A Method of Assessing the Condition of a Tubular Member

Abstract

In a method of assessing the condition of a tubular member, for example a flexible riser, an inert fluid is flowed from a fluid source through a conduit connected to a laminar flow device, which can determine volumetric and/or mass flow rate of the fluid. The fluid is flowed through the laminar flow device into the riser annulus, and flow continues until the fluid pressure within the annulus stabilises. The volume of fluid flowed into the annulus offers an indication of the integrity of the riser, for example, whether any fluid is present therein.

Claims

1. A method of assessing the condition of a tubular member, the method comprising flowing a fluid through a laminar flow device into the tubular member, wherein the laminar flow device is disposed in line with a fluid conduit delivering the fluid to the tubular member, into an annulus in the tubular member, wherein the tubular member comprises an outer and inner layer adapted to seal the annulus; and determining the remnant free volume of the annulus of the tubular member by measuring the volumetric flow rate of fluid through the laminar flow device and thus determining the volume of fluid injected into the tubular member.

2-10. (canceled)

11. The method as claimed in claim 1, including determining the remnant free volume of the annulus of the tubular member by determining the mass flow rate of fluid through the laminar flow device and thus determining and optionally recording the mass of fluid injected into the tubular member.

12. The method as claimed in claim 1, including measuring the temperature of the fluid flowing into the tubular member using at least one temperature sensor.

13. The method as claimed in claim 1, including measuring the pressure of the fluid flowing into the tubular member using at least one pressure sensor.

14-24. (canceled)

25. The method as claimed claim 1, including measuring the volume of fluid vented from the annulus of the tubular member and comparing the volume of fluid vented from the annulus of the tubular member with the volume admitted into the annulus of the tubular member.

26. The method as claimed claim 1, including calculating the mass of fluid injected into the annulus of the tubular member and the mass of fluid vented from the annulus of the tubular member and comparing the two values.

27. (canceled)

28. An apparatus for assessing the condition of a tubular member, the apparatus comprising a fluid conduit adapted to supply fluid from a fluid source to a tubular member, and a laminar flow device adapted to determine the volumetric flow rate of the fluid as it flows into the tubular member, wherein the laminar flow device is disposed in line with a fluid conduit delivering the fluid to the tubular member, into an annulus in the tubular member, wherein the tubular member comprises an outer and inner layer adapted to seal the annulus; and wherein the laminar flow device is adapted to determine the remnant free volume of the annulus of the tubular member by measuring the volumetric flow rate of fluid through the laminar flow device and thus determine the volume of fluid injected into the tubular member.

29-30. (canceled)

31. The apparatus as claimed in claim 28, including a flow rate limiting device comprising a rotameter.

32. The apparatus as claimed in claim 31, wherein the rotameter is configured to detect back pressure in the fluid conduit.

33-35. (canceled)

36. The apparatus as claimed in claim 28, wherein the laminar flow device comprises a mass flow meter.

37. (canceled)

38. The apparatus as claimed in claim 28, wherein a laminar flow device is placed on both the inlet and exhaust conduits to calculate the volume of fluid injected into the annulus of the tubular member, and the volume of fluid vented from the annulus of the tubular member.

39. The apparatus as claimed in claim 28, wherein the laminar flow device comprises a pressure sensor adapted to measure the pressure of the fluid flowing into the tubular member.

40-43. (canceled)

44. The apparatus as claimed in claim 28, wherein the tubular member comprises a riser system.

45. The apparatus as claimed in claim 28, wherein the tubular member comprises a flexible pipe.

46. The apparatus as claimed in claim 28, wherein the tubular member is adapted to provide a conduit through the water column between a subsea facility and a surface facility.

47. The apparatus as claimed in claim 28, wherein the tubular member is adapted for use with an offshore oil and gas well.

48-50. (canceled)

51. The method as claimed claim 1, including equalising the pressure between the fluid conduit and the tubular member.

52. The method as claimed in claim 1, including flowing fluid from a pressurised fluid supply into the annulus until a substantially constant pressure in the annulus is achieved and the pressure in the annulus and the pressure in the pressurised fluid supply are at equilibrium.

53. The method as claimed in claim 1, including flowing the fluid into the annulus at a flow rate that reduces to or near zero as pressure is equalized between the annulus and the fluid flowing through the fluid conduit.

54. The method as claimed in claim 1, wherein the tubular member comprises a riser of an oil or gas well providing a conduit through a water column between a subsea facility and a surface facility, and having an annulus between , and wherein the method includes determining the remnant free volume of the annulus.

55. A method of assessing the condition of a riser of an oil or gas well, wherein the riser provides a conduit through a water column between a subsea facility and a surface facility, the method comprising flowing a fluid through a laminar flow device into the riser, wherein the laminar flow device is disposed in line with a fluid conduit delivering the fluid to the riser, into an annulus in the riser, wherein the riser comprises an outer and inner layer adapted to seal the annulus; and determining the remnant free volume of the annulus of the riser by measuring the volumetric flow rate of fluid through the laminar flow device and thus determining the volume of fluid injected into the riser, wherein the method includes flowing fluid from a pressurised fluid supply into the annulus until a substantially constant pressure in the annulus is achieved and the pressure in the annulus and the pressure in the pressurised fluid supply are substantially at equilibrium, and wherein the fluid is flowed into the annulus at a flow rate that approached zero as pressure is equalized between the annulus and the fluid flowing through the fluid conduit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In the accompanying drawings:

[0042] FIG. 1 shows a generic subsea riser which does not embody the invention, and which is illustrated to provide context for understanding the later examples of the invention described in this application;

[0043] FIG. 2 shows a second generic subsea riser, which does not embody the invention, and which is also illustrated for context, to allow a better understanding of the invention;

[0044] FIG. 3 shows a schematic representation of one example of apparatus for assessing the condition of an annulus in a subsea riser with three different fluid levels indicated within the annulus.

DETAILED DESCRIPTION OF THE INVENTION

[0045] Referring to FIG. 3, this drawing shows a schematic of one example of apparatus suitable for assessing the condition of a tubular member in the form of a subsea riser 150 connected between a subsea wellhead H of an oil or gas well W and a platform P such as a drilling or production platform on which the apparatus is optionally mounted. The riser 150 provides a conduit between the platform P and the wellhead H to allow fluids from the well W to flow to the platform for export to a pipeline (not shown). Production fluids from the well typically flow in pipes and tubes such as production tubing housed within the riser 150. The apparatus described in this example comprises an annulus testing kit 101, connected to the annulus of the riser 150 via the internal vent system to external vent port The test kit 101 allows the flow of fluid into the annulus of the riser 150 for assessment of the condition of the annulus, namely, the remnant free volume in the annulus. For this purpose a test fluid is flowed into the annulus of the riser 150 to determine the remnant free volume. The components of the test kit 101 are illustrated schematically in order to illustrate one example of how the invention might be practiced, and other arrangements of components could be used.

[0046] The test kit 101 comprises a source of test fluid for flowing into the riser comprising a cylinder 102 which is optionally pressurised, and in this example is filled with pressurised inert nitrogen gas. The fluid flows through a gas line 107 between the source cylinder 102 and the annulus of the riser passing through various control valves 120a-d, a pressure relief valve 103, regulator 140, optional flow rate governor 104, and gauges 110a-d before flowing from the gas line 107 into a number of annulus ports which are in fluid communication with the annulus of the riser 150, and are generally disposed on the end termination of the riser, which is often on the platform, or is accessible from the platform. It should be noted that other examples of the invention are envisaged without any connection to a platform. The valves, regulators, governors and gauges etc. control the flow of the fluid into the annulus from the source cylinder 102, via the gas line 107. The fluid flowing into the annulus from the source cylinder 102 through the gas line 107 also passes through a laminar flow device 105, which determines the volume and/or mass of fluid flowing into the annulus. In this example, a laminar flow device in the form of a mass flow meter directly measures the volumetric flow rate passing through the mass flow meter 105 and in conjunction with a totaliser function integrated into the device, calculates the total volume and mass of the fluid flowed into the annulus.

[0047] Laminar flow provides significant improvements in accuracy due to the tight restriction of the fluid into extremely narrow fluid flow channelling elements. The elements channel the fluid particles into long, thin and parallel flow paths along the element length, removing turbulence from the flow and reducing the effects of fluctuations in temperature within the system. In order to achieve laminar flow the Reynolds number of the fluid must be maintained below a threshold of approximately 2000. The Reynolds number is defined as:

[00001]$\mathrm{Re}=\frac{v.Math..Math..Math..Math.D}{}$

where =mean velocity of the fluid; =density of the fluid; D=diameter of the elements; and =viscosity of the fluid.

[0048] If the Reynolds number exceeds 2000, the fluid flow transitions back into turbulent flow, with increasing effect as the Reynolds number increases. This creates inaccuracies in the flow measurements, which in turn produces inaccuracies in the measurements of the remnant free volume of the annulus.

[0049] Before fluid is flowed through the gas line 107 from the source cylinder 102 into the annulus of the riser 150, initial pressures in the source cylinder 102 and riser 150 are optionally recorded in order to record a baseline pressure value. Optionally the regulated pressure is set and recorded. In the present example, a leak check on the whole test spread is optionally conducted to ensure that no fluid can escape to atmosphere during the test. In this example, we also optionally allow residual pressure from the annulus 150 to build up on gauge 110d, by opening valves 120r while valve 120d (after the laminar flow meter 105) and atmospheric vent valve 130 remain closed. As the residual annulus pressure is building and registering on gauge 110d, the test spread from supply to valve 120d after the laminar flow meter 105, which at that time will be closed, can be pressurised up to a regulated 2-3 barg. Once residual annular pressure appears to be stable on gauge 110d it is recorded and the test can begin by opening valve 120d and monitoring the governor 104 and the laminar flow meter 105.

[0050] In a first example, using the general method of annulus testing, nitrogen gas cylinder 102 is opened to release nitrogen gas into the gas line 107. The gas optionally flows through a two stage regulator 140, which comprises at least one digital gauge 110a, preferably two digital gauges 110a, 110b, which optionally measure the gas pressure in the regulator 140 in barg with different scales of measurement; for example one gauge 110a can measure a range of 0-300 barg while the other gauge 110b may measure 0-30 barg. Other ranges can be adopted if desired. The high pressure gauge 110a can optionally measure the pressure in the nitrogen gas cylinder 102, while the low pressure gauge 110b measures the test pressure. The nitrogen gas flows through the gas line 107 past at least one isolation valve 120, before reaching a pressure relief valve 103.

[0051] The pressure relief valve (PRV) 103 is used to protect the integrity of the riser 150 and to allow gas to vent from the system if the gas pressure in the line 107 exceeds a threshold. The PRV 103 is typically set at a higher pressure above other reliefs in the system but below the maximum risk threshold of the riser outer sheath 150 and the other components coupled to it. This maintains another level of control over the pressure contained within the system.

[0052] Optionally the system also has a means for measuring ambient temperature, such as combined pressure and temperature gauges connected in line with the gas line 107. Optionally the gauges can record the data. In this example, the gauges can be Leo recording gauges which digitally measure pressure and ambient temperature data and store it, allowing it to be downloaded post test to plot trends.

[0053] The gas optionally flows through at least one further pressure gauge 110c (which can optionally be adapted to sense pressure in the narrower range 0-30 barg). This gauge is used to regulate the pressure down from bottle supply pressure to system feed pressure, typically 3 barg, before the fluid flows through at least one further isolation valve 120c. The flow rate of the gas in the gas line 107 is then optionally governed by a flow rate limiting device 104, which in this example can comprise a float type rotameter 104 that acts to govern or restrict or otherwise control the flow of the fluid in the line, optionally before it passes into the laminar flow meter 105. Alternatively the flow rate may be governed or restricted or otherwise controlled by one of the valves, which could, for example, comprise a needle valve. By adding the extra step of flow governance, the pressure and velocity of the fluid passing into the laminar flow meter can be controlled. This maintains the fluid flow within the tolerances of the meter and assists in reducing the Reynolds number. Use of the rotameter additionally provides a visual indicator of the flow between the source cylinder 102 and the annulus 150. With high flow rates through the gas line 107, the float of the rotameter should be observed to start at the bottom of the rotameter glass, then progressively rising in the glass as flow increases. Observation of the float at the top of the glass indicates little or no restrictions between the supply and the annulus, whereas falling of the float from the top of the glass indicates a restriction. In this example, the system can achieve 2-3 barg pressure in the annulus and if there are no restrictions in the gas line 107, the float should start at top of the glass and as gas flows through the kit into the annulus the float should progressively fall as the pressure builds in the annulus and the back pressure in the gas line 107 reduces the flow until the whole system equalises.

[0054] The rotameter 104 optionally restricts the flow to a value within the range of 0-50 litres per minute; in this example, the flow rate is initially at 30 litres per minute. The restriction of the flow rate to within a narrow range of flow rates can help to optimise the flow of the gas into the laminar flow meter, and can allow more accurate readings.

[0055] The rotameter 104 governs the rate of flow into the laminar flow meter 105, which calculates the mass of nitrogen passing a particular point within the device per unit time. As the gas is undergoing laminar flow through the meter, it is possible to use Poiseuille's Law to determine the mass and volume of gas flowing through the tube.

[0056] Alternatively the volumetric flow rate may be directly measured, that is, the volume of gas passing a particular point within the device per unit time. The laminar flow meter 105 can be used in conjunction with an accumulated totaliser function on the meter 105 that converts the volumetric measurement into a measurement in terms of total litres. The total litres injected, along with the annular pressure, is useful information during the testing of the outer sheath integrity. The accumulated totaliser can be an integrated function or component of the laminar flow meter 105, or it can be a separate device.

[0057] Once the nitrogen flows through the laminar flow meter 105, it enters the annulus of the riser 150 through at least one valve 120r connected to the riser 150. In this example three valves 120r are connected to the riser annulus ports. The nitrogen is fed into the riser annulus, optionally until the pressure equalises between the internal pressure of the gas line 107 and the annulus of the riser 150. Alternatively, the test ends when the theoretical free volume of the annulus is exceeded; if the theoretical volume is exceeded without achieving an equilibrium from regulated pressure (2-3 barg) to a stabilised annular pressure (2-3 barg), this indicates a hole in the splash zone area from 0 to 20 m at 2 barg or 30 m at 3 barg. In the event of a splash zone breach, annular pressure would always return to the hydrostatic pressure value regardless of the volume of gas flowed in. The return to hydrostatic pressure offers conclusive evidence of a splash zone breach in the riser.

[0058] The theoretical free volume of the annulus of the riser 150 is known from manufacturing specifications, and can be confirmed in a factory acceptance test for the riser 150 and therefore this can be compared to the quantity of gas that is known to have been flowed into the annulus 150 by the testing kit 101. The theoretical free volume can be divided by the length of the flexible, and offers a litres per meter measurement. This in turn can be used to detect the level of fluid based on the achieved volume of gas to bring the system to an equalised 3 barg divided by 3. The difference between the theoretical free volume and actual measured free volume is compared to assess the condition of the riser, for example, to determine whether there has been a breach in the annulus, and if so, at which level of the riser 150.

[0059] The flow rate of injected gas will reduce in an annulus with good integrity as the internal pressure builds up, as the gas supply pressure is regulated at 2-3 barg. The integrity of the riser can be determined from the total number of litres of gas used to achieve a constant 2-3 barg pressure within the annulus. This value is then compared to the theoretical free volume, and litres per meter volumes. If a high number of litres (close to the corresponding theoretical free volume value) are required to achieve a constant 3 barg stabilised pressure, this indicates that the integrity of the riser is sound. If a constant 3 barg pressure is achieved with significantly fewer litres of gas than the theoretical values, this is indicative of a subsea breach in the outer sheath of the riser. If the value is lower than the theoretical free volume, but not critically so, this indicates acceptable outer sheath integrity with condensation accumulating within the annulus. The presence of condensate is indicated by the cessation of transfer of injected gas as a constant annular pressure is achieved. This is in contrast to the above-described return to hydrostatic pressure found in risers with splash zone breaches. This difference is very important when analysing the potential for corrosion of the riser due to sea water, in comparison to the less damaging presence of condensate.

[0060] For an annulus with a theoretical free volume of 1000 litres, some possible outcomes of the testing are illustrated by the following examples:

EXAMPLE 1

[0061] In a first example, nitrogen gas fluid was flowed into the annulus of the riser 150 as described above, at a pressure of 3 barg, at an initial flow rate of 30 litres/minute and temperature of 25 C. for a period of time until the complete system equalises at 3 barg. It is useful to pressurise the annulus and generally the pressure in the annulus during testing with be somewhere between 1-3 barg. For example, although 3 barg is indicated in this example, the pressure peak in the annulus could frequently be about 2 barg. In a riser with good integrity the flow rate would ideally not be a constant and would progressively decay as the annular pressure built until an equilibrium was achieved at around 2-3 barg regulated pressure from supply and 2-3 barg or same value achieved in annulus and flow reduced to near zero levels. In the current example, the laminar flow meter measured 1000 litres of nitrogen flowed into the annulus. Thus the residual volume of the annulus according to the first example is 1000 litres. In this example, the actual volume determined by a test run on a first riser 150 is 1000 litres which conforms to the original pre calculated theoretical free volume of the flexible risers annulus. This indicates a full free volume within the annulus 150 at a level 151, well below the level of the surrounding seawater. The test therefore confirms that the integrity of the outer sheath is acceptable. Frequently, even if the riser integrity is acceptable, a low level of fluid within the annulus is found. This is indicative of condensation diffusing into the annular space from the inner liner 8. Condensation is a relatively common occurrence in the annulus of a riser 150, and it is useful to monitor the level of condensate within the annulus along with the composition. It is possible for gases such as carbon dioxide and/or hydrogen sulphide to permeate through the same layer and form an acidic solution with the condensate. Generally the condensate accumulates at the low points of any bends in the flexible pipe. Excessive accumulation of the condensate in the annulus of the riser 150 according to this example might indicate low level intervention. One possible example of low level intervention that would be suitable in the event that the low level of fluid found could be either seawater or condensate could be monitoring the annulus at repeated intervals, for example, daily, weekly, monthly or annually, depending on the severity of the fluid accumulation, optionally with a permeation rate and condensation calculation carried out. However, more usually, the presence and effects of condensation is accounted for in the design of the riser system. For example, if high levels of condensation are expected different grades of metal can be used in the manufacture of the tensile armour and intervention is generally not required.

EXAMPLE 2

[0062] In a second example, the riser is treated according to the same procedure as above, flowing nitrogen gas into the riser 150 through the mass flow meter 105 at a pressure of 3 barg, a flow rate of initially 30 litres/minute and temperature of 25 C. for a period of time until the complete system equalises at 3 barg. In this example, it was found that the residual volume of the annulus of the riser 150 was approximately 500 litres. This indicates a fluid level within the annulus at a depth below sea level within the water column 152, and suggests a build up of condensate diffusion, as the identified fluid has not free flooded to sea level. Condensate is a naturally occurring phenomenon, which is generated through the normal operation of flexible pipelines carrying bore fluids of a certain composition

[0063] Seawater ingress into the riser system could be an alternative explanation for this fluid level in the riser annulus. If the fluid below sea level is not condensate then this would tend to indicate that the riser system is in the process of flooding, for example through a breach at the sea bed. In this situation, accurate volumetric trending would be recommended.

[0064] Sea water ingress through a breach in the outer sheath, and its associated effect on the pipeline structures, is assessed on a case-by-case basis, with analytical corrosion and fatigue studies carried out. The main factors for these studies are: the location of the breach within the water column; the environmental water temperatures in that region; the working pressures and temperatures of the pipeline system; the dynamic movements of the sea water; and the levels of oxygenation of the sea water. In a worst case scenario this could lead to a serious corrosion event that could require the urgent shut down, removal, and repair or replacement of the riser system due to a high risk of corrosion and fatigue in the layers of the flexible pipe, particularly the tensile armour 4, 4 and the pressure armour 3. It is again possible for carbon dioxide and/or hydrogen sulphide to mix with the seawater and potentially exacerbate the corrosive effects of the seawater on the armour. The conditions found in example might indicate a more in-depth inspection campaign than the previous example, possibly including further investigations to confirm the possible indication of an external sheath breath below the water line. The information found would play a major part in the corrosion analysis conducted as a desk top study. Benchmark monitoring could be carried out at regular intervals over a period of 2-3 years in order to determine the rate of flooding and how rapidly intervention may be required.

EXAMPLE 3

[0065] In a third example, the riser is treated according to the same procedure as above, flowing nitrogen gas into the riser 150 through the laminar flow meter 105 at a pressure of 3 barg, flow rate initially 30 litres/minute and temperature of 25 C. for a period of time until the complete system equalises at 3 barg. In this example, it was found that the residual volume of the annulus of the riser 150 was 150 l which is equivalent to the volume of the annulus between the top of the riser 150 and sea level. This indicates fluid at the air-water interface at sea level, for example to fluid level 153. This fluid could be seawater entering at the splash zone or below the water line and flooding the annulus up to the water line. In both of these cases the annulus is at severe risk of corrosion to the armour layers and other parts of the riser structure. Seawater entering the riser from the water line reintroduces oxygen into the metallic dynamic riser structure and creates a corrosion cell, which is constantly fed with seawater which is extremely corrosive because the splash zone contains high levels of dissolved oxygen as it is close to the air-water interface and is highly agitated by waves at the surface. The seawater increases the corrosive effects, and therefore increases the corresponding risk of failure. The fluid can equally be present from a subsea breach at a depth greater than 30 meters (splash zone)), which also has a corrosive effect. The conditions found in this example, after corrosion and/or life assessment, may necessitate the urgent shut down and removal of the riser system for repair or replacement, due to the high risk of pipe failure. It is also a region where the riser is cycling through wet and dry periods as the seawater splashes against it, contributing to the corrosive action of the water.

[0066] The annulus of the riser 150 can optionally be tested on more than one occasion, for example in sequential tests performed repeatedly and spaced apart by e.g. a month or a week, depending on the severity of the findings of the previous assessment, and the residual volume compared. Sequential tests can be repeated over a testing period of months or years, or on a continuous basis. Should the volume be shown to have stayed relatively close to the theoretical free volume, this tends to indicate that the annulus is at a low risk of requiring immediate and aggressive intervention. A decrease in the residual volume indicates rising fluid levels and a more urgent problem within the pipe, potentially requiring remedial action, and/or more frequent monitoring.

[0067] The nitrogen gas may be replaced with any similarly inert gas.

[0068] Some options for addressing breaches in risers include treating with corrosion inhibitor, removing and repairing damaged sections of the outer sheath, repairing damaged sections of the flexible pipe, or replacing components. Fatigue assessment is necessary as even when the outer sheath has been repaired, there may be sufficient damage to the pipe to significantly reduce the service life.

EXAMPLE 4

[0069] In another example, the riser is treated according to the same procedure as above, flowing nitrogen gas under pressure into the riser 150 through the laminar flow meter 105 at a pressure of 3 barg, at an initial flow rate 30 litres/minute and temperature of 25 C. The annulus of the riser 150 may be filled until the pressure between the annulus testing kit 101 and the annulus itself equalises. Optionally at that stage, the valves are shut off, and the high pressure is held in the annulus, while the source of pressurised fluid is disconnected, after which the valves are opened to permit the high pressure fluid in the annulus to flow out of the fluid line, through the laminar flow meter 105, which verifies the quantity of gas flowing out of the annulus. This allows comparison between the inflow of the gas into the annulus and the outflow of gas out of the same annulus, which permits more accurate assessment of the quantity of gas and therefore the free volume of the annulus.

[0070] The calculated volume from the measured quantity of gas is then compared to the known theoretical volume. The magnitude of the difference between these two values indicates whether the annulus is subject to condensation or seawater ingress as described in examples 1-4 above.

[0071] This method can be repeated as often as required to monitor any changes in the residual volume of the riser annulus, and determine whether the fluid level is changing, and how rapidly.

[0072] Optionally in any of the above examples, the laminar flow meter can be moved from the gas inlet side of the apparatus and connected to an exhaust port to measure the volume of gas that is vented at the end of testing. The two values (input and exhaust) can then be compared for cross-checking. Due to gas permeation from the bore into the annulus, it is relatively common for the volume of vented gas to be slightly higher than the volume injected, within a range of 0-5% of the total injected volume.

[0073] Optionally a laminar flow meter can be connected at both the gas supply side and the exhaust port to measure the values without the need for reconnection. Alternatively, a laminar flow meter can be used independently at either side.