GB2474013A

GB2474013A - Measuring chain tension between a riser and buoyant tank

Info

Publication number: GB2474013A
Application number: GB0916498A
Authority: GB
Inventors: Stuart Killbourn
Original assignee: FUGRO GEOS Ltd
Current assignee: FUGRO GEOS Ltd
Priority date: 2009-09-18
Filing date: 2009-09-18
Publication date: 2011-04-06
Also published as: GB0916498D0

Abstract

A system 9 for measuring the top tension of a marine hybrid riser supported by a buoyancy tank 3 whereby the tension is inferred from natural frequencies of the connection 4, such as a chain or a cable, between the upper riser assembly 5 and the buoyancy tank 3. The system 9 includes inertial sensing means or accelerometer for sensing the motion. The motion itself may be induced by an inertial means within the system or it may be caused by ambient conditions. Measured data may be sent to the surface via a cable or by a hydro-acoustic transmission system. The apparatus may record data and also contains the necessary processing and filtering means together with a power supply.

Description

Hybrid Riser Tension Measurement

Background of the Invention

This invention relates to the measurement of tension in a marine riser where the riser weight is supported by a buoyancy tank. The buoyancy tank may leak and flood with water. This would reduce the tension applied to the top of the riser and could, ultimately, cause the riser to be dropped to the seabed. The invention is a system that measures the tension applied by the buoyancy tank to the top of the riser stack and could be used to detect flooding the buoyancy tank.

Alternatively, buoyancy tanks may provide variable tension (e.g. by means controlled of flooding or purging of compartments). In this case, the system would also verify that the intended change in tension had been achieved.

Other proposed methods for determining riser tension rely on measuring strain (e.g. using a strain gauge) or displacement (e.g. using a linear variable differential transformer). Inherently, these are measurements of a static quantity and are susceptible to drift in electronic components that may give false readings and accumulate a large error over time. Other problems that are typical of such other methods are: * Sensitive components (including cabling) are attached to the riser and exposed to sea water which compromises their reliability.

* Fixtures are required on the riser to mount the measurement equipment meaning that such systems cannot be deployed on existing risers that do not have appropriate fixtures.

* Measurement equipment is included in the load path between the riser stack and the buoyancy tank and therefore must be pre-installed and cannot be removed for maintenance or repair.

The present invention addresses these problems by proposing a completely sealed unit to record dynamic signals that can be installed on simple structural elements between the riser stack and the buoyancy tank.

Statement of Invention

The present invention determines the riser tension from the natural frequency of resonant modes of the mechanical connection that connects the top of the riser stack to the buoyancy tank. Typically, the connection might be a chain tether several metres in length.

Advantages The riser tension monitoring system will detect loss of buoyancy in the tank allowing maintenance or remedial work to be performed.

The riser tension is inferred from measurements of the natural frequency of at least one vibration mode meaning that tension readings are not susceptible to electronic drift that may occur over time or due to temperature changes.

Preferably, the riser tension monitoring system will be contained within sealed enclosures; no sensitive electronic devices or cables will be exposed to sea water.

Preferably, the riser tension monitoring system can be installed on an existing riser structure without any special interface requirements. (2)

Preferably, the riser tension monitoring system can be installed and recovered by a remotely operated vehicle or by divers.

Preferably, the riser tension would be transmitted to the floating production vessel via a wireless communications link.

Brief Description of Drawings

The invention is further described by the accompanying drawings.

Figure 1 shows a schematic diagram of a typical hybrid riser used in the offshore oil and gas industry.

This is included for reference and the invention does not relate to any items shown.

Figure 2 shows a detail near the top of a hybrid riser supported by a buoyancy tank with the riser tension monitoring system installed.

Figure 3 shows the contents of a sealed enclosure which performs the natural frequency measurements.

Figure 4 shows an alternative embodiment of the tension measurement system where multiple enclosures are used.

Figure 5 shows an alternative embodiment of the tension measurement system where a cable is run back to a surface production facility.

Detailed Description of the Invention

The invention comprises a method of determining the tension applied by a buoyancy tank to the top of a hybrid riser stack. The primary concept is the measurement of vibration signals to determine the natural frequency of the mechanical connection between the riser stack and buoyancy tank and hence to infer the tension applied. While various embodiments of the system can be envisaged, the invention is described in its preferred embodiment that (in the opinion of the inventor) provides the most reliable system for the intended purpose. The invention is not limited to this specific embodiment but the scope of the invention is as defined in the appended claims.

Figure 1 illustrates a typical hybrid riser used in the oil and gas industry. The hybrid riser comprises a floating production facility [1], a flexible riser [2], a buoyancy tank [31 connected by a tether [4] to the upper riser assembly [5] which joints the flexible riser [2] to a rigid steel riser [6] and a means of anchoring the entire structure to the seabed [7]. This figure is included for reference.

Figure 2 illustrates a measurement system [9] clamped or otherwise rigidly attached to the mechanical connection between the upper riser assembly [5] and the buoyancy tank [3]. In this illustration, the mechanical connection is a chain tether [4]. The measurement system [9] is suitable for the subsea environment and hydrostatic pressure. The enclosure contains measurement equipment and the means of processing signals to determine the natural frequency of the mechanical connection. The resulting readings are transmitted to surface production facility by means of hydro-acoustic communications [8]. Alternative wireless communications could be used.

The joints between the top of the riser stack [5] and the chain tether [4] and the chain tether [4] and the buoyancy tank [3] will likely result in mismatches in mechanical impedance. For constant tension this means a change in the mass per unit length of the elements across the joint or a change in the bending stiffness of the components. This means that resonant modes of the chain tether [4] will not be strongly coupled to motion of the buoyancy tank [3] nor the upper riser assembly [5]. Therefore, the resonant modes of the chain tether [4] will likely be localised on the chain tether which has a well-defined length. Therefore, the natural frequency of such localised modes will be proportional to the applied tension by a simple relationship. Alternatively, a more accurate relationship may be obtained using advanced structural analysis methods. (3)

Figure 3 illustrates the measurement system [9] which is largely contained within a sealed, subsea enclosure [11]. The enclosure contains inertia sensors recording linear motions [14] or alternatively angular motions could be measured. The sensor signals are digitised by a data logger [12] and processed to determine natural frequency values. The data logger [12] controls power to the sensors and other equipment by means of a power control unit [16] and all electronic parts are powered from a battery pack [17]. The natural frequency values generated by the data logger [12] and transmitted by digital means to a modem [13] connected to a hydro-acoustic transducer [10] for broadcasting through the sea water to a floating production facility [1]. Optionally, the enclosure [11] may contain an inertial excitation device [15] to apply pulsed or oscillating forces to the mechanical connection [4] for the purpose of exciting the resonant modes of the connection thus making it possible to record vibrations of these modes. Such an inertial excitation device could comprise a moving mass the reaction force from which couples rigidly via the enclosure [11] and the enclosure attachment mechanism into the chain tether [4].

Figure 4 illustrates another embodiment of the measurement system whereby a sensor enclosure [18] is rigidly attached to the mechanical connection but contains a reduced set of components necessitating only the inertial sensors [14] and, optionally, the inertial excitation device [15). A cable [19] connects the sensor enclosure [18] to a battery enclosure [20] containing at least the battery pack [17]. The embodiment may be arranged so that the battery enclosure [20] can be retrieved and replaced by a remotely operated vehicle.

Figure 5 illustrates another embodiment of the measurement system whereby a sensor enclosure [21] is rigidly attached to the mechanical connection but contains a reduced set of components necessitating only the inertial sensors [14] and, optionally, the inertial excitation device [15]. The data logger [12] would preferably be contained within the sensor enclosure [21]. A cable [22] supplying power and communications is run from the sensor enclosure [21] back to the floating production facility [1]. The cable [22] may be dedicated for this purpose. Alternatively, the cable [22] may interface with a control umbilical otherwise used in the operation of the hybrid riser allowing connection, for example, at the upper riser assembly [5]. (4)

Claims

Claims 1. A measurement system that determines the tension applied to a riser stack by measurement of at least one natural frequency of a substantially compliant mechanical connection between the riser stack and a buoyancy tank supporting the riser stack.
2. The measurement system of claim 1 wherein the tension measurements are stored internally.
3. The measurement system of claim 1 wherein the tension measurements are transmitted to a floating production facility.
4. The measurement system of claim 3 wherein said system has an electrical connection to said floating production facility allowing components of the measurement system to be located onboard the production facility.
5. The measurement system of claim 1 wherein said natural frequencies of the mechanical connection are excited by ambient forces.
6. The measurement system of claim 1 wherein said natural frequencies of the mechanical connection are excited by forces applied by the measurement system.
7. The measurement system of claim I wherein said system comprises a single subsea enclosure which can be deployed or recovered by means of a remotely operated vehicle.
8. The measurement system of claim 1 wherein said system is contained in multiple subsea enclosures at least one of which can be deployed or recovered by means of a remotely operated vehicle.