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A preliminary study on the application of Friction Welding in structural repairs

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Authors:D. Pauly G. R. Blakemore J. F. dos Santos D. Gibson

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GKSS 98/E/15

ISSN 0344-9629

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GKSS 98/E/15

A preliminary study on the application of Friction Welding in structural repairs

Authors:D. Pauly(GKSS, Institute for Materials Research, Geesthacht, Germany)

G. R. Blakemore(Pressure Products Group, Aberdeen, United Kingdom)

J. F. dos Santos(GKSS, Institute for Materials Research, Geesthacht, Germany)

D. Gibson(The National Hyperbaric Centre, Aberdeen, United Kingdom)

GKSS-Forschungszentrum Geesthacht GmbH • Geesthacht • 1998

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GKSS 98/E/15

A preliminary study on the application of Friction Welding in structural repairs

D. Pauly, G. R. Blakemore, J. F. dos Santos, D. Gibson

36 pages with 13 figures and 5 tables

Abstract

Friction Welding is characterised by the absence of a fusion zone associated with comparatively low temperatures in the weld. These features allow the application of this welding process in joining and repair of most engineering structures, especially in hazardous environments. This work presents a pre­

liminary study on different friction welding processes, including the recently developed FrictionHydro-Pillar Processing (FHPP) and Friction Stitch Welding, as joining technologies for thick-walled struc­tures. The use of these welding processes in different industrial applications, compared with the

commonly used arc welding counterparts, as well as the influence of welding parameters on the weldment integrity are discussed. A brief description of a portable friction welding equipment and its possible implementation for FHPP are presented. Stud welds produced in the commissioning phase of this equipment have been analysed and tested to access their quality.

Studie fiber die Anwendung von ReibschweiBverfahren in strukturellen Reparaturen

Zusammenfassung

Da die Schweilizonentemperatur wahrend des ReibschvVtiiBvorganges vergleichsweise niedrig ist, bildet sich kein Schmelzbad aus. Anwendbar ist dieses SchweiBverfahren zur Verbindung Oder Reparatur der meisten Metallkonstruktionen, speziell in risikobehafteter Umgebung. Diese Arbeit enthalt eine Vorstudie zu verschiedenen ReibschweiBprozessen, einschlielSlich der neu entwickelten Friction Hydro-Pillar Processing (FHPP)- und Friction Stitch Welding-Verfahren, als Fugetechniken fur dickwandige Strukturen. Die Anwendbarkeit dieser SchweiBprozesse in verschiedenen Industries verglichen mit herkommlich

verwendeten LichtbogenschweiRverfahren, sowie der EinfluR von SchweiRparametem auf die Gute derVerbindung warden diskutiert. Prasentiert wird auBerdem eine tragbare ReibschweiBmaschine und ihre mdgliche Verwendung zum FHPP-SchweiBen. BolzenschweiBungen, die wahrend der Inbetriebnahmephase dieser Maschine hergestellt wurden, sind zur Charakterisierung ihrer Qualitat analysiert und getestet

worden.

Manuscript received/ Manuskripteingang in der Redaktion: 8. Juni 1998

PREAMBLEUnderwater fatigue cracks in offshore steel structures, ships and nuclear installations are a well known problem in repair and maintenance for these industries. Repairs are often required in areas which are either hazardous or costly for human intervention. Ships usually require dry docking for underwater repairs, reactors may need to shut down and divers, operating from support ships, are required for offshore platform repairs.This European Commission sponsored project "Affordable Underwater Robotic Welding Repair System" (ROBHAZ) will develop equipment and methods for remotely operated underwater welding using the Friction Stitch Welding technique deployed and operated from Remotely Operated Vehicles (ROV's). Friction stitch welding was developed at The Welding Institute in Cambridge on a project supported by various organisations from the oil, nuclear and construction industries. Two of these sponsors are now partners in a new project to develop the process for underwater repairs.This new BRITE-EURAM III project started in June 1997 and includes participants from five European countries.

The contributors are:The National Hyperbaric Centre in Aberdeen, United Kingdom, which co-ordinates the project and provides testing facilities with its hyperbaric chambers and test tank.Stolt Comex Seaway AS in Haugesund, Norway who provide the expertise and resources for offshore repairs and ROV operation.General Robotics in Milton Keynes, United Kingdom, who will develop the man machine interface.Pressure Products Group in Aberdeen, United Kingdom, who are developing the Friction Stitch Welding Head.Neos Robotics in T_by, Sweden who will build the submersible robot for deploying the welding head.Instituto De Soldadura e Qualidade in Lisbon, Portugal, who will provide expertise in ship repair and test the system for this application.GKSS-Forschunaszentrum in Geesthacht, Germany, who are providing expertise on the design of electric robots for underwater use, repairs for nuclear applications and will perform the initial weld testing.The project will be completed in the first quarter of 2000.

TABLE OF CONTENTS

1. INTRODUCTION 91.1 The Application of Friction Stitch Welding for Underwater Repairs 9

1.1.1 Repair Needs of Offshore Structures 91.1.2 Underwater Welding Offshore - State of the Art 111.1.3 Underwater Welding in other Industries 111.1.4 Advantages of Friction Welding Underwater 12

1.2 Affordable Underwater Robotic Welding System (ROBHAZ) 121.2.1 Summary 121.2.2 Materials 141.2.3 Repair Scenarios 151.2.4 Reaction Forces of Welding Machine to Robot 15

2. BASIC PRINCIPLES OF FRICTION WELDING 162.1 Aspects of Friction Welding 16

2.1.1 Definition 162.1.2 Energy Input Methods 162.1.3 Process Characterisation 17

2.2 Friction Hydro - Pillar Processing (FHPP) 172.2.1 Definition 172.2.2 Potential Advantages of Friction Hydro-Pillar Processing 182.2.3 Advantages of FHPP for Underwater Application 19

2.3 Friction Stitch Welding (FSW) 202.3.1 Process Parameters 212.3.2 Baseplate and Stud Geometric Characteristics 22

3. WELDING SYSTEM 233.1 Stud Welding Machine 233.2 Control System 25

3.2.1 Hydraulics 253.2.2 Electronics 26

3.3 Weld Head 273.3.1 Mechanic Components 273.3.2 Electronic Components 27

4. MODIFICATION OF THE WELDING CONTROL SYSTEM 284.1 Parameter Generation and Welding 28

4.1.1 The Old Menu 284.1.2 Custom Weld Sequence 28

4.2 Calibration of the Welding System 28

5. WELDING EXPERIMENTS 295.1.1 Materials . 295.1.2 Welding Parameters 295.1.3 Macrographic Analysis and Mechanical Testing 30

5.2 Results 315.2.1 Macrographic Examination 315.2.2 Hardness Testing 315.2.3 Tensile Testing 33

6. DISCUSSION 34

7. REFERENCES 36

DEFINITIONS AND ABBREVIATIONS:

Brace A tubular member of a JacketFHPP Friction Hydro-Pillar ProcessingFSW Friction Stitch WeldingHabitat A dry underwater working chamber (under hyperbaric pressure) HAZ Heat Affected ZoneJacket The complete underwater steel structure of an offshore platformLegs The almost vertical main tubular members of a JacketLIPS Linear Inductive Position SensorNode The connection point of several bracesROV Remotely Operated Vehicle

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1. INTRODUCTION

1.1 The Application of Friction Stitch Welding for Underwater Repairs

1.1.1 Repair Needs of Offshore Structures

More than 6,500 oil and gas platforms are in operation world-wide, located on the continental shelf of some 53 countries. Many different types of platforms and Floating Production Systems (FPS) exist as each one is uniquely designed for the particular reservoir conditions, the location where it will be installed (e.g. water depth, wind, wave and current conditions, and seabed characteristics) and the method of installation. The majority of platforms are steel structures rigidly fixed to the seabed with thick steel pipes 1-2 metres in diameter that penetrate as much as 100 metres into the seabed. More than 30 piles may be required in some cases. Some platforms are gravity based structures sitting on the seabed and stabilised by their own weight; others are floating installations, tethered to the seabed by anchor chains or in the case of tension leg platforms, by "rigid" steel tubes. Fixed steel platforms and concrete platforms range in water depth from only a few metres to more than 300 metres and floating installations to more than 1000 metres.

Approximately 25%-30% of the fixed steel platforms in the North Sea are classified as large (over 4,000 tonnes weight and installed in more than 75 metres water depth), reflecting the severe environmental conditions, water depth and the size of the producing fields. They represent the greatest concentration of large jacket structures anywhere in the world (in the Gulf of Mexico less than 5% of the 4,000 platforms are large steel jackets).

A typical steel platform consists of two elements (Figure 1); the "topside" containing the processing, drilling and accommodation facilities and the "substructure" or "jacket" comprising a lattice-work of steel tubes, which at their largest may reach 3 metres in diameter with a wall thickness of up to 75 millimetres.

Installations of this type typically only have a useful life of about 20 years. At the end of that time it must be decommissioned, unless it is retrofitted, redeveloped, or repaired.

As many of the existing steel platforms in the North-Sea have reached their design life, the development of cost-effective repair techniques is becoming a major task. They need to be performed without direct human intervention (diverless) as diving is restricted to certain depths. As a matter of fact, unmanned repair techniques are presently being developed even for standard repair and maintenance work since in this way the risks to human life are substantially reduced. The availability of unmanned repair capabilities would therefore be instrumental in prolonging the expected life of structures and pipelines, safeguarding the health of those involved. Moreover, due to its intrinsic characteristics (i.e. unmanned and fully automated) such technology would help protect the environment from the consequences of catastrophic failures in oil and gas producing installations in adverse and hostile fields.

It seems clear that pipeline and platform repair concepts for water depths greater than 300m - 400m must focus on robotics and remote sensing if repair intervention is going to catch up with the capabilities that already exist in the drilling, production and transportation phases of the offshore industry/1/.

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Elf Enterprise

PIPER ‘B’ PLATFORM| | Wellheads/Drilling Services

| | Production

Compression

Utilities

Accommodation

DECK:SIZE: 89.5m x 38.0m (293' x 124')

dimensions between main grid linesWEIGHT: 24800T target dry weight

28500T target operating weight

JACKET:

BASE DIMENSIONS:

TOP DIMENSIONS:

WEIGHT AT LAUNCH:

ANODES:

BUOYANCY TANKS:

PILES:

20 off 2.43m dia. (96' dia) (5 off each corner leg)

WEIGHT:

DESIGN PENETRATION:

72.0m X 60.0m (236' X 197')

72.0m X 24.0m (236' x 79')

22145T target launch weight

600T target weight

1900T target weight (excess buoyancy 15%)

vertical skirt piles

7000T target weight

65.0M (213')

CONDUCTORS:6 off 26" dia, tie back from template 18 off 26" dia, platform installed

RISERS:30" dia Oil Export, 16 " dia Gas Export 16" dia Gas Import, 16" dia Water Injection

CAISSONS:

No 1 34“ dia containing Oil and Gas Lines No 2 28" dia containing Power Cables No 3 34" dia containing Umbilicals No 4 34" dia containing Oil and Gas Lines

Water Depth: 145m (475 )Struct Natural Period: 3.78 secsDesign Wave Height: 27.5m (90 ) 100 yearDesign Wave Period: 15.1 to 18.5 secsDesign Wind Speed: 43.4m/sec (97mph) 100 year

Figure 1 - Steel Jacket Platform /2/

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1.1.2 Underwater Welding Offshore - State of the Art

Underwater repairs to fatigue cracks on offshore tubular steel platforms in the North Sea are carried out either by dry hyperbaric welding, wet welding or by installation of clamps. These are well established techniques, but all of them rely on divers for their implementation.

Hyperbaric welding is done inside a large, purpose built, underwater dry habitat, with a typical volume of 25 cubic meters, sealed onto the damaged area and installed by divers /3/. The diver welders enter the habitat and work in a dry environment.

Wet welding (welding with the electric arc directly in contact with sea water) has generally not been used for crack repairs in the North Sea since the weldment quality so far does not meet the codes and standards applied to offshore structures.

Friction clamps or grouted clamps add weight and hydrodynamic drag to underwater structures and their performance during service life is difficult to monitor.

Dry hyperbaric welding is generally perceived as technically the most desirable repair method, but it is also by far the most costly because of the excessive time required from a construction diving support vessel with divers. The maximum water depth to which it can be used is also limited due to the need for divers. The maximum depth possible for divers is much less than the maximum feasible depth for welding /4/, /5/.

1.1.3 Underwater Welding in other Industries

In-Water Ship Repair

Fatigue cracks in ships' hulls are currently repaired by dry-docking the vessel and using conventional welding techniques. Substantial cost savings can be achieved if repairs can be done in water, preferably while the vessel is in port transferring cargo.

At the present time in water repair and inspection of ships is done using divers. The quality and reliability of these operations is questionable. The equipment is relatively low technology and manually operated. Underwater repair welds are either done with the low quality "wet" arc welding process or are performed in small dry habitats which are difficult to install on ships hulls.

Nuclear Industry

The development of a powerful, precision underwater robot capable of welding thick materials will also be of great benefit to the nuclear industry/6/.

Spent nuclear fuel is presently stored underwater in pools lined with stainless steel. Some of these linings require repair. At the present time the only way to achieve this is to removethe fuel and drain the pool. This is a very expensive exercise with a high risk of contaminating the environment.

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1.1.4 Advantages of Friction Welding Underwater

The friction stitch welding (FSW) process can be used to produce high quality welds directly in water and avoids the practical difficulties of having to install a large dry welding habitat and having to apply preheat and post weld heat treatment to an underwater weld.

The arc welding processes currently used in dry hyperbaric welding are adversely affected by increased pressure and this ultimately limits the water depth at which they can be operated. Friction stitch welding is a solid phase welding process and is not adversely affected by pressure so there is no theoretical limit to the depth at which it could be used.

A more detailed description of the process is presented in Section 2.

1.2 Affordable Underwater Robotic Welding System (ROBHAZ)

1.2.1 Summary

Initiatives to maximise safety and reduce cost are leading to the phasing out, as far as possible, of manned diving operations in the North Sea. But at the present time there is no fully mechanised or robotic, diverless underwater welding system available to the industry for use at any depth, deep or shallow /5A Welding systems for underwater repair of fatigue cracks are presently dependent on the use of divers in a dry hyperbaric habitat to manually weld complex geometries on large thick tubular intersections (called "nodes”).

This project will develop underwater robotic welding repair technology based on the friction stitch welding process for inspection and maintenance contractors, operating in the offshore, maritime and nuclear industries.

The "state of the art" hydraulic manipulators, currently in use offshore, are not capable of moving with the degree of precision required for welding operations. The necessary precision (0.2 mm) can only be achieved on a practical basis using an electric robot. Teleoperated electric robot arm welding systems developed for repairs in the nuclear industry are capable of producing only single pass welds on thin materials and have not yet been adapted for repairing cracks in thick steel (typically 25 mm thick) underwater.

In this project a submersible welding electric robot arm with a controller, will be developed to deploy and move the welding system. For offshore work the prototype robotic welding system will be delivered and clamped to underwater node sections on platforms by a large ROV (Remotely Operated Vehicle), of the type presently used by inspection and maintenance contractors (Figure 2).

This equipment will also be suitable for in-water inspection and repairs to large ships in the maritime transportation industry (Figure 3). Costly unscheduled dry-dockings can be avoided by performing inspection and repair in-water, while large vessels such as oil tankers and container carriers are transferring cargo in ports.In the future it will be possible to perform the majority of hull survey, inspection and maintenance cleaning and repair tasks using robots. This project will make an important contribution to that objective.

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The welding process used will be friction stitch welding which has recently been developed for repairing thick steels and can be used in water to produce sound welds. This technique involves drilling a hole in the crack and filling it by rotating a consumable bar in the hole and producing a friction weld. Cracks are repaired by producing a series of such welds overlapping each other. During this project a hydraulically powered stitch welding head will be developed for deployment on the submersible electric robot arm. A more detailed description of the friction welding process is presented in Section 2.

The broad project partnership includes a robot manufacturer (Sweden), a friction welding equipment manufacturer (UK), a contractor who will use the equipment in the offshore and maritime industries (Norway), centres for welding research in the offshore, maritime and nuclear industries (Germany and Portugal), a robotic software company, and a testing centre for the offshore industiy (UK). The total budget is 3.7 MECU. Cost savings of approximately 8 MECU over 5 years in the offshore industry and approximately 4 MECU in the maritime transportation industry will be possible if the technology is successfully developed.

Safety will be enhanced by replacing men in the hazardous underwater environment with a robotic system. Furthermore the quality and repeatability of welds will be improved by replacing manual welding with a fully mechanised system.

Figure 2 - The ROBHAZ system at an offshore platform P/

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1.2.2 Materials

Materials presented in Table 1 are commonly used in the offshore, maritime and nuclear industry. They shall be tested in the welding trials.

Table 1 - Common Materials

Material Type/Standard

Thickness in mm

Application

Stainless Steel 316 LUNS S300000

5/12 Nuclear

1.4571DIN

1.5 to3 Nuclear/ storage pool liner

C - Mn Steels Grade 355 EMZ (TMCR)BS 7191

12/40 Offshore

C - Mn Steels Grade 235 B (D Ship Plate)ASTMA131 -85

12 Marine

Al - Alloys Al-Mg-Mn (5074, 5086, 5083)ASTM 5000 series

up to 12 Marine

Al-Mg-Si (6082)ASTM 6000 series

up to 12 Marine

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1.2.3 Repair Scenarios

Defect location

The welds will be produced in water - depth exceeding 50 m under wet conditions.

Characteristics of the defects

The initial development shall concentrate on linear cracks in flat plate to establish basic parameters. The Hydromarine welding system (designed and manufactured by a Scottish project partner) shall be used to do some preliminary tests on 15 mm thick plate, although the new welding system should be capable of welding steel of about 30 mm thickness 18/.

Actual defects on offshore structures

Fatigue cracks mostly occur in high tensile areas close to or in the middle of a welded surface. For offshore platforms and steel structures underwater, high tensile areas are often located in and around the 'nodes' of one or more 'braces' welded into the main vertical leg of a 'jacket'.

Complex 'nodes' consisting of several 'braces' angled and welded in to one single 'node' form a complex geometry and are difficult, or even impossible to reach at some areas with tools exceeding certain volumes and shapes. Normally all 'braces' and 'legs' are tubulars in different sizes. The main 'leg' may be up to 2500 mm or more while the 'braces' may be up to 1300 mm in diameter. The angles between each member of the structure that is connected to the 'node' varies, but are typically 30 to 50 degrees.The high tensile area of such 'nodes' are the welds in the narrow angle part of the node. The weld surface and trajectory are double curved.

The cracks are normally not detected by visual methods and are only found by special sensors and tools such as magnetic sensors (Magnetic Particle Inspection tool and Eddy Current), ultrasonic sensors (Spot and area measurements) and X-ray techniques (Radioactive source and film). Cracks become visible by the human eye or photographic methods when they have penetrated the steel and the surface opening is widened or corroded. Visual detection methods may then be effective /9/. For a typical steel jacket platform see also Fehler! Verweisquelle konnte nicht gefunden werden.

1.2.4 Reaction Forces of Welding Machine to Robot

As the welding system shall be operated without using any clamp mechanism the robot arm needs to cope with the reaction forces of the weld head.

The expected thrust force when filling 10 mm dia holes is approximately 1.5 tonnes.

The maximum torque of the currently used weld head is 88 Nm when operating with 350 bar hydraulic pressure.

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2. BASIC PRINCIPLES OF FRICTION WELDING

2.1 Aspects of Friction Welding

2.1.1 Definition

Friction welding is a solid-state welding process that produces a weld under compressive force contact of workpieces rotating or moving relative to one another to produce heat and plastically displace material from the faying surfaces. While considered a solid-state welding process, under some circumstances a molten film may be produced at the interface. However, even then the final weld should not exhibit evidence of a molten state because of the extensive hot working during the final stage of the process. Filler metal, flux, and shielding gas are not required with this process. First, one workpiece is rotated and the other is held stationary. When the appropriate rotational speed is reached, the two workpieces are brought together and an axial force is applied. Rubbing at the interface heats the workpiece locally and upsetting starts. Finally, rotation of one of the workpieces stops and upsetting is complete /10/.The weld produced is characterised by a narrow heat affected zone, the presence of plastically deformed material around the weld (flash), and the absence of a fusion zone.

2.1.2 Energy Input Methods

There are two methods of supplying energy in friction welding. Direct drive friction welding, sometimes called conventional friction welding, uses a continuous input. Inertia friction welding, sometimes called flywheel friction welding, uses energy stored in a flywheel.

The following will concentrate on Direct Drive Welding, due to its the significance to thepresent work.

In direct drive friction welding, one of the workpieces is attached to a motor driven unit, while the other is restrained from rotation. The motor driven workpiece is rotated at a predetermined constant speed. The workpieces to be welded are moved together and then a friction welding force is applied. Heat is generated as the faying surfaces (weld interface) rub together. This continues for a predetermined time, or until a pre-set amount of upset takes place. The rotational driving force is discontinued, and the rotating workpiece is stopped by either the application of a braking force or by its own resistance to rotation. The friction welding force is maintained or increased (forge force) for a predetermined time after rotation ceases.

/

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2.1.3 Process Characterisation

While specific details of the bonding process are unclear, the welding cycle can be divided into two stages: the rubbing or friction stage, and the upsetting or forging stage. The welding heat is developed during the first stage, and the weld is consolidated and cooled during the second stage.

Friction Stage

The joining mechanism occurs in steps. When the pieces make contact, rubbing takes place between the faying surfaces, and strong adhesion takes place at various points of contact. The unit pressure is high. At some points, the adhesion is stronger than the metal on either side. Shearing takes place and metal is transferred from one surface to the other. As rubbing continues, the torque and interfacial temperature both increase. The sizes of transferred fragments grow until they become a continuous layer of plasticized metal. If a liquid film forms, it occurs at this point. During this period, the torque peaks and decreases to some minimum value, which remains reasonably constant as metal is heated and forced from the interface and axial shortening continues.

Forging Stage

Towards the end of the heating process, forging pressure is applied to the workpiece to cause axial shortening. This upset results in the flash. As the speed decreases, a second torque peak occurs when the interface bonds and cools from its maximum temperature. The torque then decreases as the RPM drops to zero.The bonding mechanism with dissimilar metals is more complex. A number of factors, including physical and mechanical properties, surface energy, crystal structure, mutual solubility, and intermetallic compounds, may play a role in the bonding mechanism. It is likely that some alloying will occur in a very narrow region at the interface as a result of mechanical mixing and diffusion. The properties of this layer may have a significant effect on overall joint properties. Mechanical mixing and interlocking may also contribute to bonding. The complexity makes prediction of weldability of dissimilar metals very difficult. Suitability of a particular combination should be established for each application with a series of tests designed for that purpose.

2.2 Friction Hydro - Pillar Processing (FHPP)

2.2.1 Definition

This variation of friction welding can be described as a drill and fill process /11/.A consumable rod of compatible material is rotated, then entered co-axially into a cavity. Initial contact of the rod with the bottom of the cavity will generate frictional heating and metal flow along discrete shear planes at the base of the rod. By selection of certain combinations of axial pressure and rotational speed, these shear planes can be induced to move up the rod and contact the inside surface of the cavity. Frictional heating and limited deformation occurs, which results in bonding to take place between the fill rod and the cavity (Figure 4).

-18“

In this action the consumable rod will undergo significant hot working when subjected to these severe deformations. Consequently, the formation of a very refined heat treated microstructure will result in a remarkable change of the static and dynamic properties of the consumable rod and the parent plate material.It should be remembered that all the re-processing and manufacture will take place in the absence of macroscopic melting, in other words in the SOLID PHASE, so that problems associated with solidification in a molten phase, e.g. when using arc welding, are reduced, if not almost eliminated.

Figure 4 - FHPP principle /12/

(a) Consumable(b) Plasticised zone(c) Pillar of extruded material

2.2.2 Potential Advantages of Friction Hydro-Pillar Processing

• Solid phase joining;

• Deep penetration narrow gap for repair and joining;

• Well suited for automation and remote control;

• Large section joining capacity;

• Requires less weld metal;

• Environmentally friendly;

• Comparatively low equipment costs and low cost filler material (consumable rods);

• Can be operated in hazardous environments such as underwater, explosive gases and radiation;

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2.2.3 Advantages of FHPP for Underwater Application

In all under water arc welding processes, pressure adversely affects the behaviour and performance of the welding process. For wet welding the quench (cooling) rate from the outer regions of the arc and base material is very high, and for the types of material used in offshore structures, results in poor arc performance, uneven weld bead profiles and the formation of hard brittle weld microstructure and a heat affected zone which may subsequently be prone to cracking. Hydrogen entrainment as a result of the dissociation of water within the arc increases hydrogen levels in the weld metal, enhancing susceptibility to hydrogen induced cracking (cold cracking). Porosity is also a problem and is known to be more severe as depth increases.

In hyperbaric welding, pressure influences the chemical reactions within the arc and weld pool surface, resulting in modifications of the weld pool chemistry, therefore the weld metal microstructure and mechanical properties. Increased carbon absorption in the weld pool and increased available oxygen removes deoxidizers such as manganese and silicon producing significant changes in the microstructure resulting in more brittle welds. Arc voltage increases significantly as the arc becomes progressively less stable at depth, with an increased susceptibility to magnetic fields. Thermal conductivity of helium (used as habitat breathing gas) is very high, so cooling rates are much faster than those experienced on thesurface. Thermal efficiency decreases from approximately 90% at the surface to 75% at 6 bar. Electrode burn-off rate is enhanced resulting in problems of weld pool control in positional welding.

In the case of friction welding, the material never melts, only ever reaching the plastic state. The absence of a liquid weld pool prevents diffusion of hydrogen avoiding the problem of induced cold cracking.

Porosity and weld inclusions are eliminated.

Quenching is prevented by means of a simple shrouding mechanism. The process is unaffected by environmental pressure, so a weld qualification run at the surface should be also valid at depth /13/.

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2.3 Friction Stitch Welding (FSW)

Friction Stitch Welding uses the Friction Flydro Pillar Processing method to repair linear cracks by producing a series of such welds overlapping each other. This process is schematically described in Figure 5.

Figure 5 - Friction Stitch Welding Principle /12/

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2.3.1 Process Parameters

The relevant parameters which might have a significant influence on the weldment quality and process performance are discussed below.

A. Revolution per minute of the stud / rod

This is a non-critical variable for the weld quality, as it may vary within a fairly broad tolerance to perform sound welds.

For steels, the tangential velocity should be in the range of 1.3 m/s (20 mm stud welds are currently performed with an angular velocity of 3000 rpm).

At low speed the surface velocity near the centre of the rotating bar may be too low to generate adequate frictional heating.

B. Axial pressure

Although this variable may vary in broad ranges in the heating and forging stage it controlsthe temperature gradient in the weld zone, the required drive power, and the axial shortening.

To compensate for a large mass the axial pressure should be increased to produce adequate heating (e.g. rod into bore in plate).

It must be high enough to hold the faying surfaces in intimate contact to keep detrimental substances out of the welding zone, but it should be noted that high pressure causes local heating to high temperature and rapid axial shortening which might be uncontrollable. The axial shortening rate with mild steel was investigated to be almost approximately proportional to the axial pressure.

After stopping of revolution two different methods of varying the axial pressure are commonly used. Maintaining the pressure or increasing it. The second method, in which a forging force is applied, will improve the joint quality.

C. Heating time

This variable is significantly influenced by heating pressure and speed. It is reduced as heating pressure is increased and decreases with speed at the same heating pressure.

Insufficient heating time may result in uneven heating of the weld zone, sincethermal diffusion from the outer portion of the faying surfaces must take place to ensure asound bond overall.

The heating time can be controlled in two ways :The stud rotation may be stopped after a pre-set time or a pre-set Burn-off (the axial shortening of the stud).

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D. Influence of water

So far no experiments have been carried out with Friction Hydro-Pillar Processingunderwater. Stud welding has revealed the following effects:

• The severe quenching effect of the water results in high hardness peaks in the weld zone.

• Trials at the TWI /14/ carried out with carbon-manganese steel plates (BS 4360 SOD) and carbon-manganese tapered rods (BS 2772 150 M19) resulted in a hardness up to about 500 HV5 on the bond line near the root. This value is approximately 100-150 HV5 higher than that obtained in air.

E. Controlling the quenching effect

Different efforts were made to control the hardness in the weld zone /15/ & /16/.

• protecting the stud and weld zone by a polystyrene shroud• shielding the process with foam• wrapping the stud with a heat insulating tape• performing a second friction weld on top of the first /• special welding cycle:

high initial load then reduced to a lower level to promote thermal energy development, then increased to the final forge

2.3.2 Baseplate and Stud Geometric Characteristics

Possible variations of the Baseplate and Stud geometries are listed below.Their significant influence on the above mentioned parameters will be investigated in extensive test programmes in the future.

A. Shape

• cylindrical - the ROBHAZ project proposal: 12 mm diameter

• tapered - may be necessary for materials which do not exhibit adequate plastic flow characteristics, e.g. Aluminium - Alloys

• shape of cavity bottom

B. Dimensions

• Diameter of rod / diameter of cavity in plate

• Length of rod / diameter of rod

• Diameter of rod / thickness of plate or crack of depth

• in addition when tapered:-Taper angle- Taper angle of rod / taper angle of cavity in plate

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3. WELDING SYSTEM

A System Layout is presented in Figure 6. The different components will be described in the following sections.

Topside Umbilical Hydraulic & CommunicationWATER LINE

Valve Control &Pressure Transducer Line

WELDHEAD

TOPSIDEPC

SUBSEA CONTROL POD (1 ATMOSPHERE)

HYDRAULIC VALVE PACK (PRESSURE COMPENSATED)

HYDRAULIC POWER SUPPLY TOPSIDE, SUBSEA OR ROV

IExcursion Umbilical Hydraulic & Sensors

Figure 6 - Basic System Layout

3.1 Stud Welding Machine

Features:

• designed for FRICTION STUD WELDING

• hydraulically powered

• Operation environment:

• Maximum stud rotation:

• Maximum axial pressure:

• Maximum axial movement:

air / under water

5000 rpm

6 tons

21 mm

• ability to monitor, control and adjust the rotation, welding force and the displacement of the internal piston from the surface throughout the weld cycle.

• Name and Manufacturer: HMS 3000 by Hydromarine Systems, Scotland

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Welding Cycle:

The HMS 3000 has been designed to attach mild steel studs to large engineering structures to act as secure points for the permanent attachment of secondary components, for example sacrificial anodes on offshore platforms /17/.

The welding cycle for a 16 mm mild steel stud begins by rotating the stud at the required angular velocity, typically 4000 rpm. The stud is then hydraulically lowered until it makes light contact with the substrate material. This is called the ‘touch down' phase. The pressure applied to the rotating stud at this stage is a approximately 250 psi (1.72 N / mm2). The frictional heat generated during the touch down softens the stud and the substrate material and significantly reduces their resistance to plastic deformation.

Axial pressure is then increased over a short period of time to around 900 psi (6.21 N/mm2). This is termed the 'burn - off phase where the length of the stud gradually decreases as material at the stud / substrate interface flows outward under the applied pressure forming a flash.

The material at the interface does not become molten at any time during the welding cycle!

Once the 'burn-off phase is completed stud rotation is stopped and axial pressure is increased to approximately 1100 psi (7.58 N / mm2). This is known as the 'forging pressure' and is regarded as the most critical of all friction weld parameters /17/. The pressure is then reduced to zero and the welding procedure is complete.

The forces used in each phase are dependent on material combination, stud size, and stud geometry.

-25-

3.2 Control System

3.2.1 Hydraulics

Hydraulic Valve Pack

The valve pack contains fully proportional hydraulic valves to control motor direction as well as ram direction and pressure (force acting on the stud).Pressure transducers in the system are used to monitor and (via the electronics) control these pressures (forces).

Hydraulic Power Supply

During standard offshore use the weld head is handled by a diver. The relatively small water depth (i.e. less than 50 m ) allows a top side power pack. The operation in deeper water requires the deployment of the weld head by an ROV and the use of either the ROV's own hydraulic power supply or an additional subsea power pack.

The requirements for the power pack (HPU) are indicated below :

• Minimum pressure: 210 bars• Maximum flow: 96 Itr/ min• Pressure compensated pump or bypass system

Umbilicals ( hydraulic)

There are 2 umbilicals. The surface umbilical from the HPU to the valve pack is normally up to 100 metres in length. This may be increased by use of a larger power pack and larger diameter hoses than standard.

The excursion umbilical between the valve pack and the weld head is normally 8 metres in length, allowing the diver/operator ease of movement within a reasonable distance from the main control valves. This umbilical may be increased in length, but allowances have to be made for time-lag in reaction of hydraulic forces caused by the reservoir effect of the hoses.

-26-

3.2.2 Electronics

Topside Computer

A standard Personnel Computer (PC) operating on Windows 95 is used to run the welding software.The operator controls the welding sequence by following the menu structured program step by step. The control signals are transmitted to the subsea electronic pod, which converts them to actuate the coils of the electrohydraulic valves. Signals from the pressure transducers, the proximity switch and the ram displacement sensor are converted in the electronic pod and transmitted to the PC.

The hardware requirements for the Topside Computer are as follows:Pentium Processor 133 MHz, 16 MB RAM

Subsea Control Pod

The Subsea Control Pod (Figure 7) converts the information generated by the PC to digital actuating signals for the hydraulic valves to control the ram and motor. Electronic signals from the different sensors at the Weld Head and Valve Pack are converted and transmitted to the topside PC.

r1 Atmosphere Pressure Housing ( Subsea Rod)

RS422TRANSCIEVER

POWERSUPPLY

SIGNALCONDITIONING

BOARD

EPROM

BCCBOARD

DATAACQUISITION

DRIVERBOARD

DC /DCPOWER CONVERTER

VALVE PACK:Hydraulic Control Valves and Pressure Transducers

WELD HEAD: Instrumentation Sensors

VALVES:Ram Direction / Pressure Motor Speed / Direction

UMBILICAL:DataTransmlssion

UMBILICAL:Power

Figure 7 - Subsea Control System

Electric Power Supply

The Subsea Electronic Pod is supplied with 30 V DC. Although voltage losses will occur in long cables - a high voltage would be desirable - 30 V is the maximum allowable voltage for underwater equipment used by divers.

-27-

3.3 Weld Head

3.3.1 Mechanic Components

The chuck is turned by a hydraulic motor, mounted on top of the weld head casing. The displacement of the chuck, called the RAM, is operated by an internal piston.

3.3.2 Electronic Components

The motor speed/direction, and the ram displacement/force are all monitored and controlled by the electronics system. Information is transmitted to the topside PC in real time for display and recording.

Figure 8 - Friction Weld Head

.»<M

-28-

4. MODIFICATION OF THE WELDING CONTROL SYSTEM

4.1 Parameter Generation and Welding

To establish new welding parameter sets a Custom Weld Sequence has been implemented in the new program. Details are discussed below by comparing the old and new program structures.

The database and project generation features have been optimised. The operator will be able to establish a project file much easier and the Welding and Testing Documentation for the client will be completely available on disc or as a printed out document. As these modifications are not relevant to the welding process itself they won't be discussed any further in this work.

4.1.1 The Old Menu

The old menu was designed only for friction stud welding. There were also some aspects of the user-friendliness which needed to be addressed. This posed limitations on the suitability of this menu system for the use in FHPP or Friction Stitch Welding.-

A new program and menu system were written to encompass the use of the equipment for stud welding, FHPP, and Stitch Welding.

4.1.2 Custom Weld Sequence

The 'new' Custom Weld Sequence allows the operator to control the weld cycle in real time by using the topside computer keyboard. Before proceeding with the actual weld "Start Parameters" and if required forging time (Rest Period) are set.

The Custom Weld is then started and controlled by an experienced operator, who decides on changing parameters by visually observing the welding progress. The values for motor speed, ram pressure and burn-off are presented as a graphic in real time on the PC's screen throughout the weld cycle. As soon as the pre-set burn-off is reached the motor stops automatically. After the pre-set Rest Period has elapsed the weld is completed.

When a Custom Weld has proved to be successful, i.e. the mechanical properties are satisfactory, the saved information is examined to establish a new set of parameters for this particular application.

Although a complete weld takes approximately only 10 seconds, i.e. the operators chance for intervention is very short, it is claimed that this new program sequence will save time in establishing new weld cycles for new material combinations, new stud or base plate geometries, or even in developing innovative welding processes as Friction Stitch Welding. Therefore the feasibility of this new sequence needs to be proved in extensive tests.

4.2 Calibration of the Welding System

To calibrate a new weld head and/or a new valve pack with the electronics and the computer program a password safe guarded SETUP sequence was added.

In the "old" program only the Motor High Time could be corrected by the authorised operator. All the other calibration features had to be adjusted by the system programmer.

-29-

5. WELDING EXPERIMENTS

Ordinary stud welding with 13 mm cylindrical studs was performed in the commissioningstate of new assembled welding head to prove its correct function.

The weld head was secured with its bayonet chuck in a test rig. Base Material samples (75 x 75 x 25 mm steel blocks) were clamped to the height adjustable support at the rig.

5.1.1 Materials

The following materials were used in the function test.

Stud : BS 970 - 070 M 20 (similar to DIN C 22)

COMPOSITION PROPERTIES

GRADE EnNo.

C Si Mn P and S max.

HeatTreatm.

Condi­tion

TensileStrengthN/mm2

YieldStressN/mm2

HardnessVickers

070 M 20 3A3C

0.16/0.40 0.10/0.40 0.50/0.90 0.050 N TN&G 430400

215200

160

DIN C22 0.17/0.24 <0.40 0.40/0.70 0.045Cr + Mo + Ni <0.63

470-620 290 164

Base plate: EN 10025 S 275 JR (similar to St 44-2)

COMPOSITION PROPERTIES

GRADE C Si Mn P S N TensileStrengthN/mm2

YieldStressN/mm2

HardnessVickers

St 44-2 0.19 0.210 0.83 0.016 0.018 0.003 484 311 ?

5.1.2 Welding Parameters

The presented parameter set (Table 2) has been successfully used in production. It had been proved to produce reliable welds with satisfying mechanical properties with an existing weld head working with the old control system.

Table 2 - Typical Parameter Set

Ram Pressure Burn-off Distance RPM150 2.0 4200200 2.0 4200300 2.0 4200400 2.0 4200500 0.0 0

-30-

5.1.3 Macrographic Analysis and Mechanical Testing

In order to evaluate the properties and quality of the stud welded joints a test programme consisting of metallographic examination and mechanical testing has been devised.The macrographic examination has been carried out on a polished and etched transverse section of the welded joint. This specimen was then observed in a light microscope to determine the presence of defects or lack of bonding.Due to the geometric limitations of the stud weld micro-flat tensile specimens were used to determine the strength of the joint. Four micro-flat tensile specimens were extracted by spark erosion across the welded joint, as shown in Figure 9. The micro-flat tensile specimen preparation was conducted mainly in two stages, namely: extraction of a pre-shaped block with the stud weld in the middle, and cutting out specimens from the pre-shaped block using spark erosion cutting technique (with a 0.1mm diameter Cu wire) perpendicular to the weld.Microhardness measurements were carried out on an etched and polished section of the welded joint. Two measurements profiles were performed: the first, consisting of 43 measuring points, starting at the unaffected stud material, crossing the welded region down to the substrate. The second one, consisting of 65 measuring points, was carried out across the heat affected zone of the substrate. The measurements were performed using a Vickers hardness test machine type Shimadzu HMV-200 with a pre-load of 19,6N (2 kp), with a loading time of 5 seconds.

Micro Flat Tensile Specimenst

5 mm

9 mm 2 mm

0.5 nun

4Figure 9 - Micro Flat Tensile Specimen

-31-

5.2 Results

5.2.1 Macrographic Examination

Figure 10 presents a macrograph of the welded joint. This figure clearly shows three distinct regions across the joint, namely: base/stud material, heat affected zone and bonding line. No cracks, pores or lack of bonding could be observed across the joint.

Figure 10 - Macrograph of Welded Joint

5.2.2 Hardness Testing

Figure 11 shows the hardness profile across the welded joint. This profile clearly indicates the increase in hardness in the heat affected zone and bond line resulting from the high cooling rates experienced on both regions. Since the thin liquid metal film formed during the friction stage is expelled during the forge stage there is no indication of a cast structure in the bond line and therefore, a continues increase of the hardness up to this region. The higher hardness values in the bond line indicate, as expected, that this region has undergone the highest cooling rates across the joint. The measured hardness values in the bond line are typical of martensitic structures which have been confirmed by the microscopic examination.Figure 12 presents the hardness profile across the heat affected zone. This profile also shows the increase in hardness across this region resulting from faster cooling rates. The observed micro-hardness indicates a mixed microstructure containing a minor percentage of low transformation temperature products.

-32-

Vertical Pass S144-2/C22 .

400 -

T3 300 -

Y in mm

Figure 11 - Vickers Hardness / vertical pass

- Horizontal PassSt 44-2/C 22

> 225

' I ' ' ' * I ' ' « 1 « ■i i L

X in mm

Figure 12 - Vickers Hardness / horizontal pass

-33-

5.2.3 Tensile Testing

Table 3 presents the results of the tensile tests. All specimens failed in the base material confirming the higher strength of the welded joint (see Figure 13). These results also confirm the hardness measurements and describe a „overmatch" situation (strength of the joint > strength of the base materials). Overmatched welded joints provide a so-called ..shielding effect" to small defects which may be present in the weld zone and promote gross section yielding in the base material under external loading. In other words, the toughness of the base material may control the deformation and fracture behaviour of the welded joint under static or accidental overloading. This would be beneficial since it can be assumed that finer microstructure of the base material exhibits higher fracture toughness than the HAZ and bonding line.

Table 3 - Ultimate Tensile Strengths

Ultimate Tensile Strength N/mm2

C 22 470 - 620St 44-2 484WeldedJoints1 4612 4953 4394 461

Figure 13 - Failed Tensile Specimen

-34-

6. DISCUSSION

The currently used underwater welding techniques rely on arc welding processes. Their performance is extensively effected by the wet environment and the progressively increasing pressure in greater depths.

The ROBHAZ project will develop an underwater robotic welding system based on Friction Stitch Welding. The recently developed Friction Stitch welding process will be further investigated to produce reliable parameters for different materials and welding purposes. This technique involves drilling a hole and filling it by rotating and pressing a consumable bar into it. An existing underwater Friction Stud Welding system and an electric Tripod Robot will be modified to meet the projects' objectives.

As the preliminary development of this innovative welding process did only concentrate on filling 17 mm holes without systematic investigation of the welding parameters, a prediction of welding procedures for other geometries is impossible.

Nevertheless it is believed that the existing welding system is powerful enough to perform the new Stitch Welding process. To fill a 30 mm deep hole, only the pressing devices' movement should require extension.

To develop the Friction Stitch Welding process to meet the ROBHAZ projects' requirements the following aspects demand closer investigation:

• The effect of high cooling rates when operating underwater on the welding process and the weldment quality.

• The possibility of drilling or milling the holes with the welding head by automatically changing tools.

• The reaction forces of the welding head to achieve design data for the robot arm.• The demands for positioning accuracy of the robot arm.• A Modification Concept for the existing welding equipment.

-35-

ACKNOWLEDGEMENT

This work has been carried out within the framework of the project,,Affordable Underwater Robotic Welding Repair System (ROBHAZ)" (contract no. BRPR-CT97-0422), supported by the BRITE EURAM III programme of the European Commission. The financial support of the European Commission is gratefully acknowledged.

-36-

7. REFERENCES

/M Offshore Decommissioning Communications Project, several publications in the World-Wide-Web, http://www.oilrigdisposal-options.com/back/text1.htm - /text8.htm, Oct. 12, 1997, London

121 Hull, T.: Platforms and Pipelines - Proceedings of the conference "An Overview of the North Sea Oil Industry", University of Aberdeen, 1996

f3f Gibson, D.: Hyperbaric Weld Repairs - Practical Examples. Conference on the Maintenance and Operation of Offshore Installations, Bergen, 1984

/4/ Brooke, S. T.: Identifying the Needs and Limits for Deepwater Diver Intervention. DEEPTEC '94, (Conf.) HR Conferences, Aberdeen, 1994

/5/ Gibson, D.: Achieving diverless repair of pipelines and other subsea equipment in hyperbaric environments. DEEPTEC'94, (Conf.) HR Conferences, Aberdeen, 1994

/6/ Ryder, G.: Discussions with G. Ryder - Nuclear Electric Pic. Berkley Laboratories,1996

111 The National Hyperbaric Centre, Publications for the ROBHAZ project, Aberdeen,1997

/8/ Gibson, D.: ROBHAZ (Brite) Project Minutes of Meeting. The National Hyperbaric Centre, Aberdeen, June 1997

/9/ Moldskred, S.: Brite - Euram Project Design Criteria's For Friction Stitch Welding. Stott Comex Document Re-615705-001, Haugesund, Norway, 1997

/10/ American Welding Society - Welding Handbook, Eighth Edition, Volume 2,Miami, 1991

/11/ Nicholas, E.D.: Friction Hydro Pillar Processing (FHPP)

/12/ The Welding Institute: TWI Connect. TWI, Abington, Cambridge, Issue No. 89, 1997

/13/ Blakemore, G.R.: Underwater Issues - Friction Welding. Roustabout, Jan. 1992

/14/ Andrews, R.E.: Mitchell, J.S.: Underwater Repair By Friction Stitch Welding. Metals and Materials, Dec. 1990

/15/ Nicholas, E.D.: Friction Welding Underwater. Underwater welding Proceedings of the International Conference, Norway, June 1983

/16/ Blakemore, G.R.: Underwater Application of State of the Art Portable Friction Stud Welding Machine, International Workshop on Underwater Welding of Marine Structures, New Orleans, December 1994

/17/ Brown, P., McGowan, C., Blakemore, G., Cooper, M., Bird, J.: Evaluation of Hydro Marine Systems HMS 3000 Portable Welding Machine - ASM Conference, Madrid, Spain, March 1997

Fur die Zukunftssicherung des Wirtschaftsstandortes Deutschland 1st Forschung und Entwicklung von grund- legender Bedeutung. Neben anderen Forschungs- organisationen leisten sechzehn nationale Einrichtungen der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren (HGF) hierfiir einen wichtigen Beitrag. Zu ihnen zahlt mit ca. 800 Mitarbeitern und einem Budget von 120 Mio. DM das GKSS-Forschungs- zentrum. Aufgabe dieser Zentren, die vom Bundes- ministerium fur Bildung, Wissenschaft, Forschung und Technologic (90%) und den Landern (10%) getragen warden, 1st es, fur unsere Volkswirtschaft strategische Zukunftsfelder zu eroffnen und zu gestalten. In wissenschaftlicher Autonomie warden von ihnen langfristige Forschungsziele des Staates verfolgt.

Durch Forschung und Entwicklung Grundlagen fur Technologien von morgen zu schaffen, istZiel der GKSS. Dabei bilden Forschung, Entwicklung und Anwendung eine Einheit Die Vernetzung mit Wissenschaft, Industrie und offentlichen Anwendern sowie eine Internationale Zusammenarbeit in den Forschungsschwerpunkten und den Projektfeldern, verbunden mit einer industriellen

Umsetzung und Nutzung der Ergebnisse, markierendas GKSS-Forschungsprogramm auf den Gebieten:- Materialforschung,- Trenn- und Umwelttechnik,- Umweltforschung.

Research and development is of fundamental sig­nificance to the future of Germany's advanced, industrialized economy and is in the responsibility of 16 German National Research Centres, organized in the Hermann von Helmholz-Gemeinschaft Deutscher Forschungszentren (including GKSS with its 800 employees and an annual budget of 120 million DM) and numerous other research organisations. The National Research Centres, which are funded jointly by the Federal Ministry for Education, Science, Research and Technology (90%) and the Federal States (10%), have the task of opening up and shaping new stra­tegic technological fields of benefit to the economy.

The GKSS research and development mission is to establish the basis for tomorrow's key technologies. This involves the fusion of research, development and industrial utilization. The GKSS research program is therefore characterized by close ties between science and industry, in particular in the northern German region, and international cooperation within the framework of the main research activities and project areas, thus ensuring industrial applications and utilization.The main research activities are:

- material research,- separation- and environmental technology,- environmental research.

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