OIL PLATFORMS, DESTROYERS AND FRIGATES - CASE · PDF fileOIL PLATFORMS, DESTROYERS AND...

ACMC/SAMPE Conference on Marine Composites

Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)

© Copyright QinetiQ Ltd 2003

OIL PLATFORMS, DESTROYERS AND FRIGATES - CASE STUDIES OF

QINETIQ’S MARINE COMPOSITE PATCH REPAIRS

T J Turtona, J Dalzel-Job

b, F Livingstone

b

aFuture Systems Technology, QinetiQ, Cody Technology Park,

Farnborough, Hampshire, GU14 OLX, UK

bFuture Systems Technology, QinetiQ, Rosyth Royal Dockyard,

Dunfermline, Fife, KY11 2XR, UK

ABSTRACT

Composite patch repair solutions offer many advantages over conventional repairs to

marine structures such as avoidance of hotwork and the ability to seal cracks.

QinetiQ has been carrying out patch repairs to marine structures for over 20 years.

The following case studies of QinetiQ’s marine patch repair work will show how

these advantages have produced clear benefits for the operators of the vessels

involved.

QinetiQ has patch repaired Type 21 frigates, Type 42 destroyers and offshore oil

platforms as well as developing a number of other composite repair techniques to

marine structures. During this work QinetiQ has trialed the patch fabrication

techniques of hand layup, resin infusion and prepreg, and the advantages and

disadvantages of the methods are discussed. QinetiQ is also trialling the Alternating

Current Potential Difference (ACPD) crack monitoring technique on board two Royal

Navy vessels and this technique is also discussed.

The wider use of composite patch repairs to marine structures is held back by a lack

of repair schemes approved by the classification societies. QinetiQ is working with

those classification societies and operators of marine vessels to increase our

understanding of the effectiveness of composite patches and to develop effective non-

destructive examination (NDE) techniques to inspect patch repairs. This will increase

confidence in patch repairs leading to their wider acceptance on marine structures.

Progress on this work is reported here.

INTRODUCTION

Composite patch design

Composite patches can enhance the fatigue life of a cracked plate or restore strength

of a corroded or damaged substrate. This is achieved by providing an alternative load

path for stress. Patches are commonly designed using stiffness matching criteria.

Matched stiffnesses between patch and substrate reduces shear stress at the bondline

which may cause debonding.




Composite patch dimensions

The important dimensions of a composite patch are shown in Fig 1. The dimensions

are:

H - Patch thickness, determined by matching stiffness of the patch to the stiffness of

the metal substrate.

L -Length of patch, which is the length of the defect plus taper length.

O - Overlap length, the length over which patch is full thickness and the part of the

patch which provides structural reinforcement.

T - Taper length to reduce peel stresses. Taper length depends on the patch height

and the taper angle chosen (approximately 1 in 10).

Composite patch application procedure

No matter which composite fabrication techniques is being used the first task is

preparation of the surface of the metal substrate (Fig 2). This means producing a

certain surface roughness and cleanliness. This is usually achieved by grit blasting or

with hand held grinders. The surface roughness standard of Sa2.5 is commonly used.

Next, if the patch is made of carbon fibre, then a layer of glass fibre is commonly laid

down. This separates the carbon fibre from the metal surface and so prevents

galvanic corrosion. The rest of the fabric can then be laid down. This could be in the

form of prepreg or dry fibres if resin infusion is being used. Next, the fibre stack

should be sealed in a vacuum bag. If prepreg is used it should then be allowed to

consolidate. If dry fibres are being used they are then infused with resin. Once the

infusion or consolidation is complete, then heat can be applied if necessary to cure the

patch. After curing the bagging materials are removed.

Composite patch applications

Composite patches can be used in the following applications:

Repair of fatigue cracks

Repair of corrosion damage

Sealing leaks

Reinforcement of structure against blast or due to additional loading

Advantages of composite patch repair over metallic repair

avoids stripping out surrounding compartments to carry out hot work

provides a sealing interface

can conform to complex surface geometry

reduced maintenance costs

rapid application

light weight materials, easy to transport and handle at work site




CASE STUDIES

Type 21 Frigates (Repaired 1982)

Fatigue cracks were present in the aluminium alloy superstructures of some Type 21

(Amazon class) frigates [1] (Fig 3). Attempts to weld repair these cracks invariably

resulted in failure of the weld within a short time. These repairs were therefore

routinely reinforced with bonded mild steel patches. One area of the structure where

cracking was prevalent was in the weather decks, at butt welds formed at the change

in section between 0.75" and 0.3" deck plating (Fig 4). The bonding of mild steel

patches was found to be suitable only for reasonably flat areas of deck plating due to

the need to maintain minimum bondline thickness. A comparatively rigid steel patch

is difficult to match to the contours associated with a change of section, and therefore

some alternative reinforcement was sought.

The use of a carbon fibre reinforced epoxy patch was found to be ideal for this

application. High specific strength and stiffness meant that a thin patch could be

applied, and the flexibility of the uncured materials meant that the change in deck

section could be followed. The first ship to be repaired in this way was HMS Active.

A 500mm crack was identified in the butt welds at the change in section of the

weather deck. This crack was first weld repaired and then carbon fibre-epoxy patches

were applied. These patches were 2.4m x 1m and 5mm thick and were applied to

both sides of the ship.

The patching of this first vessel was judged to be a success and the use of composite

patching in this application was then extended to all seven ships in the class. The

superstructures were patched whether cracks were present or not. No cracking was

subsequently found beneath any of the patch sites. Six of these ships were sold to

Pakistan in 1993 (one having been lost in the Falklands conflict) and are still in

service to date. At the time of sale all composite patches were operating effectively

and therefore this application shows that this type of repair is durable and can last at

least 10 years in service.

Type 42 Destroyers (Repaired 1998-03)

Two lift shafts travel up from the galleys of Type 42 destroyers (Fig 5) to carry trays

of food to the decks above. One is manually powered and goes up one deck, the other

is electrically powered and travels up two decks. Fatigue cracking occurs in these

food lifts initiating in welds in the corners of the shafts. These cracks are normally

repaired by cutting out part of the affected plate and welding in a new section.

However this is time consuming and expensive because of the need to remove

equipment from surrounding compartments that may be damaged by the hot working.

Carbon fibre reinforced epoxy patches were therefore installed. Seven ships in the

class have now been repaired with over 35 patches being installed in total.

Handlayup, resin infusion and prepreg composite fabrication techniques have been

trialed. Structural health monitoring has been installed on two ships (more later).




The lift shafts are big enough to take a tray of food only and have rails, wires and

cables which restrict space even further (see Fig 6). These repairs show how

composite patching can be used in places of restricted access. The repairs also

showed considerable time and cost saving over welded repairs as there was no need to

strip out surrounding compartments.

FPSO Oil Platforms (Repaired 2002)

FPSO’s (floating, production, storage and offloading) are oil platforms in the form of

ships permanently tethered to the sea bed (Fig 7). Oil is produced from wells on the

seabed through flexible pipes (risers). Before storage the oil must be stabilized by

removing gas and water. The gas is commonly injected back down the well to keep

the pressure up and the water is cleaned and returned to the sea. The oil is then stored

in tanks on the FPSO until being collected by shuttle tanker, approximately once a

week. FPSO’s commonly have a two skinned hull design where a ballast (seawater)

tank surrounds the cargo (crude oil) tank (Fig 8). When one tank is being filled the

other tank can be being emptied, thereby spreading the load in the tanks evenly along

the ship.

The tethering of the platform to the seabed creates a fatigue environment which is

more aggressive than if the ship was allowed to float freely. Fatigue cracking is

therefore common in these vessels. Fatigue cracks were found in an FPSO in

Norway. There were three cracks approximately 60mm long in a bulkhead separating

a cargo from a ballast tank. These cracks allowed fluid to leak from tank to tank

which meant the tanks could not being used due to safety and pollution risk. The

reduction in the storage capacity of the vessel meant a greater number of visits from a

shuttle tanker to offload oil at £40k per visit. If a shuttle tanker was not able to visit

often enough, due to bad weather for example, then this could threaten oil production.

When welding is carried out in FPSO’s in Norway two bulkheads must be present

between the site of the hot work and any stored hydrocarbons. This means that not

just the tank to be repaired must be emptied, but the surrounding five tanks must also

be emptied, severely impacting on the storage capacity of the vessel. Since the curing

of a composite patch requires only around 60°C temperature (considerably lower than

welding temperatures), only one bulkhead was needed to provide protection between

the hot work site and any hydrocarbons. Therefore only the tank to be repaired

needed to be emptied. The composite patch repair solution therefore kept the

reduction in storage capacity of the vessel to a minimum whilst the repair took place.

A number of carbon fibre-epoxy patches were applied to the surfaces of the cargo

tank using prepreg (Fig 9). The patches were applied both at sites of cracking and at

sites where cracking was expected after detailed structural modelling had been

performed. Small aluminium sheets were also bonded to the ballast tank sides of the

bulkheads where through wall cracking was present using QinetiQ developed

adhesive (Deraseal.) These sheets prevented hydrostatic pressure acting to push off

the carbon fibre-epoxy patches and were non structural in nature.

The repairs were completed successfully within two weeks, using a two man team,

despite the problems of working in tanks 22m deep with a potentially explosive




environment. The repairs demonstrated the benefits of composite patching to the ship

operator who was able to maintain full hydrocarbon production levels throughout.

The operator has since ordered a similar repair to be carried out to another of the

company’s FPSO’s. The carbon-epoxy patches must operate in an environment of

crude oil and water at 50°C and are still in operation after 18 months service.

TRIAL OF COMPOSITE PATCH FABRICATION METHODS

During the repairs to the Type 42 destroyers hand layup, resin infusion and prepreg

(fabric preimpregnated with partially cured resin) composite fabrication techniques

were trialed. The advantages and disadvantages of the methods for applying a

composite patch repair are discussed below:

Handlayup advantages

Lower level of operator skill needed (than other methods)

Vacuum bag, heater blanket and resin trap not necessary

Can be used where vacuum bagging is not practical e.g. where cables pass through

holes in substrate to be patched

Patch layers stick together so easy to apply to vertical surfaces

Material cost lower, consumables cost lower

Handlayup disadvantages

Slightly lower quality than other methods

Mixing of resin on site necessary - error may occur

Volatile emissions high

Resin infusion advantages

Better quality than hand layup

Low volatile emissions

Better suited to large patch sizes

Resin infusion disadvantages

Higher level of worker skill

Vacuum tight seal for bag can be difficult in dusty, wet or greasy environments

Mixing of resin on site necessary - error may occur

Infusion of fibres on site necessary - dry areas may occur

Dry fibres do not stick to vertical surfaces. Binder could be used to hold fibre

stack in place but too much binder can reduce properties. Fibre stack could be

stitched together but stitching may inhibit drape of fabric.

Material cost lower, consumables cost higher

Prepreg advantages:

No chance of dry patches as fibres are already infused

Patch layers adhere to each other and to vertical surfaces

Low temperature cure prepreg will cure at ambient temperature with time

No mixing of resin on site necessary

No infusion of fabric on site necessary

Low emission of volatiles




Prepreg disadvantages

Higher level of worker skill

Vacuum tight seal for bag can be difficult in dusty, wet or greasy environments

Prepreg has limited life outside freezer (6 days for low temp curing) therefore

transport and storage to work site could be difficult.

Material more expensive, consumables cost higher

PATCH REPAIR RESEARCH PROGRAMME

QinetiQ is currently leading a research programme with owners of marine structures

and classification societies to:

Demonstrate that CFRP patching can control crack growth and extend fatigue life

Trial NDE methods to monitor patch integrity, bond line and crack growth

Develop and validate predictive modelling methods

Develop patch design capability to meet specific requirements

Fatigue testing has been carried out on patched flat plate specimens containing cracks

and the results compared to a specimen without a crack. The effect of the stiffness

and thickness of the patch has been modelled. Modelling of the bondline has also

been carried out in order to try to predict debonding. Methods to predict crack growth

rate predictions are being developed. Results to date on the effectiveness of

composite patching have shown:

Patch application increases life by a factor of at least three

A patch has survived fatigue cycling equivalent to at least 12 ship years with no

sign of patch delamination

Patch application to a fatigue crack does not prevent crack growth, but does

significantly reduce crack growth rate

Work to date has looked at specific flat plate geometry. One aim of the programme is

to predict how crack growth rates will be reduced in a structural feature. This will be

carried out using FE analysis and fracture mechanics. The programme has also

looked at NDE methods and the results of this are discussed below.

NON DESTRUCTIVE EXAMINATION

Patch repair research programme

When considering NDE of a composite patch repair there are three areas of interest:

examining the quality of composite

examining the quality of the bondline

monitoring and measuring crack length




As part of the patch repair research programme four different NDE techniques have

been examined. They are all ultrasonic techniques and are carried out from the

patched sides of the specimens only. They are:

Manual pulse echo, low frequency, contact ultrasonics (to investigate the

composite patch and bondline)

Surface waves (to indicate crack length)

Time of flight diffraction (to indicate crack length)

Tandem pulse-echo (to indicate crack length)

Manual low frequency (0.5MHz), pulse echo ultrasonics is effective at inspecting the

state of the composite patch and the composite to steel bondline. This can then be

used with computer aided scanning systems, such as ANDSCAN, to give digital plan

view images (C-scans) of the inspection area for comparison with any subsequent

through-life inspections. Of the other techniques tandem pulse-echo ultrasonics is the

most promising for monitoring and sizing of cracks (Fig 10). In some cases

specimens have been sectioned after fatigue testing in order to reveal the profile of the

crack front which provides further information in assessing the NDE techniques.

NDE trials on Type 42 destroyers

A carbon fibre composite patch used to repair the hull of a marine vessel may be of

the order of 17mm thick. Such a thickness of carbon composite is difficult for any

NDE technique to penetrate in order to measure the length of a fatigue crack beneath

the patch. QinetiQ has therefore trialed the use of the ACPD (alternating current

potential difference) technique to indicate fatigue crack growth beneath a composite

patch.

ACPD involves spot-welding a pair of electrodes to the steel substrate either side of

the crack tip (Fig 11). Then spot-welding two or more further pairs of electrodes

ahead of the crack tip in a position where the crack tip is expected to grow between

them. The technique works by passing a high frequency alternating current across the

crack site (AC field) while monitoring the potential difference (PD) between the pairs

of electrodes. Once the equipment is installed the AC field can be periodically

energised and the PD between the monitoring sites recorded. If the PD measured

changes then this is an indication that the crack is growing between the electrodes.

ACPD has been installed on two Type 42 destroyers. One installation was carried out

only a few weeks before this paper was written so no data on the effectiveness of the

patch or NDE technique is available. The other patch showed no indication of crack

growth after 3 months in service. This technique has now been incorporated into two

further specimens for the Patch Repair Research Programme (Fig 12). Data from

these tests will be used to determine if it is possible to estimate the size of crack

growth rather than just detecting growth between the electrodes.

Advantages of the technique are that wires can be fed from the patch to a junction box

at a conveniently accessible location for periodic monitoring. The electrodes and

wires used in the technique are robust and generally survive the patch application and

curing process. It is possible that further developments could lead to a self contained




monitoring system which periodically (user defined time-scale) checks for crack

extension and emits a warning if crack growth exceeds a pre-set limit.

CONCLUSIONS

QinetiQ applied composite patches have been in service for at least ten years.

Hand layup, resin infusion and prepreg patch application techniques have been

trialed and advantages and disadvantages have been listed.

Composite patch repairs offer advantages over conventional repair techniques

which can bring significant benefits to operators of marine structures.

The ACPD technique is being trialed on two Type 42 destroyers as a means of

indicating crack growth beneath a composite patch.

Manual pulse echo contact ultrasonics and tandem pulse-echo ultrasonics

techniques have shown promise at checking the quality and effectiveness of a

composite patch.

Experiments show that composite patch application can increase the fatigue life of

a cracked steel plate by a factor of over three times.

A patch repair to a cracked steel plate has survived fatigue loading in the

laboratory equivalent to at least 12 ship years.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support of the UK MOD (Major Warships

IPT) for some of the work described in this paper. Thanks also to R Trask for his

contribution to the Type 42 work.

REFERENCES

1. Allan R C, Carbon fibre reinforcement of weld repairs to the aluminium alloy

superstructure of HMS Active, AMTE(S)TM83475, Nov 1983.




FIGURES

H

L

OT

Fatigue crack

T

Figure 1: Main patch dimensions. L is patch length, H is height, O is overlap length,

the length over which the patch is full thickness and T is the taper length.

substrate

carbon fibrestructuralreinforcement

glass fibre

interlayer

defecttaperlength

overlaplength

preparedsurface

Figure 2: A carbon fibre composite patch repair showing the prepared surface, glass

fibre interlayer to prevent galvanic corrosion and carbon fibre composite structural

reinforcement.




Figure 3: A Type 21 frigate

Figure 4: Position of CFRP-epoxy patch repairs to superstructure of Type21 frigates.




Figure 5: A Type 42 destroyer.

Figure 6: View within galley of access hatches to food lift shafts of a Type 42

destroyer (left) and view up the electric food lift (right) during patch repair work.




Figure 7: Showing a typical purpose built FPSO. (NB: This vessel has not been

patch repaired by QinetiQ and is given by way of illustration only.)

Figure 8: Cross section through double skinned FPSO hull showing cargo and ballast

tanks and sites of fatigue defects.

Loading

bay

Flare Turret – point of

attachment of

undersea pipes

Oil processing

plant

Accommodation

block, offices, labs,

workshops and

helideck

Oil processing

plant

Cargo (oil)

tanks

Ballast

(seawater)

tank

Sites of

defects

Storage

tanks




Figure 9: Patch applied to cargo tank onboard FPSO, size 750 x 250mm.

Figure 10: Tandem pulse-echo measurement of crack beneath composite patch from a

distance of 400-450mm. Crack size estimated at 39mm, optically measured at 47mm.




Figure 11: Position of ACPD electrodes at tips of crack in a steel structure. The

electrodes and surrounding area are then covered with a composite patch.

Figure 12: ACPD monitoring attachments on specimen surface prior to application of

composite patch. Monitoring sites in-line with crack tips and then 5 and 15mm

beyond the crack tips.

Upper monitoring positions

Crack

Lower monitoring positions

Welds from previous repairs

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