A RISER DEVICE

Abstract: A riser (18) extends between a surface vessel, e.g. a drillship (16), and an item of subsea equipment, e.g. a blowout preventer valve (12) on the seabed (10). The riser (18) is equipped with at least one thruster, for instance in the form of one or more motor driven positional propellers (201, 202), which is arranged so as to be able to exert an essentially horizontal thrust force on the riser (18) in order to counteract the lateral forces from the oceanic currents, so as to maintain the riser (18) in a position that corresponds to the desired course from the surface vessel (16) to the subsea equipment (12).

Description: A riser device The present invention regards an arrangement for risers of the type that extends between subsea equipment and a surface vessel such as a drillship. The lower end of a riser is connected to an item of subsea equipment, such as a blowout preventer valve or other subsea device, and extends to for instance a drillship or a platform, at which its upper end is attached. When drilling or performing other operations at great depths using correspondingly long risers, these will be subjected to considerable lateral forces arising from oceanic currents. This may cause the current-affected riser to follow a longitudinal course that may deviate substantially from a desired ideal position curve/line, for example a straight, vertical line between the subsea equipment and the surface vessel. A riser that is affected by lateral, current- conditional forces may in some cases assume an approximately S-shaped or spiral-like sine wave-shaped course. Even minor and moderate deviations from an ideal positioning course in the form of an imaginary line, straight or with a slight curvature, between the subsea equipment and the surface vessel, may cause complications for subsea drilling operations and other subsea operations . Angular deviations relative to the correct riser course between the subsea equipment and the surface vessel are especially critical when proximal to the subsea equipment, which may consist of a blowout preventer valve or a wellhead. Here, to all intents and purposes , no deviations exceeding three angular degrees relative to the correct riser course, which may be assumed to follow an ideal vertical line or curve, are tolerated. Moreover, light is thrown on the state of the art through the following patent publications US 4 474 129, NO 180 244 and NO 300 514. According to US 4 474 129, the problem of unwanted lateral forces that affect risers and are due to oceanic currents may be solved by fitting streamlined bodies to the riser, preferably at several locations along this. These known means of counteracting lateral forces are specially adapted to risers with buoyancy modules. The streamlined bodies are removably attached to the buoyancy modules, and are able to rotate 360° about an axis through the buoyancy modules. NO 180 244 describes bend restricting means for flexible risers, in particular risers that are integrally cast in wellheads on the seabed. The device comprises a shell that is disposed around the riser during the casting, and it is especially intended to counteract bending caused by wind and oceanic currents. NO 300 514 describes hydrodynamic elements for mounting in several locations along a rigid riser, e.g. made from steel and preferably with a J-configuration, which riser is subjected to relatively high mechanical loads due to vertical movements in the installation or vessel on the surface. For a J-riser, such loads are especially critical at the end and bend portion, and the hydrodynamically acting elements are therefore mainly arranged across this portion. The elements are designed to give a drag effect in at least one of the axial directions of the riser. By the use of simple and relatively reasonable means, an arrangement for maintaining a desired riser position will according to the present invention be arranged so as to keep the positional deviation of the riser relative to an imaginary curve or line, within acceptable values even in areas with considerable ocean currents. According to the present invention, this is obtained by a riser in conformity with the introductory part of Claim 1 being constructed in accordance with the characterising part of Claim 1. More specifically, the riser is provided with one or more positioning organs, so called thrusters, e.g. in the form of motor driven positioning propellers, arranged so as to impart to the riser an essentially horizontally oriented thrust force that may be varied by means of a control device attached to each positioning organ/thruster. The present invention presupposes that such a thruster, after being fitted to a riser, will be able to exert a thrust force that is transversely directed relative to the longitudinal direction of the riser, and which is substantially opposite of the current forces acting on the riser. The generation of one or more horizontal, laterally oriented thrust forces by the positioning organ or organs, which forces may be acting in mutually contradictory directions, will seek to counteract the force of the oceanic currents acting on the riser within the layer of water in which the positioning organ is located, and by so doing keep the riser positioned relative to the ideal course from the surface vessel to the subsea equipment. In the case of two or more vertically staggered positioning organs, and where different currents with mutually contradictory flow directions occur in different layers of water, it is the positioning organ/thruster within each respective layer of water that opposes the lateral forces acting on the riser at that depth. Thus, according to a preferred embodiment, each positioning organ/thruster is provided with a position measuring system designed to determine the position of the thruster in relation to an imaginary line/curve between the equipment on the seabed and the surface vessel. As mentioned, each thruster is provided with a control device that is designed to adjust the thrust force of each thruster relative to the adjacent portion of the riser, which portion constitutes the mounting element for the respective thruster, for the purpose of making the riser assume/maintain a desired position, independently of the influence of lateral forces due to oceanic currents. When using several thrusters, these act in concert to perform this task, regardless of whether oceanic currents in different layers of water have mutually contradictory flow directions, each thruster preferably being rotatable about the axis of the riser. A non-limiting example of an embodiment is illustrated schematically in the only figure of the accompanying drawing. The figure shows a schematic side view of a surface vessel and an item of subsea equipment, together with a riser extending between the surface vessel and said equipment. In the figure, reference number 10 denotes the seabed and 12 denotes an item of subsea equipment in the form of a blowout preventer valve. Reference number 14 denotes the surface of the sea, while 16 indicates a surface vessel, e.g. a drillship. Between the surface vessel 16 and the subsea equipment 12, a riser 18 extends in the normal fashion, following a slightly curved, vertical course. It is normally understood that the riser must not deviate from the desired vertical course by more than three angular degrees in the area of the blowout preventer valve. In order to be able to maintain the riser in at least an approximately vertical, correct position, it has in the example of an embodiment, according to the invention, been fitted with two thrusters 20ι and 20 2 each in the form of a motor driven positioning propeller, the actual propeller being indicated by 20ι a and 20 2a respectively. In order for such a thruster to function satisfactorily for realisation of the objective of the invention, it must be able to impart to the riser 18 a thrust force that is transversely oriented relative to the longitudinal direction of the riser, which force is directed against the flow direction of the oceanic current in the relevant layer of water. For illustrative purposes, the propellers 20ι a and 20 2a of the two thrusters 20ι and 20 2 respectively have been oriented in opposing directions, however this represents a special case in which two oceanic currents that are present at different depths have opposing directions of flow. Thus, the direction in which the force applied by each thruster acts with regard to the riser, must be adjustable. It is assumed that this adjustment of the desired direction of thrust and depth positioning is carried out in advance, however solutions may also be envisaged in which the thrusters 20ι, 20 2 , for example through the use of racks (not shown) attached to the riser 18 and hydraulic motors (not shown) attached to the thrusters 20τ_, 20 2 , are movable in the vertical plane. Such vertical displacement, and possibly rotation, will in that case be controllable from the surface vessel 16. A control system that is known per se, and which is not shown, must be provided for each thruster and be arranged so as to be able to vary the magnitude of the thrust of the respective thrusters, among other things in relation to the prevailing oceanic current conditions in the relevant layer of water.

Claims: C l a i m s 1. Arrangement at a riser (18) of the type extending between subsea equipment (12) and a surface vessel (16), e.g. a drillship, w h e r e i n the riser (18) is provided with at least one positioning organ/thruster (20χ, 20 2 ) arranged so as to be able to exert a transverse thrust force on the riser (18) relative to the longitudinal direction of the riser (18). 2. Arrangement according Claim 1, w h e r e i n said thruster (for instance 20ι), respectively each thruster (20ι, 20 2 ), is provided with a position measuring system designed to determine the position of the thruster relative to an imaginary straight line between the subsea equipment (12) and the surface vessel (16), and where the thruster (for instance 20ι), respectively each thruster (20ι, 20 2 ), is provided with thrust regulating positioning means designed to be able to control the thrust force which the associated thruster exerts on the riser ( 18 ) . o 3. Arrangement according to Claim 1 or 2, w h e r e i n the thruster, respectively each thruster (20ι, 20 2 ), comprises a motor driven propeller (20ι_ a , 20 2a ) and a mounting element for adjustable fastening to the riser (18), so as to bring the respective propeller to the 5 desired layer of water, and where the direction in which the thrust force applied by each propeller acts, may be changed with the aid of the rotational capability of the mounting element about the longitudinal axis of the riser ( 18) .

Inventors: GJEDEBO JON GRUDE, GJEDEBO, JON, GRUDE
Applicants: GJEDEBO JON GRUDE, HITEC ASA, GJEDEBO, JON, GRUDE
Application year: 2000
Main classification: E21B17
IPC codes: E21B17