 15 liquld chamber of the separator.

The pressure slde of the pump 3 iæ by means of a plpeline 29 ln connection wlth the connector 10. The connector 10 has three passages. A passage through the connector 10 establls~
20 hes connectlon between a well flow plpellne 1 (see Flgure 3) and a plpellne conveylng the well flow to the separator vla an ad~ustable choke 31 as shown dlagrammatically on Flgure 3 and Flgure 4. The plant shown on Flgure 1, 3 and ~ may wlth advantage be used for worklng of the method accordlng to the 25 lnventlon, but before the method or the operation of the plant will be further explalned a short descrlptlon of the so called phase dlagram for the well flow should be given.

A typlcal phase dlagram for a light hydrocarbon deposlt ls 3~ shown on Figure 2. The pressure ls glven ln bars alnng the ordlnate. The temperature, glven ln K, ls lndlcated along the abscissa.

The curve 35 represents the so called two phase envelope.
The steeply rlsing curve 36 represents the so called hydrate line. The crltlcal polnt of the llquld ls on the curve 35 indicated by 37. To the left of the critical point 37 the 9 ~Q!3~
dlagram lncludes a region referred to as "oil". To the right of the point 37 the diagram lnclude a reglon referred to as "condensate". This region covers as shown a remaining part of the two phase envelope ~5. To the right there i8 an 5 additional region whlch ln the diagram on Flgure 2 ls referred to as "gas".

These references show that there wlll be llquid outslde the envelope 35 and to the left of the crltlcal point in the ~0 dlagram. Inslde the envelope 35 in the region "condensate", condensate will be precipitated and further to the right there will be gas only at the temperature/pressure combl-natlons outslde the envelope ~5.

The hydrate llne 3h ls establlshed ln a manner known per se.
As mentloned prevlously there are avallable approved computer programs for thls work when the composition of the hydrocar-bon mlxture ls lntroduced. In the diagram, to the left of the hydrate llne, there wlll be comparatlvely low tempera-20 tures and hydrates may be formed. To the right of thehydrate line :In the diagram there wlll be now formation of hydrates. An ordlnary well flow will usually be withln the envelope for the two phase condltlon. In order to avold formation of hydrates lt ls for example posslble to in~ect 25 methanol or ethylen glycol. This has the effect that the hydrate llne wlll move to the left on the diagram and consequently provide a separatlon between the pressure/tempe-rature profile of the plpellne and the hydrate llne obtalned by the ln~ection so that formatlon of hydrates is avoided.
Alternatively an insulation of the plpeline wlll contrlbute to maintain the well flow to the right of the orlginal hydrate llne.

Over longer dlstances lt wlll usually be possible to malntain ~5 the flow in the pipellne outside the hydrate forming reglon (to the right of the hydrate llne) due to coollng and temperature drop. By initial separation of the well flow ln 21:~083;~

a liquid phase wherein most of the water wlll be present and a gas phase in whlch the llght hydrate forming hydro carbons wlll be present, these phases could be represented separately in a phase diagram. At a sufflclently low level of the 5 separation pressure the phase diagram for the liquid mlxture wlll show a hydrate llne at so low a level of temperature that the llquld flow wlll be maintained above the temperature at whlch hydrates can be formed. The phaæe dlagram for the gas wlll have a hydra~e llne at a hlgher temperature level o and the temperature of the gas flow ln the pipe line wlll sooner or later drop and intersect the natural hydrate llne.
The hydrate line for the gas phase can, however, be moved to a lower temperature level by means of the above mentioned previously known ln~ection methods as the water content in the gas phase now ls small.

With the plant described above, i.e. the subsea station comprising a separator, a pump and a compressor close to the wells, it is possible when the piping is initiated, to 20 achleve efficient control with the hydrates by separatlon of llght components from the multiphase flow near the hydrocar-bon wells. It assumed that the composltlon of the well flow is known from tests taken or in other ways. With thls background it is possible to calculate how low the pressure 25 ln the separator 2 must be to boll off a sufflclent pro-portlon of the light hydrocarons so that the hydrocarbons wlll enter the gas phase. The separation pressure is declded elther by provlslons resulting ln that at no polnt under the operatlon of the llquld pipe wlll exist temperature/pressure 30 combinatlons falllng wlthin the hydrate region (durlng lnterruptlons the plpellne wlll for example have to be pressure released to avold formatlon of hydrates), in that the gas pipeline 8 ls inJected with small quantitles of methanol of glycol, ot in that the provlsions are made to 35 maintaln the pressure so low that llght hydrocarbons wlll be transferred from the llquld phase to the gas phase to such an extent that no hydrates can be formed ln the llquid plpe line ~:()4~)83~

even at full lnterruptlons of the flow for lndefinate time ~it can be a questlon of lowerlng the pressure down to levels below 20 bar).

A condltlon for use of thls method is that the descrlbed subsea Etation 1~ available with eparator/pump/compreæsor from the flrst day hydrocarbons are conveyed through the plpeline and not after and increaæe of the presæure ls required due to a falling pressure ln the re~ervolr ln order o to malntain a sati6factory plpellne flow.

The method according to the inventlon ls naturally associa-ted with addltional costs compard to those methods which can be collectlvely referred to as "not doing anythlng". The additlonal costs are primarily due to the requlrement of an lnvestment in the subsea station several years before it normally is required due to unsatlsfactory pressure ln the reservoir. The subsea station will consume more energy than what is required for only to increase the pressure from the 20 reservolr pressure prevailing at the station the additional lowering of the pressure down to the level for correct separation pressure will also all the tlme have to be compensated. In relation to methods involvlng multiphase conveyance in one pipeline also after the subsea station, 25 investments must be made in an additional gas pipeline. The cost for this pipe~.ine will primarily be related to the length of the pipeline and its diameter. If there ls a probability for formation of hydrates, "not doing anything"
will not be a relevant means for comparison. Economy and 30 technical conditions related to the method of reducing the pressure by use of a subsea statlon will therefore have to be compared with other relevant methods in order to decide the competitive relation ln each case.

35 The method suggested accorded to the invention is considered to be of advantage and competitlve compared with other posslble methods.

Claims (8)

1.
Method of preventing formation of hydrates in flowing hydrocarbons under subsea piping of same, c h a r a c t e r i z e d 1 n that the phase diagram for the hydrocarbon flows with hydrate line is established and that the flow of hydrocarbons is separated under a pressure at which light hydrocarbons boil off the liquid phase and enters the gas phase to such an extent that no hydrates or substantially no hydrates will be formed in the liquid phase as determined by the composition of the flow of hydrocarbons and in that the gas phase is processed with known methods so that the formation of hydrates is preventing during the subsequent piping.

2.
Method as claimed in claim 1, c h a r a c t e r 1 z e d 1 n that the processing of the gas phase includes injection of compounds counteracting hydrate formation, as for example methanol or glycol.

3.
Method as claimed in claim 1, c h a r a c t e r 1 z e d 1 n that the gas and liquid involved by the separation are pressurized to the desired pressure for subsequent pipe lining.

4.
Method as claimed in claim 1, c h a r a c t e r i z e d i n that said separation pressure is provided by a controlled choking of the flow pressure in the flow of hydrocarbons.

5.
Method of subsea piping of a hydrocarbon flow, using a subsea station for pumping of a well flow, the well flow being at the subsea station conveyed through a separator and from same through a pump and a compressor respectively, c h a r a c t e r 1 z e d i n that the separator, in order to prevent formation of hydrates, is operated at a controlled pressure resulting in a boiling off of a suffici-ent proportion of light hydrocarbons from the liquid phase in order to enter the gas phase, so that hydrates will not be formed or substantially not be formed in the liquid phase in which a major part of the water is present, determined by the composition of the well flow and in that the gas phase is processed so that formation of hydrates is prevented.

6.
Method as claim in claim 5, c h a r a c t e r 1 z e d i n that said treatment of the gas phase includes injection of compounds counter acting formation of hydrates, as for example methanol or glycol.

7.
Subsea plant for processing of a well flow to prevent formation of hydrates during the subsequent piping, including a separator for separating the well flow into liquid and gas, connection means for connection of the separator, a well flow pipeline, a pump unit comprising a pump with motor and a compressor unit comprising a compressor with motor, and fluid conveying pipelines between the separator and pump resp. compressor and a pressure line from pump resp.
compressor connected to the conveying pipelines, c h a r a c t e r 1 z e d 1 n a choke device in the well flow pipeline for reduction of the pressure in the separator and control means for adjustment of the choke device in accordance with the desired pressure in the separator.

8.
Subsea plant as claimed in claim 7, c h a r a c t e r i z e d i n an injection device for injection of compounds counteracting formation of hydrate in the pressure line to the compressor.