GB1583552A

GB1583552A - Offshore pipe laying

Info

Publication number: GB1583552A
Application number: GB24770/76A
Authority: GB
Original assignee: Viking Jersey Equipment Ltd
Current assignee: Viking Jersey Equipment Ltd
Priority date: 1976-06-15
Filing date: 1976-06-15
Publication date: 1981-01-28
Also published as: ES470232A1; ZA773569B; SE7706857L; NL7706553A; DK263177A; ES459752A1; DE2726781A1; AU2607177A; JPS5326088A; BR7703854A; IT1115767B; FR2355231A1; NO772084L

Description

(54) IMPROVE MENTS IN OR RELATING TO OFFSHORE PIPE LAYING (71) We, VIKING JERSEY EQUIPMENT LIMITED, a British Company, of P.O. Box 72, Martins Chambers, St. Helier, Jersey, Channel Islands, FINN CHRISTIAN MICHELSEN, a Citizen of the United States of America, of 63 Rue de Moulin, La Hulpe, Belgium and WILLEM JAN TIMMERMANDS, a Citizen of The Netherlands, of 1011 Caspian Lane, Houston, Texas, 77090, United States of America, 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:- The present invention relates to offshore pipe laying. Usually, offshore pipe lines are laid by a pipe laying vessel. Sections of pipe are welded together on the vessel and the assembled pipe string is fed over the stern of the vessel into the water as the vessel moves forward. The pipe descends under its own weight to the sea bed and the portion of the pipe extending from the pipe laying vessel down to the point of contact with the sea bed typically has an S-shaped configuration. The upper bend of this S-shaped configuration, or "overbend", is usually supported to prevent excessive stresses in the pipe at this bend due to excessive curvature. Stingers and similar supporting apparatus are well known for this purpose. However, below the overbend, the pipe is often suspended without support; and the amount of curvature in the lower bend of the S-shaped configuration, or "sag bend", is controlled by maintaining tension in the pipe line. In another arrangement the end of the pipe string on the vessel may be supported vertically or at a relatively steep angle, so that there is no "overbend" and the pipe adopts a J-shaped configuration, with a sag bend only. In either arrangement, increased tension increases the length of suspended pipe, causing more pipe to lift off the sea bed, and reduces the curvature in the sag bend. If the tension is reduced too much, the sag bend curvature becomes excessive, eventually exceeding the stress limits of the pipe and causing a buckle or fracture.

The present invention is concerned more particularly with pipe laying including the step of monitoring one or more of the critical parameters of the suspended pipe length, with a view to reducing the risk of dmaging the pipe during laying operations. The suspended pipe parameters which are desirable for monitoring can be considered all to fall under the heading "Stress Parameters", since it is a knowledge of the stress or strain in the pipe walls that is necessary if the risk of damage to the pipe is to be minimized. However, it will be understood that a knowledge of pipe stress can be provided by pipe curvature values, bending moment and/or shear and axial forces in the pipe. Thus, when the term "suspended pipe stress parameters" is used hereinafter it should be understood to mean one or a combination of the parameters pipe curvature, bending moment shear force, and axial force.

A system for monitoring suspended pipe configuration during laying has been proposed in the article entitled "Air-Space Systems Aid Subsea Laying" in the 14th October 1974 edition of the periodical "The Oil & Gas Journal". In that article it is proposed to employ a computer to determine the suspended pipe configuration, departure angle of the pipe from the stinger, moments in the pipe and also forces and stresses along the pipe line between the point of touchdown on the sea bed up to and along the stinger system. The article goes into little detail, merely stating tht during laying operations information on water depth, pipe tension and water currents is required for the system. In order to calculate desired stress parameters in the suspended pipe, it has been found convenient to create a mathematical model of the suspended pipe line, which can be solved if certain boundary conditions are fixed. These boundary conditions determine what information describing the physical status of the suspended pipe is required to perform the necessary calculations so that pipe stress can be monitored. From the above referred published article, it appears that a knowledge of water depth and pipe tension are required to calculate stress in the suspended pipe. The article appears to assume that the sea bed is flat over the pipe laying region, so that a knoweldge of water depth for the region is sufficient. However, in practice such an assumption is not valid for typical pipe laying terrain. In practice, there is commonly a considerable distance, in the horizontal direction, between the position of the pipe laying vessel and the touchdown point of the pipe.

The water depth at the touchdown point can be significantly different from that at the laying vessel. Furthermore, the sea bed at the touchdown point is commonly not flat. It will be understood that the slope of the sea bed at the touchdown point is important since it defines the angle to the horizontal at which the pipe line rests on the bottom, and from which the suspended pipe must deviate in the sag bend. It has been found undesirable to assume that the sea bottom is flat when defining the boundary conditions for determining stress in the suspended pipe.

It is an object of the present invention to permit monitoring of stress parameters in variable depth conditions.

Accordingly, the present invention provides a method of laying an offshore pipe line with a pipe laying vessel including the step of monitoring at least one of the suspended pipe stress parameters, curvature, bending momeat, shear force and axial force, by sensing water depth at the location of the vessel, tension in the pipe at the vessel, and distance moved by the vessel along a path, storing water depth and distance moved data to provide historic values of water depth related to earlier positions of the vessel, calculating from such historic water depth values and the latest pipe tension values the or each said suspended pipe stress parameter and displaying the or each calculated parameter for monitoring.

By storing water depth and distance moved data as the vessel moves along over the sea bed, a record is provided of the water depth at various distances behind the vessel. This water depth record can then be used, in conjunction with pipe tension data to provide boundary conditions for a mathematical model used to define the suspended pipe. The model is then solved to calculate the required stress parameter or parameters.

An iterative procedures may be used in calculating the required stress parameter. A first estimation may be made of the distance behind the vessel of the touchdown point of the pipe on the sea bed. The water depth at this estimated touchdown point is then obtained from the stored data and the slope of the sea bed at this point in the direction of the pipe line is calculated from water depth values near this estimated touchdown point.

These values of water depth and bottom slope are then used to fix boundary conditions in the mathematical model defining the suspended pipe and the model is solved to provide amongst other results a more accurate position of the touchdown point. The iteration step described above may then be repeated employing this new touchdown point. How ever, it has been found that a sufficiently good first estimate of the touchdown point can be made to make further iterations after the first unnecessary.

The stress parameters which is calculated in performing the invention may be the radius of curvature of the suspended pipe length at a selected distance along the pipe from the laying vessel. However, it is preferable to calculate the curvature of the pipe at several points along the suspended length to provide an overall picture of the pipe configuration.

Clearly, pipe curvature along the suspended length is related to stresses in the pipe and the knowledge of pipe curvature can be employed to determine whether the pipe line is at risk.

However, values of actual stresses in the pipe over the suspended length can be calculated provided that characteristics, such as pipe diameter, wall thickness, etc. of the pipe being laid are known.

In accordance with a further aspect of the present invention, a pipe laying vessel has apparatus for monitoring at least one of the suspended pipe stress parameters, curvature, bending moment, shear force and axial force, during laying of an offshore pipe line, the apparatus comprising means for sensing water depth at the location of the vessel, means for determining tension in the pipe at the vessel, means for determining the distance moved by the vessel along a path, means for storing water depth and distance moved data as determined by the water depth sensing means and the distance moved determining means to provide a record of historic values of water depth related to earlier positions of the vessel and for calculating, from such historic water depth values and the latest pipe tension values as determined by the pipe tension sensing means, the or each suspended pipe stress parameter, and means for displaying the or each calculated parameter for monitoring.

The method and pipe laying vessel of the present invention as heretofore described permits only the calculation of pipe stress parameters in the vertical plane containing the path of travel of the vessel. If the vessel is moving in a straight line, laying a straight line pipe, and there is no or negligible water current in the transverse direction, it can be expected that lateral deflection of the suspended pipe line between the vessel and the sea bottom will be minimal and can be ignored. However, when the vessel is laying pipe along a curved track, or there are significant cross currents which can deflect the suspended pipe line, it may be convenient to determine the configuration of the pipe in the horizontal plane. Furthermore, situations may arise in pipe laying whereby the laying vessel is orientated at a small angle relative to the laid pipe line, although perhaps still laying pipe along the straight track. Such an orientation requires the suspended pipe between the vessel and the sea bottom to curve in the horizontal plane, thereby causing horizontal stress components in the pipe.

Thus, the method of the present invention preferably includes the steps of determining the position in the horizontal plane of the laying vessel relative to the pipe line already laid on these a bed and the difference in bearing between the laying vessel and the pipe line already laid on the sea bed. In the absence of significant transverse water currents, the data provided by these further steps permits a mathematical model of the suspended pipe length to be solved in three dimensions. The components in the horizontal plane of the parameters curvature, bending moment and shear stress can be calculated.

The method may include the still further step of sensing the component of water current -transverse relative to the heading of the laying vessel and, preferably, also the component parallel to the heading. Then, where the drag coefficient of the pipe being laid is known, the lateral force of the pipe line produced by the water current can be considered in solving the mathematical model of the suspended pipe.

Thus, the pipe laying vessel of the present invention preferably includes means for sensing the position of the vessel relative to the pipe line already laid on the sea bed and means for sensing the difference between the bearing of the vessel and that of the already laid pipe line. The means for storing and calculating may then further be adapted for solving a mathematical model of the suspended pipe line in three dimensions. Conveniently also, means are provided for sensing water current, at least transversely of the heading of the vessel, and preferably also parallel to the heading of the vessel.

The preferred method of the present invention further envisages determining movement of the laying vessel in response to wave motion and calculating the dynamic response of the the suspended pipe line to such wave response motion of the vessel. The wave response motion of the vessel may be determined by determining the wave spectrum in the surrounding sea, calculating the amplitudes of a combination of discrete regular waves of different frequencies providing the equivalent total energy of the wave spectrum and determining the vessel motion response to each of said discrete regular wave frequencies.

Thus, the spectrum of the motion of the vessel may be determined from the measured wave spectrum in the surrounding sea and the response amplitude operators of the vessel.

From the - motion spectra, a computer calculates the spectra of dynamic stresses in the suspended pipe. By using wave forecasting, anticipated future stresses can also be analysed.

When terminating a pipe laying operation, either at the end of the pipe line or temporarily during excessively heavy weather conditions, it is normal to abandon the pipe assembled so far by attaching a cable to the end of the line on the vessel and then lowering the end to the sea bed. It will be understood that, as for normal pipe laying, it is important to ensure that sufficient tension is maintained in the suspended portion of pipe between the touchdown point and the junction with the cable to prevent excessive curvature in the pipe. The method of the present invention may be used during this pipe abandonment procedure to monitor the status of the suspended portion of pipe. Furthermore, the method may be used similarly when the abandoned pipe is recovered for further pipe laying.

A better understanding of the invention may be had from the following description of an example of the invention, which is made with reference to the accompanying drawings in which; Figure 1 is a hybrid schematic representation giving two views of a pipe laying vessel fitted with pipe monitoring apparatus, together with a block schematic illustrating the general logic flow in the monitoring system; and Figure 2 is a block schematic diagram of the processing hardware of the monitoring system.

Considering firstly the Figure 1, a plan view 10 and an elevational view 20 are shown of a pipe laying vessel fitted with monitoring apparatus. The vessel is essentially a semisubmersible barge comprising a main deck 21 supported by columns 23 on a pair of spaced apart floaters 22, of which only the nearer is shown in view 20. During pipe laying operations, the buoyancy of the barge is controlled so that the sea level is at least half way up the columns 23. Then, the major submerged bulk of the vessel is below the region of wave turbulence in the sea, so that the response of the vessel to wave motion is greatly reduced.

It will be understood that it is important for pipe laying operations that the pipe laying vessel provides a stable platform on which sections of pipe can be welded together and fed over the stern.

In the vessel of Figure 1, pipe sections are welded together along a centre line 11 of the vessel. Each time a new pipe length is welded to the assembled pipe line, the vessel is moved forward by an appropriate amount and the pipe line is permitted to slide backwards off the vessel substantially by the length of the added section. In order to prevent damage to the pipe line over its suspended length between the vessel and the sea bottom, the pipe line is held under tension by tensioners 12.

The vessel itself is held in position relative to the sea bottom by a series of anchors. Moor ing systems for pipe laying vessels are well known in the art. One or more tensioners 12 may be provided and they are arranged to let pipe out rearwardly along the vessel, or pull pipe in as appropriate in order to keep the tension in the pipe between set limits. Thus, if pipe tension increases to the upper limit, the tensioner will allow the pipe line to move rearwards relative to the vessel until the tension falls back between the limits. Conversely, if the tension falls below the lower limit, the tensioner will pull in pipe onto the vessel to bring the tension back up. If the mooring system of the vessel is controlled correctly to keep the vessel stationary, little or no movement of the pipe relative to the vessel takes place, which is important when welding new sections onto the line.

The vessel shown in Figure 1 is fitted with a ramp 13 extending rearwardly of the vessel.

The ramp 13 is a rigid structure designed to support the pipe line as it leaves the vessel in the overbend. Structures for this purpose are well known in the art. After leaving the ramp 13, the pipe line is freely suspended in the water until it touches the sea bed. Between the vessel and the sea bed, the pipe has an Sshaped configuration, the upper curve of the S, or "overbend" being supported by the stern ramp 13 and the lower curve, or "sag bend", being unsupported until the pipe touches the sea bed. As explained previously herein, the present invention is concerned particularly with monitoring, during pipe laying, stress parameters in the suspended length of the pipe. By monitoring the stress parameters in the pipe during the laying operation, corrective action can be taken to alleviate stress in the suspended pipe line before the possibility of damaging the pipe line.

As explained, the essential steps for monitoring pipe stress parameters are to sense the distance moved forward by the vessel 10 simultaneously with the water depth at the vessel and also pipe tension. The water depth values are stored together with distance of movement values for the vessel, thereby providing a record of water depth at previous positions of the vessel. Such a record of the water depth behind the vessel enables stress parameters of the suspended pipe to be calculated with greater accuracy. Especially in deep water, the touchdown point of the pipe on the sea bed can be a considerable distance behind the pipe laying vessel. Thus, unless the sea bed is assumed to be flat, the water depth at the touchdown point is normally different from the water depth at the location of the vessel. The record of historic depths behind the vessel permits a more accurate estimation of the water depth at the touchdown point.

Furthermore, it will be appreciated that the angle to the horizontal at which the pipe rests when on the bottom prvides an important boundary condition for accurately determining stress parameters in the suspended pipe length. From the record of water depths, an estimation can be made of the bottom slope at the pipe touchdown point, thereby providing an indication of the angle to the horizontal of the pipe on the bottom.

The right hand side of Figure 1 is a block schematic diagram illustrating the overall logic flow in this described example of the invention. The first column of boxes in the drawing, identified by the vertical arrow 30, lists the various sources of data which are used for calculating pipe stress parameters.

The top box 31 in column 30 is identified "horizontal position reference system". In the specific example, the pipe laying vessel is provided with a navigational system which accurately determines and plots the position of the vessel. Thus, from this horizontal position reference system 31, data is provided defining the longitude and latitude of the vessel (boxes 61 and 62 in the drawing).

Also, the horizontal position reference system 31 provides a source of data describing the water depth beneath the vessel. A depth sounder 24 provides water depth information to the horizontal position reference system 31 and this water depth data is supplied to the present monitoring system (see box 63).

The water depth readings provided by the horizontal position reference system 31 are stored. In the present example, the depth under the barge is stored after each movement of the barge by a predetermined distance. A total of 50 depths may be stored. Thus, if the predetermined distance increment is set at 24 metres, depths for a distance of 200 metres behind the barge are stored.

From this record of water depths, the water depth at the touchdown point of the pipe on the sea bed is estimated in amanner which will be described later. Also, the bottom slope at the touchdown point is estimated. The water depth at touchdown point (box 64) and bottom slope at touchdown point (box 65) are employed, as will be explained, in calculating stress parameters of the suspended pipe.

Various other data is available from the horizontal position reference system 31, i.e.

the length of each section of pipe welded to the pipe line (joint length 66), the total length of pipe launched in the pipe run (pipe length 67), the number of the latest assembled joint between a new pipe section and the assembled pipe line (joint number 68), the the heading angle of the pipe laying vessel (bearing of barge 69). Some of this extra data is used in making more accurate calculations of pipe stream parameters, or for confirmatory purposes as will become apparent later.

Other data provided from source on the vessel comprise a second reading of barge bearing 70, directly from a gyro compass 32; pipe tension as measured at a load cell 33 at the pipe tensioners 12 (tension at tensioner 71); the length of pipe launched as measured by a pipe footage counter 34 at the tensioner 12 (pipe length 72); draught measurement at each of the four corners of the pipe laying vessel (box 73) as provided by pressure gauges 35 at each of the four corners of the barge; water current relative to the vessel in two directions at right angles (boxes 74 and 75) as measured by a fixed current meter 36 (and 25) beneath the barge; the time and date (box 76) as provided by a timer 37; horizontal and radial reaction forces (boxes 77 and 78) at the rollers supporting the pipe line along the launching ramp 13, as provided by load cells 38 (and 26); the mean wave period 79 and significant wave height 80 as provided by a waverider buoy 39.

Further data is provided for use in monitoring pipe stress parameters during abandonment or recovery of a pipe from the sea bed. Thus, the tension in the cable from the abandonment /recovery winch (A/R cable tension 81) is provided by a lod cell 40 at the AIR winch and the footage of cable wound from the drum of the A/R winch (A/R cable footage 82) is provided by a footage counter 41, also at the A/R winch.

Finally, data regarding the pitch 83 and heave 84 actions of the pipe laying vessel in response to wave motion are provided from accelerometers 42 and 43.

The draugh measurements 73 are employed to calculate the angle of trim 85 of the vessel, i.e. any stern down or bow down attitude, and also the freeboard 86, which data is also used in calculating pipe stress parameters.

All the data 66 to 85 is provided by sensors or sources on board the vessel so that this data represents up to date measured values, which may change as the vessel moves. However, certain further data is required for making accurate calculations of pipe stress.

Such extra data includes details of pipe parameters (box 90), i.e. pipe constants such as physical dimensions, Young's modulus for the material of the pipe, and the specific weights of the pipe material and any concrete cladding, etc. Also data defining various fixed parameters of the laying vessel (barge parameters 91) may be provided together with data defining the manner in which stress parameters are to be calculated and how displayed (control parameters 92).

Further data may be provided for calculating pipe stress parameters, for which there is a requirement to change the data only occasionally during the pipe laying operation. Such data may be altered manually and includes further control parameters 93, i.e. altering calculation or display functions, wave direction 94, i.e. the angle relative to the barge heading from which the waves appear to be coming, first joint number of new pipe quality 95, i.e. the number of the junction between two pipe sections at which a different sort of pipe begins, positions of bending moment spectra 96 (this will be explained later), current profile 97, i.e. the manner in which water current varies with depth (this may be determined from a current meter suspended below the surface on a cable but the values are input manually), and finally last joint number before A/R head 98 (this relates to calculating stress parameters during abondonment or recovery of the pipe).

In Figure 1, all the data provided and described above is fed to a block 50 indicating the calculating or computing logic flow. The data is all stored in input tables 101. Data is drawn from these tables for use in static analysis computation 102, where the configuration of the suspended pipe is calculated ignoring any dynamic effects produced by movement of the pipe laying vessel resulting from wave motion. Amongst other parameters the static analysis 102 calculates the geographical co-ordinates of the touchdown point of the pipe on the sea bed and these historic touchdown point co-ordinates 103 are also stored in the input tables 101 use in subsequent analysis. In one option the results of the static analysis of the pipe are fed directly to various display devices, i.e. printer 104, graphics display terminal 105 and hard copy unit 106. Instead of directly displaying the results of the static analysis 102, these results may be used in performing a further analysis (dynamic analysis 107) during which the effects on the suspended pipe length of movement of the barge resulting from wave motion is calculated. The results of this dynamic analysis are then displayed.

Instead of performing the static analysis 102, when abandoning or recovering a pipe from the sea bed a pipe abandonment and recovery analysis 108 may be performed to determine the configuration of the pipe suspended from the A/R winch cable. In this case, the results of analysis 108 are displayed.

All the data held in the input tables 101 may be logged (data logging 109). The log data may then be used at a later date to set the boundary conditions for analysis of an historic condition.

In the present example of the invention, the calculating of the pipe stress parameters is performed by a digital computer. Also, the storing of historic water depth values and other data for use in the calculations is also done by the computer. During pipe laying operations, the computer operates in "real time mode" regularly updating the data in its store in accordance with the values provided from the various sensors on the laying vessel.

The complete analysis of the suspended pipe does take a finite length of time, but it has been found possible to complete an analysis within ten minutes, thus enabling presentation of latest pipe stress parameters every ten minutes during laying.

Referring to Figure 2, a block schematic diagram is shown of a hardware configuration of this embodiment of the invention. Two computers 201 and 202 are provided, the control panels of these computers being designated. Computer 202 operates in a slave mode to computer 201. The various data provided by sensors on the pipe laying vessel and by the horizontal position reference system is supplied to computer 201. This computer regularly scans the input data, scaling it and filtering it as necessary and carries out the initial preparation of the data to a form suitable for analysing the suspended pipe. The fully prepared data is tabulated in computer 201. This preparing of data for analysis includes the step of choosing from the stored historic water depth values a depth value defining the water depth at the touchdown point of the pipe. This step involves estimating the distance behind the vessel of the touchdown point. This estimation is performed in computer 201 by calculating the distance behind the vessel of the point at which a tangent from the tip of the ramp 13 intersccts a horizontal plane at the same depth as the water under the vessel, and adding to this distance a predetermined amount.

This predetermined additional amount is selected to be suitable depending on the average conditions expected during the pipe laying operation and on the parameters of the pipe being held. This first estimate of the distance of the touchdown behind the vessel is used to identify the water depth at the touchdown point as chosen from the store of water depths at previous positions of the vessel. Further the water depth values either side of this estimated touchdown point are used to define the bottom slope at the estimated touchdown point. These water depths and bottom slope values are subsequently used in analyzing the configuration of the suspended pipe. From this analysis, a more accurate position for the touchdown point of the pipe is calculated. It is then possible to employ this new position of the touchdown point to identify a more accurate water depth at the touchdown point and bottom slope and then to repeat the pipe analysis. However, it has been found that the first estimate of the touchdown point can be made with sufficient accuracy to make a further iteration of the analysis unnecessary in practice.

Referring again to Figure 2, the prepared data tabulated in computer 201 is transferred to computer 202 wherein it is employed to define the boundary conditions of an analysis programme. The method of analyzing the configuration of this suspended pipe involves creating a mathematical model of the suspended pipe. The suspended portion of the pipe can be considered distance behind the vessel as abscissa, of the configuration of the suspended portion of the pipe both in the vertical and the horizontal plane, and also a graphical representation of the bending moment, shear force, axial force or equivalent stress along the length of the suspended pipe. The equivalent stress is a representation of the total stress at a position along the pipe resulting from all the loadings on the pipe at that point, Several display options are provided and the particular parameter to be displayed can be selected at a keyboard. The hard copy unit 210 provides, when required, a printed copy of the graphical display on the unit 209. The operator's control panel 211 is primarily a key panel enabling an operator to control the operation of the computer and the various display functions.

When a full analysis has been completed in computer 202 on a set of data defining the latest boundary conditions for the suspended pipe, a further analysis may be performed to determine the effect on the suspended pipe of barge movement due to wave motion. On completion of the static analysis, the calculated figures defining the configuration and stress parameters of the pipe are transferred from computer 202 back to computer 201.

Then, the computer 202 is prepared for performing the dynamic analysis, considering wave motion effects, by loading the computer store with the dynamic analysis programme from the flexible disc store unit 207. The computer 202 then performs the fresh analysis using the results of the first static analysis and using also the sensed data defining the mean wave period and significant wave height of the surrounding sea. In the analysis, this wave data is used to calculate theoretical barge response figures using response operators developed in tank model tests of the vessel. From the theoretical calculated movement of the vessel, the effect on the suspended pipe length is calculated and the values of significant stress and bending moment produced in the pipe as a result of wave motion are computed. Once again the computed results of the dynamic analysis can be displayed graphically by the display unit 209.

It will be noted from the above that the dynamic analysis programme does not employ direct readings of the pitch and heave of the pipe laying vessel in order to calculate dynamic pipe response. However, signals representing pitch and heave are supplied, as indicated in Figure 1, but are merely available for logging. These logged actual pitch and heave values may be compared with the theoretically calculated response of the vessel and used to improve the response operators employed in the theoretical response calculations.

In the above described example, apart from calculating stress parameters along the suspended length of pipe between where it departs from the ramp of the vessel and the touchdown point, the analysis also calculates the configuration and stress parameters in the pipe supported by the stern ramp in the overbend up to the deck of the laying vessel and also for a certain distance beyond the touchdown point on the sea bed. The calculation in the overbend is done assuming that the rollers on which the pipe rests along the stern ramp are positioned in certain predetermined places.

Theoretical values of load on these stern ramp rollers are also calculated and may be displayed graphically. The actual load on the stern ramp rollers is available from stern ramp load cells 26 (see Figure 1) and these measured values are provided as input to the computer system. Thus, the theoretical values of the load and the measured values can be compared and, preferably, the difference between measured and theoretical values are calculated and displayed. It will be appreciated that if one or more of the stern ramp rollers is set slightly out of position, the loading on that roller will be different from the theoretical value. Thus, the difference between measured and theoretical load values provides an indication of wrong setting of the stern ramp rollers enabling the setting to be corrected before a possibility of accidental damage either to the pipe or to the roller bearings.

A further mode of operation of the arrangement is that referred to hereinbefore as abandonment/recovery. This procedure involves lowering the end of the pipe line to the sea bed by a cable from an abandonment/ recovery winch, whilst maintaining sufficient tension in the portion of the pipe still suspended to prevent damage. When abandoning or recovering pipe the stress parameters in the suspended pipe are monitored by loading the computer 202 with a further programme stored in the flexible disc store unit 207 specifically designed to calculate the configuration of the pipe and cable between the vessel and the sea bed. Apart from data already used in the previously referred static and dynamic analysis programmes, the abandonment and recovery programme operates on two of the following three variables: length of abandonment recovery cable paid out, relative movement of the vessel over the sea bed and, of course, tension in the cable.

From any two of these variables, the programme calculates, using an iterative technique, the configuration of the suspended pipe and cable, including the depth of the connection between pipe and cable and moments, tension, stress and shear in the suspended part of the pipe. The tension along the cable is also calculated.

WHAT WE CLAIM IS: 1. A method of laying an offshore pipe

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. distance behind the vessel as abscissa, of the configuration of the suspended portion of the pipe both in the vertical and the horizontal plane, and also a graphical representation of the bending moment, shear force, axial force or equivalent stress along the length of the suspended pipe. The equivalent stress is a representation of the total stress at a position along the pipe resulting from all the loadings on the pipe at that point, Several display options are provided and the particular parameter to be displayed can be selected at a keyboard. The hard copy unit 210 provides, when required, a printed copy of the graphical display on the unit 209. The operator's control panel 211 is primarily a key panel enabling an operator to control the operation of the computer and the various display functions. When a full analysis has been completed in computer 202 on a set of data defining the latest boundary conditions for the suspended pipe, a further analysis may be performed to determine the effect on the suspended pipe of barge movement due to wave motion. On completion of the static analysis, the calculated figures defining the configuration and stress parameters of the pipe are transferred from computer 202 back to computer 201. Then, the computer 202 is prepared for performing the dynamic analysis, considering wave motion effects, by loading the computer store with the dynamic analysis programme from the flexible disc store unit 207. The computer 202 then performs the fresh analysis using the results of the first static analysis and using also the sensed data defining the mean wave period and significant wave height of the surrounding sea. In the analysis, this wave data is used to calculate theoretical barge response figures using response operators developed in tank model tests of the vessel. From the theoretical calculated movement of the vessel, the effect on the suspended pipe length is calculated and the values of significant stress and bending moment produced in the pipe as a result of wave motion are computed. Once again the computed results of the dynamic analysis can be displayed graphically by the display unit 209. It will be noted from the above that the dynamic analysis programme does not employ direct readings of the pitch and heave of the pipe laying vessel in order to calculate dynamic pipe response. However, signals representing pitch and heave are supplied, as indicated in Figure 1, but are merely available for logging. These logged actual pitch and heave values may be compared with the theoretically calculated response of the vessel and used to improve the response operators employed in the theoretical response calculations. In the above described example, apart from calculating stress parameters along the suspended length of pipe between where it departs from the ramp of the vessel and the touchdown point, the analysis also calculates the configuration and stress parameters in the pipe supported by the stern ramp in the overbend up to the deck of the laying vessel and also for a certain distance beyond the touchdown point on the sea bed. The calculation in the overbend is done assuming that the rollers on which the pipe rests along the stern ramp are positioned in certain predetermined places. Theoretical values of load on these stern ramp rollers are also calculated and may be displayed graphically. The actual load on the stern ramp rollers is available from stern ramp load cells 26 (see Figure 1) and these measured values are provided as input to the computer system. Thus, the theoretical values of the load and the measured values can be compared and, preferably, the difference between measured and theoretical values are calculated and displayed. It will be appreciated that if one or more of the stern ramp rollers is set slightly out of position, the loading on that roller will be different from the theoretical value. Thus, the difference between measured and theoretical load values provides an indication of wrong setting of the stern ramp rollers enabling the setting to be corrected before a possibility of accidental damage either to the pipe or to the roller bearings. A further mode of operation of the arrangement is that referred to hereinbefore as abandonment/recovery. This procedure involves lowering the end of the pipe line to the sea bed by a cable from an abandonment/ recovery winch, whilst maintaining sufficient tension in the portion of the pipe still suspended to prevent damage. When abandoning or recovering pipe the stress parameters in the suspended pipe are monitored by loading the computer 202 with a further programme stored in the flexible disc store unit 207 specifically designed to calculate the configuration of the pipe and cable between the vessel and the sea bed. Apart from data already used in the previously referred static and dynamic analysis programmes, the abandonment and recovery programme operates on two of the following three variables: length of abandonment recovery cable paid out, relative movement of the vessel over the sea bed and, of course, tension in the cable. From any two of these variables, the programme calculates, using an iterative technique, the configuration of the suspended pipe and cable, including the depth of the connection between pipe and cable and moments, tension, stress and shear in the suspended part of the pipe. The tension along the cable is also calculated. WHAT WE CLAIM IS:

1. A method of laying an offshore pipe

line with a pipe laying vessel including the step of monitoring at least one of the suspended pipe stress parameters, curvature, bending moment, shear force and axial force, by sensing water depth at the location of the vessel, tension in the pipe at the vessel, and distance moved by the vessel along a path, storing water depth and distance moved data to provide historic values of water depth related to earlier positions of the vessel, calculating from such historic water depth values and the latest pipe tension values the or each said suspended pipe stress parameter and displaying the or each calculated parameter for monitoring.

2. A method as claimed in claim 1 wherein the calculating step is performed by setting up a mathematical model defining the suspended pipe, using the historic water depth values and the latest pipe tension values to provide boundary conditions for the mathematical model and solving the model to calculate the required stress parameter or parameters.

3. A method as claimed in claim 2 wherein an iterative procedure is used in calculating the required stress parameter or parameters, comprising making a first estimation of the distance behind the vessel of the touchdown point of the pipe on the sea bed, obtaining the water depth at this estimated touchdown point from the store of historic water depth values, calculating from the stored historic water depth values near this estimated touchdown point, the slope of the sea bed at this point in the direction of the pipe line, using the values of water depth and bottom slope at the estimated touchdown point, together with the pipe tension values, as boundary conditions in the mathematical model and solving the model to provide a more accurate position of the touchdown point for use in further iterations of the procedure.

4. A method as claimed in claim 3 wherein the first estimation of the touchdown point position is sufficiently accurate to make further iterations after the first unnecessary.

5. A method as claimed in claim 3 or claim 4, when the pipe laying vessel has a rigid ramp for supporting the pipe in the overbend, wherein the first estimation of the touchdown point position is performed by calculating the distance behind the vessel of the point at which a tangent from the tip of the rigid ramp intersects a horizontal plane at the same depth as the water under the vessel, and adding to this distance a predetermined amount.

6. A method as claimed in any preceding claim wherein the curvature of the pipe is calculated at several points along the suspended length of the pipe.

7. A method as claimed in any preceding claim including the steps of determining the position in the horizontal plane of the laying vessel relative to the pipe line already laid on the sea bed and the difference in heading angle between the laying vessel and the pipe line already laid on the sea bed, and using this further information to perform said calculating step in three dimensions.

8. A method as claimed in claim 7 including sensing the component of water current transverse relative to the heading of the laying vessel and using this transverse water current value to modify the calculating step, the drag coefficient of the pipe being laid being known.

9. A method as claimed in claim 8 further including sensing also the longitudinal water current component relative to the vessel and using also this longitudinal value to modify the calculating step.

10. A method as claimed in either of claims 8 or 9 and including determining the profile of the variation of water current with depth and modifying the calculating step with this profile.

11. A method as claimed in any preceding claim and including the further steps of determining movement of the laying vessel in response to wave motion and calculating the dynamic response of the suspended pipe line to such wave response motion of the vessel.

12. A method as claimed in claim 11 wherein the wave response motion of the vessel is determined by determining the wave frequency spectrum in the surrounding sea, calculating the amplitudes of a combination of discrete regular waves of different frequencies providing the equivalent total energy of the wave spectrum and determining the vessel motion response to each of said discrete regular wave frequencies.

13. A method as claimed in any preceding claim and including the steps of abandoning or recovering the end of the pipe line at the end or beginning respectively of a pipe laying operation, by attaching a cable from the vessel to the end of the pipe line and lowering or raising, as the case may be, the end to or from the sea bed, keeping the cable under tension to avoid damaging the still suspended length of pipe.

14. A method as claimed in claim 13 including determining during the abandonment or recovery operation, any two of the parameters, (a) the tension in the cable, (b) displacement of the vessel relative to the sea bed, and (c) length of cable paid out, and calculating from said two parameters, together with stored historic values of water depth, the or each said pipe stress parameter.

15. A method as claimed in any preceding claim wherein the pipe laying vessel has a rigid ramp with rollers for supporting the pipe at positions along its length in the overbend and including the step of calculating the or each pipe stress parameter for the portion of the pipe in the overbend using as boundary conditions predetermined ideal positions for the rollers.

16. A method as claimed in claim 15 wherein the predetermined ideal roller positions are theoretically ideal positions for minimum loading of the rollers by the pipe.

17. A method as claimed in claim 16 wherein the step of calculating the or each pipe stress parameter for the pipe portion in the overbend includes calculating the theoretical loading of the rollers in said ideal positions, and the method further includes sensing the actual loading of the rollers, for comparing the actual loading with the theoretical loading to ascertain when rollers are wrongly positioned.

18. A pipe laying vessel having apparatus for monitoring at least one of the suspended pipe stress parameters, curvature, bending moment, shear force and axial force, during laying of an offshore pipe line, the apparatus comprising means for sensing water depth at the location of the vessel, means for determining tension in the pipe at the vessel, means for determining the distance moved by the vessel along a path, means for storing water depth and distance moved data as determined by the water depth sensing means and the distance moved determining means to provide a record of historic values of water depth related to earlier positions of the vessel and for calculating, from such historic water depth values and the latest pipe tension values as determined by the pipe tension sensing means, the or each suspended pipe stress parameter, and means for displaying the or each calculated parameter for monitoring.

19. A pipe laying vessel as claimed in claim 18, wherein said means for storing and for calculating is a digital computer with digital storage means, the computer being programmed to operate in "real time" mode during pipe laying operations and connected to receive up-to-date water depth, distance moved and pipe tension data from said sensing and determining means, and operative to store at least the water depth and distance moved data in the storage means.

20. A pipe laying vessel as claimed in claim 19 wherein the digital computer is programmed to set up a mathematical model defining the suspended pipe, to use the stored historic water depth values related to earlier vessel positions and the latest pipe tension values as boundary conditions in the mathematical model and to solve the model to calculate the required stress parameter or parameters.

21. A pipe laying vessel as claimed in any of claims 18 to 20, wherein the means for storing and for calculating is arranged to calculate the or each stress parameter at several points along the length of the supended pipe.

22. A pipe laying vessel as claimed in claim 21 wherein the means for idsplaying the or each calculated parameter comprises a graphical display unit providing a graphical display of the or each parameter against the horizontal distance behind the vessel as abscissa.

23. A pipe laying vessel as claimed in any of claims 18 to 22 and having means for determining the geographical location and heading angle of the vessel, and the means for storing and for calculating are arranged to calculate at successive intervals the geographical position of the touchdown point of the pipe on the sea bed and to store the calculated touchdown point positions, and therefrom to calculate the difference in heading angle between the vessel and the pipe line already laid on the sea bed, and further to employ the calculated heading angle difference values together with relatife positions of the vessel and the touchdown points to calculate the or each pipe stress parameter in three dimensions.

24. A pipe laying vessel as claimed in claim 23 and having means for sensing at least the transverse component of water current relative to the heading of the vessel, the means for storing and for calculating being arranged to modify the three dimensional calculation in view of at least said transverse component.

25. A pipe laying vessel as claimed in any of claims 18 to 24 and in combination with means for determining data relating to the wave motion in the surrounding sea, the means for storing and for calculating being further arranged to calculate from said wave motion data the wave response motion of the vessel and further to calculate the dynamic response of the suspended pipe line to said vessel wave response motion.

26. A pipe laying vessel as claimed in any of claims 18 to 25 and having a rigid ramp with rollers for supporting the pipe in the overbend at transversely adjustable positions along its length, the means for storing and for calculating being arranged to calculate the or each stress parameter for the portion of the pipe in the overbend using as boundary conditions predetermined ideal positions for the rollers, which are the theoretically ideal positions for minimum loading of the rollers.

27. A pipe laying vessel as claimed in claim 26 and having load cells for sensing at least the vertical loading of the rollers in the rigid ramp, the means for storing and for calculating being arranged to calculate the theoretical vertical loading of the rollers in said ideal positions and said means for displaying being arranged to display the calcu lated theoretical loadings and also the actual sensed loadings for comparison.

28. A method of laying an offshore pipe line substantially as hereinbefore described with reference to the accompanying drawings.

29. A pipe laying vessel substantially as hereinbefore described with reference to the accompanying drawings.