WO1987007949A1

WO1987007949A1 - Detection system for subsea installations, particularly pipelines

Info

Publication number: WO1987007949A1
Application number: PCT/NO1987/000049
Authority: WO
Inventors: Petter M. Bakken
Original assignee: Stiftelsen For Industriell Og Teknisk Forskning Ve
Priority date: 1986-06-18
Filing date: 1987-06-16
Publication date: 1987-12-30
Also published as: NO862422D0; NO159507C; NO159507B; NO862422L

Abstract

A system for the detection of leakages from subsea installations, particularly from high-pressure pipelines used for the transport of gas. One or preferably more sonar receivers (15) are located at a considerable distance from a transmitter (14) in order to increase the detection certainty. The system preferably incorporates a circuit for the cross-correlation of the different signals which are received with the transmitted signal in order to detect any frequency changes and thereby determine the velocity and direction of movement of the reflecting objects. It is an advantage that a number of receivers (15) are alternately positioned in rows on either side of a pipeline (13) or similar structure.

Description

DETECTION SYSTEM FOR SUBSEA INSTALLATIONS, PARTICULARLY PIPELINES

The background of the invention:

The invention consists of a detection system for subsea installations, particularly pipelines, more specifically a system as described in the introduction to Claim of Patent 1. During the production of gas from underwater wells the gas is brought to the platform in pipes for processing. Following this, the gas is led in an underwater pipeline to where it is to be used. Since the gas is under considerable pressure whilst it is passing through the pipeline, any leakage poses a severe threat to the personnel working on the platform as well as the platform itself. As escaping gas will cause the scattering of sonar waves, this factor can be used for the purpose of detec¬ tion.

It is important to detect escaping gas as quickly as possible after a leakage, preferably before the gas has reached the surface. This enables the necessary contingency measures to be put into effect on the platform such as closing the emergency valves, stopping activities which could result in a gas explosion etc. On the other hand, it is important that detection systems should avoid giving false alarms about gas leakages as such contingency measures are usually costly to implement.

Today's detection systems based on sonar use a fixed acoustic transducer for the transmission of sonar waves and the detection of the waves which are reflected back from the gas ieakage where the same transducer functions as a receiver or - hydrophone. This method is almost identical with a depth sounder and other sonar systems used onboard ships, this is often called onostatic sonar or only sonar.

It is conceivable that the similarity between the sonar scattering from shoals of fish and that from a gas leakage could be confused and thereby cause false alarms.

The purpose of the invention:

The main purpose of the invention is to design an improved detection system which can operate with a greater degree of certainty than those presently used. This involves the detection of a gas leakage as early as possible, with a low risk of confusion with shoals of fish.

The principle of the invention: The invention is mainly based on the principle described in the characterizing part of Claim of Patent 1.

Although the consistent vertical velocity of petroleum gas vapour is an excellent means of distinguishing between gas and shoals of fish, detection systems which are based on monostatic sonar which are available today are unable to measure vertical velocity components. This is partly due to the location of the transmitter and the receiver in the same unit which necessitates the use of short sonar pulses, and partly because the sonar beam is horizontal and consequently at a right-angle to the direction the gas bubbles are moving.

The transmitter in the present system will typically generate a horizontal sonar beam which will cover the volume of water above the pipeline. The scatter caused by any escaping gas will be spread in different directions and this can be detected by receivers (hydrophones) which could be located on the seabed in a pattern along the pipeline, for instance. Such a configura¬ tion is termed multistatic sonar and is already known from a variety of contexts. One of the advantages of this arrangement is that given suitable geometries and sonar signals any alterations in the total distances between the transmitter - escaping gas, and the escaping gas - receiver can be detected through frequency changes in the sonar signal (Doppler shift in the signal's requency). If a gas leakage is registered from two, three or even more separate receivers, the velocity vector of the gas bubbles can be detected within a specified margin of error. This will constitute an effective means of distinguishing between shoals of fish and escaping gas and thereby reducing the chance of false alarms caused by fish shoals.

Another advantage of this invention is that the distance which sonar waves travel is reduced in relation to existing monostatic transmitters and receivers. This reduces the transmitter output level required in a sonar system by a factor of 10 - 100 for the geometries in question. This reduction in the transmitter output level has a positive effect upon the system's cost and reliability. A third advantage of this system is that by keeping the transmitter apart from the receiver, unintensional acoustic coupling between the transmitter and the receiver is reduced (in the absence of a gas leakage). Consequently, this configura¬ tion for a multistatic sonar system enables simultaneous transmission and reception, which allows one to select long duration sonar pulses or even continuous signals and thereby obtain a better frequency resolution and a more accurate determination of objects' velocity. The ease with which sonar signals are able to determine the location and velocity of objects is dependent upon what is known as the ambiguity function. Since the transmitter and receiver are separate, modern signal processing methods allow users considerable freedom in the choice of sonar signal components enabling signals to be adjusted to specific requirements. When high-pressure gas escapes from a pipeline acoustic interference will be generated underwater. This type of noise has most of its energy concentrated at frequencies which are much lower than those used in normal sonar equipment. Sound detection is a valuable means of augmenting the system described in Claims of Patent 1 and 2 and thereby improve the certainty with which gas leakages can be detected. Sound detection should be combined with means of determining the location of the noise source so as to reduce the risk o£ confusion with noise originating on the surface, such as propeller noise and other noise generated on the surface from platforms and shipping etc. The source of the noise can be localized by sound detection from different positions and the results compared by means of cross-correlation techniques. Modern technology uses digital signal processors to make the correlations once the sound detection data has been transformed into numerical sequences. This multistatic sonar configuration is ideal for combination with passive sound detection, since the same receiver elements can be employed with both systems. In the eventuality of the limited bandwidth of hydrophones making such a combination rather undesirable, most of the expenditure associated with such a combination can be covered by building two or more hydrophones together while producing the equipment thereby completing the costly installation in a single operation.

Example:

The invention will now be described in more detail with reference to the illustrations, where:

Fig. 1 shows a schematic vertical cross-section of a fixed platform which is connected to pipeline equipped with a detection system in accordance with the invention, while

Fig. 2 shows a horizontal cross-section of the configuration depicted in Fig. 1.

The figures show a fixed platform 11 of a known design which is intended for location on the seabed 12. An incoming or outgoing pipeline 13 is connected to the platform 11 for the delivery or offtake of gas under high pressure. In the example the pipeline 13 is schematically illustrated above the seabed 12, in reality the pipeline would be sunk into the seabed. A sonar transmitter 14 has been located at the base of the platform 11. This transmitter emits sonar waves in a horizontal beam with an initial angle of <___>>✓ in a lateral direction (Fig.l) and an initial angle of <5>_v in a horizontal direction (Fig. ) .

Rows of hydrophones 15 are positioned alternately at an equal distance on each side of the pipeline 13 to receive the sonar reflections H . _n(tζ), ln the example twenty such receivers 15 are located over a distance of about 500 metres, i.e., one every 25 metres.

The electronic circuitry required to process the signals is not illustrated. On the other hand, circuitry which is already known in principle can be utilized to carry out the signal processing necessary. The most practical solution will be the use of numerical sequences to represent the signals, followed by digital processing.

Claims

Claim of Patent:

1. A system for the detection of leakages from subsea installations, particularly from high-pressure pipelines used for the transport of gas, where there is a sonar transmitter (14) of acoustic sonar waves, is c h a r a c t e r i z e d by having one or more sonar receivers (15) located at a considerable distance from the transmitter.

2. A system in accordance with Claim of Patent 1 is c h a r a c t e r i z e d by the incorporation of a circuit for the cross-correlation of the different signals which are received with the transmitted signal, in order to detect any changes of frequency, thereby being able to determine the velocity and direction of movement of the reflecting objects.

3. A system in accordance with Claim of Patent 1 or 2 is c h a r a c t e r i z e d by a receiver for the transmitted sonar signals and a receiver for acoustic noise generated by a leakage being integrated into a single unit prior to instal¬ lation.

4. A system in accordance with one of Claims 1 to 3, where a number of sonar receivers are employed, is c h a r a c t e- r i z e d by the receivers (15) being alternately positioned in rows on either side of a pipeline (13).