A system and method for determining the suitability of prevailing environmental conditions for offshore operations

The present invention relates to a method for determining the suitability of prevailing environmental conditions for offshore operations, and in particular for laying underwater pipelines from a pipeline laying vessel.

Underwater pipelines are frequently utilised for transferring petroleum products, such as oil or gas, between offshore production sites or offshore receiving or loading terminals and onshore processing and storage facilities. The term "pipeline" encompasses pipelines, cables, electric cables, waterlines, sewer lines, liquid natural gas lines or other pipelines.

For example, it is known to lay underwater pipeline between offshore and onshore terminals for the transfer of oil in either direction therebetween.

Accordingly, oil tankers anchored adjacent the offshore terminal and in oil communication therewith may either receive oil from or transfer oil to the onshore facility via the underwater pipeline and offshore terminal. Further, in the instance of offshore oil production facilities, it is customary, where the water depth and distance between the offshore and onshore facilities permit, to lay pipeline along the sea bottom between the underwater well head and the onshore oil receiving facility. The latter may comprise solely a storage facility for subsequent transfer of the oil overland or additional underwater pipeline may be provided between the onshore storage facility and an onshore terminal to an oil tanker anchored adjacent the offshore terminal.

With increasing emphasis on offshore oil production and the fact that such production sites are often located in areas remote from oil refining facilities coupled with the increasing use of offshore terminals for transfer of oil in

either direction between offshore and onshore terminals, there has arisen the need for the laying of underwater pipelines between such terminals.

Various types of vessels and companion apparatus have been developed to accomplish this. These pipeline-laying vessels or barges are, however, subject to varying sea conditions which tend to make the pipeline laying quite difficult and hazardous. During laying, the pipeline will be strung out behind the laying vessel in a generally S-shaped or J-shaped catenary configuration. When it is considered that the pipeline line is often formed of steel, frequently with an outer coating of concrete or other rigid material, it will be readily appreciated that the disposition of the pipeline in the catenary configuration causes large stresses on the pipeline which can lead the pipeline to break or become distorted such that the pipeline is no longer fit for purpose. For laying pipeline in relatively deep water from such surface floating vessels, the length of the unsupported pipeline as it is paid out increases, which may cause allowable limits, such as stress in the pipeline, to be exceeded. Thus, the bending stresses imposed on the pipeline may exceed allowable limits and result in permanent deformation, or rupture.

The present invention is particularly applicable to a self-propelled pipeline laying vessel having a reel for spooling steel pipeline thereon, pipeline working and handling means for straightening the pipeline as it is unspooled, pipeline supporting and guiding apparatus for guiding the straightened pipeline into the water at a presettable, adjustable exit angle, and means for maintaining the pipeline under a predetermined adjustable tension. However, it is equally applicable to other types of pipeline laying vessels, including anchored pipeline laying.

Once offshore, the ability of the vessel to carry out a pipeline laying operation is strongly linked to the prevailing environmental conditions as the motions of the vessel due to wave motions tend to overstress and potentially damage the pipeline as it is being laid along the sea bottom or exceed the capacity of the systems of the pipeline laying vessel, such as pipe tensioners, or station keeping capabilities, tension and bollard pull capacity of the vessel.

The chain of events that create stresses in the pipeline can be described as below:

• Motions of the sea surface due to wave;

• Transmission of this wave motion to the vessel;

• Transmission of the vessel motion to the pipeline via the mechanical connection to the moving vessel by the tensioners and support apparatus of the pipeline support apparatus of the vessel; ^β Once in the water, drag forces induced on to the pipeline catenary by currents and waves.

When weather conditions cause vessel motion inducing loads in the pipeline above a given limit, the pipeline laying vessel may have to wait for better weather conditions to avoid the risk of damage to the pipeline. Thus assessing the suitability of the prevailing environmental conditions for a pipeline laying operation is of vital importance. Optimisation of the operating weather window reduces downtime lost due to such waiting while maintaining minimal risk of damage to the pipeline during the laying operation, and optimising the planning of oil and gas resource development.

The decision to begin a pipeline laying operation is currently determined from the comparison of an allowable sea state calculated from dynamic

analysis with a visual assessment of prevailing conditions and interpretation of weather forecast.

It is a concern that this determination, generally based upon a visual assessment of wave height only, is preventing pipeline laying operations from proceeding unduly in some circumstances.

It is known to define the operating condition in which the pipeline can be laid in terms of allowable sea state based on the following:

⁹ To obtain a sufficient allowable vessel excursion » To determine limiting wave height as large as possible within the constraints of operability of the vessel. Limiting wave height and period should be based on meteocean data for the project. ^β To satisfy the requirement of the relevant code for installation of the pipeline.

In the known procedure, first a static analysis is run to create an appropriate model. Dynamic analyses are then carried out, which require various sea states, representative of the project location, to be considered. Different wave statistics and directions are input in the analysis.

Combining those sea states with the Response Amplitude Operators (RAOs) of the vessel, which depend on wave direction, induced motion of the pipeline support structures is calculated by suitable analysis software.

A dynamic Finite numerical simulation is then conducted to determine the pipeline configuration, top tension and stress corresponding to the input sea state. The obtained pipeline configuration is then compared to the applicable code acceptance criteria. The exercise is then repeated for a

number of representative sea states covering the expected weather conditions at the location of the project.

Typical limits are governed by: ^s Stress/strain at top connection and support apparatus; o Stress/strain at sag bend; ⁹ Bottom tension; ^β Top tension; • Bollard pull.

This set of analysis allows the definition of the weather conditions envelope in which the pipeline can be safely laid, from a structural integrity point of view. This is usually expressed in terms of wave height, wave period and wave direction, with wave height comprising the main determining factor.

Once offshore, the actual weather conditions, based on observation and consideration of weather forecast bulletins, are compared to the predetermined allowable sea states.

The current procedure considers the sea state as the limiting factor for installation.

Rather than relying on the analysis of the actual cause of the motion inducing loads in the pipeline (i.e. the sea state), the present invention proposes to rely on the actual motion of the pipeline support apparatus of the pipeline laying vessel.

In the following description of the present invention the coordinates X ₁ Y and Z refers to a Cartesian coordinate system used to define the motion of

the pipeline support apparatus in all degrees of freedom. σ axis refers to the vertical axis, X and Y describing the horizontal plane.

According to the present invention there is provided a method for determining the suitability of prevailing environmental conditions for an offshore operation on a vessel, the method comprising detecting the motion of at least a part of the vessel and comparing the detected motion with a predetermined maximum permissible motion or assessing numerically the effect of the measured motion to enable determination of the acceptability of the detected motion for said offshore operation.

Preferably the method includes the initial step of creating a numerical model of the vessel and utilising the simulation to establish a first data set representative of the maximum permissible motion of said part of the vessel as a function of frequency, in at least the Z or vertical axis, and storing said first data set for use in said determination.

Preferably said first data set defines a failure surface of the system as a function of the frequency of the vertical displacement of said part of the vessel (displacement in Z axis), amplitude of vertical displacement of said part of the vessel, rotation of said part of the vessel around the X axis, translation of said part of the vessel in the X axis, rotation of said part of the vessel about the Y axis and translation of said part of the vessel in the Y axis.

Preferably the motion of said part of the vessel is detected by means of at least one motion sensor, such as one or more accelerometers, capable of detecting the displacement said part.

Preferably said offshore operation comprises laying an underwater pipeline, said part of the vessel for which motion is detected comprising a pipeline support apparatus provided on the vessel. Preferably the motion sensor is provided at a lower end of the pipeline support apparatus, where the pipeline leaves the pipeline support apparatus, for detecting motion in each of the X, Y and Z axes as a function of time.

Preferably the method further comprises processing data from the motion sensor(s) to establish a second data set comprising determined values for each of the frequency of the vertical displacement of the pipeline support apparatus (displacement in Z axis), amplitude of vertical displacement of the pipeline support apparatus, rotation of the pipeline support apparatus around the X axis, translation of the pipeline support apparatus in the X axis, rotation of the pipeline support apparatus about the Y axis and translation of the pipeline support apparatus in the Y axis.

Preferably the method comprises the further step of comparing said second data set with said stored first data set to provide an output indicating the difference between the actual motion of the pipeline support apparatus and the maximum permissible motion before failure. Preferably a display of the change in this calculated difference over time is provided to provide an indication of the evolution of the prevailing environmental conditions over time.

Thus the present invention provides a clear indication of the safety envelope between the actual motion of the vessel due to the sea state and the motion which would result in damage to the pipeline and thus can provide an accurate assessment of the acceptability of the prevailing environmental conditions for a pipeline laying operation and can also provide an indication of the trend of such conditions which might impact on

the ability of the vessel to continue with a pipeline laying operation over time.

Preferably said second data set is generated by Fourier transformation to obtain a spectral description of the data from the motion sensor(s).

Preferably the method takes account of the effect of current and pipeline tension as static variables established by monitoring the shape of the pipeline catenary on the pipeline support apparatus, preferably by means of lift off sensors.

According to a second aspect of the present invention there is provided a system for determining the suitability of prevailing environmental conditions for offshore operations from a vessel, said system comprising detecting means for detecting the motion of a part of the vessel and data processing means for assessing numerically the effect of the measured motion to enable determination of the acceptability of the detected motion for said offshore operation.

Preferably said data processing means includes first data processing means for establishing a numerical model of the vessel to determine a first data set representing the maximum permissible motion of at least a part of the vessel for the carrying out of the offshore operation, and second data processing means for creating a second data set representing the actual motion of said part of the vessel and comparing the second data set with the first data set to enable determination of the acceptability of the detected motion for said offshore operation.

Preferably the detecting means comprises at least one motion sensor, such as one or more accelerometers.

Preferably the offshore operation comprises laying an underwater pipeline, said part of the vessel comprising a pipeline support apparatus provided on the vessel. Preferably said at least one motion sensor is mounted on the lower end of the pipeline support apparatus at or adjacent the point where the pipeline leaves the pipeline support apparatus.

Lift off sensors may be provided on the pipeline support apparatus for determining the shape of the pipeline catenary to establish the drag force on the pipeline caused by the current.

Preferably said first data processing means is provided onshore while said second data processing means is provided onboard the vessel. However, it is envisaged that said first data processing means may also be provided onboard and the first and second data processing means may comprise a single computer.

A system and method of determining the suitability of prevailing environmental conditions for laying underwater pipelines from a pipeline laying vessel having a pipeline support apparatus according to an embodiment of the present invention will now be described by way of example.

Firstly predetermined maximum permissible motions of the pipeline support apparatus of the vessel are determined by mathematical analysis onshore using computer models creating a simulation of the response of the actual vessel.

A simple catenary model is established and a static analysis is undertaken to find the optimum static configuration based on criteria such as stress in

the overbend or sagbend, maximum or minimum tensions, ramp lift off etc. This pipeline configuration is considered as the nominal pipeline configuration.

Figure 1 illustrates the coordinate system used for the model.

Next simulated pipeline end connection and support apparatus of the pipeline support apparatus of the vessel are displaced vertically (in the Z axis) for the model with a given frequency. The motion amplitude is then gradually increased for the simulation until the structural limit of the pipeline or some other limiting criterion is reached. The obtained limit amplitude corresponds to the maximum allowable motion amplitude at the chosen frequency. This exercise is repeated for a number of frequencies. A number of data points are obtained, representing the maximum allowable vertical motion amplitude as a function of frequency.

The limiting criteria considered include, but are not limited to: • Maintaining the required top tension below the maximum capacity of the vessel bollard pull; • Maintaining the stresses in the sag bend and in the top of the catenary within the code limitation.

This process is repeated a number of times with a gradual static change of four parameters, namely: • The pipeline end connection and support apparatus rotation around the X axis ^β The pipeline end connection and support apparatus rotation around the Y axis

⁹ The pipeline end connection and support apparatus translation in the X direction

o The pipeline end connection and support apparatus translation in the Y direction

The deliverable of the above sets of analysis is the definition of a failure surface of the system as a function of: ^β The frequency of the vertical displacement of the system;

⁹ The amplitude of the vertical displacement of the system;

• The pipeline end connection and support apparatus rotation around the X axis; • The pipeline end connection and support apparatus pitch rotation around the Y axis; ^β The pipeline end connection and support apparatus translation in the X direction; ^β The pipeline end connection and support apparatus translation in the Y direction.

This set of analyses may be repeated for specific and critical phases of operation such as critical initiation, normal pipe laying operation, laydown and flooded pipeline conditions.

The great number of simulations required to obtain the failure surface equation is managed by suitable computer software.

Once the data has been collated it can be stored for use in accurately evaluating the suitability of prevailing environmental conditions for laying underwater pipelines once the pipeline laying vessel is at sea, as described below.

The pipeline supporting ramp of the pipeline laying vessel is fitted with a motion sensor, such as a linear accelerometer, which can record the

actual displacement of the ramp in the six degrees of freedom shown in Fig 1 as a function of time.

Data from the motion sensor is supplied to an onboard computer, whereby a filter is then run on the real time data over a predetermined period of time and a fast Fourier transformation is applied to obtain a spectral description of the recorded data set. For each axis, the spectral parameters are calculated such as mean, return periods.

A representative value is determined for each of the following parameters:

• The frequency of the vertical displacement of the system ^β The amplitude of the vertical displacement of the system

• The pipeline end connection and support apparatus rotation around the X axis • The pipeline end connection and support apparatus pitch rotation around the Y axis

• The pipeline end connection and support apparatus translation in the X direction

• The pipeline end connection and support apparatus translation in the Y direction

At specified periods, new data will be added and oldest disregarded such as parameters are re-calculated.

The continuous knowledge of those data points compared with the pre- calculated failure surface provides an indication of the actual distance to the structural failure of the pipeline and thus provides a rigorous approach for calculating a probability of exceeding the limiting parameters.

A trace of this distance as a function of time also gives an indication of the operating condition evolution over time.

Current is assumed to have a static effect on the catenary. Any current will induce a drag force on the pipeline which creates a displacement of the catenaiγ from its nominal position.

Consequently, the effect of current can be captured by monitoring the shape of the pipeline catenary on the ramp using lift off sensors. Therefore the data provided by the lift off sensors can be used to monitor any effect of current on the stress applied to the top end of the pipeline. Current can then be considered as a static variable which can be evaluated. Hence it will be included in the abovementioned analysis as a supplementary parameter. In most configurations, current will have a negligible effect on the analysis.

This approach has been developed and tested with the objective of optimizing operability of the Apache reel-lay vessel (Figure 3).

It is assumed that the main source of dynamic excitation in the pipeline catenary is the motion of the pipeline top end connections and pipelay equipment supports (e.g. rollers). Therefore, if the structural response of the pipeline can be calculated as a function of these motions, and these motions can be measured, then the dynamic effect of the environmental conditions on the pipe can be determined without introduction of the uncertainties associated with approximated sea states.

Implicit in this approach is the ability to statistically analyze blocks of recorded motion data to provide a robust indication of the margin to failure and how this is changing with time.

Extensive sensitivity studies, not reported in detail here, have shown that the principal contributor to dynamic excitation of tension and stress in the pipeline catenary is due to the vertical motion of the pipeline top connection at the vessel. This vertical motion will be a combination of the heave and pitch of the vessel. For shallow water pipelay the frequency of the horizontal motion of the top connection in the fore-aft (surge) direction may also have a measurable impact but this decreases rapidly with increasing water depth. Whilst the amplitude of angular or linear displacement of the other four degrees of freedom has some effect, the frequency of motion does not have a significant impact, see Figure 4. Therefore, these directions can be considered quasi-static.

Consequently, the impact of the pipeline top end connection motion on the dynamic amplification of stress and tension in the pipeline catenary can be captured by accounting for the amplitude of motion in all six degrees of freedom and frequency of excitation in the vertical direction only. By testing only one direction to be truly dynamic the phase of motion in each direction need not be considered. Thus, the number of parameters affecting this six degrees of freedom problem can be simplified from 18 (6 amplitudes, 6 periods and 6 phases) to 7 parameters.

The method uses an office-based analysis procedure to determine a multivariate envelope of allowable top connection motions. This response surface is then used offshore as a limiting criterion against which actual measured motions can be compared. This methodology is summarized in Figure 5.

The office based study is performed before the offshore installation campaign. First, a matrix of finite element models is defined based on

project specific data, in a similar manner as the known wave-height method. The matrix of cases must reflect all key stages in the start up, pipelay and lay down process and consider any changes in pipe geometiγ or water depth. Contingency and upset conditions must also be considered.

The level of details and complexity of the matrix depends on the weather sensitivity of the operation. As a minimum, the basic matrix of cases would include initiation, normal pipelay, abandonment and accidental flooding cases.

A design of experiment (DOE) is then run over a matrix of finite element (FE) models. A design of experiment can be defined as a multi parametric sensitivity analysis aimed at defining the response of one or several parameters to a range of values and combinations of the input parameters. The DOE is performed by running several dynamic simulations on the same model. Input parameters are changed in a predefined manner for each individual simulation. The sensitivity of the model results to changes in the input parameters is therefore assessed. The DOE is performed using numerical analysis. This automates the management of the many dynamic simulations required.

In its original configuration, the model is in static equilibrium. The displacements and rotations of the pipeline top end are set to zero:-

X = O Rl = O

imt Y = 0 , R2 = 0

Z = O R3 = 0 TEP

In the first set of runs, noted Run 0, a simple harmonic motion in the vertical direction (Z direction) is imposed on the pipeline top connection. The Run 0 series of tensors can therefore be noted using the equation:-

where, t is time, Zi is the amplitude of the motion and Ti the period of the motion.

This set of runs is then repeated including successive series of static offsets for the remaining degrees of freedom. For a particular new set of runs, noted Run j, the new series of tensors can therefore be described using the equation:-

The range of values for Xj, Yj, R1j, R2j, R3j are a set of offset values chosen to cover the range of possible positions of the pipeline top end connection. All the parameters for the DOE are chosen to cover the possible ranges and combinations of those values. Those values are selected from theoretical calculations confirmed by offshore measurements.

For every single dynamic simulation, the results to be monitored are extracted for comparison against the relevant limit state equation. Allowable stress or bending strain limits may be considered in addition to equipment limits such as top tension capacity. The monitored response values are gathered in a set of DOE Data Tables which are then sent to the vessel for offshore monitoring.

Early testing of the system in the southern North Sea has confirmed that, for very shallow water lay, exclusion of the dynamic effects of surge motion can lead to some under prediction of dynamic amplification. To ensure this effect is not ignored, a surge dynamic amplification factor can be calculated for inclusion in the response surface.

In order to link the pre-campaign calculations with the actual environmental conditions in which the vessel operates, the motion of the pipelay equipment is measured, and data are available on a continuous real time basis. A motion sensor is installed on the pipelay vessel ramp, just above the exit of the main tensioner. Figure 6 is a photograph of the motion sensor mounted on the lay ramp. The motion sensor measures the displacements and rotations of the pipeline end connection in the six degrees of freedom at a pre-set frequency of 2 Hz. The data files are stored on a dedicated computer on the vessel bridge and are therefore available in real time for monitoring the pipeline structural response.

A graphical user interface is installed on the computer bridge to process the motion sensor data and to allow comparison of the measured motion against the pre-defined envelope. The user selects the office data tables corresponding to the relevant stage of operations and this data is loaded into the onboard monitoring software. A response surface is fitted on to the data points contained in the DOE output table. This operation is performed

so that interpolation between the data points can be performed. Response surfaces can be fitted to DOE data points using methods such as Least Squares Regression, Moving Least Squares or any other suitable technique.

Figure 7 is a visual representation of a maximum Von Mises stress response surface where 5 parameters (X, Y, R1 , R2 & R3) out of 7 are fixed. The pipeline end connection vertical motion amplitude (Z) and period (Tz) are kept as variables. Using this approach, the relation between a limit state and two of the seven parameters can be presented. The motion measurement time traces in all the six degrees of freedom are examined over a specified time window, typically 15 to 45 minutes long. Statistical parameters are extracted and the time trace is also transformed in the frequency domain. A stochastic approach is then used. Time correlated combinations of the 7 parameters defining the motion of the pipeline top end connection are sampled a large number of times (linked to the time window size). These combinations of input parameters are computed in the response surface to calculate series of values for the monitored parameters. Unrealistic combinations of input parameters are removed from the Monte Carlo calculation using filtering techniques based on the speed and acceleration of the measured motions.

As a result, the system provides a statistically rigorous indication of the level of impact of the vessel dynamic motions on the pipe structure. This system presents a number of advantages over the traditional Wave Height Method.

The Motion Measurement & Surface Response Method relies on two sources of information: pre-run finite element calculations and actual measurements. Therefore, it does not require the use of the vessel RAOs

and seastate modelling for the computation of the pipe structural response but relies only on measured motions of the pipelay equipment. Therefore, it removes some sources of conservatism in the assessment of the dynamic loading.

The use of this method allows offshore personnel to monitor the stress and tension state of the pipeline in real time, based on actual measurements rather than interpretation of weather bulletin and visual assessment of weather conditions. Marginal weather conditions can therefore be assessed with an additional, accurate source of information. The system does not require the installation work to have started. It can assess the impact the current environmental conditions would have on the pipeline before the catenary is formed.

As the data can be recorded and processed over time, trends in vessel motion and its effect on the pipe can be identified and used to assist in the decision making process. Therefore, operations can be planned in light of the changing environmental conditions.

Advancement in FE packages used for offshore installation analysis have made a third method of pipeline integrity monitoring a realistic option. Recently released implicit integration schemes for time domain catenary analysis are able to provide convergence solutions. The application of a third method that uses this facility is outlined below.

The method relies on two sources of real-time information: a motion sensor installed on the pipelay vessel, and an FE calculation package featuring an implicit solver. The implicit integration scheme included in the calculation package speeds up the calculation time drastically over conventional explicit methods, to the point where a time window can be

simulated in less than the actual length of this time window. In other words, it is possible to calculate the output of the FE calculation in less time than what was required to measure the input.

Consequently, the method requires two steps, one in the office prior to the offshore installation, and one during the actual offshore operations. In the office, sets of numerical models are prepared but are not fully analyzed. These models cover the usual combinations of operating parameters (job phase, water depth, pipeline properties, ramp angle, etc) see Figure 8.

On the vessel, the motion sensor is set to measure continuously the motion of the lay equipment and the data is streamed in real time into the onboard computer. An FE package featuring an implicit solver is installed on the same computer and continuously analyses the motion files created by the motion sensor. The process is automated by an additional piece of software (Figure 9). The Motion Measurement & Real Time Simulation Method offers a number of benefits over the two methods outlined above. No dynamic analysis needs to be performed in the office prior to the offshore phase, other than preliminary checks to assess the feasibility and approximately the vessel operability for a particular operation. Limiting sea states derived from this analysis would be for information only and would not govern operational decisions. The installation engineering time and resources is therefore significantly reduced to static analysis and preparation of offshore files.

Compared to the surface response method presented above, no interpolation of previously run dynamic analysis is necessary. The motions analyzed by the system correspond exactly to the motions measured on the vessel ramp. The accuracy of the calculations is therefore as good as the FE modelling can be.

In comparison to conventional wave based dynamic analysis, this method benefits from the real time measurement of the ramp motion. It is therefore believed this method introduces higher degree of accuracy in the assessment of environmental loadings.

This method does lack the statistical rigor of the surface response method in assessing the current and future margin to failure; however, it is eminently possible to combine the benefits of both techniques.

The monitoring of the dynamic response of a pipeline during installation from a pipelay ship is linked to the extremely complex problem of the interaction of the vessel to its environment. In some cases, the pipeline structural response can dictate the environmental conditions in which offshore work can be carried out. This can lead to significantly longer installation time driven by long waiting on weather period. Conventional wave based methods have been used in the past to link the weather to the pipe structural response, but are believed to be unduly conservative in some circumstances. Introducing new methods, based on proven accurate motion measurement systems and advanced computerized FE calculations is seen as an attractive way of monitoring safely and accurately the pipeline integrity.

Various modifications and variations to the described embodiments of the inventions will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.