OIL PLATFORMS, DESTROYERS AND FRIGATES - CASE STUDIES OF QINETIQ S MARINE COMPOSITE PATCH REPAIRS. Farnborough, Hampshire, GU14 OLX, UK

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1 OIL PLATFORMS, DESTROYERS AND FRIGATES - CASE STUDIES OF QINETIQ S MARINE COMPOSITE PATCH REPAIRS ABSTRACT T J Turton a, J Dalzel-Job b, F Livingstone b a Future Systems Technology, QinetiQ, Cody Technology Park, Farnborough, Hampshire, GU14 OLX, UK b Future Systems Technology, QinetiQ, Rosyth Royal Dockyard, Dunfermline, Fife, KY11 2XR, UK 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 nondestructive 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.

2 Composite patch dimensions ACMC/SAMPE Conference on Marine Composites 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

3 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 ) 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).

4 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

5 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

6 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

7 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

8 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.

9 FIGURES Fatigue crack L H T O 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. prepared surface substrate carbon fibre structural reinforcement glass fibre interlayer taper length overlap length defect 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.

10 Figure 3: A Type 21 frigate Figure 4: Position of CFRP-epoxy patch repairs to superstructure of Type21 frigates.

11 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.

12 Flare Turret point of attachment of undersea pipes Oil processing plant Storage tanks Loading bay Accommodation block, offices, labs, workshops and helideck 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.) Oil processing plant Cargo (oil) tanks Ballast (seawater) tank Sites of defects Figure 8: Cross section through double skinned FPSO hull showing cargo and ballast tanks and sites of fatigue defects.

13 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 mm. Crack size estimated at 39mm, optically measured at 47mm.

14 Upper monitoring positions Crack Lower monitoring positions Welds from previous repairs 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.