Hull Inspection Techniques & Strategy

8/10/2019 Hull Inspection Techniques & Strategy

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HULL INSPECTION TECHNIQUES & STRATEGY

R. Caldwell, Marine Integrity Management Solutions Ltd., UK

SUMMARY

As field life extends, managing the hull integrity of a Floating Offshore Installation (FOI) has become more critical.Employing Class entails the hull undergoing a robust five yearly survey cycle to assure its integrity. Part of that cycleincludes drydocking the vessel. FOIs tend not to drydock resulting in significant challenges to operators in managingthe hull integrity of their floating installations. Consequently, some operators have chosen to extend the concepts ofstructural integrity management and are seeking to develop new techniques to support risk based inspection of thesubmerged hull and the internal cargo and ballast tank structure. Inspection techniques and the information obtainedtherefrom are key parameters in the assessment of structural integrity. But are these parameters well identified? Whatdata is needed to assure hull integrity and how can we best obtain this data?

1. INTRODUCTION

As understanding in the management of Floating

Offshore Installations (FOI) has grown across theoffshore Oil & Gas (O&G) industry, so too has anappreciation of the criticality of FOI hull structures.Previously the hull was seen by many operators asmerely the platform upon which the process equipmentwas located. The hull condition was considered as lowrisk. The hull had, after all, been designed andconstructed to Classification Society Rules, some ofwhich stemmed from over 250 years of experience withocean going vessels. However, the offshore O&G sectoris increasingly challenging Classs prescriptive surveyrequirements for floating installations in service asoperators seek to implement risk based inspection andmaintenance strategies which better fit their operationalgoals. This project seeks to consolidate known anddeveloping inspection technology with an objective todevelop guidelines on their application to the offshorefloating industry, the qualifications of personnel

providing these services and specifications for equipmentwhich can be used.

2. BACKGROUND

The offshore O&G industry has developed verydifferently from the more traditional marine industry. Aship operates under the legislative and regulatoryframework of an adopted Flag State who as signatories toIMO Conventions, impose Statutory regulations andClassification. This results in Classification beingmandatory for all ocean-going trading vessels. Incontrast, the legislative and regulatory framework for anFOI varies throughout the world and is as a rulegoverned by the National regulations of the Coastal Statein which it will be operating. As a result, whereas in theMarine Industry, Class is considered as an important partof the regulatory framework, it does not enjoy this staturein the O&G industry.At the outset, all operators build their FOI to Class Rulessince these continue to be the most suitable buildstandard for FOI and are expected to remain so for theforeseeable future. This is, in part, because of the

building yard's technical and commercial familiarity withthe Class concept and their Rules. However, it is duringthe operational life of an FOI that the maintenance of

Class becomes more challenging.The UK model is that of self regulation and one whichhas been adopted to varying degrees globally. Under theUK's Safety Case Regulations (SCR) [1], the asset owneris required to identify major accident hazards (MAH) andfrom these, Safety Critical Elements (SCE). They arethen required to produce a Performance Standard foreach SCE outlining how they will assure the integrity andreliability of the SCE. This model does not mandateClassification although the maintenance of Class is seen

by some operators as a means of achieving theassurances for the Hull Performance Standard.The basic premise behind Class Rules is that following

build, the hull undergoes a robust five yearly cycle ofsurveys during which every aspect of the hull structureand marine systems is examined to confirm and assuretheir continued integrity. Part of that survey regimeincludes drydocking the vessel. This allows the ownersto carry out external hull maintenance such as cleaningand painting. Where items are found deficient, thereexists an opportunity, either during such drydockings orat port calls, for the owner to carry out repairs.Unfortunately an FOI is afforded neither the luxury of

port calls nor regular drydockings. As a result, someoperators employ a more risk based approach in the formof integrity management.

3. OFFSHORE PRACTICE

The primary focus of the offshore operator is to achieve arespectable return on their capital and operational costs.They achieve this by maximising continuous safe

production with the minimum of through life inspection,maintenance and repair disruption. This results in allinspection, maintenance and repair work having to bescheduled around production requirements with theminimum of risk. The planning and scheduling of theseinspection and maintenance tasks are considered

paramount to the efficient operation of the asset.Operators have developed a mature methodology forensuring the integrity of SCE from their experience of


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operating fixed platforms. The operator normallyemploys an integrity contractor that provides technicianswho carry out inspections based on a developed integritymanagement strategy. The strategy defines the extent,type and periodicity of inspections to be carried out.Most inspection schedules employ a risk-based approach.

This methodology is typically adopted on their floatingassets too. However, the complexity and variety of thehull structural configuration of FOI have led tochallenges on how this approach is adopted particularlyon areas such as the submerged hull, cargo and ballasttanks whilst ensuring uninterrupted continuous

production.FOIs normally remain on location for the duration offield life, often up to 20 years. With the advent of newextraction technology, in many cases, this is beingextended even further. During this time, they experiencesignificantly different loadings and contain differentfluids than their marine counterparts. An FOI in a fixed

location cannot divert around bad weather: Green seasand slamming damage can and has occurred. A typicalFPSO loads continuously. The loading / offloading cyclemay be as short as 7 days. The consequence is that thehull girder experiences significant stress fluctuations andlow cycle fatigue, as well as wave loading high cyclefatigue, is a known threat to the hull structure of thesevessels. Structural damage can have high consequence to

production, the environment and not least the safety of personnel on board which can number over a hundred.Repairs on location may be technically too demandingresulting in the asset having to leave field with theassociated significant economic implications.

To date, the Class Societies have not fully addressedthese issues in-so-far as their Rules relating to FOIs haveyet to properly acknowledge and account for thesefundamental differences between the operation of aseagoing vessel and an FOI.

4. CARGO TANKS

Class Regulations presently require an exclusive Classsurveyor to carry out the surveys. However, Classsurveyors are not in a position to comply with their ownregulations when carrying out close-up surveys, definedas "normally within reach of hand" [2]. Scaffolding a

tank on an FOI on location to gain access to all areasrequiring inspection is prohibitively expensive as well asimpractical and rafting does not meet offshore industrysafety criteria. Assuch, it has becomecommon practice forRope AccessTechnicians (RATs) toaccess structures, carryout inspections andreport findings to theClass surveyor.Concurrent production

operations present anumber of unique

challenges predominantly centred around isolations toadjacent cargo tanks and deck piping containing crude oiland/or inert gas. The practicalities of tank cleaning onlocation and dealing with slops have to be carefullymanaged to avoid any detrimental environmentalconsequences. The level of cleaning must be complete in

order to not only be able to carry out the inspectionsafely but to be able to do so thoroughly. Cargo tankscan suffer from isolated bottom pitting (Figure 1),grooving and accelerated underdeck corrosion, which arenot easy to detect if the tank is not properly clean.

Figure 1: Bottom Pitting in a cargo tank

All of the inspection techniques that involve close visualinspection of the higher level structures incur a safetyrisk whether using rope access, scaffolding, or rafting.

The inspectors are more often than not part of thecontracted integrity inspection team permanentlyonboard the asset and may not have had formal certifiedtraining in inspection of ships structures leaving theoperator and classification societies exposed to risk.A tank such as that shown in figure 2 has considerablestructural elements all of which have to be given dueconsideration at the inspection outset.

Figure 2: Typical single bottom tank configuration

The penetrations of bottom longitudinals through theforward and aft bulkheads as well as their connectionthrough the bottom web frames are areas of interest asare the web frame toes, the centre girder bracket toes and

the stringer toes to name but a few. Also clearly visibleis the piping located in the tank: It too requires to be


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integrity managed. In the tank shown in figure 3, onlythe transverse bracket toes remain from the list of areasabove.

Figure 3: Typical double skinned cargo tank

Presently both tanks require to be inspected in the sameway even though their differing structural configurations

provide an opportunity, with the correct techniques andmethodology, to inspect the latter without entering thetank.

5. BALLAST TANKS

A number of the same challenges that exist for cargotank inspections on FOI exist for the ballast tanks too.

These are likewise inspected concurrent with continuedoperations resulting in similar isolation threats. Cleaningmay not be as big an issue but the confined space threatof these double side/double bottom tanks is significantwith adjacent cargo tanks in use.A greater challenge is the integrity management of

ballast tank coatings. Grit blasting and recoating entiretanks is difficult to achieve offshore. The accurateassessment of the extent of coating deterioration andcorrosion is therefore paramount in order to effectivelymitigate these critical hull structural integrity degradationthreats. It is not sufficient to rate an entire tanks coatingcondition as good, fair or poor. The coating and

corrosion condition of all tank areas must be recorded.

6. MARINE SYSTEMS

Tank inspections may be driven by the management ofcomponents located within ballast and cargo tanks suchas piping, valves, deepwell pumps, level and other safetydevices. Such access could allow the opportunity to gaininspection data on adjacent tanks, if suitable inspectiontechniques were acceptable.

7. CORROSION & COATING ASESSMENT

In the marine industry, the assessment of corrosion iseffectively dealt with by thickness gauging of plating and

stiffening. This is required to be carried out bycompanies approved by the Class Societies. Theapproval requires the company to provide technicianswith experience o f ships structure as well as beingcompetent in using the gauging equipment and requisitereporting software. For classed FOI, a similar

philosophy is applied. However, the O&G industry doesnot lend itself well to this philosophy being effective. Asintimated earlier, the operator normally employs anintegrity contractor that provides technicians usually withrope access qualifications who carry out all forms ofintegrity inspections based on developed integritymanagement strategies. Whilst they normally have thenecessary NDT competence, they lack fundamental hullstructural knowledge, most commonly of thenomenclature. This has resulted in structure beingincorrectly identified which can lead to ineffectiveintegrity assessment. Employing such an integrityinspection regime can also result in confusion when it

comes to reporting of corrosion and coating condition.Many of the NDT technicians are more familiar withinternational standards with which they report on thetopside infrastructure. The marine industry and ClassRegulations however adopt the International Associationof Classification Societies (IACS) RecommendedGuidelines for Coating Maintenance & Repairs forBallast tanks and Combined Cargo/Ballast tanks on OilTankers [3]. Whilst this document goes some way tocompare international standards with its own guidance,the scope for misinterpretation is great and, as previouslyintimated, it falls short of O&G industry expectationswith respect to its use as a tool for integrity management

purposes. These issues are particularly prevalent onaccount of most inspection companies having a highturnover of personnel. To aggravate this further, it has

become increasingly common for the industry tochallenge the Class Societies to justify a surveyoronboard who will not physically carry out the survey butonly review inspection reports, an activity, they suggest,that could be equally effectively carried out ashore.Where bed space is at a premium, this results in thesetechnicians carrying out the Class inspections themselvesand reporting their findings to Class.There is presently only very limited guidance as to theextent of supervision or assistance that may be given to

the RAT or other inspectors and none to their hullstructural competence. For classed assets, IACS

presently permits remote inspection techniques providedthey meet with its recommendation No. 42 [4]. Thisrecommendation provides for an assessment of thetechnique proposed on a case by case basis. Thetechnique is to provide the survey results normallyobtained by the Surveyor. However, it stipulates thatconfirmatory close-up surveys are to be carried out bythe Surveyor at selected locations where close up surveysare required to verify the results of the RemoteInspection Technique . This would appear to negate thevery reason for adopting the remote technique.


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8. SUBMERGED HULL

The submerged hull of an FOI differs in a number ofimportant aspects from a ship hull. A ships hull isshaped to maximise fuel efficiency. To facilitate this, theowner will normally remove marine growth and reapply

antifouling during drydocking. An FOI will not drydockand will not be afforded the luxury of antifoulingreinstatement. In contrast, marine growth on thesubmerged hull of an FOI does not have a significantdetrimental operational impact although it can preventeffective inspection of the external hull plating, weldsand coating.The risks to the underwater portion of an FOI differ froma trading tanker too. An FOI is highly unlikely to sufferfrom grounding damage: This was recognised by anumber of designs being double sided but only single

bottomed. Neither do FOI suffer from sterntube sealwear or pintle wear to the same extent as trading tankers.

The majority of modern purpose built FOI now minimiseappurtenances such as the number of hull penetrations inorder to reduce the problems and costs associate withshipside valve and seachest grillage maintenance.Ships now routinely carry out in-water surveys in lieu oftheir intermediate docking. These are carried out indesignated sheltered waters affording suitable visibility

by divers employing CCTV and guided by a Classsurveyor. For an FOI, this work has to be carried out onlocation. Since the O&G industry has steered away fromdivers where possible because of the safety risks, thiswill generally result in a work class ROV being deployedfrom a support vessel with considerable associated costs.

As a consequence, FOI operators challenge the periodicity of these costly subsea inspections when itappears that the degradation mechanisms are much lesscritical to their FOI, even more so when a number oftechniques for items such as shipside valves have beendeveloped allowing them to be effectively integritymanaged from inboard. This has resulted in their seekingalternative inspection techniques, some from inboard, toinspect the external submerged hull.

9. INSPECTION PROVISION

The purpose of carrying out a structural survey of any

tank is to determine the extent of corrosion wastage andstructural defects present in the tank. In terms ofguidance on the provision of inspection in the O&Gindustry and FOI in particular, the operator is directed toISO 19904: Petroleum and natural gas industries -Floating offshore structures - Part 1: Monohulls, semi-submersibles and spars [5]. The standard states that forPersonnel Qualifications, Only suitably qualified

personnel shall be assigned to perform inspections .With respect to the equipment used, Any equipment ormeasuring instruments used as part of a structuralinspection shall be provided with current, validcalibration certificates or a ready means of confirming

that they remain within acceptable calibration standards and that inspection programmes ensure, The

reliability, accuracy, precision and tolerance of the system, including operating personnel, shall beestablished .

10. THE CHALLENGE

FOI operators agree that Class surveys and their initial periodicity are on the whole sufficiently robust tomitigate hull structural risks. However, as these assetsreach their end of design life, the probability of failureswill increase. The challenge of this project is to providea variety of alternative inspection techniques acceptableto the O&G industry and the Classification Societieswhich will allow operators the flexibility to implement amore pragmatic approach to inspections and inspectionscheduling and in so doing maintaining or potentiallyreducing further the level of risk of hull structural failure.In so doing, the project will attempt to quantify the

present level of acceptable structural risk. Both the O&G

industry and the Classification Societies have beeninvited to participate. During this phase, the failuremechanisms associated with FOI hull internal andexternal structure will be identified and defect sizecriteria developed. This criteria will in turn be used toestablish a probability of detection criteria against whichall the investigated inspection technologies will begraded.The deliverables will be guidelines on a variety ofinspection techniques, the equipment used there with andtraining & certification to ensure inspector competence.A matrix of inspection technique and periodicity betweeninspections will be developed.

The remainder of this paper describes a number ofexisting techniques, their advantages and disadvantagesand their opportunities to develop further.

11. MANUAL TOOLS TECHNOLOGY

Contracting companies providing inspection techniciansand/or equipment are regularly challenged on their

personnel competence and equipment certification butthere is presently only general NDT guidance rather thanspecific inspection technique procedures. Presentlyinspection technicians are required primarily to have ropeaccess training and NDT qualifications generally

unspecified. There is a heavy dependency on integrityengineers providing quality workbooks for thetechnicians to use. These generally contain drawings ofthe structure to be inspected and criteria to be employedwhen reporting findings. There is obviously some

practical constraints to using these workbooks in tankswhen suspended from ropes. The technicians aregenerally encouraged to take photos but this at theirdiscretion. Ideally continuous contact with an integrityengineer or Class surveyor is sought. The Head UpDisplay (HUD) is a development which assists thetechnician in seeing information from remoteinstruments or prior reports whils t sending inspection

data to the surveyor or integrity engineer (Figure 4).


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Figure 4: Rope Access Technician using HUD (Courtesyof EM&I)

Common equipment such as cameras and ultrasonicthickness gauging probes have been developed tofacilitate inspecting bellmouths without their removal.

Figure 5: Cobra Equipment (Courtesy of EM&I)

Mobile phones and other mobile technology availabletoday are increasingly becoming commonplace ineveryday life. They are capable of taking photographsand video footage with voice overlay. With the correctapps, such devices could be employed in a number of

ways. They could be made to facilitate an appropriatelevel of technical overview during a remote inspection byenabling technical drawings to be electronically to handduring the inspection. The devices are also equippedwith a bank of accelerometers to detect the orientation ofthe handset. It may be possible to use this functionalityto control a surveying robot and immediately analyse thedata retrieved. Transponders are already used in someoperations for locating ROVs. All this information could

be transmitted to onshore in real time permitting animmediate appropriate technical assessment.On-line execution of image processing algorithms andresults are being developed which can be applied to

images immediately available from field providingcorrosion assessment features (Figure 6).

Figure 6: Corrosion Image Processing Algorithm(Courtesy of MINOAS)

12. NON-DESTRUCTIVE TESTING (NDT)TECHNOLOGY

As far as surveying methods are concerned theAlternating Current Field Measurement (ACFM)technique is increasingly used in the offshore industry

both in underwater and topside structural parts, for thedetection of fatigue cracks. Processes have been put in

place to distinguish between transverse and longitudinaldefects, as well as cracks located on the outer and theinner surface of the specimens, although at present this islimited. Furthermore the results are not affected by thecoatings of the structure. In general, the ACFM

technique is considered a reliable, cost-effective, time-effective surveying method to detect defects anddetermine their dimensions.Presently Time of Flight and other ultrasonic methodsare capable of inspecting external plating and the fullvolume of welds from inside the hull thus eliminating theneed to employ expensive workclass ROV and standbyvessels.Further techniques utilising eddy current such as theSaturated Low Frequency Eddy Current (SLOFEC)technique are now being used for rapid scanning to detectthrough-wall discontinuities in steel materials up to35mm and these methods have the advantage of being

able to inspect through marine growth in many instances.Whilst this technique is one of the inside looking out,some operators prefer to inspect from the outside in.These techniques could be equally developed to inspectcargo tank bottom plating pitting from within a water

ballast double bottom tank. If they were marinised theycould potentially be used attached to ROVs.

13. ROBOTIC TECHNOLOGY

Crawlers of various types have been deployed to carryinspection tools such as ultrasonic thickness probes andCCTV whilst in contact with the hull structure and have

the advantage of being relatively inexpensive to deploy.An existing example is Snoopy, an underwater, line-


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operated vehicle, launched or operated from the FOI. Aline is deployed around the hull to guide and tow Snoopyaround hull obstructions, such as the bilge keels. Once in

position, the unit's buoyancy is adjusted to bring it intoclose contact with the hull. Snoopy then tracks along thehull, continuously monitoring the condition of the plating

thickness, coatings and cathodic potential. Reports are produced on site, using 3D color graphics mapping bottom plate condition such as the presence of corrosion pits, condition of coatings and the cathodic potential. Atypical case history is the Snoopy device illustrated

below (Figure 7) which was used to inspect slops tank bottom plating from outside the hull this being beneficial to the operator who had concerns about pitting but who had operational difficulties in taking the tank outof service and cleaning it sufficiently.

Figure 7: Snoopy at work (Courtesy of EM&I)

In this instance the Snoopy device carried an array ofultrasonic probes and a CCTV pan and tilt camera inorder to inspect selected areas of the tank from outsideand to examine the condition of the external coating.Future developments include improved accuracy oflocation and wider area coverage with larger probearrays.

14. ROV TECHNOLOGY

Due to the safety concerns related to diving operations,considerable investment in ROV development by the

offshore industry has resulted in a myriad of ROVcapable of performingvery nearly all underwatertasks. ROV are nowcommonly used forsubsea hull inspections.The need to employ acostly work-class ROVfor underwater hullinspection is beingincreasingly challenged

by some of the newer,smaller ROVs which are

able to be launched fromthe FOI itself.

With the advent of the suitcase-sized micro ROV, someoperators are opting to employ these devices to carry out

ballast tank inspections (Figure 8). Dependent upon thetank configuration, the lightening holes are generally thelimiting constraint on the size of the ROV that can beemployed. These may be as small as 350 x 225 x 210

mm.

Figure 8: Micro ROV (Courtesy of HPR)

Another operating constraint for the micro ROV is that atypical double skinned FOI having common wing anddouble bottom water ballast tanks some 50m long, 30mwide and 30m deep will require the micro ROV tooperate on an umbilical of length 150m.Unfortunately there is presently no industry guidance ontheir use and the quality of inspection can vary

considerably. Figure 9 below is an example of what may be construed as good photo quality.

Figure 9: ROV photo of crack-like indication

However there is no location identification, no defectsizing and no associated conclusive NDT to confirmwhether this is a coating crack or a structural crack.Experience to date has found that a minimum of oneROV operator should hold a valid CSWIP 3.3Ucertificate as well as have knowledge and experience of

hull structure or have been through a Class providedtraining scheme. The equipment provided must be fully


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serviced and correctly calibrated. Ideally, the ROVshould be capable of the following functions:

Navigating through 400 x 600 mm access holesOperating with a suitable illumination deviceOperating with a video camera

Carrying out thickness gauging using a UT probeCarrying out ACFM using an ACFM probe (presentlynot possible see later comments)Measuring defect sizesMarking defect locationsDepth rating not less than 50m

The ROV control console and equipment within should be suitably waterproof and 'Ex' rated (e.g. IP67) such thatthe equipment has no spark potential. A provision torecord and overlay the video with date, time and depthshould be provided. Successful operations have shownthat 40 W of illumination is acceptable. The videocamera should have a still photo image facility withminimum resolution capability of 550 x 0.1lux. Asuitable type approved underwater UT probe capable ofattachment to the ROV should be available for use. Inorder to evaluate surface defects, a suitable typeapproved underwater ACFM probe capable ofattachment to the micro ROV should be developed:Present ACFM equipment is too heavy to be fitted to amicro ROV without the micro ROV losing some of itsmanoeuvrability. A mechanical or optical (laser) devicefrom which defect sizes can be measured should beavailable for use. A capability to measure depth toevaluate pitting should be developed and available foruse. A capability to identify and return to a specificlocation is imperative. Whilst it is appreciated that this ismost readily achieved through the skill of the operator, itis considered that a method of marking the location of adefect should be available for use.

15. BENEFITS

Industry guidance on inspection techniques and operatorqualifications as well as equipment specifications will

provide significant benefits to owners and operators ofFOIs as well as integrity contractors and theClassification Societies working within O&G industry.Some of these benefits will include:

Improved Safety reduced HSE riskReduced cost & budget uncertaintyImproved inspection qualityConsistent approach to training, management and

best practiceFitness for Service recognition globallyImprove adequacy in managing FOI assets globallyMotivate industry and public authorities to improvesafety by standardized approachRecognition and use by international authorities,safety and environmental bodies globally

16. CONCLUSION

This paper has described some of the challenges faced byoperators of floating offshore installations to managetheir structural integrity. It highlights the dearth ofguidance available to the operators on applying existing

inspection technology. It describes some of thesetechnologies, their operational constraints and futuredevelopment opportunities. Upon completion, the HITS

project aims to provide the offshore oil & gas industrywith comprehensive guidelines on the application of anumber of inspection techniques, including operatorcompetency levels, equipment specifications and relativeinspection frequencies thereby providing operators withchoices of inspection schemes which best suit theiroperations.

17. ACKNOWLEDGEMENTS

The Author would like to acknowledge permissiongranted by EM&I and the EU funded MINOAS projectfor content and photographs related to inspectiontechnologies.

18. REFERENCES

1. The Offshore Installations (Safety Case)Regulations 2005, Statutory Instrument 2005 No. 3117Offshore Installations2. IACS Rec 82: Surveyors Glossary Hull Terms& Hull Survey Terms3. IACS Rec 87: Guidelines for Coating

Maintenance & Repairs for Ballast tanks and CombinedCargo/Ballast tanks on Oil Tankers4. IACS Rec 42: Guidelines for Use of RemoteSurvey Techniques5. ISO 19904-1:200 6 P etroleum and natural gasindustries - Floating offshore structures - Part 1:Monohulls, semi-submersibles and spars

19. AUTHORS BIOGRAPHY

Raymond Caldwell is a Chartered Marine Consultantwith Marine Integrity Management Solutions Ltd. Heconsults as Chevron Nor th Sea Ltd.s Marine Technical

Authority providing specialist hull, mooring & marinesystems advice and is the present Chairman of Oil & GasUKs FPSO Network. He also provides marine integritymanagement and marine consultative advice for anumber of other FOI operators. Consulting for LREnergys Technical Directorate, he provides advice ondevelopment of their FOI Class Rules and in-housemarine integrity procedures. He represented LR at theMooring Integrity II JIP and LRs technical participationin EU funded MINOAS & Ship Inspector JIPs.
http://www.iso.org/iso/rss.xml?csnumber=22995&rss=detailhttp://www.iso.org/iso/rss.xml?csnumber=22995&rss=detail

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