GB1587712A

GB1587712A - Method and apparatus for determining marine riser shape

Info

Publication number: GB1587712A
Application number: GB4521975A
Authority: GB
Original assignee: STRUCTURAL DYNAMICS Ltd
Current assignee: STRUCTURAL DYNAMICS Ltd
Priority date: 1977-01-20
Filing date: 1977-01-20
Publication date: 1981-04-08

Description

(54) METHOD AND APPARATUS FOR DETERMINING MARINE RISER SHAPE (71) We, STRUCTURAL DYNAMICS LIMITED (formerly Structural Dynamics (Offshore) Limited, a British Company of, 19 Archers Road, Southampton, formerly of 58 The Avenue, Southampton SO1 2ET, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is concerned with method and apparatus for the determination of the spatial location of a point or points along a line source of wave energy. In this specification, the term "line source" means a linear source distribution resembling a point source in section which distribution is not necessarily straight.

The line source may consist of a leaky wave energy propagating member along which wave energy is propagated as a pulse or train of pulses.

From one aspect, the invention provides a method for the determination of the shape or other properties of a line source of wave energy or of a medium in which said line source is supported, said line source radiating energy at a plurality of positions along its length, comprising the steps of detecting with transducer means said energy radiated at each of said plurality of positions, said transducer means being arranged to determine the angle of incidence of the wave energy from each said position, and computing the spatial coordinates of said positions on the line source from said angles of incidence. The radiated energy field is monitored by the transducer means and the information thus obtained is processed to yield the position of the line source, or parts of the line source, relative to the transducer means.

From another aspect, the invention provides apparatus for determining the shape or other properties of a line source of wave energy or of a medium in which said line source is supported, said line source being arranged to radiate energy at a plurality of positions along its length, transducer means arranged to receive energy from said plurality of positions and arranged to determine the angle of incidence of the wave energy from each said position, and means for computing the spatial coordinates of said positions on the line source from said angles of incidence.

The selection of transducer means depends upon the nature of the medium into which the line source is radiating and also upon the characteristics of the radiated energy (i.e. whether acoustic, electromagnetic etc.). Conversely, the radiation field of a known straight line source may be used to determine properties of the medium supporting the radiation, since deviation from the expected radiation field of the straight line source is representative of discontinuities in the intervening medium.

The invention is particularly though not essentially applicable to the determination of the shape of a marine riser system. A marine riser is the semi-flexible link between a floating drilling rig and a fixed wellhead on the sea bed.

A floating drilling rig is subject to more or less constant motion under the influence of wind and weather, resulting in variable distortion and stressing of the riser components.

After an unspecified time, the riser is bound to fail due to the effects of fatigue and corrosion and has to be replaced.

It is therefore important for the operator to know the useful life of a particular riser system with sufficient accuracy to be able to predict the point of failure and avoid unscheduled riser manipulations.

It is possible to predict the failure of a riser system from the study of its stress-cycles graph, but application to a production system is difficult due to the need for constant stress monitoring in the riser system.

If the riser shape is accurately known at any point in time, then it is possible to calculate the stresses at that time. Thus, by constantly monitoring the shape of a riser in a production system, continuous stress values may be obtained using real-time computation methods.

In the method of the invention as applied to a marine riser system, the line source is the riser, the supporting medium is water and the radiated wave field is the radiation field of a train of acoustic pulses introduced into the riser from an external source, for example by piezo-electric or electromechanical means. The characteristics of the input signal chain are controlled by the receiver output computer.

Preferably the apparatus further includes software packages for compensating for drill rig motion and consequent off-setting of the transducer array so as to give greater accuracy.

Further desirable adjuncts are a real-time software riser modelling package and a stress graph tracer.

The above and other features of the invention are embodied in the following description of the system, having reference to the accompanying informal drawings in which: Figure I is a diagrammatic representation of the components of the system; Figure 2 is a diagrammatic representation of the marine riser and the forces acting thereupon; Figure 3 is a diagram illustrating the principle of determining the incidence angle of a signal from a source of wave energy using a pair of transducers; Figure 4 is a diagram illustrating the principle of determining a second angle of incidence of a signal from a source wave energy and employing an. array of three transducers; Figure 5 is a diagrammatic representation of a typical equipment arrangement for the marine riser monitoring system.

Referring to Figure 1, the basic components of the system consist of a line source (not necessarily straight) of wave energy, a suitable array of transducers and a computer-based data analysis and storage system. The computer is also used to control an input transducer to externally excite the source.

In Figure 2, a marine riser is shown connected at its head to a drilling rig and at its lower end to the guide base of a wellhead.

The riser is subject to stresses from wind and sea as well as from its own weight and that of contained drilling fluid, as shown in the drawing.

Wind, current and vessel attitude are all more or less continuously changing, so that the shape of the riser and the consequent stresses are likewise variable.

Continuous monitoring of riser shape enables stress concentrations to be computed and plotted, so that imminent failure can be reasonable accurately predicted.

The principles of operation of a suitable transducer system for signal reception in the marine riser system are described below.

Referring to Figure 3, the relative phase of the wavefronts of the incident pulse carrier frequency (fo) between two transducers located close together is measured, leading to an angle of incidence Om of the pulse on the transducer base line.

8m = Cos1 R where A 8 e is the electrical phase difference Om is the mechanical incidence angle R = 2 z fo b/c b is the base line of the sensors and c is the speed of propagation of the wave energy in the transmitting mediums.

A similar determination performed on an orthogonal pair of transducers, as shown in Figure 4, yields another angle of incidence; the combination of these two angles giving spatial co-ordinates in terms of separation of the source from the receiver array.

Xs = (1- h Cos Omx Xs = (1 Cos2 Omx - Cos2 0my) h = h Cos f)my (1- Cos2 ûmx - Cos2 0my); In one embodiment of this invention, the source point is derived from the line source by a pulse or train of pulses introduced into the riser head by a suitable transducer and propagating down the riser.

The receiver array will receive a continuous signal as the pulse radiates from the riser during its passage to the wellhead. This signal is chopped electronically into pulse lengths, each of which represents the signal radiated by the input pulse at a particular position on the riser. Thus, the received pulses represent a linear array of source points distributed down the riser. The positions of these points on the riser are calculated from the phase speed of the pulse in the riser and the elapsed time since pulse initiation at the riser head. The separation of the source points is controlled by the electronic chopping network in the receiver instrumentation.

It is now possible to compute the spatial co-ordinates of each source point on the riser in terms of the vertical separation (h) between source and receiver array. This vertical separation may be adequately approximated by the distance of the source point down the riser; a valid approximation for moderate riser deflections, although severe deflections will lead to greater errors in the spatial co-ordinates of the source points.

The instrumentation required for this system comprises an array of transducers (typically contained within an area of 0.2m2) a digital phase-coherent receiver and a computer in which software packages are implemented to compensate for ship motion and transducer array offsets, to control the characteristics of the input pulse chain and to perform the selection of incoming pulse information. Some form of input transducer (piezo-electric or electro-mechanical in principle) will also be required for signal input Figure 5).

The accuracy of the system is in the order of 1% of the vertical separation (h).

The intensity of the initial pulse must be sufficient to permit sufficient radiation from the riser at all depths to overcome the sea-state noise. This requirement is, of course, influenced by the sensitivity of the hydrophone array.

The frequency content of the pulse or pulse train must be selected to optimise the radiation from the riser and the signal to noise ratio at the transducer array. The selection is made with reference to the sea-state noise spectrum.

In order to derive the stresses at any point in the riser, the shape information may be utilized in a laboratory simulation using an instrumented riser, or in a real-time soft-ware riser modelling package. The latter permits a continuous display of riser shape, stresses and fatigue life to be available on board the drill rig.

WHAT WE CLAIM IS: 1. A method for the determination of the shape or other properties of a line source of wave energy or of a medium in which said line source is supported, said line source radiating energy at a plurality of positions along its length, comprising the steps of detecting with transducer means said energy radiated at each of said plurality of positions, said transducer means being arranged to determine the angle of incidence of the wave energy from each said position, and computing the spatial coordinates of said positions on the line source from said angles of incidence.

2. A method as claimed in Claim 1, wherein wave energy is propagated along said line source as a pulse or train of pulses, and the output from said transducer means is chopped into time-separated pulses, each pulse representing wave energy from one of said positions.

3. A method as claimed in Claim 1, in which the radiated energy is produced by an externally applied source of excitation.

4. A method as claimed in any of the preceding claims, in which the line source is a leaky wave propagating member along which energy is propagated as a pulse or train of pulses.

5. A method according to Claim 4, applied to the determination of the shape of a marine riser, in which the radiated energy is the radiation field of a train of acoustic pulses introduced into the riser from an external source.

6. A method according to Claim 5, in which the characteristics of the input signal trains are controlled by a receiver output computer.

7. Apparatus for determining the shape or other properties of a line source of wave energy or of a medium in which said line source is supported, said line source being arranged to radiate energy at a plurality of positions along its length, transducer means arranged to receive energy from said plurality of positions and arranged to determine the angle of incidence of the wave energy from each said position, and means for computing the spatial coordinates of said positions on the line source from said angles of incidence.

8. Apparatus as claimed in Claim 7 including a phase coherent receiver connected to said transducer means.

9. Apparatus as claimed in Claim 7 for determining the shape of a marine riser, including software packages for compensating for drill rig motion and consequent off-setting of the transducer array.

10. Apparent as claimed in Claim 7 for determining the shape of a marine riser, including a real-time software riser modelling package and a stress graph tracer.

11. A method for the determination of the shape or other properties of a line source or of a medium in which said line source is supported substantially as described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. each of which represents the signal radiated by the input pulse at a particular position on the riser. Thus, the received pulses represent a linear array of source points distributed down the riser. The positions of these points on the riser are calculated from the phase speed of the pulse in the riser and the elapsed time since pulse initiation at the riser head. The separation of the source points is controlled by the electronic chopping network in the receiver instrumentation. It is now possible to compute the spatial co-ordinates of each source point on the riser in terms of the vertical separation (h) between source and receiver array. This vertical separation may be adequately approximated by the distance of the source point down the riser; a valid approximation for moderate riser deflections, although severe deflections will lead to greater errors in the spatial co-ordinates of the source points. The instrumentation required for this system comprises an array of transducers (typically contained within an area of 0.2m2) a digital phase-coherent receiver and a computer in which software packages are implemented to compensate for ship motion and transducer array offsets, to control the characteristics of the input pulse chain and to perform the selection of incoming pulse information. Some form of input transducer (piezo-electric or electro-mechanical in principle) will also be required for signal input Figure 5). The accuracy of the system is in the order of 1% of the vertical separation (h). The intensity of the initial pulse must be sufficient to permit sufficient radiation from the riser at all depths to overcome the sea-state noise. This requirement is, of course, influenced by the sensitivity of the hydrophone array. The frequency content of the pulse or pulse train must be selected to optimise the radiation from the riser and the signal to noise ratio at the transducer array. The selection is made with reference to the sea-state noise spectrum. In order to derive the stresses at any point in the riser, the shape information may be utilized in a laboratory simulation using an instrumented riser, or in a real-time soft-ware riser modelling package. The latter permits a continuous display of riser shape, stresses and fatigue life to be available on board the drill rig. WHAT WE CLAIM IS:

1. A method for the determination of the shape or other properties of a line source of wave energy or of a medium in which said line source is supported, said line source radiating energy at a plurality of positions along its length, comprising the steps of detecting with transducer means said energy radiated at each of said plurality of positions, said transducer means being arranged to determine the angle of incidence of the wave energy from each said position, and computing the spatial coordinates of said positions on the line source from said angles of incidence.

12. Apparatus for the determination of the shape or other properties of a line source or

of a medium in which said line source is supported substantially as described with reference to the accompanying drawings.