IN SITU AND REAL TIME X-RAY COMPUTED TOMOGRAPHY FOR THE MICROMECHANICS BASED CONSTITUTIVE MODELLING OF THE UNBONDED

FLEXIBLE RISER

Ketan PancholiAquaterra Energy Limited

2 Alkamaar Way, Norwich, NR6 6BFUnited Kingdom

Vineet JhaGE Oil and Gas UK

Wellstream Flexibles, Subsea Systems, Newcastle upon Tyne, NE6 3PF, United Kingdom

Neville Dodds GE Oil and Gas UK

Wellstream Flexibles, Subsea Systems, Newcastle upon Tyne,

NE6 3PF, United Kingdom

Mehul PancholiCity University

London, EC1V 0HB,

United Kingdom

James Latto GE Oil and Gas UK

Wellstream Flexibles, Subsea Systems, Newcastle upon Tyne,

NE6 3PF, United Kingdom

ABSTRACT

The failure mechanism of the composite flexible riser, comprising a pipe with melt fused carbon fiber tape or pultruded composite rods, is not well understood. As there is change in the configuration of the composite layers and its manufacturing methods, so the bulk material property also changes significantly. To capture the correct material model for global FE analysis, real time x-ray computed tomography was performed while the flexible pipe was being compressed. For developing a constitutive model for the composites, a time series of 3D volume images were analyzed quantifying the local strains responsible for the debonding of the layers and the crack development. These values were then used to understand the inter-layer adhesion leading to correlation between the FE global modelling and experiments capable of capturing the progressive delamination. The resulting global modelling was used to determine the area under compressive loading. The effect of global sea conditions and cumulative damage was noted. A correlation between the global model and experiments can be used to optimize riser performance. This method hopes to capture the overall behavior of flexible pipe under compressive loading.

1. INTRODUCTION

Lightweight materials like carbon fiber reinforced polymer (CFRP) composites are increasingly in demand for various industries, such as aerospace, oil and gas, automotive, bioengineering, etc. Special applications include deepwater flexible risers, panels for satellites, ailerons, to name but a few. The main interest of using CFRP composites is based on weight reduction, durability, and low thermal expansion.

Global analysis using Orcaflex has been used to study the response of wave loading on flexible pipes made from both composite and metallic materials (1, 2, 3, 4). Most of these papers study the global response and some studies compare the behavior with scaled flexible pipe structures (5). It is known

that shallow water risers experience a more dynamic response; both at connection to top infrastructure and at the touchdown point (6, 7). For deeper water conditions, dynamic responses are equally important. These effects are seen at the connection to top infrastructure, bend in pipe, buoyancy attachment and in the vicinity of the touchdown point (8, 9, and 10). Parameters such as installation methods, top tension, curvature, material model, hysteresis, mudline/sea bed contact point, shear strength and local stress can greatly influence the behaviors and overall performance of the riser system (1, 6, 11, 12).

CFRP composites can offer high specific tensile strength, however the strength is relatively low in compression, as the fiber in particular is sensitive to compressive loading (8,14). For deepwater flexible risers, the compressive loads can be experienced by these layers under external hydrostatic pressure, bending and inter-layer interaction (13, 14). So far the failure mechanism, especially at micro scale level, has not been fully understood. To understand its failure mechanism, both global modelling (as discussed) and experimental work are needed to identify the failure modes of the CFRP composite pipes under radially compressive loads.

Among several non-destructive testing (NDT) methods, the X-ray computed tomography (CT) as an imaging test procedure has many advantages over other NDT methods for applications involving defect analysis at the micro scale. This is attributed to its good spatial resolution and fast 3D scanning capability. Our previous work has demonstrated that the capability of using X-ray CT techniques in detecting and quantifying the development of micro-cracks in the composite layers during in-situ compression testing (13). Obtained results have clearly shown the possibility of using this technique further; especially to investigate the detailed failure mechanism of the composite pipe. Crack propagation and delamination have been observed in testing, and delamination was identified as a source of the crack initiation. The crack also formed a continuous connection with the delamination within the volume. More interestingly, such a crack initiation

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and delamination was observed at stresses less than previously thought. However, a time step for the obtaining of such 3D images was too large to determine the exact reason of the crack initiations. Additionally, due to the relatively large diameter test rings used, early crack initiation can be attributed to the stress concentration caused by the application of loads on the smaller area.

In order to establish better understanding of the crack development during compressive loading of the ring, it is necessary to reduce the time between two consecutive imaging scans. This will allows detection of the initial, smallest crack in composite materials at a much finer time step than previous work. Fast scanning of the ring during in-situ compression gives a detailed picture of the failure modes and crack growth in relation to the applied stress and strain. Furthermore, the role of interlayer bonding strength in failure of the composite can clearly be established if crack growth is observed in a slower time frame. In order to capture the details of crack growth at a finer time scale, a compression testing rig was used to initiate a crack and it was placed inside the imaging area during compression. The compression test was repeated to observe the crack development. The data was stored in the data logger attached to a testing rig.

Further, small diameter test rings were used for the current work and hence the compressive load was applied to a larger area depicting uniform loading from all directions. This solves the problem of stress concentration and, therefore, it is likely that it provides a more accurate correlation of failure with the acquired stress-strain data.

This work employed a method based on advanced CT technology to facilitate the precise evaluation of the micro-mechanical failure, manufacturing defects and interaction of various micro-failure modes with such defects. The distinctive objectives of the work included:

a) Using the best resolution to investigate the minimal initial crack size

b) Revealing the failure mode, i.e. the crack propagation modes (e.g. along or across the layers)

c) Investigating the correlation between micro local stress/strain, crack initiation and global analysis

d) Investigating the influence of pre-existing defects on the crack initiation and propagation.

2. Materials and Experimental Method2.1. The test ring

The compression test ring used in the X-ray CT experiment is carbon fiber reinforced polymer (CFRP) composite. Laser processing was used to improve the bonding strength between layers. The test ring has an outer diameter of 40 mm, an inner diameter of 30 mm and a length of 20 mm. Figure 1 shows a test ring siting on the compression rig on the machine before loading.

Figure 1: X-ray computed tomography facility

A 3D non-destructive technique using high-resolution X-ray tomography was applied in the current experiment to investigate the damage during the compression tests. The Nikon Metris Custom Bay (Figure 1 & 2) is the most frequently used imaging machine at the Manchester X-ray Imaging Facility (MXIF) due to its multi-functionality. There are many advantages to this machine such as the large bay area, heavy-duty manipulator, and its ability to perform serial scans enabling the Custom Bay to be used to scan large samples such as turbine blades or geological core samples. The system is adopted to scan specimens between 5 mm and 230 mm in cross-section, and higher resolution scanning for the interest region. During tomography scanning, the X-ray generator and detector remain stationary. The specimen remains stationary relative to the turntable and is placed in the origin of the virtual coordinate system. The X-ray generator scans a slice of the object as a very high precision turntable rotates 360˚. All images were taken at 200-220 amp current and 185-200 kVA voltage. Sufficient care was taken to avoid the metal parts being seen in the field of view as metal parts have a very high density compared to polymer-based composites and appear dark, blocking the details of the specimen. The ring specimen with a metal clamp capable of applying the compression load in-situ is designed in such a way that its jaw will not interfere in the imaging of the ring. The plates that are in direct contact with the ring are made of a polymer matching the composite ring’s density (as shown in Figure 1). This helps to prevent the high density material appearing in the field of the view.

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Figure 2: Nikon custom bay X-ray tomography facility set-up

2.2. The in-situ loading rig The loading rig produced by Deben UK Ltd is available

for in-situ deformation studies in either tension or compression modes, and is specifically for use with the Nikon Custom 320kV bay. The rig can operate in displacement and load control and with 4 different load cells depending on the material/sample under investigation. The available load cells are 1.25 kN, 2.5 kN, 10 kN and 25 kN. The top grips with load cell are supported on a polycarbonate tube to allow a clear view through the sample around a 360° rotation during tomography scanning. The loading platens (for compression mode) have a maximum diameter of 30 mm and the maximum sample height is 80 mm. Figure 3. Shows the Deben loading rig used in the experiment.

2.3. Procedures

The experiments in this work include specimen centring, loading tube material correction, initial crack identification, full rotation CT scanning, post processing of the images, etc. The detailed procedures are as follows:

1. The CFRP composite test ring was placed between a pair of parallel transparent PMMA blocks, as shown in Figure 1 and 3. The PMMA blocks are placed between the steel parallel plates of the loading rig. The benefit of using transparent PMMA blocks is to avoid steel plates appearing in the field of view, as the metal parts have a higher density compared with polymer-based materials and are displayed as a dark area in the imaging field, thus blocking the details of the specimen to be measured.

2. To increase the grip between the specimen and the PMMA blocks, the PMMA block surface was filed with sand paper to deliberately reduce its surface roughness.

Figure 3: The Deben loading rig

3. The test ring specimen was carefully positioned in the centre of both X and Y coordinates to allow 360° rotation tomography scanning.

4. Set the Z position to allow the test ring is in the centre of Z direction.

5. Since the density of the Deben loading rig (its cylindrical tube is made of transparent polycarbonate, as shown in Figure. 3) is similar to that of the test ring specimen, the imaging quality will be reduced. A correction process is therefore conducted before imaging the test ring. A full rotation X-ray CT scanning was conducted on the polycarbonate tube, i.e. Without the test ring inside. The image data was then stored and used to correct the images to be taken with the test ring later on.

6. Set the time step size and resolution, based on the size of the test ring (the whole ring needs to be imaged) and other parameter settings, the best resolution was determined to be a voxel size of 6 μm and a spatial resolution of 12 μm.

7. Set other parameters – X-ray power (current 55 µAmp, 150 kV)

8. Before applying any compression load, a full revolution scan was first performed using the above parameter settings on the test ring. The purpose of this scan is to set the reference images and identify any pre-existing defects in the materials which could be introduced by the manufacturing process.

9. Apply initial load, there are two methods to apply loads, one is applying constant force and the other is applying constant displacement. The latter method was used to apply load on the test ring. The deformation was controlled, but the varying forces were also recorded for later analysis.

10. Observe initial crack initiation, if no cracks are found, increase loads and repeat the above step until cracks are found.

11. With the cracks observed in the test ring, adjust the display (zoom in to a certain region), use the best spatial resolution, and then perform a 360° rotation tomography scanning.

12. Further, increase the loading, adjust the interested area, and perform a new 360° rotation tomography scanning.

13. Apply incremental loads, until the test ring is broken.14. Data were stored for the above operations, and

analyzed by the 3D image processing software

3. Global modelling

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Global analysis was carried out using Orcaflex to understanding the effect of wave loading on flexible composite pipe. The operating condition was assumed to be the Gulf of Mexico Deepwater field having a depth of 3000 m. The pipe size was 8-inches having a pressure rating 40 MPa, operating temperature of 120°C and life of 25 years. Weather data used for analysis is shown below in the Table 1. This includes wave current speed data.Table 1: 1000 year wave hurricane dataParameter Sector Compass direction Omni-directional Hs (ft.) 57.7Tp (s) 14 ±1.4Hmax (ft.) 101.3Crest elevation 56.2Spectral shape Jon-swap

Current speed (kt)Surface 2.9200 (ft.) 2.9300(ft.) 0.2Mud-line 0.1

Table 2: Vessel parameterParameter Units Ballaste

d Loaded

LOA M 292.2LPP M 277.0Breadth M 45.5Depth M 28.0Draft(at turret) M 12.34 18.31LCG( aft of turret centerline) M 65.98 69.23VCG(above keel) M 17.76 13.38Turret center position (aft FP)

M 74.0

Environmental condition and input vessel data (RAO) was used as provided for GOM conditions. In the present case, varied vessel offset between 8%-10% of water depth was used. The vessel direction was varied between ±30° and all load cases were analyzed under conservative 1000 year omni-directional wave conditions. The declination angle at the hang-off point was varied between 4°and 16°. The density of the inside fluid was varied between no fluid and 800 kg/m.3 The motion of the vessel was prescribed by the RAO and tension was applied from the top of the flexible riser. Static analysis was first carried out with inside fluid to perform effective tension optimization in relation to declination angle of pipe and applied top tension.

Figure4: Schematic of model used for global analysisTable 3: Pipe data used for Global modeling

Pressure : 40MPa

Temp : 120°C

Water Depth : 3000m

ID : 8"

Composite Bonded LinerWeight in air (kg/m) 152.45Weight in seawater (kg/m)

189.12

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Buoyancy section 1

Buoyancy section 2

Mid section

Bottom section

Front section

Figure 5: Effective tension vs Pipe length for various design cases

Figure 6: Bend radius vs Pipe length for various design cases

Figure 7: RZ stress vs pipe length for various design cases

Figure 8: CZ stress vs pipe length for various design cases

Figure 9: Max Shear vs Pipe length for various design cases The material was modelled using the Saevik stick-slip

model (15) in an area where large bending curvature was expected. The stiffness of the remaining section of the composite pipe was calculated by considering the stiffness of each individual layer. The friction model for the seabed-pipe interaction was used to match the seabed soil condition at the GOM. Dynamic analysis was performed to study the sensitivity to buoyancy and buoyancy length in many cases. In this work, four different cases are explained to study this effect. For each case effective tension, bend radius and shear stress have been studied and co-related with experimental results.

4. Results and Discussion:

Many configurations for the flexible pipe were studied to find the effect of the parameters such as buoyancy, length, vessel offset and its location on the pipe. The buoyancy modules with different buoyancy values, buoyancy module length were modelled and parameters such as effective tension and bending radius were extracted. These values were used as a selection criterion for determining the flexible riser configuration. After selecting the favorable configuration, the

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shear stress and compressive stress were extracted or calculated to check whether flexible pipe fails under the compressive or shear load at the critical location such as the touchdown point. Furthermore, the obtained results were compared with the conventional pipe results to observe deviation in stresses due to the use of lighter composite pipe.

After taking the water depth (3000 m) and 1000 year environmental condition of the location into account, the steep riser configuration is considered for further numerical experimentation. Since the new pipe with low mass per length was used instead of the conventional heavier pipe, the effect on the choice of buoys, its length and finding out the stresses under compression was the main aim of the work.

4.1. Optimization of Effective tension and Bending Radius:

During the analysis, it was found that achieving acceptable effective tension whilst not exceeding the maximum allowable bending radius is relatively easy compared with the conventional pipe (Figure 5). As seen in Figure 5, the maximum effective tension value was observed at the hang-off point. With installation of the buoys one section away from the hang-off point, the tension in the section of the flexible pipe between the hang-off point and starting of the buoys will decrease. However, as we move away from the curvature (and buoys, towards the touchdown point), the effective tension along the length of buoy section 1 gradually increases depending upon the radius of the curvature (Figure 6). Because of the high depth, shallow curvature is difficult to achieve with the amount of buoyancy required to lift the pipe.

During the optimization process, the length and buoyancy was varied to understand its effect. In this study, it is considered that the highest point of curvature formed due to buoys acts as a new hang-off point for the bottom pipe section going towards the mudline. However, it also reduces the tension in the front section of pipe preceding the installed buoy. If the cumulative buoyancy of the entire length of buoy section 1 is 1.5-2 times the cumulative buoyancy of the front pipe section preceding it, the maximum effective tension drops considerably. However, considerably longer pipe in this configuration requires two sections of buoys installed to reduce the tension and control the bending radius at the touchdown point. The length of mid-section of the pipe between the front and bottom sections of the buoyancy modules has to be relatively smaller than the front and bottom sections of the pipe. It is found that the length of this middle section of pipe should approximately be 0.5 times the other section of the pipe. If the rule of thumb (1.5 time buoyancy) is followed for buoyancy sections, the effective tension peak can be brought down further to a desirable level.

Instead of two separate sections of buoyancy modules, if single large sections (~800m) of buoyancy modules are installed at the midspan of the pipe (case 4), the maximum effective tension reaches 1.7 times the effective tension observed in configuration with two buoyancy sections. Before recording this observation, many configurations with single modules of different buoyancy were tested to obtain the best possible results.

Installing buoyancy section 2 with buoyancy more than 2 times the buoyancy of the mid pipe section also increases the effective tension along the length. The similar buoyancy relation also found to be important for the bottom section of the pipe (at mudline) and buoyancy section 2 module attached at the length preceding bottom pipe section.

In all cases, it is desirable to make sure that the buoyancy modules are located where the maximum of the current is not present. Additionally, longer front and bottom pipe (1000 m approx.) is required to be available for the larger offset (300 m) of the vessel. With shorter front pipe sections and larger offset of the vessel, the effective tension due to lateral pull and dynamic wave loading tends to increase. This also affects the bending radius.

Figures 7, 8 and 9 show shear stress at different pipe lengths for four different cases (each case having a different buoyancy ratio). It is clear from graphs shears RZ, CZ and Max shear are reasonably low in strength varying between 140KPa to 350 KPa. It should be noted the highest shear stress value occurs at zero, the highest curvature point and the touchdown point. These areas of pipe are therefore more prone to delamination or material compressive failure. Later, these values have been correlated with experimental shear stress values.

4.2. Comparison between conventional and lighter pipe

The study suggests that effective tension can be reduced to the desired level with the buoyancy module alone while keeping the bending radius below the maximum allowable limit. To reduce the amount of buoyancy and bending radius, it is advisable to apply external tension. For a conventional pipe, (with similar configuration as the lighter pipe) results in effective tension being more than the allowable (without safety factor). Moreover, the lighter pipe does not exceed its limit under compressive load at the touchdown point.

5. Micro scale testing Imaging

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Figure 10: An image showing cross-section of the composite pipe before compression testing.

The images were acquired in real time during the compression test. Approximately ¼ of the bottom and top of the composite pipe section was subjected to compressive load at a rate of 4 mm/min. Thus, it was not considered as the point load for the purpose of the analysis. The strain values were obtained using image analysis in Imagej plotted along the thickness of the layers. The above figure shows the development of the strain at layers during the compression. The strain increased at 26 mm radius from the center of the ring section. At 2.2 minutes, the strain starts to appear at the middle of the composite section layers.

After an additional 2.1 minutes, the location of the strain shifted with further increases. The shift in the location was observed due to the physical deformation of the ring during the compression. Resulting strains were measured and used to calculate the bonding strength of the layers. It was considered that the applied compression load was 3 times higher in the area where stresses are concentrated. These geometrical nonlinearities can be fed into the 2D model to predict any micro-shearing in the composite section. This can also confirm the suitability of the lighter pipe for the deepwater application without apprehensive compression failure.

Figure 11: The typical temporal lapsed images showing the composite ring section under compressive load at different time step. All images were taken in real time while compression load is being applied to the composite ring at a constant speed of 4 mm/minutes. The image (i) depicts the state of the composite ring at 1.7 m after the test was initiated. Subsequent images were obtained at (ii) 2.2 minutes (iii) 3.7 minutes (iv) 4.3 minutes

During the ring compression test, no material compressive failure was observed before delamination (a function of shear stress; in the present case a thermoplastic composite material was tested) which occurs at the strain shown in Figure 12, in which micro strain varies from 0-70 (overall difference of images under consideration). This trend was consistent with shear testing performed later and in-situ ring testing confirmed overall trend and details of failure mechanism.

Figure 12: Micro strains as function of radial distance from center of ring

Combining Global and Micro-scale testing: Shear stress as a predictor

Specimens were cut out based on ASTM D2344. The testing fixture used is shown in Figure 13. Results of the experiments are shown in Figure 14. It can be seen from the results of ILSS that the tests are repeatable. All specimens show a maximum shear stress of 25 – 27 MPa and displacement at maximum shear stress: ≈ 2000 μm. To get a broad overview of the failure mechanism, microscopic was first carried out. This was followed by detailed 3D tomography

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testing. Figure 15 and Figure 16 show a microscopic cross-section of the tested specimen. This shows fiber fractures occur along a detectable line, beginning from the outer surface ending at the middle of the specimen.

There were no fiber fractures between the middle of the tape and the inner surface. Changing fiber direction was caused by the bending test in the area of fiber fractures. Micro buckling of fibers occurs under compression. Surface deformation of the thermoplastic matrix was also observed. Multiple fiber fractures were seen in the same fiber at two different positions (typical shear fracture for FRPs). Local inception fiber cracking was seen. The specimen was nearly free of delamination in microscopic testing or showed only small areas of delamination. However, 3D tomography was performed to check for delamination in the 3D volume of the specimen.

Figure 13: Short beam shear fixture

Figure 14: Shear stress vs displacement

Figure 15: Microscopy of cross-section

Figure 16: Micro-buckling of fiber under compression

Figure 17: 3D Tomography images of short beam shear before and after testing showing area of delamination

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Outer surface

Inner surface

Surface deformation due to application of force

Black spots are artefacts

Micro buckling

Figure 18: Showing Delamination (occurs first) and material compressive failure

Figure 17 and Figure 18 show 3D tomography images. This shows minor pores could be present showing as white points occurring in the CT scan. Clear delamination can be observed in the top-right area which lies in the 3D volume of this particular specimen. It seems the delamination starts at the line of fiber fractions. In the present case delamination occurs before the fiber fracture, or fiber fracture (material compressive failure) occurs just after or at point of delamination. Delamination is strongly related to the shear stress value shown in Figure 14. Delamination occurs at the top end to experimental shear stress value or 25 MPa- 27 MPa. This value of shear stress is much higher than RZ,CZ or max shear reported in earlier Figures 7, 8, and 9. Therefore:

1. Delamination occurs before material compression failure

2. Shear stress is more important when relating bulk stress to micro failure

3. Shear stress in global modelling is much lower than experimentally observed (Figures 7, 8, 9 and 14). However, shear stress value in global modelling is strongly related to effective tension, curvature and buoyancy location. It must be noted, as previously mentioned, the areas of pipe more prone to failure include the top connection point, highest curvature point and touchdown point.

4. As long as the shear stress value of the produced composite is high, material compressive failure is unlikely to occur or will occur very close to the point of delamination. It must be noted this observation is related to softer thermoplastic material and might not be applicable for all composite materials.

Therefore shear stress value combined with global modelling is recommended as a good predictor for compressive failure. This can be followed by the relevant mid-scale bend or compression test as final verification (and 2D FE modelling). It is hoped this can be carried out in the near term for the present sample and product.

4. Conclusions

Composite material offers to expand the working design envelope of flexible pipes. Local and global analysis is key for understanding the overall performance of the proposed composite flexible riser. Overall effective tension, buoyancy selection, buoyancy distribution, and bend radius are key parameters that can influence the local behavior.

3D Image analysis has shown the ability to capture details of the failure mechanism which is normally not visible in microscopy analysis. It has been shown that micro-strain can be worked out and correlated with the failure mechanism.

In the present paper shear stress, calculated form global analysis, was correlated with shear testing resulting and local material compressive failure. It has been shown as long as shear stress remains within the limit, material compressive failure is not expected. Therefore, shear stress can be used as a predictor for failure. Shear stress value strongly depends on the production quality of the manufactured composite material. Thus, careful consideration must be given to ensure a functional quality assurance system is implemented whilst producing long lengths of composite riser systems.

References

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(11) Randolph M. and Quiggin P. ‘Non-Linear Hysteretic Seabed Model for Catenary Pipeline Contact’ 28th International Conference on Ocean, Offshore and Arctic Engineering, Honolulu, Hawaii, USA, May 31–June 5, 2009(12) Janssen E. F. ‘Comparison study of deepwater installation methods’ Master thesis, University of Stavanger, Norway 2014 (13) Jha V., Dodds N., Finch D. and. Latto J.R., Anderson T.A. and Vermilyea M.E., RPSEA UDW ‘Research Efforts - Floaters and Risers: Flexible Fibre-Reinforced Pipe for 10,000 Foot Water Depths: Performance Assessments and Future Challenges’, Offshore Technology Conference, Houston, Texas, USA, 6–9 May 2014.(14) N. Dodds, K. Pancholi, N.Dodds, V.Jha et al. , In Situ Investigation of Microstructural Changes In Thermoplastic Composite Pipe under Compressive Load, 33rd International Conference on Ocean, Offshore and Arctic Engineering, OMAE, 2014.(15) Saevik, S. ‘A finite element model for predicting stresses and slip in flexible pipe armouring tendons’, Computers & Structures, 46, 2 219-230,1993.

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