Patent Application
20080304916
Not Applicable
Not Applicable
This invention relates generally to the field of floating offshore platforms or vessels for the exploitation of undersea deposits of petroleum and natural gas. More specifically, it relates to a system and apparatus for tensioning risers that extend from a subsea wellhead or subsurface structure to a floating platform or vessel.
Offshore platforms for the exploitation of undersea petroleum and natural gas deposits typically support production risers that extend to the platform from one or more wellheads or structures on the seabed. In deep water applications floating platforms (such as spars, tension leg platforms, extended draft platforms, and semi-submersible platforms) are typically used. These platforms are subject to motion due to wind, waves, and currents. Consequently, the risers employed with such platforms must be tensioned so as to permit the platform to move relative to the risers. Also, riser tension must be maintained so that the riser does not buckle under its own weight. Accordingly, the tensioning mechanism must exert a substantially continuous tension force to the riser within a well-defined range.
One broad class of risers is the category called “Top Tensioned Risers” or TTRs. Such risers extend from the subsea wellheads below the hull of the platform substantially vertically to the deck area of the platform, where they are supported by a tensioning mechanism; hence the term “Top Tensioned Riser.” Each TTR typically extends from a riser tension point up into the production deck levels of the platform with the use of a heavy wall conduit or stem joint. At the top of the conduit or stem joint is an upper riser termination where a surface wellhead and a production tree or flow control device are mounted. (Platforms with such an arrangement are called “dry tree” platforms.) A flexible jumper attached to the production tree enables the produced well fluids to be transferred to the topside processing facilities.
Passive buoyancy cans are a well-known type of riser tensioning mechanism that is used primarily on spars. The buoyancy cans independently support each TTR, which allows the platform to move up and down relative to the riser. This isolates the risers from the heave motion of the platform and eliminates any increased riser tension caused by the horizontal offset of the platform in response to the marine environment.
Hydro-pneumatic tensioner systems are another form of riser tensioning mechanism used to support TTRs on various dry tree platforms. Hydro-pneumatic riser tensioning has its origins in the support of drilling risers of MODUs (mobile offshore drilling units). A plurality of active hydraulic cylinders with pneumatic accumulators is connected between the platform and the riser to provide and maintain the necessary riser tension. Platform responses to environmental conditions, mainly heave and horizontal motions causing hull set-down, necessitate changes in riser length relative to the platform, which causes the tensioning cylinders to stroke in and out. The spring effect caused by the gas compression or expansion during riser stroke partially isolates the riser from the low heave platform motions while maintaining a nearly constant riser tension. However, when the platform takes a significant horizontal offset, the compression of the gas in the cylinders causes increased cylinder pressure and thus increased riser tension. The magnitude of this increased riser tension is a function of the stiffness of the riser and the tensioning system.
Two major types of hydro-pneumatic tensioner systems are currently in use: the “push” or compression style system and the “pull” or tension style system. Both systems use hydraulic cylinders having pistons with piston rods connected to the riser by a tension ring device. Push-style cylinders are mounted with the piston rods looking up, and they use pressure applied to the piston side of the cylinders to provide riser tension. The piston rods effectively push up on the riser, putting the rods in compression while providing the necessary riser tension. The pull-style cylinders, by contrast, are mounted with the piston rods looking down. Pressure applied to the rod side of the cylinders puts the piston rods in tension while pulling up on the riser to generate the riser tension.
Pull-style tensioner systems have to date been used predominately on tension leg platforms (TLPs) to support TTRs. The tensioner cylinders may be symmetrically mounted under the well deck, outboard of the riser, using padeyes and shackles, or they can be mounted in a similar manner in a cassette frame that is then mounted to the well deck. The cylinders are angled inboard to riser attachment points on a tension ring. Generally, a roller assembly mounted at the well deck level above the tension ring is used to provide lateral support to the riser as it passes through the tensioner.
The pull-style tensioners on TLPs are designed for short strokes due to the low heave characteristics of the hull, combined with the relatively small riser length changes associated with small hull set down due to the parallelogram arrangement formed by the platform, tendons, risers, and the seafloor well pattern. The advantage is that the surface production tree or flow control device at the top of the riser on a TLP can be mounted closer to the tensioning point of the riser, and the well spacing inside the platform can be reduced. This reduces the bending loads induced in the portion of the riser above the tension point, i.e., the upper riser stem joint, from the dynamic motions of the surface production equipment. However, the production equipment for other hull types and riser system configurations may be located some distance away from the tensioning point. Because there is generally only one set of lateral motion restraining devices (such as rollers) to restrain the riser laterally, dynamic bending moments from the production equipment are transferred across the rollers and the tension ring into the riser pipe below the tension point. Also, riser vortex induced vibration (VIV) oscillations can be transferred across the tension ring and into the upper riser stem joint, possibly affecting its fatigue life.
If a tension cylinder failure occurs, the eccentric load generated by the unequal application of cylinder forces at the tension ring may also cause additional bending moments that must be reacted to by the riser pipe. The unbalanced cylinder forces can also cause the riser and the surface tree to lean to one side. The occurrence of dynamic bending moments from the production equipment and the failed cylinder scenario dictate that the tensioning cylinders be mounted so as to allow pivoting, such as with the use of padeyes and shackles. Pivot mounting eliminates the need for the cylinders and cylinder supports to react to the various loads. However, because the cylinders are generally hung from above to pull up and are also angled inboard to the riser, failed cylinder change-out is made more difficult because of the location of the cylinders below the hang-off deck.
Push-style tensioner systems are a more recent approach to riser tensioning and have been used on deepwater spars to support TTRs and drilling risers. Typically, four to six push-style cylinders are vertically mounted to the platform deck. A piston is journaled in each of the cylinders, each of the pistons being connected to an upwardly-extending piston rod that is attached to a structural top frame. The structural top frame, in turn, supports a large diameter conductor pipe and contains the tension ring attachment to the riser. The piston rods push up on the top frame, which, in turn, pushes up on the riser via a tension ring. The conductor pipe, with two sets of reaction rollers, creates a two-point force coupling to react to riser dynamic bending moments generated from the production equipment and failed cylinder-induced bending moments. The conductor pipe and the associated anti-rotation devices also resist riser torque induced by platform or vessel yaw motions. Because the rods are in compression and are required to resist buckling under very large loads, the rod diameters are larger than those of a pull-style tensioner system.
In general, while conventional pull-style tensioners, as described above, are generally smaller, less expensive, and more widely available than push-style tensioners, the typical pull-style tensioner system generally exhibits one or more of the following disadvantages: (1) It may not provide two-point reaction to riser dynamic bending moments generated by surface production equipment located above the riser tension point. (2) The lack of two-point reaction also allows riser VIV oscillations below the tension point to excite the surface equipment above the tension point, thus adversely affecting its fatigue life. (3) It may not react adequately to failed cylinder eccentric loads, thus creating additional riser bending moments. (4) It may not sufficiently resist riser rotation (torque) created by platform yaw motions. (5) Failed cylinder replacement is made more difficult by below-deck work requirements.
Broadly, the present invention is a pull-style, hydro-pneumatic tensioner system for a riser in a floating platform, comprising a riser support conductor coaxially surrounding the riser and operatively coupled to an upper end of the riser; and a plurality of hydro-pneumatic tensioners operatively coupled between the platform and a lower end of the riser support conductor so as to exert a pull-type tensional force on the riser support conductor, whereby the riser support conductor conveys the pull-type tensional force to the upper portion of the riser. The tensioner system of the present invention provides a two point reaction to riser loads, and also resists riser rotation from, e.g., platform yaw motions.
More specifically, a tensioner system for a top-tensioned riser in a floating platform, in accordance with an exemplary embodiment of the present invention, comprises a plurality of hydro-pneumatic tensioners, each comprising a hydraulically-actuated piston disposed for reciprocation within a hydraulic cylinder and including a piston rod having a lower end operatively coupled to the lower end of a riser support conductor by means of a support conductor coupling assembly; a riser tension joint support assembly operatively coupling an upper end of the riser support conductor to an upper end of the riser; and a support conductor reactive load assembly operatively coupling the support conductor to the platform so as to react to lateral loads and bending moments in the support conductor, and to resist the rotation of the support conductor about its longitudinal axis.
Hydro-pneumatic retraction of the tensioner rods in response to platform motion applies an upward tension force to the support conductor coupling assembly. Axial tension loads are thereby conveyed from the tensioners to the lower end of the support conductor by the support conductor coupling assembly, and then from the upper end of the support conductor to the upper end of the riser by the riser tension joint support assembly, thereby tensioning the riser.
The tensioner system of the present invention is intended primarily for use on spars, extended draft platforms (EDPs), and semi-submersibles to support top-tensioned risers. Nominal operating strokes of about 28 feet (about 9 meters) and nominal operating tension loads of about 1,500 to 2000 kips are typical, but can be varied to suit particular system applications.
As used herein, the terms “invention” and “present invention” are to be understood as encompassing the invention described herein in its various embodiments and aspects, as well as any equivalents that may suggest themselves to those skilled in the pertinent arts.
Referring to the drawings,
Turning to
In general, the top-tensioned riser 101 is connected in a dry-tree arrangement to drilling and production equipment (not shown) disposed, for example, on or above the main deck 112. The tensioner system of the present invention, as described below, supports the top-tensioned riser 101 in alignment with a vertical axis 105, relative to the floating platform 100.
In accordance with an exemplary embodiment of the present invention, the tensioner system for the top-tensioned riser 101 comprises a plurality of pull-style hydro-pneumatic tensioners 120 (preferably four in number), a riser support conductor 150, a reactive load assembly 400 (
The hydro-pneumatic tensioners 120 provide the riser support conductor 150 with tensional forces used to stabilize the riser 101 with respect to the platform 100 by way of the conductor coupling assembly 500 and the riser tension joint assembly 700. The conductor coupling assembly 500 communicates the tensional forces from the hydro-pneumatic tensioners 120 to the riser support conductor 150 and the riser tension joint assembly 700. The riser tension joint assembly 700, in turn, may use its rigidity (bending resistance) to resist side-to-side (lateral) bending and rotational (tortional) movement by the riser 101, and to offset static riser forces, including the weight of the riser 101. Advantageously, the reactive load assembly 400 provides a compensatory reactive force to loads imposed on the riser 101 and related structures, including, without limitation, loads producing bending moments and lateral forces.
Each of the hydro-pneumatic tensioners 120 is a pull-style hydro-pneumatic tensioner that exerts a pull-type tensional force to the upper portion of the riser 101. Depending on the requirements of a particular application, there may be four or six or more of the hydro-pneumatic tensioners 120 resiliently mounted to the floating platform in a generally symmetric arrangement. Each hydro-pneumatic tensioner 120 includes a cylinder or barrel 125 and a piston rod 130 having a first or upper end connected to a piston 136 (
Referring to
Selected embodiments of the tensioners 120 can be configured to produce total nominal operating tension loads of about 1,500 kips, with about 2,000 kips maximum. However, the tensioners 120 also may be configured to produce greater or lesser tensional loads, in accordance with the application requirements. Desirably, the hydro-pneumatic tensioners 120 are passive devices, in which the internal tensioner pressure can be monitored and adjusted through a local pneumatic control panel (not shown), of conventional design, which may communicate with a variety of sensors (not shown), such as pressure and rod stroke sensors, that generate signals that are transmitted back to the control panel. The control panel also is used in the initial riser installation to adjust the internal tensioner pressures to achieve the correct riser tension. Thereafter, it is used for monitoring only, unless there is an operational need to increase or decrease the cylinder pressures and thus the riser tension.
As shown in
The riser support conductor 150 is a vertical pipe with an inside diameter that is greater than outside diameter of the riser 101. The support conductor 150 is positioned generally coaxially around the riser 101, relative to the riser axis 105, and it extends downward from the platform 100 toward the seabed. In general, the riser 101 is run through and landed on the support conductor 150, so that the riser 101 is supported coaxially within the support conductor 150. The riser support conductor 150 communicates tensional forces from the floating platform 100 to the riser 101; restrains the riser 101 from translational and rotational motions; and reacts to bending and lateral loads placed on the riser 101 using the lateral load reaction elements 400 described below. The riser support conductor 150 is advantageously configured with a conductor tension ring interface (described below with reference to
The interior surface of the tension ring 510 is advantageously configured as a bearing surface that mates with a conductor/tension ring interface. In an exemplary embodiment, the conductor/tension ring interface comprises a plurality (e.g. eight) female J-slots 570 machined into the support conductor 150, and a like number of mating male lugs 580 projecting from the surface of the conductor tension ring body 510. The conductor J-slots 570 may be aligned with and receive the conductor mating lugs 580, after which the support conductor 150 is rotated by ⅛ turn clockwise (looking down), and is made to securely but releasably engage the conductor tension ring 510. In this way, the tension loads generated from the piston rods 130 may be transferred respectively from the lower rod ends 131 to the tension ring arms 520 extending from the tension ring 510. The tension loads then may be transferred to the support conductor 150 via the mating bearing surface formed between the conductor tension ring lugs 580 and the top of the J-slots 570 in the support conductor 150.
In general, the tension joint support head 705 engages the tension joint donut 710, which, in turn, circumferentially engages (indirectly, as discussed below) the riser tension joint 715. Specifically, a plurality of retractable load shoulder dogs 707 are pivotably attached around the upper end of the tension joint support head 705. The retractable load shoulder dogs 707 are configured to rotate radially inward and outward relative to the axis 105. When the load shoulder dogs 707 are retracted by rotating them radially outward, access is provided to the interior of the support conductor 150 to enable, for example, the installation of the riser 101 by running it through the riser support conductor 150. When landed by rotating them radially inward, the load shoulder dogs 707 provide a load shoulder for engagement by a mating shoulder on the outer periphery of the adjustable tension joint donut 710.
The inner periphery of the donut 710 is sloped radially inwardly from top to bottom so as to mate with similarly sloped or tapered outer surfaces of a pair of semi-annular engagement segments 711 that are received within the inner periphery of the donut 710. The inner surfaces of the engagement segments 711 are configured to engage and mate with a threaded or grooved section 725 in the riser tension joint 715. The tension joint donut 710 is removably fixed to the engagement segments 711 by a pair of semi-annular capture plates 712, each of which is secured to the donut 710 by an attachment member, such as a cap screw or bolt 713. The inner periphery of each of the capture plates 712 is retained in a slotted plate retainer element 714 on the upper surface of each of the engagement segments 711. By removing the cap screws or bolts 713 and thus loosening the capture plates 712, the position of the tension joint donut 710 and the engagement segments 711 may be adjusted, relative to the tension joint 715, to provide a proper riser space out, relative to the subsea wellhead (not shown), the top of the riser support conductor 150, and the tension joint support head 705. The outer surface of each of the engagement segments 711 is advantageously provided with at least one anti-rotation block 716 that is received in a mating slot 717 in the inner periphery of the donut 710, so that the donut 710 cannot rotate relative to the engagement segments 711. As shown in
In the embodiment of
In the
From the foregoing description, it will be appreciated that the riser axial load path from the upper portion of the riser 101 to the spar tensioner support deck (i.e., the main deck 112) is through the riser tension joint support assembly 700, then to the upper end of the support conductor 150. From there, the axial load is transmitted through the support conductor wall down to the attachment points between the piston rod lower ends 131 and the tension ring arms 520. The riser tension is provided by the tensioner piston rods 130 that are actually riding on the hydraulic pressure provided by the tensioner cylinder or barrel 125 charged with nitrogen or dry air from the interconnected high pressure pneumatic accumulator 138. The same pressure is pulling the cylinder or barrel 125 down against the platform support structure (such as the main deck 112), thus completing the load path from the upper portion of the riser to the platform support structure. By contrast, prior art tensioners only subject the support conductor to a pair of lateral loads and the bending moment imposed at the top of the support conductor through the flag pole effect of the surface equipment. The present invention, on the other hand, uses the large cross sectional area of the support conductor 150 to support the riser axial load in a compressive load fashion, in addition to providing the lateral support to the upper portion of the riser near its top or upper end.
As will be appreciated from the detailed description above, the present invention offers significant advantages, including, without limitation: (1) the stroke and tensioning capacity can be adjustable to suit a wide range of riser systems; (2) the cylinders or barrels of the hydro-pneumatic tensioners are installed and operate vertically, which enables a failed tensioner to be removed easily from service for repair, requiring limited below-deck activity; (3) the support conductor can be installed vertically and can be connected to the conductor tension ring by a simple ⅛ turn breech-lock connection; (4) a piston rod can be attached to the conductor tension ring using a simple spherical bearing tension nut: (5) the use of the support conductor allows the riser to be centralized prior to engaging the tension ring during installation, which also advantageously extends riser fatigue life during operation; (6) the support conductor and the lateral load reaction elements resist riser rotation and riser conductor bending moments induced from riser loads, the “flagpole” effect of equipment above the tension ring, or a failed tensioner; (7) the compliant lower riser centralizer provides a mechanism for VIV suppression; (8) the compliant flex-bearing support members 135 absorb the impact load in the event a piston rod bottoms out during, for example an extreme environmental event; and (9) the tension joint support assembly 700 (specifically the tension joint donut 710 and the shoulder dogs 707) allows for piston rod top-out without damaging the riser support conductor, with a consequent possible release of the riser.
The above described example embodiments of the present invention are intended as teaching examples only. These example embodiments are in no way intended to be exhaustive of the scope of the present invention, as defined in the claims that follow.
