EP2886786A1

EP2886786A1 - Steel production riser, offshore hydrocarbon production system, and method of producing a hydrocarbon stream

Info

Publication number: EP2886786A1
Application number: EP13198736.4A
Authority: EP
Inventors: Kok-Chieng LIM; Jan Willem Van De Graaf
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2013-12-20
Filing date: 2013-12-20
Publication date: 2015-06-24
Also published as: BR112016014076B1; AP2016009250A0; AU2014368814A1; WO2015091616A2; BR112016014076A2; AU2014368814B2; WO2015091616A3

Abstract

A steel production riser (100) provided with an auxiliary buoyancy section (106) around the touchdown point (115) wherein the production riser is provided with a first set of external buoyancy modules (140). As a result of the first set of external buoyancy modules, the upward buoyancy force on the auxiliary buoyancy section in the body of water is smaller than the downward gravity force. The touchdown point is located within the auxiliary buoyancy section. The steel production riser may be used in a method for producing a hydrocarbon stream, whereby mineral hydrocarbon fluids produced from a subsea hydrocarbon reservoir are conveyed to a floating structure via the steel production riser.

Description

The present invention relates to a steel production riser. In a further aspect, the present invention relates to an offshore hydrocarbon production system provided with such a steel production riser. In another aspect the present invention relates to a method of producing a hydrocarbon stream.
A steel production is a steel riser, typically formed out of a string of pipes made out of steel, arranged to convey mineral hydrocarbon fluids produced from a subsea hydrocarbon reservoir to a floating structure such as a floating production platform, a floating production storage and offloading (FPSO) structure, a semi-submersible.
There are various types of such steel production risers, including catenary risers and lazy wave risers, that have a configuration wherein the riser gradually approaches the sea bed and touches down on the seabed in a downwardly convex curve whereby in the touchdown point the riser is in tangential alignment with the seabed. In the context of this disclosure, such riser configurations are called "lazy". In contrast, steep risers typically approach the sea bed steeply and touch down at a pronounced non-tangent angle (typically vertical or near-vertical).
Amongst numerous lazy configurations, there are three common main types: so-called catenary risers, lazy wave risers, and shaped catenary risers. In lazy wave riser and shaped catenary risers, a segment of the length along the riser is provided with a set of external buoyancy modules to create a primary buoyancy section wherein an upward buoyancy force on the riser in the body of water is greater than a downward gravity force. As a result, part of the riser is raised in the water thereby an upwardly convex curved section (so-called hog bend or arch bend) is formed in the primary buoyancy section. The primary buoyancy section lifts up parts of the riser adjacent to the primary buoyancy section, whereby a downwardly convex section (so-called sag bend) can form hanging between the floating structure and the primary buoyancy section. This causes a waved or shaped path. The presence of both a hog and a sag bend distinguish a lazy wave riser from a steel catenary riser or a shaped steel catenary riser. The shaped steel catenary riser can be seen as a transitional form between the catenary and the lazy wave riser: it also has a buoyancy section, similar to the primary buoyancy section and which also changes the trajectory of the riser in the body of water, but the amount of buoyancy is not enough to raise the buoyancy section high enough to form actual hog and arch bends.
The use of steel production risers, catenary and lazy wave risers in particular, has been proposed in the past for connecting a floating structure to a pipeline or well head located under water on a seabed. The hog and sag bends in steel lazy wave risers help to decouple the touchdown point from horizontal and vertical motion of the floating structure as a result of factors such as wind, currents, waves and tides. Generally, fatigue and strength performance in the touchdown zone is improved by the waved modification. According to a paper by Songcheng Li and Chau Nguyen, presented at Deep Offshore Technology (DOT) International conference (Amsterdam, 30 November to 2 December 2010), the lazy wave configuration has gained popularity as a viable solution to improve fatigue life and strength performance at the touchdown zone of a simple catenary riser.
In a first aspect of the present invention, there is provided a steel production riser, steel production riser, comprising a string of pipes made out of steel, which string of pipes is suspended from a floating structure into a body of water above a seabed, on which body of water the structure floats, wherein at a hang-off end of the riser the string of pipes is connected to the floating structure in a hang-off point, and extending to the seabed wherein, as seen along the string of pipes starting from the hang-off point, distal to a touchdown point which corresponds to a first point of contact of the production riser with the sea bed the production riser comprises a touchdown section wherein the pipes rest on the seabed, whereby the string of pipes in the touchdown point are tangentially aligned with the seabed, further comprising an auxiliary buoyancy section extending from a point between the hang-off point and the touchdown point into the touchdown section, whereby the touchdown point is located within the auxiliary buoyancy section, in which auxiliary buoyancy section the production riser is provided with a first set of external buoyancy modules and wherein the upward buoyancy force in the body of water is smaller than the downward gravity force.
There is also provided an offshore hydrocarbon production system, comprising a floating structure floating on a body of water above a seabed, and a steel production riser according to any aspect of the present invention suspended from said floating structure into said body of water.
In another aspect there is provided a method of producing a hydrocarbon stream, comprising conveying mineral hydrocarbon fluids produced from a subsea hydrocarbon reservoir to a floating structure via a steel production riser in accordance with the first aspect of the invention, and processing the mineral hydrocarbon fluids on the floating structure whereby forming the hydrocarbon stream out of the mineral hydrocarbon fluids.
The invention will be further illustrated hereinafter by way of example only, and with reference to the nonlimiting drawing in which:

Fig. 1 schematically shows a not-to-scale side view of an offshore hydrocarbon production system including a steel catenary riser;
Fig. 2 schematically shows a not-to-scale side view of an offshore hydrocarbon production system including a steel lazy wave riser; and
Fig. 3 schematically shows a not-to-scale side view of an offshore hydrocarbon production system including a steel shaped catenary riser.

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.
A production riser is presently proposed, which comprises an auxiliary buoyancy section around the touchdown point, in which auxiliary buoyancy section the riser is provided with a first set of external buoyancy modules as a result of which the upward buoyancy force on the auxiliary buoyancy section in the body of water is smaller than the downward gravity force. The touchdown point is located in the auxiliary buoyancy section.
The auxiliary buoyancy section helps to reduce the curvature in the touchdown zone compared to the same riser having no auxiliary buoyancy section around the touchdown point. Fatiguing of the riser pipes around the touchdown point transition can be reduced by reducing the downwardly convex curvature in this area.
The sets of external buoyancy modules each comprise a plurality of external buoyancy modules. The external buoyancy modules in any of the first, and second set may be embodied in distributed buoyancy configuration, whereby distinct external buoyancy modules are attached to the riser with a selected spacing between successive adjacent external buoyancy modules. This includes a so-called full coverage configuration, whereby the spacing is zero or close to zero and the successive adjacent external buoyancy modules are configured in a physically abutting configuration.
The term "gravity force" in any named section of the steel production riser refers to the downward force exerted by gravity on the mass of the production riser in the section, normalized to a unit of length, including contents of the production riser and any external buoyancy modules. Contents include the fluids that are being conveyed through the production riser, typically from the seabed to the floating structure. Preferably, these fluids comprise mineral hydrocarbon fluids produced from a subsea hydrocarbon reservoir.
The term "upward buoyancy force" in any named section of the waved steel production riser refers to the upward force imposed on the production riser in the section by the weight of water from the body of water that is displaced by that section of the production riser (including the riser pipes and the external buoyancy modules), normalized to the same unit of length.
The upward buoyancy force on the primary buoyancy section is generally higher than the upward buoyancy force on the auxiliary buoyancy section. This can be achieved for instance by selecting external buoyancy modules in the second set that per external buoyancy module have more buoyancy than the external buoyancy modules per module in the first set (per module). This may be achieved by selecting external buoyancy modules with lower density and/or larger volume for use in the second set compared to those for use in the first set. Alternatively, or in addition thereto, the spacing between successive adjacent external buoyancy modules in the primary buoyancy section may be selected smaller than the spacing between successive adjacent external buoyancy modules in the auxiliary buoyancy section. In this case, the external buoyancy modules in the second set can be exact copies of those used in the first set.
The steel production riser according to the present invention can be used on any type offshore hydrocarbon production system on any type of floating structure. Examples of floating structure include a floating production platform, a floating production storage and offloading (FPSO) structure, a semi-submersible structure, and a SPAR. A tension leg platform (TLP) may also be considered a floating structure on which the steel production riser of the invention can be beneficial. A floating liquefied natural gas (FLNG) barge is a special example of FPSO, and it contains process equipment and utilities by which natural gas can be produced from a subsea reservoir, treated, and finally cooled down to produce liquefied natural gas (LNG) at a pressure of less than 2 bar absolute.
Fig. 1 shows an offshore hydrocarbon production system including a steel production riser 100 embodied in the preferred form of a steel catenary riser. The system comprises a floating structure 10, which floats on the surface 25 of a body of water 20, above a seabed 30. The steel production riser 100 is suspended from the floating structure 10, into the body of water 20. The steel production riser 100 is generally constructed in the form of a string of pipes made out of steel.
At a hang-off end of the steel production riser 100, generally indicated at 101, the string of pipes is connected to the floating structure 10 in a hang-off point 110. The steel production riser 100 extends all the way to the seabed 30. Distal to a touchdown point, which corresponds to a first point of contact of the production riser 100 with the seabed 30 as seen from the hang-off point, the production riser 100 comprises a touchdown section 105 wherein the pipes rest on the seabed 30. The string of pipes in the touchdown point are tangentially aligned with the seabed 30. In the context of the present disclosure, this tangent alignment characterizes the term "lazy" in the configuration of the riser.
The first point of contact 115 is dynamic as motion of the floating structure 10 causes the riser to be lifted off from or laid down on the seabed 30. Fatiguing of the riser pipes around the transition between the touchdown section 105 and the riser sections proximal to the floating structure relative to the first point of contact 115 can be reduced by reducing the downwardly convex curvature in this area. To this end, the steel production riser 100 comprises an auxiliary buoyancy section 106, which extends from within the touchdown section 105 a point between the hang-off point 110 and the touchdown point in the first point of contact 115. Thus, the touchdown point is located within the auxiliary buoyancy section 106. In this auxiliary buoyancy section 106 the production riser 100 is provided with a first set of external buoyancy modules 140, whereby the upward buoyancy force on the auxiliary buoyancy section 106 within the body of water is smaller than the downward gravity force. Moreover, the upward buoyancy force in the body of water within the auxiliary buoyancy section 106 is higher than the upward buoyancy force in the body of water of the string of pipes made out of steel without any external buoyancy modules. The net force, however, remains downwardly directed (sinking).
The upward buoyancy force in the body of water within the auxiliary buoyancy section 106 of the production riser 100 is preferably selected between 40% and 99% of the downward gravity force in the same section. At 99% the riser section is considered neutrally buoyant for practical purposes. More preferably the upward buoyancy force in the body of water within the auxiliary buoyancy section 106 of the production riser 100 is selected between 40% and 90% of the downward gravity force in the same section. The range of between 40% and 90% is preferred over the range between 90% and 99% in order to keep some more strain on the riser and less transverse movement of the riser over the sea bed. Still more preferably, the upward buoyancy force in the body of water within the auxiliary buoyancy section 106 of the production riser 100 is selected between 50% and 90% of the downward gravity force in the same section, and most preferably between 60% and 90% of the downward gravity force in the same section. This is generally achieved by purposely selecting the spacing between the external buoyancy modules in the first set and/or their sizes. Thus, most preferably the spacing between the external buoyancy modules in the first set and/or their sizes are sized such that the pipe weight is reduced to between 10% and 40% of the bare pipe weight including its contents.
Suitably the external buoyancy modules are provided in distributed buoyancy configuration. Each of the modules may consist of parts (usually two halves provided with an internal recess) and a clamping system that can be clamped around the pipes in the riser. The parts suitably comprise a syntactic foam. Suitable external buoyancy modules are available from a variety of vendors. One example is Balmoral Offshore Engineering, Aberdeen, Scotland. Reference is made to pages 29-31 of a Balmoral Offshore Engineering full brochure about Buoyancy, insulation and elastomer products (document number BOE-0410-REV00), for examples.
Alternatively, buoyancy modules may be applied pendant to the riser pipes whereby the external buoyancy modules are anchored to the riser pipes by anchor lines. These external buoyancy modules would still be configured fully submerged to benefit from maximal buoyancy force. This alternative may be preferred if contact of the external buoyancy modules in the auxiliary buoyancy section would cause unacceptable abrasion as a result of physical contact with the seabed 30. Such physical contact would be avoided using the pendant buoys.
In the example as shown in Fig. 2, the steel production riser 100 is a steel lazy wave riser. In the example of Fig. 3 the steel production riser 100 is a shaped catenary riser. In both cases, as seen from the floating structure 10, and as described along the string of pipes starting from the hang-off point 110, the production riser 100comprises:

a hanging section 102;
a primary buoyancy section 103, wherein the production riser 100 is provided with a second set of external buoyancy modules 130;
a landing section 104, extending between the primary buoyancy section 103 and a first point of contact 115 with the seabed 30; and
the touchdown section 105 wherein the pipes rest on the seabed 30.

The touchdown section 105, as seen from the hang-off point 110, is distal from a touchdown point which coincides with the first point of contact 115. As is the case for any of the lazy wave and shaped catenary risers discussed herein, in the transition between the landing section 104 and the touchdown section 105 the steel lazy wave riser 100 is curved with a downwardly convex curvature.
The major distinction of the lazy wave riser of Fig. 2, and the shaped catenary riser of Fig. 3, compared to the catenary riser of Fig. 1, resides in the presence of the primary buoyancy section 103.
The totality of the external buoyancy modules 130 in the second set cause an upward buoyancy force on the primary buoyancy section 103 within the body of water 20, that is greater than a downward gravity force in the primary buoyancy section 103. Therefore, within the primary buoyancy section 103 the steel lazy wave riser 100 floats. The primary buoyancy section 103 generally does not reach the surface 25 of the body of water as it is pulled down by the hanging section 102 and the landing section 104.
In the embodiments comprising such a primary buoyancy section 103, the auxiliary buoyancy section extends from a point within the landing section 104 (between the primary buoyancy section 103 and the first point of contact 115) and into the touchdown section 105.
The hanging section 102 hangs between the floating structure 10 and the primary buoyancy section 103. In the case of the steel lazy wave riser of Fig. 2, a downwardly convex curved section is formed in the hanging section 103. A sag point 125 is defined in the lowest point on the downwardly convex curved section, there where the riser has a tangent 127 in a horizontal direction and parallel to an imaginary vertical plane, which spans between the hang-off point 110 and the first point of contact 115. In the case of the shaped catenary riser of Fig. 3, the buoyancy force in the primary buoyancy section 103 is not high enough to actually form the arch bend.
In either case, the fact that the upward buoyancy force in the auxiliary buoyancy section 106 is kept smaller than the downward gravity force is a distinct difference of the auxiliary buoyancy section 106 compared to the primary buoyancy section 103.
The external buoyancy modules in the first and second sets may be of the same types as those discussed above with reference to Fig. 1. The external buoyancy modules in the second set may be of the same type as those used in the first set. However, generally the clamped distributed configuration may be preferred for the primary buoyancy section 103.
The invention is applicable on steel production risers having pipes of any outer diameter, including steel production risers having pipes of which the outer diameter exceeds 199 mm (which includes 8-inch pipes). Notwithstanding, fatigue phenomena in the touchdown area are generally more of concern for larger diameters. Hence the invention has more benefit on steel production risers of which the outer diameter exceeds 249 mm (which includes 10-inch pipes), and even more when the outer diameter exceeds 299 mm (which includes 12-inch pipes, and up).
Preferably the water depth exceeds 500 m. It is envisaged that below this depth the choice of steel for the risers would generally be outcompeted by alternatives, such as flexible risers.
The steel production riser described herein can be used in a variety of methods of producing a hydrocarbon stream. In such methods, mineral hydrocarbon fluids may be produced from a subsea hydrocarbon reservoir to the floating structure via the steel production riser. Subsequently, on the floating structure, the mineral hydrocarbon fluids are processed whereby the hydrocarbon stream is formed out of the mineral hydrocarbon fluids. Processing may include any kind of known hydrocarbon processing steps, including separation steps to remove undesired components from the hydrocarbon fluids such as water, acids, hydrate inhibitors, sulphur components, mercury. Processing may further include (field) stabilization of hydrocarbon liquids, and purification of hydrocarbon gases.
In cases of an FPSO or a SPAR, the produced hydrocarbon stream may be stored and off-loaded in batches (bulk transportation). In case the mineral hydrocarbon fluids comprise natural gas, process steps may be applied by which the natural gas is treated, and finally cooled down to produce the hydrocarbon stream in the form of liquefied natural gas (LNG). Such LNG is typically also stored in (or on) the floating structure, and off-loaded in batches like described for FPSO and SPAR.
The person skilled in the art will understand that the present invention can be applied and/or carried out in many various ways without departing from the scope of the appended claims.

Claims

A steel production riser, comprising a string of pipes made out of steel, which string of pipes is suspended from a floating structure into a body of water above a seabed, on which body of water the structure floats, wherein at a hang-off end of the riser the string of pipes is connected to the floating structure in a hang-off point, and extending to the seabed wherein, as seen along the string of pipes starting from the hang-off point, distal to a touchdown point which corresponds to a first point of contact of the production riser with the sea bed the production riser comprises a touchdown section wherein the pipes rest on the seabed, whereby the string of pipes in the touchdown point are tangentially aligned with the seabed, further comprising an auxiliary buoyancy section extending from a point between the hang-off point and the touchdown point into the touchdown section, whereby the touchdown point is located within the auxiliary buoyancy section, in which auxiliary buoyancy section the production riser is provided with a first set of external buoyancy modules and wherein the upward buoyancy force in the body of water is smaller than the downward gravity force.
The steel production riser of claim 1, wherein the upward buoyancy force in the body of water within the auxiliary buoyancy section is higher than the upward buoyancy force in the body of water of the string of pipes made out of steel without any external buoyancy modules.
The steel production riser of claim 1 or 2, wherein the upward buoyancy force in the body of water within the auxiliary buoyancy section of the production riser is between 40% and 99% of the downward gravity force.
The steel production riser of any one of the preceding claims, wherein in addition to the auxiliary buoyancy section further comprises a primary buoyancy section wherein the production riser is provided with a second set of external buoyancy modules causing an upward buoyancy force on the primary buoyancy section in the body of water that is greater than a downward gravity force in the primary buoyancy section, wherein a hanging section is formed between the hang-off point and the primary buoyancy section whereby the hanging section hangs between the floating structure and the primary buoyancy section, and wherein as described from the hang-off point distal to the primary buoyancy section but proximal to the touchdown section a landing section extends between the buoyancy section and a first point of contact with the seabed, whereby the auxiliary buoyancy section extends from a point in the landing section the touchdown section.
The steel production riser of claim 4, wherein the production riser is a lazy wave riser, wherein a vertical riser plane is defined, which contains both the hang-off point and the first point of contact and extends parallel to a vertical direction, and wherein a downwardly convex curved section is formed in the hanging section, whereby a sag point is defined in the downwardly convex curved section there where the production riser has a tangent in a horizontal direction and parallel to the vertical plane.
The steel production riser of any one of the preceding claims, wherein the pipes each have an outer diameter that exceeds 199 mm, preferably 249 mm, more preferably 299 mm.
An offshore hydrocarbon production system, comprising a floating structure floating on a body of water above a seabed, and a steel production riser according to any one of the preceding claims suspended from said floating structure into said body of water.
A method of producing a hydrocarbon stream, comprising conveying mineral hydrocarbon fluids produced from a subsea hydrocarbon reservoir to a floating structure via a steel production riser according to any one of claims 1 to 6, and processing the mineral hydrocarbon fluids on the floating structure whereby forming the hydrocarbon stream out of the mineral hydrocarbon fluids.