NO20141060A1 - Offshore system with underwater riser - Google Patents

Abstract

An offshore system with an underwater riser, including a floating platform and an underwater riser consisting of pipe pieces. A riser stretching system compensates for movement of the platform while providing tension in the riser. At least two of the riser pieces are connected to a connector so that a proportion of the riser can be pressurized without bulging.

Description

Background

[0001] Drilling and production from oil and gas wells on the seabed includes the use of offshore platforms for the extraction of undersea petroleum and natural gas deposits. In deep water operations, floating platforms are typically used (such as spar platforms, tension stay platforms, "extended draft" platforms and semi-submersible platforms). One type of offshore platform, a tension leg platform ("TLP" - Tension Leg Platform), is a vertically anchored floating structure used for offshore oil and gas production. The TLP platform is permanently anchored by groups of moorings, called tension rods or tension cables, which remove virtually all vertical movement of the TLP platform as a result of wind, waves and currents. The tension cables are kept under tension at all times by ensuring net positive buoyancy on the TLP platform under all environmental conditions. The tensioning cables hold the TLP platform rigidly against vertical displacement and thus essentially prevent heave, bump and roll movement, but to a lesser extent prevent lateral displacement of the TLP platform and thus allow a limited yaw, sway and yaw movement. Another type of platform is a spar platform, which typically consists of a single large-diameter vertical cylinder that extends into the water and supports a deck. Spar platforms are anchored to the seabed in a similar way to TLP platforms, but while a TLP platform has vertical tension moorings, a spar platform has more traditional mooring lines.

[0002] The offshore platforms typically support risers that extend from one or more wellheads or other structures on the seabed to the platform on the sea surface. The risers connect the subsea well to the platform to protect the well's fluid integrity and to provide a fluid channel to and from the wellbore. During drilling operations, a drill riser is used to maintain fluid integrity in the well. After drilling is completed, a production riser is installed.

[0003] The risers that connect the surface wellhead to the seabed wellhead can be more than a thousand meters long and extremely heavy. To keep the risers as light as possible, they are designed so that they are not able to support their own weight, even when in water. The connecting links used to connect lengths of individual risers, e.g. production risers, are designed so that they are weaker than the riser pieces themselves. An example of such a coupling joint is a socket coupling where the ends of two adjacent riser pipe pieces are both screwed into the coupling joint. When the riser is exposed to conditions that exceed operating limits, the coupling joints will be the first components to fail.

[0004] To prevent the risers from bulging under their own weight or from placing too much stress on the subsea wellhead, an upward tension is applied, or the riser is lifted, to relieve part of the riser's weight. Since offshore platforms are subject to movement due to wind, waves and currents, the risers must be tensioned to allow the platform to move relative to the risers. The biasing mechanism must therefore exert an essentially continuous tensile force on the riser within a well-defined range to compensate for the movement of the platform.

[0005] Hydro-pneumatic tensioning systems are an example of a riser biasing mechanism used to support risers. A plurality of active hydraulic cylinders with compressed air accumulators are connected between the platform and the riser to create and maintain the necessary tension in the riser. Platform responses to environmental conditions that cause changes in riser length relative to the platform are compensated for by the bias cylinders adjusting for movement.

[0006] Regardless of the type of tension system used, the system must be designed to accommodate weight and movement tension for each riser. However, some risers may require so much prestressing that the loads transferred to the platform exceed the lower allowable load requirements for the platform. A way to accommodate risers when load requirements exceed the platform's limits is needed.

Brief description of the drawings

[0007] For a detailed description of the preferred embodiments of the invention, reference will now be made to the attached drawings, where:

[0008] Figure 1 shows an offshore drilling or production system in accordance with various embodiments;

[0009] Figure 2 shows views of different parts of the riser system in Figure 1;

[0010] Figure 3 shows a first example of a connecting link in accordance with different embodiments; and

[0011] Figure 4 shows another example of a connecting link in accordance with different embodiments.

Detailed description

[0012] The description that follows deals with various embodiments of the invention. The figures in the drawings are not necessarily accurate. Certain features of the embodiments may be shown with an exaggerated size or in a somewhat schematic form, and some details of traditional elements may be omitted to improve the overview and make the description more concise. Although one or more of these embodiments may be preferred, the embodiments shown should not be understood, or otherwise used, as limiting the scope of the invention, including the claims. It should be understood that the various ideas in the embodiments discussed below may be used individually or in any suitable combination to achieve desired results. Furthermore, the person skilled in the art will understand that the following description has general validity, and that the description of a given embodiment is only intended as an example of this embodiment and is not intended to imply that the scope of the invention, including the claims, is limited to this embodiment.

[0013] Various words and designations are used throughout the following description and in the claims to refer to specific features or components. As the person skilled in the art will understand, different people may refer to the same feature or component with different names. This document does not intend to distinguish between components or features that differ in name but not in function. The figures in the drawings are not necessarily accurate. Some features and components herein may be shown with an exaggerated size or in a somewhat schematic form, and certain details of traditional elements may be omitted to improve the overview and make the description more concise.

[0014] In the following description and in the claims, the words "include", "include" and "comprise", and variants thereof, are used in an inclusive manner, and are thus to be understood to mean "includes, but is not limited to. .." Further, the word "connect" or "connects" is intended to refer to either an indirect or a direct connection. If a first device is connected to a second device, this connection can thus be through a direct connection, or through an indirect connection via other devices, components and connections. Furthermore, as used herein, the words "axial" and "axial" generally mean along or parallel to a central axis (eg, the central axis of a body or port), while the words "radial" and "radial" generally mean perpendicular to the center axis. For example, an axial distance refers to a distance measured along or parallel to the center axis, and a radial distance means a distance measured perpendicular to the center axis.

[0015] Figure 1 shows a schematic view of an offshore system 10.1 this example, the system 10 is an offshore production system and includes a riser 14 between a floating platform or a vessel 16 and an underwater wellhead 12 on the seabed 13. Since the example shown is a production system, the riser is designed as a production riser. However, it must be understood that the offshore system 10 and the riser 14 can also be made and arranged for drilling operations in accordance with different embodiments. As shown in Figure 1, mooring lines or tension cables 15 can be provided to connect the floating platform 16 to the seabed.

[0016] In the example shown in Figure 1, the riser 14 is connected to the platform 16 (in this example a SPAR-type platform). Other types of floating structures 16 that may be used with the invention include floating production, storage, and offloading (FPSO) systems, semi-submersible platforms, tie rod platforms (TLP platforms) and others known to those skilled in the art. The connection between the subsea wellhead 12 and the platform 16 provided by the riser 14 enables fluid communication therebetween.

[0017] In Figure 2, the riser 14 is shown broken up to be able to incorporate details of specific parts, but it must be understood that the riser 14 maintains fluid integrity from the subsea wellhead 12 to the production equipment on the platform 16.

[0018] The platform 16 has a mezzanine deck 20, tension application deck 22 and a production deck 24 located above sea level 21. As shown, the riser 14 includes a tension piece 34 and a transition piece 36. The riser 14 is connected at its lower end to the subsea wellhead 12 using a suitable connection. For example, the riser 14 may include a wellhead coupling 40 with an integrated load piece as shown. As an example, the wellhead coupling 40 may be a "tie-back" coupling. Alternatively, the load piece may be separate from the wellhead coupling 40. The riser 14 may, but need not, include other specific riser lengths, such as riser lengths 42 with strakes or fairings and splash zone lengths 44. The upper end of the riser 14 terminates in a surface wellhead and production tree 50 on the mezzanine deck 20.

[0019] A riser tension system 60 is connected to the riser 14 in the tension piece 34 by using a tension ring 62 on the riser 14. The riser tension system 60 is supported on the tension application deck 22 and applies tension to the riser 14 dynamically. This enables the tensioning system 60 to adjust for the movement of the platform 16 while keeping at least part of the riser 14 under tension. The riser tensioning system 60 may be any suitable system, such as a hydro-pneumatic tensioning system with bias cylinders 64 as shown. The number of bias cylinders used may vary depending on the design of the system 10.

[0020] Although the tension system 60 is capable of compensating for the movement of the platform 16, the tension system 60 is not designed to provide all the tension necessary in the riser 14 to prevent buckling. In order to prevent the riser from bulging, the riser 14 includes pressure connections 80 which are designed so that they are strong enough that at least part of the riser 14 is able to take up a pressure load. The number and positioning of pressure connections will depend on the design loads and the deformation of the system 10 and the riser 14. It must be understood that pressure connections 80 need not be used over the entire length of the riser 14. Instead, the riser 14 need only include at least one pressure connection 80 as that a part of the riser 14 can withstand being put under pressure. In this way, the tension system 60 does not need to provide the entire pretension that the riser 14 would otherwise need to prevent bulging. The full load necessary to support the riser 14 therefore need not be transferred to the platform 16, and the platform 16 can be used to support a riser 14 that is heavier than it would otherwise be capable of.

[0021] Figure 3 shows an example of a pressure connection. In this example, the pressure coupling 80a is a male and female coupling including a pin 82 and a socket 84. The pressure coupling 80a can be designed so that it is even stronger than the riser piece itself.

[0022] Figure 4 shows another example of a pressure connection. In this example, the pressure coupling 80b is a flange coupling, which is typically designed so that it is stronger than other types of coupling joints. The flange coupling 80b includes a body with a flange 88 and a neck 86 for each riser end. The riser end is connected to the neck 86 either by welding, crimp fitting, or some other suitable joining method. When connected, the riser pieces are joined by tension bolts which are passed through the adjacent flanges of the connecting link 80b.

[0023] Although the present invention has been described with respect to specific details, these details are not intended to be considered limitations of the scope of the invention, except to the extent included in the appended claims.

Claims (10)

1. Offshore system for a subsea well, comprising: an underwater riser including pipe pieces; a riser tension system capable of dynamically compensating for movement while providing tension in the riser; and where at least two of the riser pipe pieces are connected together with a connecting link so that a portion of the riser pipe is able to withstand a pressure load. 2. System according to claim 1, further including several connecting links. 3. System according to claim 1, where connecting links join pieces of the underwater riser from the bottom of the riser and as high up as the riser is designed to be under pressure when installed. 4. System according to claim 1, where each connecting link includes a pin and a sleeve. 5. System according to claim 1, where each connecting link includes a body including a flange and a neck projecting from the flange. 6. System according to claim 1, where at least some of the support load for the riser is transferred to the connecting link and the tension system provides all the necessary prestressing that the riser would otherwise need to prevent buckling without the connecting link. 7. System according to claim 1, where the tensioning system can be designed to only support a weight that is less than necessary to put the underwater riser in tension. 8. System according to claim 1, wherein the tension system is capable of supporting a riser that is heavier than the tension system is designed to support. 9. System according to claim 1, where the riser is one of a production riser or a drilling riser. 10. System according to claim 1, where the riser tensioner system includes a dynamic riser tensioner.