Marine flexible elongate element and method of installation

Abstract

A flexible elongate element for installation in a body of water to extend between a seabed connection device and a unit arranged at the opposite end of the flexible elongate element. At least one lateral displacement device is connected to a portion of the flexible elongate element and configured to displace at least a portion of the flexible elongate element a lateral distance away from a first axis extending between the seabed connection device and the unit, the at least one lateral displacement device including a spring member. The flexible elongate element may be a flexible riser, umbilical, hose, or cable.

Claims

1. A flexible elongate element installed in a body of water and extending between a seabed connection device and a first unit arranged at a first end of the flexible elongate element; the flexible elongate element being capable of a wave configuration in a vertical direction via at least one of a plurality of buoyancy members or inherent buoyancy of the flexible elongate element wherein at least a first_lateral displacement device is connected to a first portion of the flexible elongate element and configured to displace at least a portion of the flexible elongate element a lateral distance away from a first axis extending between the seabed connection device and the first unit such that the flexible elongate element exhibits a sideways wave in addition to a vertical wave, the at least a first lateral displacement device comprising a_spring and damper member, wherein the flexible elongate element further comprises at least a second lateral displacement device, wherein at least the first and second lateral displacement devices are connected to the first portion and a second portion of the flexible elongate element, respectively, and are configured to displace at least a portion of the flexible elongate element the lateral distance away from the first axis, wherein the first and second portions are offset longitudinally along the first axis.

2. The flexible elongate element of claim 1, wherein the at least a first and second lateral displacement devices are configured to act in generally opposite directions.

3. The flexible elongate element of claim 1, wherein the at least a first lateral displacement device is further configured to displace at least a portion of the flexible elongate element a vertical distance below the surface of the body of water.

4. The flexible elongate element of claim 1, wherein the at least a first lateral displacement device further comprises a seabed anchor portion.

5. The flexible elongate element of claim 1, wherein the flexible elongate element further comprises at least one support member, wherein at least a portion of the flexible elongate element is suspended above the seabed.

6. The flexible elongate element of claim 5, wherein the at least one support member further comprises at least one buoyancy member.

7. The flexible elongate element of claim 1, wherein the at least a first lateral displacement device comprises tethers connecting the flexible elongate element and a seabed anchor portion via the spring and damper member, and the spring and damper member further comprise: a buoy submerged or floating in the body of water; and a clump weight.

8. The flexible elongate element of claim 1, wherein the first unit is a plug, or a vessel floating in the body of water.

9. The flexible elongate element of claim 1, wherein at least a portion of the flexible elongate element is configured to curve the lateral distance.

10. The flexible elongate element of claim 1, wherein the flexible elongate element is a flexible riser, a cable, an umbilical, or a hose.

11. The flexible elongate element of claim 1, wherein the spring and damper member comprises a spring and damper device dimensioned to provide a desired restoring force.

12. A method of installing a flexible elongate element in a body of water, wherein a first flexible elongate element end is connected to a seabed connection device and second flexible elongate element end is connected to a first unit the flexible elongate element comprising buoyancy members or inherent buoyancy to provide a wave configuration in a verical vertical direction; the method further comprising: connecting at least one first lateral displacement device comprising a spring and damper member to a first portion of the flexible elongate element and operating the at_least one first lateral displacement device to displace at least a portion of the flexible elongate element a lateral distance away from a first axis extending between the seabed connection device and the first unit such that the flexible elongate element exhibits a sideways wave in addition to a vertical wave, wherein the method further comprises: connecting at least the first lateral displacement device and a second lateral displacement device to a first and second portion of the flexible elongate element so that the first and second portions are offset longitudinally along the first axis; and operating the at least the first and second lateral displacement devices to displace at least a respective portion of the flexible elongate element the lateral distance in generally opposite directions, away from the first axis.

13. The method of claim 12, wherein the device is operated to displace at least a portion of the flexible elongate element a vertical distance below the surface of the body of water.

14. The method of claim 12, wherein the method further comprises the connection of a tether system between the flexible elongate element and a seabed anchor portion.

15. The method of claim 12, wherein the flexible elongate element comprises at least one support member, wherein the at least one support member is a buoyancy member, wherein at least a portion of the flexible elongate element is suspended above the seabed.

16. The method of claim 12, wherein the first unit is a plug, or a vessel floating in the body of water.

17. The method of claim 12, wherein the spring and damper member further comprise: a buoy submerged or floating in the body of water; and a clump weight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other characteristics of the invention will become clear from the following description of an embodiment, given as a non-restrictive example, with reference to the attached schematic drawings, wherein:

(2) FIG. 1 is a schematic view along an imaginary x-axis of an embodiment of the invented flexible elongate element, installed between a seabed base and a floating vessel;

(3) FIG. 2 is plan view of the embodiment shown in FIG. 1;

(4) FIG. 3 is a side view of the embodiment illustrated in FIG. 1 and FIG. 2;

(5) FIGS. 4a and 4b are a schematic side view and plan view, respectively, of a flexible elongate element installed between a base and a plug, prior to connection of lateral displacement devices;

(6) FIGS. 5a and 5b are a schematic side view and plan view, respectively, of a flexible elongate element installed between a base and a plug, with a first lateral displacement device connected to the elongate element, causing a first lateral displacement;

(7) FIGS. 6a and 6b are a schematic side view and plan view, respectively, of a flexible elongate element installed between a base and a plug, with a first lateral displacement device connected to a first part of the elongate element, causing a first lateral displacement, and a second lateral displacement device connected to a second part of the elongate element, causing a second lateral displacement;

(8) FIGS. 7a to 7d are schematic side views illustrating an installation sequence for a flexible elongate element; and

(9) FIG. 7e is a plan view of FIG. 7d.

DETAILED DESCRIPTION OF AN EMBODIMENT

(10) The following description may use terms such as horizontal, vertical, lateral, back and forth, up and down, upper, lower, inner, outer, forward, rear, etc. These terms generally refer to the views and orientations as shown in the drawings and that are associated with a normal use of the invention. The terms are used for the reader's convenience only and shall not be limiting.

(11) FIG. 1 is an illustration of an embodiment of the invented flexible elongate element configuration in a body of water W, between a water surface S and a seabed B. In the following description, the flexible elongate element will be referred to as a flexible riser.

(12) A flexible riser 1 comprises a plurality of buoyancy members 7a, 7b, generally arranged in two groups in order to provide a wave configuration. Such buoyancy members, and their application on and connection to flexible risers, are well known and need therefore not be described in more detain here. The flexible riser 1 may also be inherently buoyant. The flexible riser may be any known flexible riser, umbilical or cable type known in the art, for examplebut not necessarily limited tofor transmitting power, electrical/optical signals, and/or fluids between the seabed and a topsides vessel. The term flexible riser shall therefore not be limiting, but encompass other elongate flexible elements as well. The riser comprises a seabed connector 14, by means of which the riser is connected to a riser base 5, for example a PLEM (pipeline end manifold). The riser base 5 may for example be a gravity structure, a piled structure, or a suction/anchor pad. The other end of the flexible riser comprises topsides connector 4, which is connected to a floating vessel 6, for example an FPSO (floating production, storing and offloading) vessel. The topsides connector 4 typically comprises a bend stiffener and may for example be connected to the vessel 6 via turret. The various means and devices for connecting the flexible riser 1 to the riser base 5 and to the vessel 6 are well known in the art and need therefore not be described in more detail here. The invention shall not, however, be limited to connection to a floating vessel.

(13) Connected to the flexible riser 1 are two lateral tether systems 2a, 2b (in the following also referred to as lateral displacement devices). In the illustrated embodiment, each tether system 2a, 2b comprises a tether 3a-c connected between respective portions 8a,b on the riser and respective tether anchors 11 on the seabed B. The anchors 11 may for example be a gravity-based anchor, a piled anchor, or a suction anchor. Each tether comprises a first portion 3a, which is connected between its respective portion 8a,b (via e.g. a bridle connection 13) and a clump weight 9. A second tether portion 3b is connected between the clump weight 9 and a buoy 12, and a third tether portion 3c is connected between the buoy 12 and the tether anchor 11 on the seabed. The tether material is of a kind which per se is known in the art (e.g. polyester). In FIG. 1, the buoys 12 are shown as being submerged in the water W, i.e. below the surface S. It should be understood, however, that the illustration in FIG. 1 may show only a transient state. The skilled person will understand that whether the buoys are submerged or floating is in fact determined by the buoyancy generated by the buoys and the overall forces acting on the system (i.e. riser, vessel, and tether systems). These forces may vary considerably, depending on water currents waves, etc. The combination of the buoy 12, clump weight 9 and the connection to the respective riser portion 8a,b, provides an effective spring, anddue to resistance provided by the water when the parts are movingserves to dampen the riser movements. It should therefore be understood that the buoy and clump weight may be substituted by other spring and damper means. The spring means may be dimensioned to provide the desired restoring force (i.e. spring constant). In the illustrated embodiment, this spring constant may be designed by an appropriate dimensioning of the mass of the clump weight 9 and the buoyancy of the buoy 12.

(14) The tether systems 2a, 2b thus act as lateral displacement devices, serving to pull its respective riser portion 8a, 8b sideways (i.e. laterally), indicated by the force arrows L in FIG. 1, away from the (imaginary) line between the riser base 5 and the floating vessel 6. The lateral tether systems thus generate a riser in a three-dimensional (3D) wave), in that the flexible riser exhibits a sideways wave, in addition to the more or less vertical (2D; prior art) wave. The lateral displacement devices may be applied to any known 2D wave configuration (e.g. lazy wave, steep wave, pliant wave) and generate augmented, 3D, riser configurations, that are particularly useful in shallow water.

(15) The invented riser configuration is further illustrated in FIGS. 2 and 3. The first tether system 2a (comprising the 3a-c, clump weight 9, buoy 12 and anchor 11) serves as a displacement device to pull a first portion 8a of the flexible riser 1 a lateral distance y.sub.a away from the x-axis extending between the riser base 5 and the vessel 6, and a vertical distance d.sub.a below the water surface S. Similarly, the second tether system 2b (comprising a tether 3a-c, clump weight 9, buoy 12 and anchor 11) serves as a displacement device to pull a second portion 8b of the flexible riser 1 a lateral distance y.sub.b away from the x-axis, and a vertical distance d.sub.b below the water surface. It should be understood that these distances are not necessarily constant values, as the flexible riser is a prone to move in the water when subjected to waves, currents and varying loading. Thus, the lateral displacement devices (e.g. tether system) may also in fact control the vertical position (cf. d.sub.a, d.sub.b) of the riser in the water column. This may allow for variations in the density of the internal product over time which may be a challenge for shallow water riser configurations in combination with maintaining offset flexibility. The configuration also allows for piggyback of umbilicals, multiple riser configurations etc.

(16) FIGS. 4a to 6b illustrate a typical installation sequence for the invented riser configuration: FIG. 4a and FIG. 4b (side view and plan view, respectively): The flexible riser 1 has been connected to the riser base 5 at one end and to a plug 10 (via the topsides connector 4) at the other end. The riser generally exhibits a long wave in the water, by virtue of its buoyancy members 7a,b (see FIG. 1, not shown in FIGS. 4a, 4b). Lateral displacement devices (i.e. tether systems, as described above) 2a, 2b have been installed on seabed but not connected to riser. FIG. 5a and FIG. 5b (side view and plan view, respectively): The first tether system 2a has been connected to a first portion of the flexible riser, displacing a portion of the riser a lateral distance y.sub.a with respect to the x-axis and a vertical distance d.sub.a below the water surface S. FIG. 6a and FIG. 6b (side view and plan view, respectively): The second tether system 2.sub.b has been connected to a second portion of the flexible riser, displacing a portion of the riser a lateral distance y.sub.b with respect to the x-axis and a vertical distance d.sub.b below the water surface S.

(17) Upon completion of the operations illustrated in FIGS. 6a and 6b, the topsides connector 4 and the plug 10 may be retrieved and connected to a vessel (in a manner well known in the art), resulting in a configuration similar to those illustrated by FIGS. 1, 2, 3.

(18) FIG. 7 illustrates a similar installation sequence to that described above with reference to FIGS. 4a to 6b. In FIG. 7, drawing (a) shows the preinstalled plug 10, riser base 5 and two tether anchors 11. In drawing (b), both tether systems 2a, 2b have been connected to its respective anchor 11 but have not been attached to the flexible riser. In drawing (c), the first tether system 2a has been connected to a first portion of the riser. In drawing (d), the second tether system 2b has been connected to a second portion of the riser, thus completing the riser installation. Drawing (e) is a plan (top) view of drawing (d).

(19) Although the figures indicate that the tethers are connected to the flexible riser below water, it should be understood that the tethers may be connected to the riser while the riser is floating in the water surface S, or be preinstalled prior to riser installation. The installation of tether anchors, and connected of the tethers to the riser and to the seabed anchors, may be performed by known methods and equipment.

(20) Although the invention has been described with two lateral tether systems, it should be understood that other numbers are possible. For example, using only one tether system, the resulting wave configuration will be as illustrated in FIGS. 5a and 5b.

(21) Although the invention has been described with reference to a flexible riser, it should be understood that the invention is equally applicable to other flexible, elongate, elements installed in a body of water. Examples of such elements are hoses, umbilicals, and cables. The cables may for example be telecommunications cables or power cables, and comprise metal conductors or fiber optic conductors.

(22) Although the invention has been described with reference to tethers, it should be understood that other lateral displacement systems are possible and conceivable.