infrastructure access report - marinet2€¦ · as wave energy, tidal energy, offshore-wind energy...

Infrastructure Access Report

Infrastructure: UNEXE Dynamic Marine Component Test Facility

User-Project: HDPC4FMEC

Development of new highly dynamic power cables design solutions for floating offshore renewable

energy applications

Norddeutsche Seekabelwerke GmbH

Marine Renewables Infrastructure Network

Status: Final Version: 1.1 Date: 30-Sep-2015

EC FP7 “Capacities” Specific Programme Research Infrastructure Action

Infrastructure Access Report: HDPC4FMEC

Rev. 1.1, 30-Sep-2015 of 18

ABOUT MARINET MARINET (Marine Renewables Infrastructure Network for emerging Energy Technologies) is an EC-funded network of research centres and organisations that are working together to accelerate the development of marine renewable energy - wave, tidal & offshore-wind. The initiative is funded through the EC's Seventh Framework Programme (FP7) and runs for four years until 2015. The network of 29 partners with 42 specialist marine research facilities is spread across 11 EU countries and 1 International Cooperation Partner Country (Brazil). MARINET offers periods of free-of-charge access to test facilities at a range of world-class research centres. Companies and research groups can avail of this Transnational Access (TA) to test devices at any scale in areas such as wave energy, tidal energy, offshore-wind energy and environmental data or to conduct tests on cross-cutting areas such as power take-off systems, grid integration, materials or moorings. In total, over 700 weeks of access is available to an estimated 300 projects and 800 external users, with at least four calls for access applications over the 4-year initiative. MARINET partners are also working to implement common standards for testing in order to streamline the development process, conducting research to improve testing capabilities across the network, providing training at various facilities in the network in order to enhance personnel expertise and organising industry networking events in order to facilitate partnerships and knowledge exchange. The aim of the initiative is to streamline the capabilities of test infrastructures in order to enhance their impact and accelerate the commercialisation of marine renewable energy. See www.fp7-marinet.eu for more details.

Partners

Ireland University College Cork, HMRC (UCC_HMRC)

Coordinator

Sustainable Energy Authority of Ireland (SEAI_OEDU)

Denmark Aalborg Universitet (AAU)

Danmarks Tekniske Universitet (RISOE)

France Ecole Centrale de Nantes (ECN)

Institut Français de Recherche Pour l'Exploitation de la Mer (IFREMER)

United Kingdom National Renewable Energy Centre Ltd. (NAREC)

The University of Exeter (UNEXE)

European Marine Energy Centre Ltd. (EMEC)

University of Strathclyde (UNI_STRATH)

The University of Edinburgh (UEDIN)

Queen’s University Belfast (QUB)

Plymouth University(PU)

Spain Ente Vasco de la Energía (EVE)

Tecnalia Research & Innovation Foundation (TECNALIA)

Belgium 1-Tech (1_TECH)

Netherlands Stichting Tidal Testing Centre (TTC)

Stichting Energieonderzoek Centrum Nederland (ECNeth)

Germany Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V (Fh_IWES)

Gottfried Wilhelm Leibniz Universität Hannover (LUH)

Universitaet Stuttgart (USTUTT)

Portugal Wave Energy Centre – Centro de Energia das Ondas (WavEC)

Italy Università degli Studi di Firenze (UNIFI-CRIACIV)

Università degli Studi di Firenze (UNIFI-PIN)

Università degli Studi della Tuscia (UNI_TUS)

Consiglio Nazionale delle Ricerche (CNR-INSEAN)

Brazil Instituto de Pesquisas Tecnológicas do Estado de São Paulo S.A. (IPT)

Norway Sintef Energi AS (SINTEF)

Norges Teknisk-Naturvitenskapelige Universitet (NTNU)

http://www.fp7-marinet.eu/


Rev. 1.1, 30-Sep-2015 of 18

DOCUMENT INFORMATION Title Development of new highly dynamic power cables design solutions for floating offshore

renewable energy applications

Distribution Public

Document Reference MARINET-TA1-HDPC4FMEC

User-Group Leader, Lead Author

Sven Mueller-Schuetze Norddeutsche Seekabelwerke GmbH

User-Group Members, Contributing Authors

Carsten Suhr Norddeutsche Seekabelwerke GmbH Marco Marta Norddeutsche Seekabelwerke GmbH Heiner Ottersberg Norddeutsche Seekabelwerke GmbH Daniel Isus Feu General Cable Lars Johanning University of Exeter Philipp Thies University of Exeter

Infrastructure Accessed: UNEXE Dynamic Marine Component Test Facility

Infrastructure Manager (or Main Contact)

Lars Johanning

REVISION HISTORY Rev. Date Description Prepared by

(Name) Approved By Infrastructure

Manager

Status (Draft/Final)

0.1 First draft Marco Marta Draft

1.0 15/09/15 Second draft including UoE comments Marco Marta Draft

1.1 30/09/15 Final including NSW comments Marco Marta Lars Johanning Final


Rev. 1.1, 30-Sep-2015 of 18

ABOUT THIS REPORT One of the requirements of the EC in enabling a user group to benefit from free-of-charge access to an infrastructure is that the user group must be entitled to disseminate the foreground (information and results) that they have generated under the project in order to progress the state-of-the-art of the sector. Notwithstanding this, the EC also state that dissemination activities shall be compatible with the protection of intellectual property rights, confidentiality obligations and the legitimate interests of the owner(s) of the foreground. The aim of this report is therefore to meet the first requirement of publicly disseminating the knowledge generated through this MARINET infrastructure access project in an accessible format in order to:

progress the state-of-the-art

publicise resulting progress made for the technology/industry

provide evidence of progress made along the Structured Development Plan

provide due diligence material for potential future investment and financing

share lessons learned

avoid potential future replication by others

provide opportunities for future collaboration

etc. In some cases, the user group may wish to protect some of this information which they deem commercially sensitive, and so may choose to present results in a normalised (non-dimensional) format or withhold certain design data – this is acceptable and allowed for in the second requirement outlined above.

ACKNOWLEDGEMENT The work described in this publication has received support from MARINET, a European Community - Research Infrastructure Action under the FP7 “Capacities” Specific Programme.

LEGAL DISCLAIMER The views expressed, and responsibility for the content of this publication, lie solely with the authors. The European Commission is not liable for any use that may be made of the information contained herein. This work may rely on data from sources external to the MARINET project Consortium. Members of the Consortium do not accept liability for loss or damage suffered by any third party as a result of errors or inaccuracies in such data. The information in this document is provided “as is” and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and neither the European Commission nor any member of the MARINET Consortium is liable for any use that may be made of the information.


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EXECUTIVE SUMMARY The test work carried out at the DMaC test facility site between July and August 2014 is part of an ongoing project lead by Norddeutsche Seekabelwerke GmbH (NSW) aimed at developing a new range of highly dynamic inter-array medium voltage (MV) power cables suitable for typical floating offshore renewable energy (ORE) operating conditions, namely compliant riser cable configurations with very dynamic mechanical loading regime. The test program at DMaC focused on two of the critical aspects of a dynamic cable design, namely bend stiffness characterization and fatigue life estimation. Bend stiffness measurements under dynamic load regimes produced useful information on the non-linear bend stiffness behaviour and hysteretic effects under varying bending loads that lead to the reassessment of part of the applied cable mechanical modelling methods. Cable samples were also subjected to severe dynamic loading to the limit of their expected fatigue life. The type and distribution of damage identified on the cable components during the subsequent dissection and analysis was mostly in line with expectations hence supporting the validity of the applied failure mode analysis method. The fatigue life of the cable specimens was also largely in line with expectation. However, only a limited number of samples could be tested in the allocated time and further testing is required for full validation of fatigue life estimation procedure. Overall the test results contributed to the review and calibration of cable mechanical modelling practices as well as providing critical information supporting design choices for a novel highly dynamic test cable that has already been manufactured and is now subjected to further testing for performance assessment and full design validation.


Rev. 1.1, 30-Sep-2015 of 18

CONTENTS

1 INTRODUCTION & BACKGROUND ...................................................................................................................7

1.1 INTRODUCTION .................................................................................................................................................... 7 1.2 DEVELOPMENT SO FAR .......................................................................................................................................... 7 1.2.1 Stage Gate Progress .................................................................................................................................... 7 1.2.2 Plan For This Access ..................................................................................................................................... 9

2 OUTLINE OF WORK CARRIED OUT ...................................................................................................................9

2.1 SETUP ................................................................................................................................................................. 9 2.1.1 Materials and equipment ............................................................................................................................ 9

2.2 TESTS ............................................................................................................................................................... 11 2.2.1 Test Plan .................................................................................................................................................... 11

2.3 RESULTS ............................................................................................................................................................ 13 2.3.1 Bend stiffness and structural damping ...................................................................................................... 13 2.3.2 Fatigue testing ........................................................................................................................................... 13

2.4 ANALYSIS & CONCLUSIONS................................................................................................................................... 15 2.4.1 Bend stiffness and structural damping ...................................................................................................... 15 2.4.2 Fatigue testing ........................................................................................................................................... 15

3 MAIN LEARNING OUTCOMES ....................................................................................................................... 16

3.1 PROGRESS MADE ............................................................................................................................................... 16 3.1.1 Progress Made: For This User-Group or Technology ................................................................................. 16 3.1.2 Progress Made: For Marine Renewable Energy Industry .......................................................................... 16

3.2 KEY LESSONS LEARNED ........................................................................................................................................ 16

4 FURTHER INFORMATION .............................................................................................................................. 16

4.1 SCIENTIFIC PUBLICATIONS .................................................................................................................................... 16

5 REFERENCES ................................................................................................................................................ 17

6 APPENDICES ................................................................................................................................................ 17

6.1 STAGE DEVELOPMENT SUMMARY TABLE ................................................................................................................ 17


Rev. 1.1, 30-Sep-2015 of 18

1 INTRODUCTION & BACKGROUND

1.1 INTRODUCTION Norddeutsche Seekabelwerke GmbH (NSW), a wholly-owned subsidiary of General Cable Corporation, is a leading manufacturer of communications, submarine, overhead, power and offshore cables, including dynamic power cables for the oil & gas sector. The work carried out at the University of Exeter, Dynamic Marine Component Test Facility (DMaC) and described in this report is part of an ongoing project whose aim is to develop a new range of highly dynamic inter-array medium voltage (MV) power cables suitable for typical floating offshore renewable energy (ORE) operating conditions, namely compliant riser cable configurations with very dynamic mechanical loading regime. The comprehensive project involves reviewing and strengthening the hydrodynamic and mechanical modelling practices, designing and manufacturing of a highly dynamic test cable incorporating both best practices and novel solutions and finally assessing design choices and the overall cable performance through a range of mechanical testing and analysis. A number of workshop based tests were carried out to support the assessment and calibration of modelling and design procedures mainly focusing on calculating submarine power cables mechanical properties, assessing failure modes and effects under cyclic loads as well as strength and fatigue life. They included quasi-static tensile and bending tests as well as dynamic cyclic bending against template type fatigue testing according to industry recommendations (CIGRE, 1997; DNV, 2012). The test work conducted at the DMaC test facility site between July and August 2014 contributed to the calibration of cable mechanical modelling practice and provided critical information that was used to support the test cable design choices. Testing focused on two of the critical aspects of a dynamic cable design, namely bend stiffness characterization and fatigue life estimation. DMaC based measurements of the test cable bend stiffness under dynamic loads well complemented the quasi-static measurements carried out in-house. The results were then used to assess of the reliability of the presently used cable mechanical modelling tools. The test also provided an indication of the structural damping effects due to bending. The fatigue testing carried out could take full advantage of the DMaC test rig ability to apply complex loading regimes that replicate dynamic cables operating conditions in a fully controlled test environment. Loads conditions exceeding normal operations were applied in order to accelerate components degradation and the development of failures.

1.2 DEVELOPMENT SO FAR

1.2.1 Stage Gate Progress Relevant stage gate progress criteria are highlighted in the table below. However, the reader should note that the listed criteria tend to refer to full device development and hence not always suitably describe dynamic power cable development stages.

Previously completed: Planned for this project:

STAGE GATE CRITERIA Status

Stage 1 – Concept Validation

Linear monochromatic waves to validate or calibrate numerical models of the system (25 – 100 waves)

Finite monochromatic waves to include higher order effects (25 –100 waves)

Hull(s) sea worthiness in real seas (scaled duration at 3 hours)

Restricted degrees of freedom (DofF) if required by the early mathematical models

Provide the empirical hydrodynamic co-efficient associated with the device (for mathematical modelling tuning)


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Investigate physical process governing device response. May not be well defined theoretically or numerically solvable

Real seaway productivity (scaled duration at 20-30 minutes)

Initially 2-D (flume) test programme

Short crested seas need only be run at this early stage if the devices anticipated performance would be significantly affected by them

Evidence of the device seaworthiness

Initial indication of the full system load regimes

Stage 2 – Design Validation

Accurately simulated PTO characteristics

Performance in real seaways (long and short crested)

Survival loading and extreme motion behaviour.

Active damping control (may be deferred to Stage 3)

Device design changes and modifications

Mooring arrangements and effects on motion

Data for proposed PTO design and bench testing (Stage 3)

Engineering Design (Prototype), feasibility and costing

Site Review for Stage 3 and Stage 4 deployments

Over topping rates

Stage 3 – Sub-Systems Validation

To investigate physical properties not well scaled & validate performance figures

To employ a realistic/actual PTO and generating system & develop control strategies

To qualify environmental factors (i.e. the device on the environment and vice versa) e.g. marine growth, corrosion, windage and current drag

To validate electrical supply quality and power electronic requirements.

To quantify survival conditions, mooring behaviour and hull seaworthiness

Manufacturing, deployment, recovery and O&M (component reliability)

Project planning and management, including licensing, certification, insurance etc.

Stage 4 – Solo Device Validation

Hull seaworthiness and survival strategies

Mooring and cable connection issues, including failure modes

PTO performance and reliability

Component and assembly longevity

Electricity supply quality (absorbed/pneumatic power-converted/electrical power)

Application in local wave climate conditions

Project management, manufacturing, deployment, recovery, etc

Service, maintenance and operational experience [O&M]

Accepted EIA

Stage 5 – Multi-Device Demonstration

Economic Feasibility/Profitability

Multiple units performance

Device array interactions

Power supply interaction & quality


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Environmental impact issues

Full technical and economic due diligence

Compliance of all operations with existing legal requirements

1.2.2 Plan For This Access

1.2.2.1 Objectives

The objectives of the test activities carried out at the DMaC test facility can be summarized as follows:

Bend stiffness measurements

Measurements of varying bend stiffness of a submarine power cable under dynamic bending load regimes representative of floating offshore renewable energy system operating loads. Comparison with estimated results from presently applied numerical modelling method for reliability assessment and calibration.

Assessment of structural damping effects

Assessment of magnitude of structural damping effects due to the relative motion of adjacent internal cable components and the frictional forces acting between the contact surfaces when the cable is subjected to bending loads.

Validating failure mode assumptions

Applying accelerated stress testing technique to fatigue testing in order to validate and/or calibrate failure modes and effects assumptions for the internal cable components – and in turn for the full cable – under severe dynamic loading regime. Dissection of cable samples and detailed visual inspection to be carried out at NSW premises to identify and evaluate components degradation and development of failures

Assessing validity of fatigue life estimation method

To apply cyclic loading up to the limit of the estimated fatigue life to cable samples and then carry out dissection and analysis of components and material conditions in order to verify if the incidence of failures as well as their distribution within the cable structure and components is in agreement with expectations. The result is intended to only provide an indication of the validity of fatigue life estimation method as the limited number of samples that can be tested in the available time are insufficient to account for the non-deterministic nature of the fatigue failure process(Ellyin, 1996). Dissection and analysis carried out at NSW premises.

2 OUTLINE OF WORK CARRIED OUT

2.1 SETUP

2.1.1 Materials and equipment

2.1.1.1 Submarine MV power cable test samples

Test specimens were 5.5m long sections of a three-core x 50mm2 conductors submarine MV power cable with rated voltage 12/20 (24) kV designed according to IEC60502-2 standard(IEC, 2014). A basic layout and components list is shown in Figure 2.1. Please note that while this cable was not designed for dynamic applications it was chosen for two main reasons. On the one hand the design includes few relevant candidate solutions for the new highly dynamic test cable design. On the other, the relatively simple layout was selected for a more efficient result analysis.


Rev. 1.1, 30-Sep-2015 of 18

Figure 2.1 - Submarine MV power cable test sample

2.1.1.2 Dynamic Marine Component test facility

The DMaC is a purpose built test rig that aims to replicate the forces and motions acting on system components in offshore applications. At one end of the test rig, named the tailstock, a linear hydraulic cylinder can apply tension and compression force (replicating heave). At the opposite end, the headstock can move with two degrees of freedom (replicating pitch and roll). The cable specimens were installed on the test rig using custom made mechanical terminations. The equipment and test arrangement are shown in Figure 2.2. Fixtures are described in the following section.

Figure 2.2 - DMaC test rig in operation during the cable tests

2.1.1.3 Fixtures

Cable samples were installed on the DMaC test rig using custom made mechanical terminations. At both ends, the cable was fixed through its steel armoring wires. The load at the tailstock end was expected to be acting mainly along the direction of the cable longitudinal axis for all the planned test cases. Consequently a relatively simple mechanical termination was chosen (Figure 2.3).

Copper conductors

Polyethylene filling and jacket

Insulation system (XPLE)

Galvanized steel wires armoring

Cladding


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Figure 2.3 - Tailstock fixture assembly

More complex loading was expected at the headstock end where bending was also applied to the cable specimen through the inclination of the headstock plate. The headstock attachment included a cylinder head ending with a bellmouth to ensure that the section subjected to maximum curvature during testing is sufficiently far from the cable end to minimize the influence of the mechanical termination (Figure 2.2).

2.2 TESTS

2.2.1 Test Plan

2.2.1.1 Bend stiffness and structural damping measurements

Cable bend stiffness – a measure of its resistance to bending – is function of the modulus of elasticity E and the second moment of area I. Further basic beam theory, bend stiffness can be calculated as the ratio of applied moment M and resulting curvature κ. That is:

𝑬𝑰 =𝑴

𝜿

[1]

In other words, bend stiffness is given by the slope of the curve produced by plotting bend moment vs. resulting curvature. The cable mechanical model presently used by this user group only provides a single bend stiffness value and assumes a simple linear relation between bend moment and curvature. However, as both material and structural damping effects are expected to occur during cyclic bending, the plotted measurements are expected to produce a hysteresis curve. The area enclosed by the curve is proportional to the energy dissipated by damping. Under bend stiffness/damping testing, the cable is bent cyclically to a maximum specified curvature by applying headstock inclination about the vertical axis while maintaining a constant tension along the cable longitudinal axis. Curvature is calculated as a function of headstock inclination while the bend moment is directly recorded by DMaC. Measurements were carried out with different combinations of maximum curvature κMAX and cycles duration T as shown in Table 2.1.

κMAX[m-1] 0.20 0.33 0.45

T [s]

8 3 cycles 3 cycles 3 cycles



Table 2.1 - Bend stiffness/Structural damping test cases


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2.2.1.2 Fatigue testing

Converting global loads to internal stress distribution

In order to calculate the fatigue life of the cable critical components (and consequently the cable fatigue life), global bending, torsional and tensile loads need to be combined and converted into effective stresses acting locally on internal cable components. Numerical modelling was used to calculate resultant von Mises (maximum effective) stress for individual components when external loads are applied within the cable operating loads range. The range for each loading mode (i.e. bend, torsion and tension) was defined between minimum loads below which further decreases result in very marginal changes contribution to effective stress and maximum loads that must not be exceeded as likely to cause failures. Within the restricted ranges, it was possible to define linear conversion relations linking to a good approximation inner cable components maximum effective stress (σMES) to both curvatures (ρ) and axial load (T) as follows:

𝝈𝑴𝑬𝑺[𝑴𝑷𝒂] = 𝑨 × 𝝆[𝒎−𝟏] + 𝑩 × 𝑻 [𝒌𝑵]

[2]

Where both A and B are coefficients calculated for each cable component.

DMaC input loads time series

Applied loading regimes were based on the results of numerical modelling carried out using OrcaFlex simulations that included a submarine power cable linking a floating host to a point on the seabed and operating under storm conditions. The cable configuration followed the specimens’ mechanical properties while the selected cable route type in the water column was lazy wave including a suitable stiffener and floating elements. The cable section subjected to the most severe load regime was identified, a 250 seconds long period was selected and both curvature and axial loading time series were extracted to be used during the fatigue test. The procedure was intended to ensure that the applied loading reflected the full spectrum of combined axial and bending variable loading acting in offshore field deployments. The following procedure was followed to estimate the cable fatigue life:

- Combination of global loading time series as per [2] to calculate each component maximum effective stress time series 𝜎𝑀𝐸𝑆

- Filtering of 𝜎𝑀𝐸𝑆 time series through the Rainflow counting algorithm in order to convert irregular varying stresses into simple stress reversal cycles.

- Calculating cable components fatigue life by applying the standard S-N curve and Miner-Palmgren rule of damage accumulation approach using the Rainflow counting output

The DMaC input loads time series based on the global cable loads were calibrated so that both cable samples reached the estimated fatigue life limit within the time allocated for the test. Figure 2.4 show tensile and curvature load signals. They are shown as percentage of minimum breaking load (MBL) and recommended cable maximum curvature (𝜅𝑀𝐴𝑋) respectively.

Figure 2.4 – DMaC input loads time series

0%

10%

20%

30%

0 50 100 150 200 250

Ten

sile

load

[%

of

MB

L]

time [s]

Tensile load input

0%

10%

20%

30%

40%

0 50 100 150 200 250

Cu

rvat

ure

[%

of

κ MA

X]

time [s]

Curvature input


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Scheduling

Fatigue testing was carried out on two cable samples. For each sample, dynamic loading was applied continually for 13 days. After loading, both specimens were then shipped to NSW premises for dissection and material analysis.

2.3 RESULTS

2.3.1 Bend stiffness and structural damping Figure 2.5 shows the bending moment against curvature as measured by the DMaC for the three selected maximum curvature cycles all on a 16 seconds period. The plots are compared to the (linear) moment calculated through the cable mechanical model. Changes in the cycle duration produced no significant difference in the measurements and practically identical plots.

Figure 2.5 - DMaC measured bending moment vs. curvature under cyclic bending (T=16s)

2.3.2 Fatigue testing Visual inspection during cable dissection identified significant wear along the lines of contact between power cores in both specimens. The damage is shown in Figure 2.6, with the two yellow arrows marking the line of contact.

-1

-0.5

0

0.5

1

-0.5 -0.3 -0.1 0.1 0.3 0.5M/M

BM

AX

κ [m-1]

Bending Moment vs Curvature

M (κ<0.20)

M (κ<0.33)

M (κ<0.45)

M (as modelled)


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Figure 2.6 - Example of wear identified on outer power cores surface

A total number of four conductor copper wires failed due to fatigue in specimen 2. Figure 2.7 shows the dissection of one of specimen 2 power cores where two fracture wires can be identified as indicated by the yellow arrows.

Figure 2.7 - Specimen 2 conductor copper wires fatigue failure

Additionally, thick oxide patches were present at the copper wires contact points between layers. Matching oxide dark patches are commonly seen when inspecting conductors subjected to cyclic loading. However, in the specimens section subjected to maximum bending load the oxide layer thickness was found to increase with brittle debris flaking off. Deep pitting and cracks were identified on the wires surface under the oxide patches.

Figure 2.8 - Pitting (left) and cracks (right) under oxide patches on specimen 1 conductor copper wire surface


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No copper wires failure was identified in the specimen 1 conductors. Thick flaky oxide patches, pitting and cracks were also present on copper wires surface. Figure 2.8 shows two examples from specimen 1. No further significant damage was identified on any of the cable specimens components.

2.4 ANALYSIS & CONCLUSIONS

2.4.1 Bend stiffness and structural damping The bend moment vs. curvature graph in Figure 2.5 above show the cable bend stiffness – that is, the slope of the curve – varying under the cyclic bending loads. It also shows a slightly different behaviour between cycles where maximum curvature is κ=0.20 m-1 and the two other test cases. Starting from the latter cases, the measured bend stiffness is relatively low in the low curvature area of the graph highlighted by the central grey square. In this part of the bending cycles, the measured bend stiffness is practically constant and in good agreement with the value produced by the cable mechanical model as shown by the very similar lines slopes. However, the measured bend stiffness increases markedly when the cycle moves towards higher curvature values and further increases when there is a change in the direction of bending at the extremes of the cycles. It can be assumed that when low curvature is applied to the cable, the magnitudes of the internal frictional forces are relatively low. Consequently, the internal cable components are able to move freely relative to each other. With the increase of curvature, bend stiffness increases due to the larger internal friction effects. The effect is further increased when there is a change in bending direction. The higher bend stiffness measured for low curvature loading cycles can arguably have a similar explanation as it can be assumed that due to minimal or no relative motion regime, static friction will be acting between at least some of the internal cable components The plotted moment vs. curvature measurements produced a hysteresis curve in each one of the test cases confirming that power cables structural damping under bending could be significant, depending on the cable design.

2.4.2 Fatigue testing Numerical analysis identified the cable copper conductor wires as the critical elements governing the fatigue life of the selected cable design. The result of the tests and following dissection and analysis supported the assumption by finding significant damage in the copper conductors while no significant failures were identified on any of the other components. Wear damage exceeding expectation was found on the power cores polyethylene outer layer. While the damage is not expected to compromise the cable functionality, the failure mode analysis procedure will be reviewed so that the issue could be considered in future designs. On fatigue life estimation, the results were considered to be largely in line with expectation. However, due to the limited number of samples testes and the non-deterministic nature of the fatigue failure process a high level of uncertainty still remain on the validity of the fatigue life estimation procedure.


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3 MAIN LEARNING OUTCOMES

3.1 PROGRESS MADE

3.1.1 Progress Made: For This User-Group or Technology Bend stiffness measurements confirmed expectations that the assumption of simple linear bend stiffness behaviour in mechanical models of power cables under dynamic bending loads may provide unreliable results as variation of bend stiffness under varying bending loads can be significant, depending on the design. As both bend stiffness and structural damping significantly affect a dynamic cable hydrodynamic behaviour, a more accurate estimation would enable the selection of less conservative cable designs. Fatigue test results largely confirmed failure modes assumptions and provided supporting evidence on the suitability of some of the design choices considered for the new highly dynamic test cable design. The cable fatigue life estimation method proved largely effective during these tests, although further work is still required to confirm its validity.

3.1.1.1 Next Steps for Research or Staged Development Plan – Exit/Change & Retest/Proceed?

Further work to be carried out to investigate improvements of the applied cable mechanical modelling methods. A new highly dynamic test cable designed and manufactured with the objective of testing the suitability of a range of design solutions for typical floating ORE operating conditions.

3.1.2 Progress Made: For Marine Renewable Energy Industry The work carried out at the DMaC test facility is part of a development project aimed at designing and manufacturing highly dynamic MV submarine power cables that can meet the marine renewable energy industry demands.

3.2 KEY LESSONS LEARNED - Dynamic loading regimes including severe bending can potentially develop fretting fatigue in copper

conductors. - Material and structural damping effects in a submarine dynamic power cable under dynamic bending can be

significant, depending on design. Their impact on cable hydrodynamic behaviour may lead to the reduction of global hydrodynamic loads.

- Applying non-linear bend stiffness behaviour and accounting for hysteresis effects in hydrodynamic modelling would be beneficial as it would yield less conservative cable designs

4 FURTHER INFORMATION

4.1 SCIENTIFIC PUBLICATIONS List of any scientific publications made (already or planned) as a result of this work: Partially contribution for the following conference paper:

MARTA,M., MUELLER-SCHUETZE,S., OTTERSBERG,H., ISUS FEU,D., JOHANNING,L., THIES,P.R., (2015) Development of dynamic submarine MV power cable design solutions for floating offshore renewable energy application. Paper presented at the JICABLE'15, Versailles, France.


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5 REFERENCES CIGRE. (1997). ELECTRA 171 - Recommendations for Mechanical Tests on Sub-Marine Cables (pp. 59-65). DNV. (2012). DNV-RP-F401: Electrical Power Cables in Subsea Applications: DNV. Ellyin, F. (1996). Fatigue Damage, Crack Growth and Life Prediction: Springer. IEC. (2014). IEC 60502-2 - Power cables with extruded insulation and their accessories for rated voltages from 1kV up

to 30kV.

6 APPENDICES

6.1 STAGE DEVELOPMENT SUMMARY TABLE The table following offers an overview of the test programmes recommended by IEA-OES for each Technology Readiness Level. This is only offered as a guide and is in no way extensive of the full test programme that should be committed to at each TRL.


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