GB2221033A

GB2221033A - Method for measuring the natural vibrational frequency of a submarine pipeline free span

Info

Publication number: GB2221033A
Application number: GB8913745A
Authority: GB
Inventors: Richard John Holt
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1988-06-16
Filing date: 1989-06-15
Publication date: 1990-01-24
Also published as: GB8913745D0; GB8814336D0

Abstract

A method for measuring the natural vibrational frequency of a submarine pipeline free span involves using the thrust of a suitably positioned remotely operated vehicle (ROV) to excite the pipeline at its free span. The vibration response of the span is measured by an accelerometer (1) mounted on the ROV. The accelerometer (1) is positioned within a pressure housing (4) and is linked by cable (5) and receptacle (6) via the ROV umbilical to recording equipment on the surface. By measuring the natural frequency the susceptability of a pipeline free span to potentially damaging vibrations e.g. caused by vortex shedding, may be determined. The signal processing may include a correction to take account of the added mass of the ROV. <IMAGE>

Description

METHOD FOR MEASURING THE NATURAL FREQUENCY OF A SUBMARINE PIPELINE PE SPAN The invention relates to a method for measuring the natural frequency of submarine pipeline free spans.

In the developmeat of offshore oil and gas fields, submarine pipelines are used for many purposes. Small diameter pipelines, for example, are used as flowlines or gathering lines linking satellite wells or sussa complexes with production platforms. Subsea risers are pipes running between installations on the sea bed and the deck level of offshore platforms. Lager pipelines generally are used to transport crude oil or gas from offshore platforms to land based installations.

The pipelines may rest on the sea bed or be partly buried in trenches excavated out of the sea bed.

Depending on the geometry of the sea bed the pipelines may also rest on supports situated at intervals along their length.

In time erosion of the sea bed beneath the pipeline, for example through scouring caused bp the action of tidal currents, leads to discontinuous support of the pipeline and the formation of free spans.

This is potentiAlly also a problem with supported pipels whereby the pipeline may lose contact with the support.

It is well known that such unsupported lengths of subsea a pipeline are potentially liable to damaging vibrations as a result of vortex shedding induced by current action. Under these circumstances pulsating forces are applied. to the pipe spans in both the vertical direction (normal to the current flow) and the horizontal direction (parallel to the current flow). It is important that the natural frequencies of the pipe spans are remote fram these excitation frequencies if excessive amplitudes of span vibration are to be avoided.

Bp natural frequency we mean the frequency with which the pipe oscillates in the absence of external forces.

Vortex shedding can also lead to both high and low pressure variations on subsea risers which can set up potentially damaging vibrations in the structure particularly at the mid-span position. By span in respect of a riser we mean the distance between any two successive support guides.

It is therefore desirable to know the susceptibility of a submarine PiPeline span to vortex induced vibration in order to assess the stress levels placed an the pipe and the need for remedial action to eliminate the risk of the excessive vibration amplitudes than can be caused by such vortex shedding.

In determining the susceptibility of a pipeline free span to such vibration the following parameters are critical: 1. The current velocity normal to the pipe span.

2. The natural frequency of the pipe span.

3. The daztping of the pipe span.

The current velocity normal to the span is either well known or can be easily measured using available equipment.

At present the natural frequency of a pipe span is estimated by visually observing the unsupported length either by divers or during pipeline survey Using remotely operated vehicles (ROV's).

Visual procedures however are unsatisfactory.

The build up of algae, sand, silt, etc., around the pipeline makes it difficult to measure the span length acourately and also to determine the end conditions of the span. Depending on the environment of the seabed, the end conditions of a supported pipeline will be somewhere between fully supported and simply fixed. Fully supported pipelines are restrained against both vertical and rotational displacement at the ends while simply fined pipelines are restrained against only vertical displacement at the ends. These end conditions or end fixity have a substantial influence on the natural frequency.The level of vamping present affects the amplitude of vibration. A heavily damped line may be satisfactory at vortex shedding frequencies approaching the natural frequency while a lightly damped line may be subject to severe vibration.

It is also difficult to ascertain visually where the pipeline is supported, for example, by boulders on the sea-bed or trench bottom.

We have now discoered that by exciting the free span and measuring the vibration response the span natural frequency can be determined.

Thus according to the present invention there is provided a method of measuring the natural frequency of a submarine pipeline free span which method comprises the steps of: (a) esoiting the pipe at the free-span by Using the thrust of a suitably positioned ROV, (b) measuring the vibration response of the pipe span, and (c) analysing the vibration response to determine the natural frequency.

The excitation may be by the application of vertical thrust transients or a steady downthrust.

The downthurst supplied by the ROV is typically in the range 20 to 100% of the available ROV vertical thrust depending on the pipeline udder investigation.

The vibration response of the pipe span may be measured Using an accelerometer suitably mounted on the ROV.

The fundamental natural frequency is expected to be in the range 0.05 - 10 Hz depending on the end conditions and length of span.

Accelerometers are commercially available designed for very low frequency response ( < 20Hz). These are generally used for seismological measurements.

Suitable models for the present invention include the Sensonics SP3 accelerometer supplied by Sensonics Ltd.

The accelerometer may be mounted on the ROV on a vibration isolated mount in pressure proof pods to prevent both water borne and structural borne vibration from masking the vibration signals of interest.

Accelerometers nay be positioned on the ROV to record vibrations in both the vertical and horizontal direction.

The suspension system on ROV's during pipeline surveys is relatively soft to allow the vehicle to be driven over irregularities on the pipes surface.

In order to make reliable vibration measurements the ROV is employed with rigid feet which are jacked into place to give a rigid pipeline/vehicle contact.

Preferably two feet are deployed by nean5 of hydraulic cylinders.

A strain gauge load pin is fitted to ane of these cylinders to record the dynamic foroes applied by the ROV to the pipeline.

During vibration monitoring a continuous downthrust may be required to ensure the ROV remains in contact with the pipe span at all times.

The signal output from both the accelerometer and the strain gauge load pin is fed to the ROV electronics box and then via the ROV umbilical to the surface for recording on a spectrum analyser.

A suitable analyser is the Hewlett Packard 3582A spectrum analyser.

It is a further feature of the invention that the accelerometer and housing is mounted permanently on the ROV and causes no special requirements in the course of a pipeline survey.

The measured natural frequency is reduced below the actual span frequency as a result of the mass of the ROV an the span. In this context the mass of the ROV includes aided mass due to entrained and surrounding water.

This is particularly evident with smIl diameter pipelines.

To compensate for this when analysing the signal from the accelerometer a correction based on the theoretical influence of a point mass at mid span may be used.

Alternatively, plots of frequency against position of ROV on the span or against vibration amplitudes along the span may be made and ettrapolated to the span end or to zero amplitude respectively.

The invention nay be illustrated by the following operational procedure.

Prior to the survey, calculations are required to estimate the maximum permissible thrust level, the anticipated pipe span frequency and the appropriate ROV mass correction.

The ROV identifies a pipe span for investigation and locates the ends approximately using viral methods.

The ROV taees up position at mid span and the hydraulic jacks are deployed.

ROV thrust is set at the predetermined level and vibration monitoring equipment activated.

Vibration data are recorded for a set time or until well defined spectral peaks are obtained.

The natural frequency recorded is compared with the maximum investigation span frequency to assess whether a serious resonant response can oocur in the free span.

By maximum investigation frequency we mean the known vortex shedding frequency or excitation frequency for the pipe Udder investigation.

The amount of damping present may be determined bp measuring the rate of decay of oscillation. This is conveniently expressed as the logarithm decrement which is defined as the natural logarithm of the ratio of any two successive amplitudes.

By also measuring the span length the end fixity may be derived and the span characterised.

The invention is further illustrated with reference to the Figure which is a sectional drawing of the accelerometer housing arrangement.

The Sensonics SP3 accelerometer 1 is mounted on a steel mass 2 supported by rubber isolation mounts 3 within an air filled aluminium pressure housing 4. The accelerometer is linked by the shielded cable 5 throught the bulkhead receptable 6 to the ROV electronics box.

An in-line amplifier, if required, may be fitted inside the pressure housing.

The accelerometer housing arrangement is attached either vertically or horizontally on the ROV.

Claims

Claims:

1. A method of measuring the natural frequency of a submarine pipeline free span which method comprises the steps of: (a) exciting the pipe at the free span by using the thrust of a suitably positioned ROV, (b) measuring the vibration response of the pipe span, and (c) analysing the vibration response to determine the natural frequency.

2. A method according to claim 1 in which the thrust is by means of vertical thrust transients of the ROV.

3. A method according to claim 1 in which the thrust is by means of a steady downthrust of the ROV.

4. A method according to claim 1 in which the vibration response is measured by means of an accelerometer.

5. A method according to claim 4 in which the accelerometer is mounted on the ROV.

6. A method according to claim 1 in which the ROV is provided with rigid feet.

7. A method according to claim 6 in which the number of rigid feet is two.

8. A method according to claim 6 in which the rigid feet are deployed by means of hydraulic cylinders.

9. A method according to claim 6 in which a strain gauge load pin is fitted to one of the rigid feet.

10. A method for measuring the natural frequency of a submarine pipeline free span as hereinbefore described and with reference to the accompanying drawings.