WO2010069992A2

WO2010069992A2 - Riser valve

Info

Publication number: WO2010069992A2
Application number: PCT/EP2009/067280
Authority: WO
Inventors: Knut Tore LJØSNE
Original assignee: Aker Subsea As
Priority date: 2008-12-16
Filing date: 2009-12-16
Publication date: 2010-06-24
Also published as: WO2010069992A9; NO20085258L; WO2010069992A3; NO329168B1; WO2010069992A4

Abstract

A marine riser fill-up valve adapted to be connected to a riser extending through a body of water and to selectively open and close communication between the interior of the riser and the surrounding water. The valve comprises a housing (21) and a spool (22) within the housing (21). The spool (22) is adapted to move within the housing (21) between a closed position preventing communication between the interior of the riser and the water and an open position allowing communication between the interior of the riser and the water. The spool (22) and the valve housing (21) have respective one of a seal surface (29) and a seat (39). The seal surface (29) and seat are (39) adapted to form a sealing interface in a closed position of the valve. The valve comprises a water back pressure chamber (28) which is communicating with the riser interior when the valve is open and with the surrounding water when the valve is closed.

Description

RISER VALVE

The present invention relates in general to the field of marine oil and gas well drilling and in particular to a valve according to the preamble of claim 1 , which is placed in a deep water riser string to prevent collapse of the riser tube by the pressure of the sea water when the internal pressure of the riser drops, for example due to expansion of rising formation gas or loss of circulation and subsequent reduction height of the mud column. If the internal pressure drops significantly below the external hydrostatic pressure the valve is opened, permitting sea water to enter the riser via ports, thus equalizes the internal and the external pressures.

A prior art riser fill-up valve of this type is shown in US 4719937 and is also described in detail in Operators Manual dated March 2008. It is intended to be installed near the surface and consists of four hydraulically controlled spool valves 1 (if which one is shown in Figure 1 ). When open, these valves 1 allow the riser 20 to fill with water to prevent the collapse of the riser string due to pressure differences caused by sea water pressing on the exterior of the riser string if drilling mud exits the riser during operations. Each spool valve 1 consists of a spool 2 that slides up and down on a stationary valve stem 3. The valve stem 3 has hydraulic ports 4a, 4b for directing opening and closing hydraulic pressure to chambers 5, 6, on opposite sides of a piston 13, inside the body of the valve spool 2. A seal gland 7 seals the chambers 5, 6 so that the hydraulic pressure can act upon the spool 2 to open or close the valve as desired. The interior of the valve is open to the interior of the riser string by a port 8 and is exposed to the drilling mud. Each end of the spool 2 slides inside a sleeve 9, 10. These sleeves 9, 10 are open to the sea. Seals in each end of the spool 2 prevent mud from escaping from the riser and sea water from entering the riser when the valve is in the closed position. A protective rod boot 11 protects the valve stem.

Figure 2 shows the valve 1 in open position. Since figure 2 is a figure from the operations manual and figure 2 is a figure from the patent, the structures are slightly different. In open position the spool 2 has shifted upwards and opened a flow channel illustrated by arrows 12 from the sea into the riser. When the pressure has been equalized the valve can be closed again by applying a hydraulic pressure in port 4b, thereby increasing the pressure in chamber 5 and forcing the spool 2 downwards.

Although, this riser fill-up valve in most cases is expected to perform well, there is a potential risk involved. The valve will usually stay in the closed position, in which it will prevent mud from escaping to the sea, for a very long time. If the internal pressure drops below a certain differential pressure relative to the seawater pressure, a pressure sensor will activate an alarm at the surface structure. The operator will then, based on his judgment of the seriousness of the situation decide if the riser fill-up valve should be opened. If he decides to do this a hydraulic pressure is applied on port 4a, which opens the valve. If for the operator, based on his judgment or for some other reason, does not open the valve and the pressure differential across the valve is exceeding a predetermined pressure, the valve should nevertheless open automatically.

This automatic opening is done by the seawater pressure acting on the valve. If the valve for some reason does not open even though the seawater pressure is very high relative to the internal pressure in the riser, there is a potential for collapse of the riser, and consequently great damages, substantial delay in drilling, large economical loss and risk of pollution. It is therefore of utmost importance that the valve works even if it has been standing in a closed position for a long time and has been subject to marine growth and corrosion. It is also important that the valve closes firmly when the pressure has been equalized and drilling is to be resumed. If it does not close tightly when a hydraulic pressure is applied in port 4b, mud and other pollutions will escape to the sea. There is also a risk of deterioration of the protective rod boot 1 1 and subsequent leakage of seawater into the boot. If this happens the rod is subject to fouling and the movement of the spool will be hindered. The result may be that the valve will not open, at least not to the required extent. For these reasons the prior art valve involves some risks for malfunction in respect of both the automatic opening function and closing function. The prior art valve has radial seals which must enter a bore. These seals must fit snugly into the bore. It the clearance is too tight, the valve will resist opening and an increased opening force must be applied. The result may be that the valve will open too late. If the seals fit too loosely into the bore, dirt and fouling may enter into the space between the seal and the bore and the result may be that the valve will get stuck in the bore and be difficult to move.

It is suspected that the prior art valve is depending on an operator control in order to be positively opened or closed. Although, the valve has been installed on some riser it has to our knowledge not been operated in practice.

The present invention has as object to reduce these risks for malfunction. This is achieved by the features of the characterising clause of the subsequent claim 1 .

A preferred and illustrative embodiment of the present invention will now be described, referring to the following figures:

Figure 3a, showing the valve of the invention in a closed position,

Figure 3b, showing the valve of the invention in a partially open position,

Figure 3c, showing the valve of the invention in an open position, and

Figure 4, showing an assembly of four valves mounted on a riser.

The structural principles of the valve of the present invention will be described referring to figure 3a. The valve generally consists of a valve housing 21 and a valve spool 22. The housing comprises three parts: a main part 23, a sleeve part 24 and a cover 25. The main part has an opening 26, which is adapted to communicate with the interior of the riser.

The spool 22 has a generally sleeve shaped part 27 with a relatively large diameter and an internal axial bore 28, which is also denoted a valve back pressure chamber, as it contains fluid that acts as a back pressure to the valve to act against opening of the valve.. The sleeve shaped part 27 is open at a first end where its interior is open to the inside of the cover 25. At its second end the sleeve shaped part 27 has an external shoulder 29. At its second end the sleeve shaped part 27 is also integrated with a stem 30, which extends axially from the sleeve part 27. At the transition between the sleeve shaped part 27 and the stem 30, the spool has a plurality of valve back pressure channels 31 , which sets the interior of the sleeve part and the back pressure chamber 28 in communication with a seawater chamber 32. The seawater chamber 32 is in communication with the surrounding seawater via four ports 33.

The cover is equipped with a small opening 50 to allow trapped air to pass out of the cover during installation.

The outer end of the stem 30 is received in a first hydraulic chamber 36 in a reduced diameter part 34 of the valve housing 21 . The first hydraulic chamber

36 is sealed off from the seawater chamber 32 by a seal case 35, which encircles the stem 30 The first hydraulic chamber 36 has a first hydraulic port

37 for communication of hydraulic pressure, which will be explained in the following.

The shoulder 29 is at its side facing the seawater chamber 33 equipped with an O-ring seal 38 and may thus act as a seal surface. The O-ring seal is adapted to seal against a valve seat 39 situated on a ledge 40 in the housing 21. At the side of the shoulder 29 facing the seawater chamber 33 the spool 22 is also equipped with a choke ring 41 of a material resistant to abrasion, which is adapted to enter within the seat 39 with small clearance. The choke ring 41 ensures that the valve spool 22 continues to open when it is first lifted off the valve seat 39. Since the choke ring 41 prevents sea water from flowing past the seat 39, the pressure differential will not drop instantly. The choke ring 41 also acts to protect the seal 38 from the flow of seawater both during opening of the valve and when the valve is fully open. Between a sealing 43, about mid way along the sleeve shaped part 27 of the spool 22, and a seal 44 at the end of the housing 21 proximal the cover 25, is defined a second hydraulic chamber 45. A ledge 48 is formed on the spool, on which a pressure in the second hydraulic chamber 45 can act. As is shown in figure 4, the valve housing 21 has a second hydraulic port 42 for communication of hydraulic fluid to the second hydraulic chamber 45. An annular groove 47 is formed in the wall of the sleeve part 24 of the housing 21 , to distribute the hydraulic pressure across the cross section of the second hydraulic chamber 45.

The valve is one of a cluster of valve, preferably four identical valves, which are attached to the riser, as shown in figure 4. The four valves are adapted to operate in parallel to fill the riser with sea water if the mud column for some reason decreases out of control. The valves are designed to operate automatically, only based on differential pressure between the interior and exterior of the riser. In addition the valves also have the option of remote operation, as will be explained in detail later. An important feature to ensure proper operation of the valves is that the valve is a seat valve.

In figure 3a the valve is in the closed position, where the sealing between the o- ring 38 and the seat 39 effectively prevents communication of seawater into the riser. The fluid in the riser acts on the shoulder 29 over an annular area A, while the seawater acts on two opposite areas, an net annular area B between the housing 21 and the stem 30 in the seawater chamber 33, and a circular area C corresponding with the cross section of the sleeve shaped part 27 of the spool 22. Area B is slightly larger that area C, which results in a net water ring area D. The net water ring area D is slightly smaller than the annular area A. Since the pressure on both areas B and C is the pressure of the seawater, hence equal, the spool 22 is biased towards the open position of the valve shown in figure 4 by the pressure acting on the net water ring area D. To keep the valve in the closed position shown in figure 3, the pressure within the riser, acting on area A has to exert a force, which is at least equal to the force provided by the differential area B-C=D. In the show embodiment, the pressure in the riser therefore has to drop a certain amount below the seawater pressure before the force acting to open the valve exceeds the force acting to close the valve. If the pressure in the riser drops to a level below the pressure sufficient to balance the forces, the spool 22 will start to move towards the open position. The pressure differential at which this happens can be denoted ΔPi .

As soon as the spool 22 has moved away from the seat 39 and the choke ring 41 has cleared the bore of the seawater chamber 32, the area on which the opening force is acting, will increase by the area that has been covered by the seat 39, as shown in figure 3b. Consequently, the opening force will increase and the differential force will act to rapidly open the valve.

As the valve opens the seawater will flow past the salve seat 39 and into the interior of the riser as shown by the arrows 51 in figure 3c. As this happens the pressure in the area of the seat 39 will drop, as fluids in motion has a lower pressure than stationary fluids. To prevent that this pressure drop does not prevent the valve from opening further or even close again, which could result in an undesired fluttering of the valve, the back pressure chamber 28 is in communication with the seawater chamber 32 via the back pressure channels 31 . This ensures that the fluid in the back pressure chamber will flow towards the lower pressure in the seawater chamber until the two pressures are equalized, as show by the arrow 52. Hence the back pressure of the valve will adapt to the pressure near the valve seat and stabilize the valve in fully open position. This provision ensures that the valve will stay open even when the pressure inside the riser starts to increase. It also ensures that all the four parallel valves open fully, even if they should not open simultaneously.

When the valve has opened completely the shoulder 29 of the spool 22 will rest against an abutment 46 of the sleeve part 24 of the housing 21 . The result of this is that the area A on which the pressure inside the riser acts on will be significantly reduced, and the pressure necessary to hold the valve open will be correspondingly reduced. The net force acting to close the valve will be approximate to the weight of the valve spool 22. As a result the valve will stay in the open as long as there is any significant flow through the valve. When the flow decreases the weight of the valve spool 22 will overcome the reaction forces of the flow into the riser, and the valve will start to close. As soon as the choke ring 41 reaches the bore of the seawater chamber, the ring area A is re-established and the area on which the seawater acts to open the valve is reduced. Thus the valve shuts firmly.

The above explained opening by the differential pressure between the seawater and the riser is a failsafe feature, to ensure that the valve opens before the pressure in the riser drops to a level where there is a risk of collapse. The valve may also be operated hydraulically. In that case a sensor (not shown) monitors the pressure differential between the riser and the surrounding seawater. If this pressure, which drops below a certain pressure (hereinafter denoted ΔP₂, an alarm will be triggered at the operator's desk. ΔP₂ is a higher pressure than ΔPi and is at a level which makes it possible for the operator to try other options to increase the pressure in the riser before flooding the riser with seawater. If the pressure in the riser drops further the operator may decide to open the riser valves. He then activates a supply of hydraulic fluid on the first hydraulic port 37, which in turn increases the hydraulic pressure in the first hydraulic chamber 36. This forces the spool 22 to move in the direction of opening the valve. As soon as the O-ring 38 has cleared the seat 39, the force from the seawater pressure will increase (as explained above) and aid in the opening of the valve.

When the riser has been flooded with seawater (either by initiation from the operator or by the failsafe mechanism) so that the pressure in the riser is equalized with the seawater pressure and the cause of the pressure drop in the riser has been removed, the valve can be closed again. If the valve has been opened hydraulically, the operator activates a supply of hydraulic fluid on the second hydraulic port 42, this in turn increases the hydraulic pressure in the second hydraulic chamber 45. Thereby the spool 22 is forced towards the closed position. As soon as the O-ring 38 in the shoulder 29 reaches the seat 39, the area on which the surrounding seawater acts will decrease and hence the force acting to open the valve will decrease. As a consequence the valve will stay in the closed position even when the hydraulic pressure in the second hydraulic chamber is relieved. If the valve has been opened automatically by the pressure differential, the valve will also automatically close when the pressure differential has been equalized.

By adjusting hydraulic pressure at the ports 37, 42 it is also possible to adjust the opening and closing force, and hence bias the valve in one or both of the opening and closing directions. It is also possible to short between the ports 37, 42 to apply the same pressure on both ports.

Since the valve will stay motionless in the closed position for prolonged periods of time, there is a high risk of fouling on the parts which is exposed to seawater and a risk for the parts exposed to the riser fluids are subject to abrasion and deposits adhering to the part. To prevent these effects from having a detrimental consequence for the operation of the valve, measures have been taken to protect crucial parts from exposure.

The sleeve shaped part 27 of the spool is completely surrounded by the sleeve part 24 of the housing 21 from the sealing 43 to the end of the sleeve shaped part 27. This means that this part of the spool 22 is exposed neither to the seawater not the riser fluids, but to hydraulic fluids only. The same applies for most of the inner surface of the sleeve part 24 of the housing 21 . Hydraulic fluids can be used that protects the material from degradation.

The sleeve shaped part 27 of the spool 22 has a slightly reduced outer diameter between the shoulder 29 and the sealing 43. This part of the spool 22 is exposed to the seawater. However, due to the reduced diameter this part of the spool 22 can experience quite some degree of fouling without this having any detrimental effect on the ability for the spool to shift when the valve is supposed to open.

The sealing between the spool 22 and the valve seat 39 of the housing 21 is an axial seal. Even though fouling or deposition may occur to a very large extent around the sealing area, which is exposed in part to the riser fluids and in part to seawater, this fouling will not have any major influence on the sealing effect or the ability for the spool to move away from the seat. The sealing is effected by the O-ring 38 pressing against an annular rib 49. Due to the force acting to keep the valve closed there is very little chance for any debris or biological material to enter in between these parts

The sealing area is fairly small so that even if the shoulder and seat to some degree should become stuck together, the opening force of the valve will be sufficiently large to overcome the adhesion.

The stem 30 is surrounded by the seal 35 and the first hydraulic chamber 36 along its entire length. As for the part of the spool 22 surrounded by the second hydraulic chamber 45, the stem and the inner surface of the reduced diameter part 34 of the housing 21 is protected from the deteriorating seawater and riser fluids. Consequently, the stem will remain smooth even through a very long time of stand still.

The valve is not intended to stay in the open position for a long time. Consequently, fouling and abrasion in this position will be insignificant. It is, however, important that fouling or abrasion that have taken place during the long time the valve has been in the closed position, does not prevent the valve from closing properly again.

Fouling on the spool surface between the shoulder 29 and the seal 43 will be drawn into the bore of the sleeve part 24 of the housing 21 . However, since this part of the spool has a reduced diameter, there will be space enough between the inner surface of the housing and the spool to accommodate any fouling that is not scraped off during the movement of the spool. As a consequence, the chances for the fouling resulting in the spool becoming stuck in the housing are very small.

Fouling around the sealing area between the shoulder 29 and the seat 39 may become dislodged and end up covering parts of the sealing surfaces. However, since the sealing surfaces are fairly small, the sealing pressure will be quite high and the fouling will most likely be forced out of the sealing area or compressed to a degree where it does not contribute to a leak. The wear ring 41 will also scrape off some of the fouling and deposition that may cover the sealing area.

The openings which are exposed to seawater or the riser fluids are large enough to not become clogged by fouling or debris.

Claims

C l a i m s

1.

A marine riser fill-up valve adapted to be connected to a riser extending through a body of water and to open and close communication between the interior of the riser and the surrounding water, comprising a housing and a spool within the housing, the spool being adapted to move within the housing between a closed position preventing communication between the interior of the riser and the water and an open position allowing communication between the interior of the riser and the water, the spool and the valve housing having respective one of a seal surface and a seat, the seal surface and seat being adapted to form a sealing interface in a closed position of the valve, characterized i n that the valve comprises a water back pressure chamber which is communicating with the riser interior when the valve is open and with the surrounding water when the valve is closed.

2.

Valve according to claim 1, c h a ra c t e r i s e d i n that effective mud area on which the pressure in the riser acts to maintain the valve in the closed position is larger than the effective water area on which the pressure from the surrounding water acts on to force the valve open, when the valve is in the closed position.

3.

Valve according to claim 1 or 2, c h a ra c t e r i s e d i n that effective mud area on which the pressure in the riser acts to force the valve into the closed position is smaller than the effective water area on which the pressure from the surrounding water acts on maintain the valve in the open position, when the valve is in the open position.

4.

Valve according to any of the preceding claims, c h a ra c t e r i s e d i n that the spool is adapted to move in a direction transverse to the plane of the sealing interface to open and close the valve.

5.

Valve according to any of the preceding claims, c h a ra ct e r i ze d i n that the seal surface comprises an O-ring.

6

Valve according to any of the preceding claims, ch aracterized i n that it further comprises a first hydraulic chamber at a first end of the spool, the first hydraulic chamber having at least one seal surrounding a part of the spool, the first hydraulic chamber being adapted to receive a hydraulic pressure to open the valve.

7.

Valve according to any of the preceding claims, ch aracterized i n that it further comprises a second hydraulic chamber at a second end of the spool, the second hydraulic chamber having at least one seal surrounding a part of the spool, the second hydraulic chamber being adapted to receive a hydraulic pressure to close the valve.

8. Valve according to claim 6 or 7, c h a ra ct e r i ze d i n that the hydraulic chamber and its seal is completely enclosing a part of the spool surface, which surface is adapted to slide in contact with the seals when the spool is moved from a closed to an open position.

9.

Valve according to claim 8, characterized i n that a part of the spool, which is adapted to move into a bore of the housing that encloses the second hydraulic chamber when the valve is opened, has a reduced diameter compared to the part of the spool that is situated within the second hydraulic chamber when the valve is in the closed position.

10.

Valve according to any of the claims 6 -9, ch a ra cterized i n that the spool comprises a stem, which is received in the first hydraulic chamber.

11.

Valve according to any of the preceding claims, ch aracterized i n that the spool comprises a bore that provides seawater communication across the sealing interface.

12

Valve according to any of the preceding claims, ch aracterized i n that seawater pressure acting on the spool to close the valve is acting on a smaller area than the seawater pressure acting on the spool to open the valve, thus creating a biasing force in the opening direction of the valve, the biasing force determining the minimum force from the pressure of the fluid content of the riser to hold the valve in the closed position.

13.

Valve according to any of the preceding claims, c h a ra c t e r i s e d i n that it comprises a choke ring on the spool, the choke ring being near the valve seat when the valve is in the closed position, the choke ring preventing an instant pressure drop during the initial opening phase of the valve.