Pipeline protection system

[0001] This application claims the benefit of United Kingdom Patent Application No. 0610069.7, filed on May 20, 2006, which hereby is incorporated by reference in its entirety.

[0002] This invention relates to a protection apparatus for detecting and reducing overpressure in a fluid pipeline and a method for resetting a pipeline protection system after tripping.

[0003] Hydrocarbon wells, for example subsea oil wells, require a pipeline through which the hydrocarbon fluid is transported. In the case of subsea wells, these may be of substantial length, sometimes several tens of kilometres long. Consequently, the pipeline is a major cost element of the fluid extraction system. Many subsea wells have to contend with very high fluid pressures, for example as high as 700 bar. For a pipeline to withstand such pressure would require it to have a substantial wall thickness, and it is not cost effective to implement this for long pipelines. To decrease cost, it is preferable instead to reduce the maximum operating pressure of the pipeline, typically to about 200 bar, using devices such as valves or chokes to reduce the fluid pressure from the well. However, a failure of the pressure reducing device may result in overpressure in the pipeline with disastrous results. To prevent this happening, a pipeline protection system is typically incorporated into the pipeline proximate the well. Such a system must have high integrity and virtually guarantee to prevent any overpressure from the well from reaching the pipeline.

[0004] FIG. 1 shows a typical conventional high integrity pipeline protection system (HIPPS). Two hydraulically operated ‘HIPPS’ barrier valves 1 and 2 are inserted in the fluid extraction flow line 3, prior to the thinner walled pipeline 4. These valves are opened when their operating hydraulic cylinders are fed with hydraulic pressure, and closed, typically under spring pressure, when the hydraulic pressure is removed and vented, i.e. they are failsafe. Pressure transducers 5, 6 and 7 are fitted between the valves 1 and 2. The pressure transducers 5, 6 and 7 are connected to a subsea control module (SCM) 8, which houses a hard-wired and thus also high integrity, electronic safety critical control board. This board produces an output that energises directional control valves (DCVs) which in turn operate the valves 1 and 2. The DCVs are also failsafe in that they close the hydraulic pressure source, and open the valve 1 and 2 actuating cylinders to vent when de-energised. Thus loss of electrical or hydraulic power causes the valves 1 and 2 to close. The safety critical control board in the SEM 8 contains logic that de-energises the DCVs if two out of the three transducers 5, 6 and 7 indicate a pressure level that exceeds a pre-set limit. This limit is set to a pressure that is less than the safe operating pressure of the pipeline 4. The use of two failsafe HIPPS valves, three pressure transducers and failsafe DCVs ensures high integrity of the system.

[0005] Employment of a HIPPS system not only reduces installation costs, but also allows for reduced rating of downstream equipment, such as risers, separators and process plant and permits high pressure fields to be commercially viable by allowing them to tie into existing infrastructures.

[0006] Although such known HIPPS systems are effective at protecting the pipeline, they are relatively basic, and do not address the problem of reducing the overpressure in the fluid system, which should be reduced before the HIPPS valves are re-opened and normal operation resumed. It is of course preferable that the overpressure is reduced in a controlled manner which minimizes the risk of component damage.

[0007] It is an aim of the present invention to provide a HIPPS system which enables a controlled start-up and shutdown which improves the integrity of the system.

[0008] According to a first aspect of the present invention there is provided a protection apparatus for detecting and reducing overpressure in a fluid pipeline having a fluid input end and a fluid output end, the fluid input end being connected in use to a fluid source, as set out in the accompanying claims.

[0009] According to a second aspect of the invention there is provided a method for resetting a pipeline protection system after tripping, as set out in the accompanying claims.

[0014] FIG. 2 shows an embodiment of the present invention in which the system is in normal, i.e. fault-free, operation. As shown, fluid enters the system from four wells connected via individual isolation valves 13, 14, 15, and 16 to a relatively thick-walled high pressure fluid extraction input flowline 3. This in turn connects to a relatively thin-walled lower pressure output pipeline 4. The fluid flow from flowline 3 to pipeline 4 is regulated by series-connected pipeline HIPPS barrier valves 1 and 2. Connected between pipeline HIPPS valves 1 and 2 are three pressure transducers 5, 6 and 7. A fluid bypass line is connected between flowline 3 and a location between valves 1 and 2, the bypass line including a by-pass valve 9. A further bypass branch is connected between the bypass line and pipeline 4, which includes a bypass valve 11. The bore of the bypass line and branch pipework is relatively small, typically having a diameter of about 2 inches (5.08 cm). This compares to a typical flowline 3 diameter of about 9 inches (22.86 cm) and pipeline 4 diameter of about 11 inches (27.94 cm). The bypass line is further connected to a venting means 8 via a vent line. This is connected between bypass valve 9 and the bypass branch and leads to a venting system. The vent line includes a vent valve 12. An optional second bypass valve 10 may be connected between the vent line and the pressure transducers 5, 6 and 7. All of the valves are individually switchable to allow or block fluid flow therethrough. Switching is actuated by means of individual respective hydraulic actuators which are fed with hydraulic power via additional DCVs in an SCM (not shown) which are controlled by additional hard wired electronics on a safety critical control board. The valves 1 and 2 and the pressure transducers 5, 6 and 7 are similar to those described with respect to FIG. 1.

[0015] As shown in FIG. 2, the system is in normal, fault-free operation, in which all fluid pressures are below the threshold limits for the pipeline 4. valves 1, 2 and 13-16 are all open, so 1 that fluid can flow from all the wells through to pipeline 4. The remaining valves 9, 10, 11 and 12 are all closed, so that no fluid is diverted through the bypass line.

[0016] The mode of operation of the system to provide a controlled system start up following a HIPPS trip caused by fluid overpressure is now described with reference to FIGS. 3 to 9.

[0017] FIG. 3 shows the system following a “HIPPS trip”. Here, overpressure from the topmost well as shown in the Figure has been detected by transducers 5, 6, 7, which cause HIPPS pipeline barrier valves 1 and 2 to trip, i.e. to close to prevent overpressure fluid from reaching pipeline 4. The cause of the overpressure, i.e. the topmost well, is determined by a pressure sensing device on the well-side of valve 16 (not shown). The fault-free well valves 13, 14 and 15 are also caused to close, to prevent backflow of fluid to their respective wells.

[0018] Following such a trip, which could for example have been caused by the failure of a production flow control choke at the topmost well, the pressure at that well output is reduced. This may be achieved by, for example, closing a valve (not shown) in the flowline at the well tree.

[0019] FIG. 4 shows the first step in resetting the system. Valve 11 is opened so that the overpressure between valves 1 and 2 is able to fall to normal, by bleeding the overpressure fluid through the bypass line and bypass branch to the pipeline 4. The pipe between valve 11 and pipeline 4 has a relatively small bore. Thus the rate of fluid flow, via valve 11, is constrained by the smaller bore pipe and the temporary increase in pressure from the bleeding of the residual overpressure in the pipe between valves 1 and 2 is gradual, and due to the relatively huge length of the pipeline 4 will result in a negligible increase in pressure of the pipeline 4. Although it may appear from the figure that the overpressure between valves 1 and 2 could instead simply be relieved by opening valve 2, this would result in unacceptable overpressure in pipeline 4.

[0020] FIG. 5 shows the second reset step. Here, valve 11 is closed and valve 2 is opened. This step enables the pressure transducers 5, 6, 7 to sense the pressure in pipeline 4.

[0021] FIG. 6 shows the third reset step. Here, valves 9 and 12 are opened so that the region of overpressure between valve 1 and the well valves is connected to the vent. This allows the overpressure to fall.

[0022] FIG. 7 shows the fourth reset step. Here, valve 9 is closed when the pressure transducers 5, 6 and 7 sense normal pressure, i.e. within operating limits. This closes the well to vent.

[0023] FIG. 8 shows the fifth reset step. Valve 12 is closed to close pressure transducers 5, 6 and 7 to vent.

[0024] FIG. 9 shows the sixth reset step. Valve 1 is opened, which allows the pressure transducers 5, 6 and 7 to sense the pressure in the inlet flowline 3.

[0025] The final reset step is to open valves 13, 14 and 15, which restores the system to normal operation, as shown in FIG. 2.

[0026] Ideally, the closed HIPPS barrier valve 1 should not be bypassed when high pressure is potentially available to the low pressure pipeline 4. In FIG. 6, showing the third reset step, valve 9 is shown open, which appears to contravene this rule. In practice however, this valve being open is not a problem, since the bore of the bypass pipework around the valves 9, 11 and 12 is typically much smaller than that of the production pipeline 4, and pipeline 4 is typically many kilometres long, so that raising the pressure to unsafe limits in the pipeline 4 would take a substantial period of time. An alternative system configuration could be used to allow for pressure bleeding of an overpressure well via the well tree and the vent lines, which would then permit valve 9 to be closed for the duration of the bleeding. In such an arrangement, the bypass branch and valve 11 may be omitted.

[0027] The figures and above description describe an embodiment of the invention only, and it will be apparent to those skilled in the art that various alternatives are possible within the scope of the invention.

[0028] For example, it is not necessary to use three transducers, one or more would be sufficient if it was of adequate reliability and integrity.

[0029] Although inputs from four wells are shown, the invention may be applicable to systems with any number of wells.

[0030] It may be advantageous in some situations to incorporate a further valve in the bypass line between the vent connection and the pressure transducers. This arrangement would be useful if it is necessary to bleed the pressure in the pipeline 4 side of the HIPPS valves 1, 2. This may be necessary in the case of gas wells for example.