pelamis joint system test

8/9/2019 Pelamis Joint System Test

1/49

PELAMIS WEC – FULL-SCALE

JOINT SYSTEM TEST

V/06/00191/00/00/REPDTI URN 03/1435

ContractorOcean Power Delivery Ltd.

Prepared by Dr. Richard Yemm

The work described in this report was carried

out under contract as part of the DTI New

and Renewable Energy Programme, which is

managed by Future Energy Solutions. Theviews and judgements expressed in this

report are those of the contractor and do not

necessarily reflect those of the DTI or Future

Energy Solutions.

First Published 2003© Crown Copyright 2003


2/49

I

EXECUTIVE SUMMARY

INTRODUCTION & BACKGROUND

The Pelamis Wave energy Converter (WEC) is an innovative concept for extracting

energy from ocean waves and converting it into a useful product such as electricity,direct hydraulic pressure or potable water. The system is a semi-submerged,

articulated structure composed of cylindrical sections linked by hinged joints. The

wave-induced motion of these joints is resisted by hydraulic rams that pump high-

pressure oil through hydraulic motors via smoothing accumulators. The hydraulic

motors drive electrical generators to produce electricity. The complete machine isflexibly moored so as to swing head-on to the incoming waves and derives its

'reference' from spanning successive wave crests.

OPD has systematically tackled design of the first full-scale machine through a

rigorous RD&D programme including concept development, extensive numerical and

experimental work, demonstration of the whole machine concept at 7th scale (through project Reference V/06/00188), preparation of several full machine designs, and a

thorough analysis of the initial and future economics of the system. The programme

reported here has extended this to integrated testing of all key components at full-

scale using a fully functional joint test rig in the laboratory.

The programme reported here met its key objectives, and is seen as a major step inreducing the technical risk of the programme. It paves the way for the first full-scale

prototype machine, and onward optimisation of the performance and reliability of the

entire system.

PROJECT OBJECTIVES

The project had the following key objectives:

1. Build and test a full-scale Pelamis WEC two-axis joint system

2. Confirm functionality of active and passive ('failsafe') joint control modes3. Confirm functionality of power conversion and electrical systems

4. Determine pressure drops through all hydraulic flow paths to ensure inlet

cavitation and local overpressure are avoided

5. Confirm the thermal stability of the system for of normal and failed operating

conditions6. Determine full-cycle conversion efficiency of the complete system at a range

of mean power levels

7. Conduct a three-month cycle test to increase confidence in reliability beforethe first offshore test

8. Confirm suitability of the chosen hydraulic fluid9. Preliminary assessment of static and dynamic seal performance and likely

service life.

10. Preliminary assessment of the suitability of the proposed prototype structure

11. Assess space and accessibility of components and demonstrate procedures for

installation and removal of system components

12. Assess health and safety issues for work on the full-scale system


3/49

II

13. Allow the OPD team to work closely with prospective full-scale contractorsand to gain experience of assembling, testing, operating and maintaining the

full joint system

SUMMARY OF THE WORK PROGRAMME

A full-scale two-axis joint system was built, commissioned and tested. A general

view of the rig is shown below.

The rig comprises

1. A fully functional joint module unit (housing all the systems for two joint

axes)2. Two pairs of external actuation rams with 500kW hydraulic actuation power

packs3. Two end frames to couple the actuation rams to the internal systems

4. A purpose built test facility with a full set of electrical, control and monitoring

equipment.

The rig was built and commissioned over an intensive 8 month period, using most ofthe OPD’s engineering resources.

Work was split into the following six main work packages:

Work Package (a) – Final design of all rig components. Adapting the joint moduledesign to include attachment points for the external actuation

system, and specification/design of all elements of ancillary

equipment.

Work Package (b) – Rig build, including set up of the test facility and infrastructure.

Also included a critical assessment of packaging, space,accessibility and health and safety issues before and after build.

Heave actuation

ram (1 of 2)

Joint module

structure

Sway actuation

ram (1 of 2)

Floor stand

End frame

Main bearing


4/49

III

Work Package (c) – Commissioning of the complete facility and test rig

Work Package (d) – Functionality test programme to confirm that all systems were

working to specification, including an initial inspection

programme to confirm any component failures.

Work Package (e) – An extended cycle test programme to improve confidence in

the reliability and longevity of the system.

Work Package (f) – A detailed inspection programme following the cycle test toassess likely long term wear-rates and reliability.

This work programme was completed, the key results are reported below.

KEY CONCLUSIONS

The following overall project conclusions have been drawn:

1. A full-scale, two-axis joint module system has been successfully built,

commissioned and tested in the laboratory. The unit is representative of thefull-scale prototype specification and design, and where possible uses

components identical to those specified for the full-scale prototype machine.2. Actuating the system posed a number of technical challenges. The system

chosen was a pair of large closed circuit hydraulic systems. These generally

performed satisfactorily though at high operating pressures problems were

encountered with limited drive rigidity due to compressibility of the oil and

‘stretchiness’ of the hoses. However, it is not felt that this significantly

compromised the tests.

3. All rig systems were proven to function satisfactorily, and in line with

predictions.4. Active and passive control modes were demonstrated, though active

algorithms had to be modified at elevated pressures to address the issuesrelating to drive rigidity.

5. Pressure drops and flow losses through the system were proven to be similar

or less than predicted. It is estimated that generally flow losses will be less

that 1-2%.

6. It has been confirmed that cavitation at the inlets to the ram chambers will not be a problem. Pressure drops on inlet were estimated to be less than 1 bar at

peak flow-rates – significantly less than anticipated. The positive result of this

is that a lower reservoir pre-charge is required, this will prolong the life ofstatic and dynamic seals.

7. Oil cooling provided was found to be adequate. However, estimation of thethermal stability of the module as a whole was not practical. Despite this it is

not felt that there will be a problem in this regard.

8. The measured conversion efficiency was 75-85% across the range of power

levels. This is in line with predictions. It is essential for the economics of the

system that the captured primary energy is converted efficiently. The

achieved conversion efficiency is seen as adequate for initial commercial


5/49

IV

machines though there are several avenues to improve performance – thesewill be actively pursued in the onward programme.

9. The rig was cycle tested for a period of one and a half months. This was lessthat the three months originally envisaged. It was felt that the best use of the

rig was testing and commissioning actual components and sub-systems for the

full-scale machine. However, it is felt that sufficient testing was carried out tomeet the objectives of the programme.

10. There is nothing to indicate that the biodegradable oil chosen is not suitable as

a working fluid. However, only extended testing in the full-scale prototype

and subsequent machines will determine any long term issues.

11. Static and dynamic seal performance was generally shown to be satisfactory.The only issues arising were the front rod seals on the rams, and a damaged

static seal on one of the system valves. I was concluded that neither of thesewas serious, and that both were probably caused by external factors.

However, long-term performance will only become apparent through extended

testing in the real environment.

12. It was concluded that the structural configuration chosen for the joint moduleis satisfactory, there are no signs of cracking or yielding of the rig structure. It

was not possible to directly confirm suitability of structural components due to

the more severe load regime of the test rig.

13. Experience gained with the test rig regarding packaging, installation ofcomponents, accessibility and general health and safety issues has been

invaluable for the detailed design of the full-scale prototype. The layout andinstallation procedures have been fully revised for the prototype machine.

14. The rig has provided invaluable experience working with full-scale

components and systems, and with assessing and working with the intended

suppliers for the full-scale machine.

15. Overall, the project is seen as a major success – it has certainly achieved its

primary objective of significantly reducing the technical risk of the onward

programme.


6/49

V

TABLE OF CONTENTS

EXECUTIVE SUMMARY I

TABLE OF CONTENTS V

1. INTRODUCTION…………………………………………………….. 1

1.1 Background 1

1.2 Scope of Work 2

1.3 This report 3

2. TEST RIG SPECIFICATION & DESIGN…………………………. 42.1 Joint system concept 4

2.2 General specification 6

2.3 Structure 8

2.4 Actuation system 102.5 Power conversion system 11

2.6 Test facility 15

3. TEST & INSPECTION PROGRAMME…………………………….. 163.1 Commissioning Tests 16

3.2 Functionality Tests 183.2.1 Primary power take-off 18

3.2.2 Power conversion system 22

3.3 Cycle Testing 25

3.3.1 Details of test programme 25

3.3.2 Anticipated future testing 26

3.3.2 Other cycle testing 26

3.4 Inspection Activities 28

3.4.1 Structure 283.4.2 Hydraulic rams 28

3.4.3 Accumulators & reservoirs 323.4.4 System valves 33

3.4.5 Hoses & fittings 33

3.4.6 Motors & generators 35

3.4.7 Oil quality 35

4. ONWARD DEVELOPMENT CONSIDERATIONS……………....... 37 4.1 General & structure 37

4.2 Power take-off system 374.3 Electrical & control systems 40

4.4 Conclusions 40

5. OVERALL PROJECT CONCLUSIONS………………………..….. 41

6. REFERENCES………………………………………………………… 43


7/49

Page 1

1. INTRODUCTION

1.1 BACKGROUND

Ocean Power Delivery Ltd (OPD) has been actively developing the Pelamis WEC

concept since January 1998.

The Pelamis WEC is a semi-submerged, articulated structure composed of cylindrical

sections linked by hinged joints (Figure 1.1). The wave-induced motion of these joints

is resisted by hydraulic rams which pump high pressure fluid through hydraulic

motors via smoothing accumulators. The hydraulic motors drive electrical generatorsto produce electricity. The complete device is flexibly moored so as to swing head-on

to the incoming waves and derives its 'reference' from spanning successive wavecrests.

Figure 1.1 – Artists impression of a Pelamis WEC wave-farm

A novel joint configuration is used to induce a tuneable cross-coupled resonant

response that greatly increases power capture in small seas. Control of the restraint

applied to the joints allows the resonant response to be 'turned-up' in small seas wherecapture efficiency must be maximised or 'turned-down' to limit loads and motions in

survival conditions. Electrical power from the joints is fed down a single umbilical

cable to a junction on the seabed. Several machines can be connected together and

linked to shore through a single seabed cable.

The core theme of the Pelamis concept is survivability. The fundamental survivability

mechanisms employed are the use of length as the source of reaction (to allow the

system to de-reference in long storm waves) in conjunction with a finite diameter toinduce full submergence and emergence in large, steep waves, thereby limiting loads


8/49

Page 2

and motions. The system is slack moored and does not use mooring reaction in orderto absorb power. The moorings have a motion envelope large enough to

accommodate extreme wave motions in addition to the low frequency wave-groupinduced response.

As part of the overall RD&D programme, OPD has previously been supported by theDTI Sustainable Energy Programme under grants V/06/00181 (to conclude primary

R&D considerations) and V/06/00188 (to demonstrate the Pelamis WEC concept at

intermediate scale). The former project looked at various aspects of the design and

consolidated previous work on small-scale wave-tank models. The latter developed a

1/7th

scale, full-systems model which included fully functional hydraulics and controlsystems. This successfully demonstrated the whole machine concept in real-sea

conditions.

From the outset OPD has been committed to a responsible, staged development

programme to systematically tackle all major elements of technical risk before

committing to a full-scale prototype. A complex electro-hydraulic system such as thePelamis WEC joint unit would not, for example, be cleared for service in the

aerospace or offshore industries without extensive dry-land tests. OPD feels that

wave energy systems should be treated in the same way.

Consequently, the purpose of this project has been to design, build and test a full-scale

model of the Pelamis joint system, with the aim of improving understanding of thesystem’s function, performance and control, and to further reduce the technical risk of

the Pelamis WEC full-scale prototype programme.

1.2 SCOPE OF WORK

The project’s main tasks were as follows:

1. Build and test a full-scale Pelamis WEC joint system2. Confirm functionality of active and passive ('failsafe') joint control modes

3. Confirm functionality of power conversion and electrical systems4. Determine pressure drops through all hydraulic flow paths to ensure inlet

cavitation and local overpressure are avoided

5. Confirm the thermal stability of the system for of normal and failed operating

conditions

6. Determine full-cycle conversion efficiency of the complete system at a rangeof mean power levels

7. Conduct a three-month cycle test to increase confidence in reliability before

the first offshore test8. Confirm suitability of the chosen hydraulic fluid

9. Preliminary assessment of static and dynamic seal performance and likelyservice life.

10. Preliminary assessment of the suitability of the proposed prototype structure

11. Assess space and accessibility of components and demonstrate procedures for

installation and removal of system components

12. Assess health and safety issues for work on the full-scale system


9/49

Page 3

13. Allow the OPD team to work closely with prospective full-scale contractorsand to gain experience of assembling, testing, operating and maintaining the

full joint system

At the outset of the project, it was planned to base the rig on a simplified version of

the full-scale joint that would mainly allow testing of the system components.However, in parallel with advancements to the power take-off design, it was realised

that testing the complete, dual-axis system would be of greater benefit, giving greater

confidence in the structure, mounting hardware, access, operability, maintainability,

thermal stability and safety of the joint system. Although it was recognised that a

dual-axis rig would be more expensive, the extra cost was considered well-justified onthe grounds of increased test realism and scope. The project therefore proceeded on

this basis.

1.3 THIS REPORT

This report summarises all aspects of work carried out under the project. The main body is set out as follows:

Section 2: A general description of the full-scale joint rig and test facility

specification, configuration, and design.

Section 3: A description and discussion of the test and inspection programmeincluding commissioning, functionality, and cycle testing.

Section 4: A discussion of onward development consideration.

Section 5: Project conclusions


10/49

Page 4

2. TEST RIG SPECIFICATION & DESIGN

2.1 JOINT SYSTEM CONCEPT

The Pelamis WEC power take-off system design underwent a major revision at the

end of 2001. Each joint between adjacent units was formerly of a conventionaluniversal joint configuration with a 'spider' linking the main tube elements (Figure

2.1). Opposing hydraulic rams mounted in the ends of each tube acted on the spider

to restrain the joint about its two axes. Bellows seals were used to prevent water

ingress as the ram exits, while the main bearings were water lubricated.

Figure 2.1 – Old Pelamis joint configuration

The revised configuration houses all of the systems for both joint axes within an

elongated universal joint spider, as shown in Figure 2.2. Consequently, all of the

complex power conversion components are contained in a single, compact space.

This has the advantages of:

• Allowing the system to be fully assembled and tested before movement to the

intended launch site.

• Minimising final assembly and commissioning operations at the launch site.

• Separating the main structural and power take-off elements. The ‘high-tech’

joint module can be manufactured centrally, while the ‘low-tech’ main tubes

could be fabricated close to launch sites. This is beneficial from thecommercial perspectives of IPR protection and export sales. In this context,

the 'joint-module' is analogous to the nacelle of a wind-turbine.

The new configuration also has a number of significant technical advantages over the

previous layout, some of which are summarised below.

• All of the power take-off and conversion systems are now in one place

allowing the heave and sway hydraulic systems to be linked. This reduces the

total accumulator volume required to generate smooth power across the full

range of wave conditions.

• Two generators are retained to allow one to be run at part load. Thisdramatically improves efficiency at power levels less that 50% of rated, gives


11/49

Page 5

a level of redundancy in the event of partial failure, and increases the servicelife of the bearings and hydraulic motors.

• The hydraulic circuits are designed to split into two independent systems in

the event of failure. One heave and one sway ram are on each half of the split

system, allowing joint restraint to be maintained even in the event of complete

failure of the other half of the system. A good analogy for this is dual circuit braking on cars.

Figure 2.2 – Joint-module configuration (ram seals not shown)

• All bearings are now internal to either the joint-module or the main tube ends,

thus allowing bearing pins to be removed for inspection without surface divers

or water ingress.• The main bearings are now dry, sealed with ‘O’rings.

• The structural duty of the joint-module is low as all major bending loads between the main tubes to pass through the hydraulic rams.

A result of this change in configuration was that, to be a representative test, the full-scale joint test rig must be a full 2 degree-of-freedom unit, rather than the single axis

unit originally proposed (Figure 2.3).

Figure 2.3 - Single axis test rig as originally proposed


12/49

Page 6

2.2 GENERAL SPECIFICATION

The dual-axis test rig configuration is shown in Figure 2.4. Two sets of actuationrams are mounted outside the module to simulate wave action and drive the internal

systems. The main tube elements are replaced with rigid, triangulated open truss

structures on each end of the rig to couple the drive and power take-off rams.

Figure 2.4 – Test rig configuration as built

For meaningful tests the system must be able to run both joint axes up to full rated

moment, angle and power. In addition it was important to ensure that the inlet valves

to the rams would not cavitate at extreme angular velocities. The system was

therefore designed to run in two modes, one for all normal testing where angular

velocity was restricted to ~7.5 degrees/second, and a ‘high-speed’ mode with one

chamber of each actuation ram isolated giving peak angular velocities of >15

degrees/second, (which is consistent with the highest values observed in model

testing).

While the rated, steady output power of each axis is ~125kW, the instantaneous peakinput power can be up to 1MW per axis in extreme seas. This overdrive ratio of >8:1

was not possible using available hydraulic systems so a compromise was made. A

pair of the largest displacement Denison Goldcup units were chosen to give a peak

instantaneous power of >500kW per axis. At higher instantaneous powers the only

important difference is the increased pressure drop in the flow path. As long as it isnot extreme this is not a problem on the high pressure side of the system, on the low

pressure side cavitation at inlet is a potential problem as mentioned above. If the inlet

cavitiates there will be no resistance to the return stroke, and oil foaming may causefurther problems due to oil compressibility in this state. The high speed mode

allowed it to be confirmed that this would not be a problem.

The test rig was therefore specified as follows:

• Peak actuation power 500kW per axis

• Mean actuation power 350kW per axis

• Rated steady output power 125kW per axis• Angle limits +/- ~15 degrees


13/49

Page 7

• Angular velocity limits (normal) +/- ~7.5 degrees/second

• Angular velocity limits (anti-cavitation tests) +/- ~15 degrees/second

Further details of the rig are described in the following sections.


14/49

Page 8

2.3 STRUCTURE

The rig structure (Figure 2.5) is based on the full-scale prototype structural design,with the principle differences being to accommodate the external actuation rams. The

loading in the rig is actually much more severe than for the module in service due to

the large moments generated between the actuation and power-take off rams, butcareful design and the use of extra stiffening plates solved this problem. The rig

structure was been the subject of a full finite-element analysis using a similar

methodology to that used for the full-scale prototype design. An example plot is also

shown in Figure 2.5, with the deflection exaggerated 100 times.

Figure 2.5 – Cutaway view of the joint test rig structure & finite element analysis

results

The structure comprises the main module shell and the two end frames to provide the

reaction between the externally mounted actuation rams and the internal power take-

off rams. The main shell is an assembly of prefabricated parts to form a rigid shell. It

is intended that the ram and main bearing attachment points are castings in the full-scale prototype to remove welds from highly stressed areas. However, for the test rig

heavy fabrications were used as castings were not practical for the test rig as only fourwere required. Various frames, end plates, internal stiffening rings and four rolled

closing plates complete the structure. All internal system components are mounted on

elements of the main structure such as the ring frames or end walls to minimise part

count and fabrication costs.

The structure was built by Ross Deeptech Initiatives Ltd in Stonehaven. Several

pictures of the structure during fabrication are shown in Figure 2.6 on the following

page. The finished structure was assembled at Ross Deeptech before shipment toOPD’s test facility for population with components and subsequent commissioning

and testing.


15/49

Page 9

Figure 2.6 – Test rig structure: Ram attachment inserts during fabrication (top left),fabrication of the main shell in process (top right), welding out the completed shell

(middle left), machining the bores on the end rigs (middle right), final finishing work

on the completed module (bottom left), the complete, painted, assembled structureinstalled at the OPD test facility (bottom right).


16/49

Page 10

2.4 ACTUATION SYSTEM

Each axis of the rig is driven by an individual closed circuit hydraulic system with a peak rating of ~500kW. A large Denison Goldcup 500cc/rev over-centre pump is

directly coupled to each set of opposing hydraulic actuation rams. The manifolding

system allows the power unit to deliver in excess of ~350kW of r.m.s. power intoeach axis with a peak of ~500kW. Valving allows two of the actuation ram chambers

to be isolated to allow the system to be actuated at over twice the angular velocity at a

reduced moment. This is to confirm that the inlets to the power take-off rams will

not cavitate in extreme conditions.

Each unit is mounted on an independent skid measuring approximately 4.5x2x2.5m.

These units were designed, fabricated and assembled by Hytec Hydraulics Ltd on aturn-key basis. Various views of the actuation packs during build and installed at the

OPD test facility are shown below in Figure 2.7 below.

Insert pics of power packs

Figure 2.7 - 500kW closed circuit hydraulic actuation packs: Trial fitting of drive

motor and cooler at Hytec (top left), Denison Goldcup 500cc/rev actuation pump (topright), views of the two completed packs installed at the OPD test facility (bottom).


17/49

Page 11

2.5 POWER CONVERSION SYSTEM

The general layout within the joint module is shown schematically in Figure 2.8. Ageneral schematic of the power take-off and conversion system is shown in Figure

2.9. Certain details have been omitted for commercial reasons.

Figure 2.8 – Joint module layout

Figure 2.9 – Power conversion system schematic

The PTO system is a development of the previous configurations described in the

final reports for projects V/06/00181 [1] and V/06/00188 [2] (see Introduction). Themain changes are integration of the heave and sway systems into a single unit as

L.P.

H.P.

A

B

AH.P.

B

AXIS 2AXIS 1

M

L.P.

M

MANIFOLD

MANIFOLD

MANIFOLD

MANIFOLD

FILTER

125kW GENERATOR

HEAT X

HEAT X

FILTER125kW GENERATOR

PTO RAM

PTO RAM

PTO RAM

PTO RAM


18/49

Page 12

described earlier. Where possible the major components were dual-sourced to allowOPD to assess what the best supplier options are for the full prototype machine.

The heave and sway rams are specially made units designed for the required load and

motion envelope. The rams (one-off each heave and sway) were sourced from two

different suppliers. The inlet and outlet control manifolds are bolted directly to theram bodies to minimise compressibility effects. A semi-transparent view of the full

manifold is shown below in Figure 2.10, and a picture of an actual unit mounted to a

ram body in Figure 2.11. Before the main order was placed, a single chamber

prototype was built to confirm satisfactory operation over the required pressure and

flow envelope.

Figure 2.10 – Ram manifold schematic

Figure 2.11 - ram manifold installed on the ram body

Central to the performance and reliability of the power conversion system are the high

and low-pressure accumulators. The low-pressure circuit is maintained at a raised pre-charge pressure bar by a pair of 1000 litre low pressure bladder accumulators


19/49

Page 13

shown prior to installation in Figure 2.12. These use a low permeability nitrile bladder to separate the oil and gas volumes to prevent the nitrogen from being

absorbed into the oil. The pre-charge ensures that the inlet valves do not cavitate atextreme joint velocities and that there will be a minimum of dirt and water ingress

into the hydraulic circuit. After much iteration is was decided that the Pelamis system

should use large piston accumulators as the high-pressure reservoirs instead of bladderaccumulators as shown in Figure 2.13. This minimises the required pipe-work and

reduces the probability of serious problems associated with rapid gas release into the

system (for example catastrophic failure of a bladder).

Figure 2.12 – Hydraulic reservoirs

Figure 2.13 – High pressure piston accumulators

Power conversion is performed by a pair of hydraulic-motor/induction-generator

units. OPD has chosen simple induction generator units for the prototype machine to

minimise the complexity of the control and switch gear required. Each unit can be

run independently from the common accumulator bank to minimise exposure to

failure of a single unit, and to allow conversion efficiency to be maximised andrunning hours minimised at part load. OPD dual sourced the motors and generators.


20/49

Page 14

All of the main components are plumbed together using flexible hose and SAE

standard flange fittings. Hard pipe work was considered, but ultimately rejected dueto cost, the difficulty of meeting the required positional tolerances, and the conclusion

it would be unreliable due to thermal expansion and flexing of the joint module

structure. Also, connecting and disconnecting individual components is much easierusing flexible piping. A central 'hydraulic-hub' manifold is used to make all the key

connections (Figure 2.14). This is located at one end of the joint module.

Figure 2.14 – Main connections manifold

All system components were installed by OPD personnel. One of the key objectivesof this programme was to give OPD experience of working with the full-scale

technology, it is felt by OPD that it is important for the designers of the system toexperience issues relating to assembly so that that this can inform the design of future

systems. The experience gained from packaging and assembly of the full-scale joint

rig has led to a number of major improvements and simplifications for the full-scale

prototype joint modules.

The system was originally commissioned and tested using the 7th scale control and

data acquisition system. However, a second revision of the control hardware has been

built and retro fitted into the test rig since completion of the cycle tests. This hasallowed the actual control hardware to be fitted to the full-scale prototype to be tested

on a realistic platform.


21/49

Page 15

2.6 TEST FACILITY

Given the nature of the testing and the need for test supervision during the initialstages, OPD felt that the most effective route was to commission its own facility for

the purpose, close to its offices. OPD leased and upgraded a 450m2 industrial unit

from Forth Ports plc for the test rig. Upgrade work included thorough cleaning and painting, installation of an office, installation of a hydraulics assembly area, a

mechanical workshop area, and the required 5tonne overhead gantry crane and 2MVA

electrical supply. Also provided was a control area with provision for the joint

computers and hardware, including independent condition monitoring and emergency

shut-down system, as well as a dedicated independent 64 channel data acquisitionsuite.

The costs of this were significant within the overall project, but were justified on the

grounds that the location’s convenience meant that travel costs were minimised, and

in any case, it proved impossible to source an alternative facility that did not require

electrical upgrade.

Various views of the facility during preparation are shown below in Figure 2.16.

Figure 2.16 – The OPD test facility: During preparation (top left), installation of the

overhead gantry crane (top right), during installation of the full-scale joint rig (bottom

left), and fully commissioned (bottom right).


22/49

Page 16

3. TEST & INSPECTION PROGRAMME

Testing of the full-scale joint rig fell under three main headings:

• Commissioning Tests – to confirm satisfactory operation of all test facility and

rig systems• Functionality Tests – to characterise all joint power take-off systems, and

confirm performance was as anticipated

• Cycle Testing – to improve confidence in the initial reliability of the system

prior to the first offshore tests, and highlight any early wear issues

The scope and results from each of these are set out below.

3.1 COMMISSIONING TESTS

During December 2002 the test facility and test rig were successfully commissioned.

Test included confirmation of satisfactory operation of:

• The grid connection and test facility electrical systems

• The actuation power-packs

• The joint module instrumentation, control and electrical systems

• The overall test facility data acquisition suite

The 2MVA 6.6kV/415V grid connection was witness tested by Scottish Power to

confirm satisfactory operation of the fault trip and earthing systems. This is a near

identical process to that that will be required at the test centre prior to installation ofthe full-scale prototype, and gave valuable experience.

Each of the actuation packs and the joint module are served by independent electrical

panels. Each of these was commissioned on-site as a suitable connection was not

available at the manufacturer's facility. The actuation packs were run-up to break in

the pumps and were subsequently commissioned using a flow restrictor valve rather

than on the rig itself. This was seen as a prudent measure as the actuation system is

extremely powerful. Once the actuation pump controllers had been commissioned

work concentrated on setting the closed loop control around each joint axis in turn. A

closed loop system is required to properly simulate the action of waves on the real

system. The control loops were closed and the system parameters optimised to givethe best response. There were a number of problems with commissioning the

actuation power packs themselves, these were mostly concerning the boost system for

the Denison pumps. The rapidly reversing load and rapid pressure changes in the

system initially gave problems until a small accumulator was included in the boost

circuit.

The joint module systems include full electrical panels, including back-up battery

supply. Soft-starters are included for the induction generators to reduce the inrush-

current, a power factor correction unit is also included. The complete electricalsystem was commissioned at the manufacturer prior to shipment, however, the system

was re-commissioned on-site to confirm wiring and local voltage drops. The pair of


23/49

Page 17

generators and hydraulic motors were run up, enabled and tripped to confirmsatisfactory operation.

The joint instrumentation and control systems are essentially identical to the 7 th scale

system. However, a number of extra channels have been introduced to allow full

performance appraisal and better diagnostics. The full system was tested, andcalibrated where appropriate. Commissioning then moved to confirming correct

operation of the joint control systems. This process was directly analogous to that

carried out using the 7th scale joint test rig. Many of the tests and analysis tools are

common to both systems, proving the value of the earlier work.

A National Instruments Labview based system is used to collect data from both the

test rig itself and the actuation power packs. A total of 64 channels are available.During the majority of testing a total of approximately 40 channels were used.


24/49

Page 18

3.2 FUNCTIONALITY TESTS

The most important tests carried out were the functionality trials. These were to:

• Confirm satisfactory operation of the actuation system across the design

load/motion envelope• Confirm satisfactory operation of the active joint control algorithms across the

design joint restraint envelope

• Characterise flow induced pressure drops through the complete system

• Confirm that there was no cavitation at the inlets to the ram chambers

• Confirm satisfactory operation of the hydraulic motors, induction generators

and controls

• Confirm that the full-cycle conversion efficiency is as anticipated

The primary hydraulic system comprises the rams, manifolds, flow control valves,

reservoirs, accumulators and pressure relief valves. Testing of the primary hydraulicsystem was the most important set as they proved the more innovative parts of the

system. While the system is an assembly of proven components, the joint module

assembly is a new application of this technology – it was a key objective of this

programme to confirm the satisfactory operation of the system as a whole.

The power generation system is conventional by comparison. Early tests were carried

out at reduced pressure while problems with the actuation system were resolved, and

to reduce the stored energy in the system while confidence built. After various

problems during commissioning were solved, the actuation system the worked

satisfactorily during all subsequent testing. It should be noted that most of the

problems experienced during commissioning and operation have been due to theactuation system. It can be concluded that simulating the North Atlantic is not an

easy task!

3.2.1 Primary power-take-off

A large number of active and passive tests were run to confirm functionality of the

primary power take-off system. The results can not be fully presented here for

commercial reasons. However, the key results are discussed in some detail below.

The active primary power take-off system functionality tests were carried out using a

control program previously proven on the 7th scale system. Indeed, most of thealgorithms proved to be directly portable between the two systems once corrections

had been made for geometry, pressure and the time-constants of active components.

This proves the value of the 7th

scale test rig and machine programmes. Example

results are shown in Figures 3.1 & 3.2.

Active algorithms were applied to cover the intended range of joint restraint. Initially,tests were conducted at reduced pressure to lesson the amount of stored energy in the

system. These tests proved that the main algorithms were sound. However, tests athigher pressures caused problems due to compressibility of oil and the stiffness of the

hoses in the actuation system. Tests at these levels had to be conducted with modified

algorithms to prevent rapid depressurisation of the actuation system leading toundesirable pressure spikes in the inlet manifold of the primary power take-off


25/49

Page 19

system. This was not an issue on the 7th

scale joint test rig as the drive system wasmore rigid. It will not be a significant issue with the full-scale prototype machine at

the structure is of adequate rigidity. However, initial sea trials will quantify the effect.

Figure 3.1 – Results from an example test showing high-pressure accumulator,

reservoir, and ram chamber pressures (top), and applied moment records (bottom).

The control algorithms used for these tests were identical to those implemented on the

7th scale machine – proving the value of the intermediate scale programme.

Figure 3.2 – Test results for typical stiffness (left), damping (middle) and complex(right) joint impedances. The damping and complex results show more disturbances

and oscillation than the stiffness results due to the compliance of the rig actuationsystem (note however that the traces are repeatable). However, quantised moment

levels are still clearly visible.

The results from the joint test rig were correlated the Pel power take-off system

numerical model. The methodology used was identical to that used for the 7th scale

system under V/06/00188, proving the value of the earlier work. Catalogue values forall system impedances, response times, volumes, compressibility etc were first entered

y = 12.051x

-1.6

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

1.6

-0.1 -0.05 0 0.05 0.1

y = 21.444x-1.6

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

1.6

-0.08 -0.04 0 0.04 0.08

y = 15.373x-1.6

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

1.6

-0.08 -0.04 0 0.04 0.08

Upper Chamber Pressures

0

10

20

30

40

50

60

70

80

90

100

10 12 14 16 18 20 22 24 26 28 30

Time (s)

P r e s s u r e ( b a r )

4.1

4.2

3.1

3.2

State

HP acc

LP res

Total Applied Momemt

-1.5

-1

-0.5

0

0.5

1

1.5

10 12 14 16 18 20 22 24 26 28 30

T ime (s )

M o m e n t ( M N m )

Drive


26/49

Page 20

into the model. Agreement was found to be poor for most cases. Measured valuesderived from results from the rig were subsequently used – agreement was much

improved. The power take-off model in the Pel machine simulations is now a veryaccurate representation of the system for all operating cases studied, an example of

the level of agreement achieved is shown below in Figure 3.3. The good level of

agreement means that the power take-off model can now be used to predict theresponse and performance of the full machine with a good degree of confidence.

Figure 3.3 – Example chamber pressure records from the test rig, plotted withcorresponding simulation results. The results show good agreement between the test

rig and the simulation down to the level of individual pressure transient features.

Previous analyses, and experience with the 7th scale machine have shown that ultimate

joint restraint levels achievable are inherently limited by the frequency response of the

control valves. OPD have been in close discussion with the valve manufacturers to

characterise and improve the performance of the valves to be used on the full-scale prototype machine. This led to an iterative test procedure using the full-scale joint rig

at the end of the programme. Various valves were modified and tested by OPD and

the manufacturer. They were installed in the rig and tested under real operatingconditions. A good solution was found that gives acceptable frequency response

characteristics without major modification to the valves. The manufacturer has since produced a batch of modified valves for installation in the full-scale prototype. It was

precisely this kind of issue that the full-scale joint was conceived of to address. A

large amount of time and money has been saved by identifying and addressing this

issue before the full-scale prototype is built and deployed.

A further key objective of the test programme was to determine the fluid resistancesin the system to quantify losses and to confirm that there would be no cavitation on


27/49

Page 21

inlet and no extreme over pressure on outlet across all operating conditions. Thegraphs in Figure 3.4 (below) show passive fluid resistances. The left hand graph

shows the inlet characteristics.

Figure 3.4 – Ram inlet (left) and outlet (right) fluid resistances

On induction (negative half of the x-axis) the control valves are designed to ensure

that the resistance is low, so that cavitation is avoided at high flow rates. The pressuredrop lies below 1 bar even at flows of >350litres/minute. It should be noted that the

small pressure differences measured on induction are obscured by the datum accuracy

of the 400bar transducers used in the ram chambers. However, the chamber pressures

can be seen to be flat on induction at a pressure less than 1 bar different from the

reservoir pressure. The induction pressure drop is considerably lower than has been

assumed in the past, this is a very positive result as it will allow the system pre-charge

pressure to be reduced, reducing the cost of the reservoir and increasing the life of all

dynamic and static seals.

Resistance to flow through the outlet control valves (positive half of the x-axis) ishigher due to the different flow-path, this is not a problem as cavitation is not possible

in this case. The graph on the right of Figure 3.4 is the resistance of the high pressure

valve as oil is pumped from the ram. A minimum of around 3.5bar is dropped due to

the cracking pressure of the valve, the pressure drop rises to approximately 8 bar at

300litres/minute. Again this is less than was previously anticipated due to thespecification of a larger valve.

The pressure measurements on which the pressure drops shown above are based weretaken inside the ram chambers and at the high or low pressure reservoirs. They

therefore include the drops over the hoses and manifold. The results imply that around1-2% of absorbed power would be lost due to pressure drops through the flow path

depending on the operating conditions. This agrees well with previous estimates of

flow losses made in V/06/00181. The majority of the losses are due to the cracking

pressure of the high pressure non-return valve, the design and choice of this valve is

currently being reviewed.

HP (outlet) valve pressure drop vs flow for

chamber 1.1

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300

(lt/min)

( b a r )

LP (inlet) valve Pressure drop vs flow for

chamber 1.1

-2

0

2

4

6

8

10

-400 -200 0 200 400

(lt/min)

( b a r )


28/49

Page 22

Further tests showed that variation of fluid resistance with oil temperature was notsignificant (see Figure 3.5).

Figure 3.5 – Effect of oil temperature on fluid resistances (13.5oC (left), 34.3

oC

(right)) showing the expected small reduction in pressure drops with risingtemperature as fluid viscosity falls.

To put losses in context, 1-2% losses due to flow restrictions would equate to £1,000-

£2,000 per annum at Renewables Obligation prices. This clearly shows why allaspects of system efficiency must be optimised. Considerable effort will be focussed

on optimising the flow paths for future machines.

3.2.2 Power conversion system

A broad range of tests have been conducted on the power generation system and

controls. The functionality of the electrical control panels has been fully tested with

interlocked start sequences implemented on the FPGA controller. The most important

interlock is ensuring that the hydraulic flow cannot be enabled before the generator is

up to speed with the power factor correction enabled. This has been fullydemonstrated along with the other more minor interlocks.

Active over-speed shutdown has been tested using a signal generator to simulate the

speed encoder. The system also incorporates a mechanical over-speed shutdown in

the form of a ‘flow-fuse’ on the inlet that shuts at 1.5-2 times the full load flow-rate.

This system is seen as a last ditch saver to prevent the hydraulic motor or generator

being damaged in the event of the electrical load being removed while the hydraulicsare still active (the hydraulic motors are rate for up to three times rated speed). The

flow-fuse has been tested with the hydraulic motor disconnected.

Motor-generator performance has been as expected. Figure 3.6 shows example test

records from the full-scale joint rig. The results show a test where both generators arerunning with an average system pressure of ~300bar. The test includes a simulated

emergency shutdown at 50 seconds. Fluctuations in pressure and power are due to the

fact that the input to the rig is a large sine wave. Further refinements to the control

algorithm prior to deployment in the full-scale prototype will significantly improve

the output smoothness.


chamber 1.1

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300

(lt/min)

( b a r )


chamber 1.1

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300

(lt/min)

( b a r )


29/49

Page 23

Figure 3.6 – Example power conversion system results for a short sweep of pressure

and swash angle. The top graph shows the motor-generator rpm, the generator is aninduction machine and as such increases in speed with torque (and therefore power),

this can clearly be seen on the trace. The spike at 50seconds corresponds to thegenerators being tripped. The next plot shows oil pressure in the HP accumulator and

LP reservoir. The lumpiness is due to the unsteady nature of the input power. The

following graph shows the demand and achieved swash angles for the two motors.

The next plot shows individual hydraulic and electrical power, and total power

electrical output power for the same test. The final graph shows overall conversion

efficiency for the two motor-generator sets, calculated as the ratio of electrical powerout to hydraulic power in (pressure x flow-rate).

Motor swash angles (%)

0

20

40

60

80

100

0 10 20 30 40 50 60

Low er Upper low er demand upper demand

Generator speeds (rpm)

1400

1450

1500

1550

1600

0 10 20 30 40 50 60

Low er Upper

Oil Pressures (bar)

0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60

accumulator reservoir

Module 1 Powers (kW)

0

50

100

150

200

0 10 20 30 40 50 60

Pow m1 upp

Pow m1 low

Pow m1 Total

hyd powe upp

hyd powe low

Conversion efficiency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60

Time

upper

lower


30/49

Page 24

Efficiencies have been calculated using input power from high pressure accumulator pressure and flow-rate, and output power from the electrical panel. Measured

efficiencies are typically ~75% at 150bar operating pressure and full-flow (~35-40kW per generator) which is in line with motor and generator data. The combined

efficiency rises to 85% - 90% at 300 bar (~100kW per generator). Again this is in

line with motor and generator manufacturer’s data. The results are conservative dueto losses through the long pipe work of the test rig, the motor/generator sets and heat

exchangers were mounted outside the joint module for all testing to allow safe access

to the equipment while the rig is running. This adds a total of ~60metres of extra

hose, and several couplings in the circuit, giving an extra ~2-3bar of pressure drop at

full flow. The actual module will have much shorter pipe work and overall efficiencycan be expected to rise by 1-2% from the figures presented. The power conversion

packs and heat exchangers on the full-scale prototype have been redesigned and re-sited to minimise hose length and connections.

The measured efficiencies of all of the components show that the overall conversion

efficiency of the system will be better than 80% for most operating conditions. Thisis a very encouraging result as it confirms that the performance of the OPD power

take-off and conversion system is good enough (even in its current form) for direct

application in future commercial machines. However, improvements to the various

flow paths are expected to lead to modest improvements of the order of 1% at high power levels, at no extra cost, with no new technology required. Larger gains are

possible through the development of specialist hydraulic motors optimised for theapplication. OPD are in discussions with manufacturers regarding this.


31/49

Page 25

3.3 CYCLE TESTING

Cycle testing was carried out over a period of three months from February – April2003. Within this, a total of approximately one-and a half months running time was

accumulated. While this is less than anticipated (due to heavy commitments on the

full-scale prototype design/build programme) sufficient testing has now beenconducted to meet the original project objectives – specifically, to confirm the

satisfactory extended operation of the system and that there are no 'show-stopping'

issues regarding reliability or component wear.

Rather than continuing the cycle tests into May & June 2003 the rig has beenextensively utilised to test additional components for the full-scale prototype and to

commission the actual power conversion systems to be used in the full-scale prototype. These tests are briefly discussed within this report for completeness.

3.3.1 Details of test programme

All cycle testing was to a common format, the testing was monitored throughout with

continuous data logging of 38 data channels (see below) from the rig and actuation

system, and a check of all system parameters every half-hour to confirm satisfactory

operation. The test format was as follows:

• Start up test rig

• Bring system up to operating pressure

• Start up both generators

• Ramp up to test power levels

•Run at steady state conditions, logging test data and monitoring every halfhour

• Change steady state conditions as required

• Shut down at end of test period (typically 10-12 hours continuous)

• Perform post shutdown checks and inspection

• View sampled test data to confirm satisfactory operation of all components

The system proved highly reliable in operation with no un-commanded shutdowns

due to the joint systems. On three occasions one of the generators tripped off during

testing, due to the power limit settings on the generator soft starter. These settingswere adjusted and the problem did not recur. The soft-starter was included to limit

the in-rush current at start-up. This was due to concerns about voltage drops in thesystem causing undesirable interactions between modules. Subsequent testing has

shown that the voltage drops will be manageable and the soft starters have now been

removed altogether.

Several problems were experienced with the actuation-system during early cycle

testing, these were traced to faulty transducers or connections on the actuation packs.In all cases the rig safety shut-down systems and procedures worked correctly.


32/49

Page 26

The following data was collected for all cycle tests:

Description Type Number of

channels

Heave axis angle Unipolar, differential 1

Sway axis angle Unipolar, differential 1

Heave drive signal Bipolar, differential 1

Sway drive signal Bipolar, differential 1

Heave actuation pressure Unipolar, differential 2

Sway actuation pressure Unipolar, differential 2

Main manifold HP & LP pressure Unipolar, single-ended 4

Ram manifold (x4) HP & LP Unipolar, single-ended 8

Ram chamber pressures Unipolar, single-ended 16

Heave controller demand Digital 1

Sway controller demand Digital 1

TOTAL: 38

Data was collected using a National Instruments sampling system and Labview

software – this is the same system as will be used for full-scale prototype structural

monitoring. Configuring and using the system and managing the considerable

volumes of data provided useful experience for the forthcoming full-scale prototype programme.

The data was collected to allow diagnostic analysis to be performed in the event of

problems with system components. However, as there we no significant problems

with the test rig systems, little actual in-depth analysis of the sampled data was carried

out. Analysis was limited to on-line checks while the rig was running to confirm that

the system was running to specifications.

3.3.2 Anticipated future testing

Subsequent to the main cycle tests, the rig has been used to test the full-scale

prototype power conversion packs. All three units are being thoroughly tested prior toinstallation in the joint modules. The final versions of the control system hardware

and software are also being tested. At the time of writing, the rig is operational with a

single joint control system installed, the other two systems are set up on the bench

with dummy inputs to allow the whole system to be tested.

Further cycle tests will be run once commissioning test on full-scale prototype

systems have been completed. It is anticipated that a further months test will be runahead of deployment of the prototype.

Thereafter, the rig will become a test-bed for new valves, components and controlalgorithms, and used to simulate any failures experienced on the full-scale prototype

in operation. The joint rig represents a considerable investment for the company and

we are keen to utilise it fully in the future.

3.3.3 Other cycle testing

It was originally intended that the flexible cable unions between segments of the

Pelamis would be tested on the FSJ rig. However, it was realised early on that theenvisaged rig cycle tests would give a maximum of the order of 1 or 2 x 105 cycles


33/49

Page 27

through moderate angles. It therefore was deemed preferable to build an separate,dedicated rig for this purpose. This rig incorporated both power and signal cabling

including the correct geometry, conduits and glanding. It used a simple gearedinduction motor to drive the flexible union system through its maximum operating

angles (rather than average) at ~3Hz, allowing tests to be conducted at approximately

100 times the real time flexure duty. The rig was run for 1.2 x 106

cycles, giving a predicted flexural test greater than 15 years in normal service. The individual cables

were then examined for signs of damage – none was evident indicating that the

system is performing adequately.

While it was possible to address flexural life and wear with the rig, the tests did notassess the long term sealing and corrosion performance of the system. The flexible

union system incorporates two independent seal paths giving a high degree ofintegrity in the event of a single failure (as is standard practice for marine and

offshore systems). Also, in the event of failure of the external barriers the individual

wire cores are sheathed, giving the system an effective third barrier. Long-term

performance of the complete system will be monitored on the full-scale prototype andfuture machines during offshore trials. However, the multiple independent seal paths

are expected to give the system a high degree of integrity, and will afford a good

degree of failure tolerance. OPD therefore have a high degree of confidence in the

performance and robustness of the flexible joint union system. The sealing andcorrosion performance of the system will be monitored in service on the full-scale

prototype.


34/49

Page 28

3.4 INSPECTION ACTIVITIES

The rig has been thoroughly inspected during and after the cycle test programme.These inspection activities were predominantly aimed at confirming the satisfactory

(or otherwise) operation of all system components, and identifying any components

that were exhibiting early signs of loss of performance or wear. The tests andinspections proved highly useful. One of the aims of the FSJ programme was to

assess alternative suppliers for the main components, the results have proved

invaluable for specifying components for the full-scale prototype machine.

Inspection activities and results are summarised below.

3.4.1 Structure

During the test programme, the rig was visually inspected while stationary and also

during operation. No cracking or distortion has been found to date. The rotational

locking plates for the ram pins were found to loosen during extended cycle testing,their design has been changed for the full-scale prototype to ensure this does not

recur.

3.4.2 Hydraulic rams

The rams have been thoroughly visually inspected throughout the test programme.Inspection activities have concentrated on the following:

• Wear of external rods

•

Rod exit seal leakage• Backlash in rod-end bearings & pins

• Signs of movement in unions between ram barrels and blocks, including tierods and anchors

External rods

The external ram rods have been monitored for signs of early degradation, wear orundue burnishing. None of the rams shows serious wear, though the rams from one

supplier (Supplier A) appear to be in generally better condition than the other

(Supplier B). The visible wear on the front rods of the Supplier B rams may be due to

higher level of dust/dirt contamination prior to installation of an effective dust shield(no dust ingress will be experienced on the full-scale prototype). However, the upper

Supplier A heave and sway actuation rams were most exposed and shows little sign of

wear other than normal polishing and a few shallow scores. The Supplier B sway ram

rod looks the worst – wear here seems unlikely to be attributable to dirt and appears to be more like a degree of binding in the front rod bearing. From the tests and

inspections conducted it appears that the design, build and performance of the unitsfrom Supplier A is superior, these have been specified for the full-scale prototype.

Close-up pictures of each power take-off ram rod are shown on the following two

pages in Figure 3.7(a)-(d).


35/49

Page 29

Figure 3.7(a) - Lower heave power take-off ram external rod condition showing normal degree of polishing

Figure 3.7(b) – Upper heave power take-off ram external rod condition showing normal degree of polishing on underside (right) but some

shallow scoring on upper face (left), possibly due to dirt contamination during early runs


36/49

Page 30

Figure 3.7(c) – Lower sway power take-off ram external rod condition showing normal degree of polishing

Figure 3.7(d) – Upper sway power take-off ram showing more significant surface wear on rod (NB – the wear is not serious but is

significantly more than on other rams and is of a different type inconsistent with scoring due to dirt ingress)


37/49

Page 31

Piston rod front seals

The performance of the internal piston seals is not as critical as that of the front pistonrod seal, where the piston rod exits the ram body. All such seals will leak slowly as

they leave a thin oil film on the rod as part of their normal operation. However, this

should be a maximum of an (estimated) 1-2 drops/day. While it does not pose a problems in terms of oil release into the environment as the rods are inside the robust

outer bellows seal, excessive leakage here is still highly undesirable as it will result in

oil being lost from the system, and will cause a build up in the seal.

All front seals are performing satisfactorily, apart from on the upper heave ram whichis showing significant leakage in operation (4-5 drops/minute) (Figure 3.8). This may

again be due to contamination during early tests but again it is observed that the upperheave actuation ram was in the worst location for contamination, and is showing no

signs of leakage. The poor performance of the upper heave ram front seal was

another key reason why the other manufacturer’s units were specified for the full-

scale prototype machine.

Figure 3.8 – Modest oil leakage from the front seal of the upper heave power take-off

ram

Rod-end bearings

The rod end bearings used on the full-scale joint rig were the recommended wideseries steel-on-steel spherical bearing units, typically used on hydraulic rams. In

service it was found necessary to lubricate these units at regular (~daily) intervals. Atfirst OPD concluded that this would require a centralised greasing system, however,

individual automatic greasing systems were found to be a cheaper, more reliable

alternative.

The frequency of greasing required led to a further detailed study into rod-end bearing

requirements and options. The result of this was specification of larger PTFE-on-steelspherical units for the full-scale prototype, which have a higher dynamic load rating


38/49

Page 32

and maintenance free characteristics. It was not possible to retrofit these units ontothe test rig due to the jaw spacing of the front and rear mounts. However, the

manufacturer has been consulted in detail about their use and in-service lifeexpectancy.

Therefore, the lubrication requirements of the rod end bearings on the test rig cannot be taken as representative of the full-scale prototype in service, as larger PTFE-on-

steel units have been specified. In-service performance will be determined by regular

inspection of the front and rear units on the full-scale prototype machine.

Ram bodies

Careful inspection of the ram bodies was carried out at regular intervals. Allinspection was visual, mainly concentrating on whether the paint line between the

various components had become cracked, crazed or damaged. No signs of movement

were found (see Figure 3.9). The rams specified for the full-scale prototype are more

conservative in terms of structural design, and this gives further confidence.

Figure 3.9 – Two views of the ram barrel end/mid block unions showing no signs of

movement

3.4.3 Accumulators & reservoirs

The accumulators and reservoirs have been examined for external signs of damage,

and none has been found. The main criterion is confirmation of gas retention, assignificant loss of gas would seriously affect performance. While short term

monitoring is impossible due to fluctuations in temperature, long-term monitoring has

found no perceptible loss of gas from either the accumulators or reservoirs – either tothe outside atmosphere or into the oil.

The only other significant potential weak point in the system is the gas connection

between the piston accumulators and their gas back-up bottle as shown in Figure 3.10.

If there is significant motion of either unit in the module this link may be

compromised. While the gas link has been designed to give sufficient compliance to

cope with small motions, gross movements are likely to result in a failure. There has

been no sign of movement during the tests but the accumulator mounting system forthe full-scale prototype has been upgraded.


39/49

Page 33

Figure 3.10 – Accumulator to gas bottle connection

The reservoir is a self contained unit, so has no problems with gas piping. However,there were concerns over the vulnerability of the gas filling valve. A more robust unit

is fitted to the reservoirs for the full-scale prototype.

OPD therefore currently has no concerns over the performance of the accumulator andreservoir units. Though no motion of the units has been evident, OPD have upgraded

the mounting system for the full-scale prototype. The gas filling valves on the

reservoirs have been upgraded to a more robust design for the full-scale prototype

units.

3.4.4 System valves

The Pelamis power take-off system uses a number of valves to control oil flow

through the primary circuits. A number of other valves are used in the rest of the

system, including some large pressure relief valves to limit system pressure in large

seas. Through their life, the valves will see a very large number of cycles (>107),

manufacturers have conducted tests up to 106 cycles which gives a high degree of

confidence for the short-medium term. However, OPD will be monitoring the

condition of the various system valves in service to ascertain whether there is likely to be a serious longer-term problem.

Several valves have been removed for inspection following the main block of cycle

tests. This has included at least one of each type of valve. In general, the valves have

been found to be in near as-new condition with little or no sign of operation. One

valve has been found with a damaged 'O' ring seal, but this damage looks consistent

with pinching of the seal during installation rather than pressure related extrusion.This has been discussed with the manifold supplier. Two views of system valves are

shown in Figure 3.11.

3.4.5 Hoses & fittings

The Pelamis power take-off system uses a significant number of flexible hoses to link

the main components in the hydraulic circuit. As discussed earlier, flexible hoses are

used throughout as they are felt to be more reliable with regard to thermal expansion

and flexing of the joint module structure and are easier to route and install. However,

the use of flexible hoses increases the risk of failures due to pipe chafing. Inspection

of the flexible pipe-work in the full-scale joint rig showed that one pipe had chafed inthe supporting steelwork during the cycle tests. The hose was re-routed and clamped


40/49

Page 34

to prevent further damage. Damage of this type is possibly the highest risk facing thefull-scale prototype hydraulic system, and consequently very careful thought has been

applied to the routing and mounting of all flexible hoses in the full-scale prototype joint modules. Also, the system has been fully reconfigured to minimise the number

and length of flexible pipe work used to further reduce risks.

Figure 3.11 – One of the system check valves showing no signs of wear (left) and the

damaged seal on one of the solenoid pilot valves (right). This looks like damage oninsertion rather than extrusion damage to the seal.

In general the full-scale joint rig used SAE flange fitting for all hose terminations.

However, due to compatibility issues some large BSP cone fittings and a couple of

BSP face fittings using bonded seals were used. Both types are shown in Figure 3.12.

We have had no problems with the SAE flange system at either high or low pressure,

and the 'O'ring element provides effective sealing and the flange bolts have shown no

sign of loosening or moving. However, it has been found that both of the BSP types

are unreliable in these large sizes, perhaps due the difficulty of torquing the two

halves of the joint against each other. Therefore, all BSP fittings requiring in-situassembly have replaced with SAE flanges.

Figure 3.12 – Fittings used were almost exclusively SAE flange type couplings

(shown left on main manifold block), however, a few large BSP fittings using bonded

seals were also used (right). The latter proved less reliable and were prone to slowleakage.


41/49

Page 35

Pipe work has been further minimised by careful grouping of components, with due

consideration to hose routing and clamping to prevent chafing. All large fittings arenow SAE flanges.

3.4.6 Motors & generators

The motors and generators have not been dismantled for inspection. This was deemed

unnecessary as they are 100% standard off-the-shelf components. There was no

change in the sound of the units during the test period (a very good indication of

health), and oil samples taken downstream of the motors indicated no significantcontamination (see following section). This has shown that there are no short-term

problems with the hydraulic circuit. However, the longevity of the motors, as withother components, remains a subject for the in-service full-scale prototype trials.

OPD is however confident that the hydraulic motors and induction generators are

performing satisfactorily.

3.4.7 Oil quality

The oil used for all tests was BP Biohyd SE-S 32, a biodegradable synthetic ester

based fluid. One of the aims of the full-scale joint programme was to assess thesuitability of Biohyd as the working fluid. Nothing from any of the testing has

indicated anything to the contrary.

One of the most powerful tools for determining the 'health' or otherwise of a hydraulic

system is to examine the level of contamination in the working fluid (surprisingly

similar in nature and scope to a blood test in medicine!). It is a generally accepted

view in the industry that an estimated 80% of all failures in hydraulic systems are due

to oil contamination. Various technologies are available for assessing the condition of

the system oil – of most relevance to the early testing of a system such as the full-

scale joint rig is examining the level of solid particulate contamination of the oil.

There are various industry standards used for specifying the permissible level of oilcontamination, and for assessing sample oil against this. One of the most universal is

the NAS Class system, and OPD have adopted this for use with the full-scale joint rig

and full-scale prototype. OPD has purchased a MP Filtri LPA2 laser particle analyser

for monitoring the oil quality in the test rig as shown in Figure 3.13. The laser

particle analyser takes three independent oil samples from the system, counts thenumber of particles of different sizes in each sample, and averages the results to arrive

at an overall NAS Class rating for the cleanliness of the oil. This is the simplest,

cheapest, most reliable way of determining whether there are problems in the circuit,and is the industry standard method.

During and after cycle testing, oil was sampled from key locations. Although there

have been a few 'rogue' readings (possibly attributable to bubbles in the oil sample) in

almost all cases the oil has achieved a cleanliness of better than NAS Class 8

(adequate), normally all samples were better than NAS class 6 or 7 which is as good

as required in aerospace applications. OPD will be using the laser particle analyser

as the primary method for assessing the condition of the FSP hydraulic systemsduring maintenance in-service.


42/49

Page 36

Figure 3.14 shows scans of the analyser printouts from three consecutive readings,

from oil samples taken from the lower heave ram. The results show a NAS rating of 6or 7 which is excellent for this type of system.

Figure 3.13 – Portable MP Filtri Laser Particle Analyser

Figure 3.14 - Example of three consecutive oil quality tests from the full-scale joint

rig while running, each test is the average of three automated samples. The rating of NAS 6 or 7 shows that the oil is running clean enough for an aerospace application.


43/49

Page 37

4. ONWARD DEVELOPMENT CONSIDERATIONS

Overall conclusions and development considerations are discussed below withreference to the main sub-systems.

4.1 General & structure

The general configuration, structure, and equipment mounting points proved

satisfactory. Stress ranges in critical components around the load points are high.

Through discussions with WS Atkins (full-scale prototype design verifiers) is has

been decided to manufacture the main load bearing components from discrete castingsrather than the heavy fabrications used for the test rig. This is to remove welds from

high stress areas to improve fatigue performance.

Actuating the system posed a number of technical challenges. The system chosen was

a pair of large closed circuit hydraulic systems. These generally performed

satisfactorily though at high operating pressures problems were encountered with therigidity of the drive system, due to compressibility of the oil and ‘stretchiness’ of the

hoses. This meant that the control algorithms implemented had to be modified to

prevent rapid decompressions as these induce unacceptable angular accelerations

resulting in large pressure transients in the low pressure circuit. However, it is not feltthat this significantly compromised the tests.

Thermal stability of the oil was difficult to assess. The rig was fitted with external

plate heat exchangers, cooled by force circulation of water. There proved to be no

problem cooling the oil with the units chosen, it is felt that the amount of cooling

capacity provided is conservative. The direct oil water heat exchangers for the full-

scale prototype have a similar rating to those used for the test rig. Thermal stability of

the entire module was impossible to assess as the heat transfer path is not

representative. OPD has carried out preliminary modelling of the likely internal air

temperature at peak output. Temperature rise is expected to be manageable and will be monitored in service on the prototype machine.

As a direct result of experience with the rig, individual component siting and

mounting has been re-examined for the full-scale prototype to minimise the number

and length of hydraulic and electrical interconnections.

4.2 Power take-off system

The power take-off rams in the test rig performed satisfactorily. However, it was

evident that rams from one supplier were superior to those from the alternativesupplier. The former were specified for the full-scale prototype. In addition, a fuller

fatigue analysis has been carried out on the ram design leading to adoption of anumber of detail design changes. While being of identical configuration, the rams to

be installed in the prototype are of a generally stronger construction. Due to robust

construction and some non-standard bore sizes etc the rams ended up being

considerably more expensive than originally budgeted. While it is anticipated that

there will be significant economies of scale associated with volume production, there

is a need to re-engineer the units to drive down the cost. This will be addressed as part of the onward development programme.


44/49

Page 38

The accumulators performed satisfactorily throughout the testing. There was nothing

to split the products from the two suppliers. The accumulators were therefore selectedfor the full-scale prototype only the basis of cost and delivery. Cost reduction will be

a priority in the onward programme.

The reservoirs also performed faultlessly during commissioning and trials. Similar

units have been specified for the full-scale prototype. However, costs for the

reservoirs are again high. Also, the reservoirs are bulky and take up lots of space in

the module. In the future it is anticipated that the reservoirs can be integrated into the

joint module structure to save cost and space– once again this will be an objective ofthe onward programme.

System valves generally performed adequately. In the future work will focus on

working with the manufacturers to improve performance in terms of reducing flow

losses and decreasing response time. The first steps in this process are underway as

discussed in Section 3.4.4.

Static and dynamic seal performance was generally shown to be satisfactory. The

only issues arising were the front rod seals on the rams, and a damaged static seal on

one of the system valves. Performance of the front rod seals was generally goodthough all such seals will leak slowly. For most of the rams this leakage was barely

perceptible. However, one of the ram seals developed a significant leak during thecycle tests (~4-5 drops/minute). It is not clear whether this is premature wear of the

seal or damage due to contamination from the test facility roof falling onto the ram

rod. Performance of the front seals on the full-scale prototype will be monitored

closely as a result. Following close examination, it was concluded that the damaged

‘O’ring on one of the system valves was caused during installation of the valve, rather

than through extrusion of the seal under pressure. The matter was raised with the

manifold manufacturer.

The dominant source of inefficiency in the system is the hydraulic motor/generator

sets. It is estimated that the combination results in 10-15% loss depending on the power level. The use of dual motor-generator sets running off the same circuit goes

some way to mitigating this as it allows a single generator to be run at higher capacity

at times of part load. However, it is estimated that motor-generator inefficiency will

result in a revenue loss of approximately £15,000-£20,000 per annum per 750kW

machine assuming UK Renewables Obligation prices. There is clearly a strong case toimprove the performance of the power conversion system, and significantly higher

component costs may be tolerable if they lead to proportionately larger increases in

energy capture. Fortunately, there remains considerable scope for improvement ofthis system. These are off the shelf items as it is seen as essential that the prototype

and early commercial machines use 100% available technology.

The hydraulic motors are drawn from the mobile hydraulics industry where power to

weight and size ratio is of prime importance and efficiency a secondary consideration.

OPD are already in consultation with a number of motor manufacturers to develop

improved machines optimised for efficiency, particularly at part load. Optimisation of

generator efficiency has less potential for improvement. The generators used in thefull-scale joint rig and prototype are conventional induction units, wound for


45/49

Page 39

maximum efficiency as generators. Their nominal efficiency is ~95% at full load.Small improvements to this can be expected through further optimisation but gains are

expected to be small. Larger gains are possible by moving to synchronous generators.This would confer other advantages in terms of power quality and reactive power

control and it is anticipated that a move will be made to this technology in the near

future. It is anticipated that this will be first tested on the full-scale joint rig.

It was recognised in the design of the full-scale joint rig that there are a large number

of interconnections to be made between the various system components. Each

interconnection is a source of cost and unreliability. This was to some degree

desirable as the system was by definition a prototype. Assessment of the optimumlayout for components for ease of

pelamis joint system test

Documents