WO2009109747A1

WO2009109747A1 - Subsea pipeline slug measurement and control

Info

Publication number: WO2009109747A1
Application number: PCT/GB2009/000588
Authority: WO
Inventors: Gary Martin Oddie; Stephen J. Kimminau
Original assignee: Schlumberger Holdings Limited; Schlumberger Canada Limited; Prad Research And Development Limited; Services Petroliers Schlumberger; Schlumberger Technology B.V.
Priority date: 2008-03-04
Filing date: 2009-03-03
Publication date: 2009-09-11
Also published as: GB2458125B; GB2458125A; GB0804020D0

Abstract

A subsea pipeline slug measurement system (30) comprising a hoop strain sensor in the form of an optical fibre distributed strain sensor (32) coupled to a subsea pipeline (36). The fibre sensor (32) is provided within a sensor carrier, which is adapted to be mechanically coupled to the pipeline (36). The system (30) also comprises a fibre optic strain sensor interrogation apparatus (34) optically coupled to the fibre optic strain sensor (32) and operable to detect changes in the strain sensed by the fibre optic strain sensor. A central processor adapted to receive strain data from the interrogation apparatus is also provided, and is operable to determine from the received data when a slug is present within a multiphase flow in the pipeline (36) and the magnitude of the slug.

Description

Subsea pipeline slug measurement and control

Technical field

The invention relates to a subsea pipeline slug measurement and control system using strain sensors, and specifically to a slug measurement system and a method of measuring and controlling slugs in subsea multiphase pipelines.

Background art

Offshore hydrocarbon production from the seabed to sea-surface facilities involves the rapid flow of oil, water and gas through flowlines, risers and other components (referred to jointly herein as pipelines). During the extraction of hydrocarbons from subsea production facilities, multiphase flow of various phases differing in density can often occur in the pipeline. Under certain conditions gas flow becomes segregated from the heavier liquids which travel along the pipes as "slugs'. Once the slug has formed, pressure builds up behind the slug until it is high enough to push the slug through the pipeline, which can then involve large amounts of liquid moving at high speed along the pipeline. The inertial loads caused by these slugs can damage facilities and overload the processing capacity; hence the importance of controlling slugs by various active and passive measures. When not properly managed, the arrival of a large liquid slug can overflow or damage the surface separator of the pipeline, causing significant production and QHSE (Quality Safety Heath Environment) problems. Repeated slugging events, even of an small amplitude may cause fatigue damage to pipelines and other components. Typically slug measurement and control systems rely on pressure sensors in direct contact with the flowing fluids, mounted at limited positions, often not ideal, and which do not measure directly the strain loads imposed upon pipelines.

Summary of the invention

It is an object of the invention to provide an improved subsea pipeline slug measurement system.

According to a first aspect of the present invention there is provided a subsea pipeline slug measurement system comprising: a fibre optic hoop strain sensor adapted to be externally coupled to a subsea pipeline, the fibre optic hoop strain sensor comprising a sensor carrier adapted to be mechanically coupled to the pipeline and at least one fibre optic strain sensor mechanically coupled to the sensor carrier; fibre optic strain sensor interrogation apparatus optically coupled to the fibre optic strain sensor and operable to detect changes in the strain sensed by the fibre optic strain sensor; and a central processor system adapted to receive strain data from the interrogation apparatus and operable to determine from the received data when a slug is present within a multiphase flow in the pipeline and the magnitude of the slug.

The hoop strain sensor senses changes in the diameter of the pipeline due to changes in the pressure within the pipeline, but the response is directly proportional to the amount of strain experienced by the pipeline pressure-bearing structure. The invention therefore provides a slug measurement system which is non-invasive to the pipeline and may be retrofitted onto existing pipeline installations.

In addition to hoop strain, other sensors configured to measure tension, bending, or torsion of pipelines caused by slugging or external loads may be provided at certain places. Strain is the basic physical parameter most important to the fatigue lifetime of the pipeline. In flexible or highly-stressed pipelines the strain, rather than the internal pressure, is often of most interest to the pipeline operator.

The fibre optic strain sensor may comprise an optical fibre forming a distributed fibre optic strain sensor. The optical fibre is preferably arranged within the sensor carrier such that in use it is provided in a wound configuration around the outside of the pipeline. The optical fibre is preferably provided in a wound configuration having a pitch which is small relative to the diameter of the pipeline, in order to create maximum response sensitivity.

The fibre optic strain sensor interrogation apparatus may comprise a Brillouin or Rayleigh scattering fibre optic strain sensor interrogation apparatus.

The fibre optic strain sensor may alternatively comprise a fibre optic grating sensor, and most preferably comprise a fibre Bragg grating strain sensors. The fibre optic strain sensor interrogation apparatus may comprise a wavelength division multiplexed fibre grating interrogation apparatus or a time division multiplexed fibre grating interrogation apparatus.

The system preferably comprises a plurality of hoop strain sensors. The fibre optic sensors of the plurality of hoop strain sensors may be provided within a single optical fibre.

The system is thereby able to both identify the presence and magnitude of a slug and to determine its location within the pipeline, and the total strain being imposed upon the pipeline, composed of the dynamic load caused by the slug and other loads to which the pipeline is subjected, which may be static or dynamic.

The central processor system is preferably further operable to determine the speed of movement of the slug. The central processor system is preferably further operable to generate an alarm signal. The central processor system is preferably further operable to generate a control signal for transmission to a slug management system. The central processor system may be operable to decrease or increase the total flow rate within the pipeline, inject gas into the pipeline, divert gas within the multiphase flow, or take other action as appropriate to ameliorate the effects of slugs.

Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings.

Brief description of the drawings

Figure 1 illustrates the mechanism associated with slug formation in a subsea pipeline;

Figure 2 is an illustration of a subsea pipeline installation provided with a subsea pipeline slug measurement system according to a first embodiment of the invention;

Figure 3 is a diagrammatic side view of a fibre optic hoop strain sensor of a subsea pipeline slug measurement system according to a second embodiment of the invention; and

Figure 4 is an illustration of a subsea pipeline installation provided with a subsea pipeline slug measurement system according to a third embodiment of the invention. Description of embodiments of the invention

The mechanism associated with slug formation is shown in Figure 1. The liquid slug 20 builds-up (Figure 1 (a)) until it completely fills the pipe 22 (Figure 1(b)) and prevents gas 24 in the pipe from flowing. Due to the restriction created in the pipe 22 by the slug 20, the pressure of the gas 24 builds up until it is high enough to move the slug 20, pushing the slug 20 along the pipe 22. The slug 20 accelerates until it reaches the surface.

Once moving, liquid slugs 20 create large inertial forces at changes in direction in the pipeline 22, and by means of the strain created in the pipeline 22 these inertial forces can be measured and hence controlled.

The present invention consists in monitoring the pressure built-up behind the slug 20 as the slug progresses along the installation (pipe 22). The monitoring is done using noninvasive pressure sensors. In one embodiment, an optical fibre configured as a distributed strain sensor provides a distributed pressure measurement along the pipe 22. In another embodiment a network of localized pressure sensors, in the form of optical fibre Bragg grating (FBG) strain sensors 13, is provided along the pipe 13, as shown in Figure 2.

As the slug 20 progresses to the surface, the FBG sensors 13 will see an increase of pressure as the slug passes nearby them, and respond to inertial loads in curved sections of the pipeline 22. Based on these measurements it is possible to give an estimation of the slug location. A fully distributed measurement, using the distributed fibre strain sensor provided along the pipeline 22, can provide a truly real time estimation of the slug position and magnitude.

Referring to Figures 2 and 3, a first embodiment of the invention provides a subsea pipeline slug measurement system according to a first embodiment of the invention. The system is for monitoring slug formation and movement through a subsea pipe 22, such as flowlines 22a and risers 22b, of a subsea installation. A flowline 22a is typically the horizontal pipes at the sea bed. Risers 22b are the vertical lines. The flowline and risers can be either of flexible type or rigid. The system comprises a plurality of hoop strain measurement devices 10 as shown in Figure 3 positioned along the pipe 22 at selected positions, as shown in Figure 2. The hoop strain measurement devices 10 comprise a clamp 11 that in use is placed to embrace the pipeline 22 to be monitored and a strain sensor 13 attached to the clamp and arranged so as to detect strain in the direction of the perimeter of the pipeline, around the periphery of the pipeline. The clamp can be a strap or belt or a compliant material, and may be shaped to the outside shape of the structure. In a preferred embodiment, the clamp 11 is made using composite material. The strain sensor can embedded into the clamp in one preferred embodiment. In the example shown in Figure 3, the clamp comprises a ring-type clamp 11 , typically made out of a composite material. In this embodiment, the clamp 11 is formed in two semicircular halves secured together using a securing system 12. In this case, the securing system comprises two nut and bolt arrangements on opposite sides of the clamp. Other releasable securing systems can also be used and it is also possible to replace one securing system with a hinge. The shape of the clamp 11 in Figure 3 is circular, although other shapes can be used depending on the cross-sectional shape of the pipeline to be monitored. A strain sensor 13 is located on the clamp 11 or embedded into the clamp material (as is shown in Figure 3). As the pipe 22 expands the fibre grating 13 response varies due to the varying hoop strain which it experiences.

The spacing of the hoop strain measurement devices 10 should be along the whole length of the pipe 22 as shown in figure 2, and may be located near areas of concern where is known that slugs may form and propagate. So, for example, in a long flowline 22a an area of concern would be around a bend. Therefore, by positioning a plurality of sensors before, on and after the bend the system will be able to detect slug formation and movement. At a minimum the system should be two sensors strategically placed but in most cases multiple sensors will be used.

The hoop strain sensors 10 are connected to a central processor located either subsea in a subsea communications hub or preferably at the surface via a communications line (a umbilical) adapted to receive and transmit the hoop strain data (local deformation of the pipe) from each sensor to the central processor. The central processor is operable to plot pipe deformation data received from each hoop strain sensor 10 with time. So for example if a slug begins building in the vicinity of a sensor 10, that sensor will indicate a pipe diameter change (we call it pressure change for convenience) while the remaining sensors indicate steady pressure data. The central processor system initially receives signals from each of the multiple sensors 10 and establishes a pressure baseline response characteristic of non slugging behaviour. In operation, the central processor receives the pressure change signals from the sensors 10 and matches these signals to a characteristic response indicative of a slug formation. Thus, as a slug is forming the operator receives an indication from the output of the central processor, where it is forming and of the magnitude of the slug. In the event of a pressure build up the operator will have advance warning to take appropriate action. For instance in the event of a large slug the operator will be ready to divert the flow from that particular flowline away from the surface production facilities to a slug management system facility. The slug can also be controlled by adjusting chokes located on the installation or by activating surface safety equipment that will slow down the slug before it arrives at surface.

The operator can take other remedial actions as the case be. This way, catastrophic failures of surface production facilities due to slugs can be avoided. The system also provides an indication of when the slug begins to move after formation. Thus, for example if a slug buildup is forming near the vicinity of sensor X1 when the slug begins to move through the system the operator will see a pressure buildup through the next sensor X2 and so on and so on.

The system comprises means for detecting the movement of a slug and the speed at which a particular slug is moving and means for notifying the operator of such events in a digital form rather than relying on the operator to follow the pressure data visually. For instance, as persons skilled in this art would appreciate, the system will measure the time difference between two consecutive pressure buildups between two consecutive pressure/hoop strain sensors 10 and by also knowing the distance between the sensors 10 the speed of the slug can be readily estimated and provide an estimate of the time when a slug will reach the surface equipment. Thus, the operator or an automated system will divert the flow from that particular flowline from the surface production facility to a slug management system.

The central processor system is also operable to generate a control signal for transmission to a slug management system, to cause the slug management system to decrease or increase the total flow rate within the pipeline, inject gas into the pipeline, divert gas within the multiphase flow, or take other action as appropriate to ameliorate the effects of slugs. The above steps taken by an operator may therefore be automatically implemented by the central processor system.

Referring to Figure 4, a second embodiment of the invention provides a subsea pipeline slug measurement system 30 comprising a fibre optic hoop strain sensor 32 in the form of an optical fibre configured as a distributed strain sensor for interrogation using Brillouin scattering, and fibre optic strain sensor interrogation apparatus 34 in the form of a Brillouin scattering detection hub. The fibre 32 is provided within a sensor carrier in the form of a carrier rod.

The sensor carrier containing the fibre 32 is helically wound around, and is thus coupled to, a pipeline 36, which may be a flowline or riser, to be monitored. The fibre 32 is wound with a smaller pitch (i.e. more tightly wound) in areas where slug formation is more likely. The tight pitch renders the fibre 32 more sensitive to hoop strain rather than tensile strain in the pipeline 36. One possible configuration as shown in Figure 4 is a tight pitch in the areas of concern, and a looser pitch to the next point of measurement, tight pitch again and similarly all the way to the scattering detection hub 34, giving the system a daisy-chain arrangement of sections of tightly wound fibre 32. The measured hoop strain data from the scattering detection hub is transmitted via a communications line to a central processor as described above.

The present invention uses measurements from fibre-optic strain sensors to establish baseline dynamic loading on a flexible riser pipeline system subject to additional loads from slugs. When an active slug control system is activated the change in loading in the riser can then be measured in real-time. This can confirm that the system has moved from a potentially hazardous state to a safe operating region, or in favourable cases production could even be increased whilst confirming that riser loads are still within their safe operating range.

In addition to point pressure, temperature and flowrate sensors at certain nodes, the present invention provides a fibre-optic strain sensor system which measures strain and stress on flowing flexible pipes. Typical flowline and riser systems have a point pressure and temperature sensor at both the inlet and outlet, but the flowing length of pipe which can be hundreds or thousands of metres long, has no sensors. The strain sensor systems described allow measurements to be made at critically stressed positions at almost any location. The systems can be retrofitted onto installations which were not originally designed to be monitored.

Slug mitigation systems typically rely on the inlet and outlet pressure and flowrate sensors to detect and then control slugs. The change in pressures and flowrates when the slug control system is working can be measured and hence the effectiveness of the system could be estimated, but with pressure and flowrate sensing alone the actual stresses imposed on the flexible pipeline are not directly measured. The systems described however allow direct measurements of maximum pipeline stress to be made in real time and combined with the slug management system of an installation.

Two modes of operation of the described subsea pipeline slug measurement systems are envisioned; either simply detecting the maximum stress and strain at a point of maximum loading, and hence controlling the system based upon this simple parameter alone, or by measuring the time-varying loading over a period of more than a single cycle, deriving a total effective cycle load or fatigue load.

The systems described enable pressure events due to slugs to be monitored by measurements of hoop strain added at key points in the flow. Hoop strain measurements are non-intrusive and can be attached at many places not accessible to conventional pressure sensors. Hoop strain pressure measurements are applicable to all types of pipelines and risers, both solid and composite.

The described embodiments use non-invasive strain and pressure measurement distributed between seabed and surface to monitor slugging multiphase flow in a subsea pipeline installation. The described embodiments use non-invasive pressure sensors or a fully distributed system to locate and follow the formation of liquid slugs in multiphase flow.

Claims

1. A subsea pipeline slug measurement system comprising: a fibre optic hoop strain sensor adapted to be externally coupled to a subsea pipeline, the fibre optic hoop strain sensor comprising a sensor carrier adapted to be mechanically coupled to the pipeline and at least one fibre optic strain sensor mechanically coupled to the sensor carrier; fibre optic strain sensor interrogation apparatus optically coupled to the fibre optic strain sensor and operable to detect changes in the strain sensed by the fibre optic strain sensor; and a central processor system adapted to receive strain data from the interrogation apparatus and operable to determine from the received data when a slug is present within a multiphase flow in the pipeline and the magnitude of the slug.

2. A subsea pipeline slug measurement system as claimed in claim 1 , wherein the fibre optic strain sensor comprises an optical fibre forming a distributed fibre optic strain sensor.

3. A subsea pipeline slug measurement system as claimed in claim 2, wherein the optical fibre is arranged within the sensor carrier such that in use it is provided in a wound configuration around the outside of the pipeline.

4. A subsea pipeline slug measurement system as claimed in claim 3, wherein the optical fibre is provided in a wound configuration having a pitch which is small relative to the diameter of the pipeline.

5. A subsea pipeline slug measurement system as claimed in any of claims 2 to 4, wherein the fibre optic strain sensor interrogation apparatus comprises a Brillouin or Rayleigh scattering fibre optic strain sensor interrogation apparatus.

6. A subsea pipeline slug measurement system as claimed in claim 1 , wherein the fibre optic strain sensor comprises a fibre optic grating sensor.

7. A subsea pipeline slug measurement system as claimed in any preceding claim, wherein the system comprises a plurality of hoop strain sensors.

8. A subsea pipeline slug measurement system as claimed in claim 7, wherein the fibre optic sensors of the plurality of hoop strain sensors are provided within a single optical fibre.

9. A subsea pipeline slug measurement system as claimed in any preceding claim, wherein the central processor system is further operable to determine the speed of movement of the slug.

10. A subsea pipeline slug measurement system as claimed in any preceding claim, wherein the central processor system is further operable to generate an alarm signal.

11.A subsea pipeline slug measurement system as claimed in any preceding claim, wherein the central processor system is further operable to generate a control signal for transmission to a slug management system.

12.A subsea pipeline slug measurement system as claimed in any preceding claim, wherein the central processor system is operable to decrease or increase the total flow rate within the pipeline, inject gas into the pipeline, divert gas within the multiphase flow, or take other action as appropriate to ameliorate the effects of slugs.

13. A subsea pipeline slug measurement system substantially as described above with reference to the accompanying drawings.