DNV OS 121 - Classificatio Based on Performance Criteria Det

OFFSHORE SERVICE SPECIFICATION

DET NORSKE VERITAS

DNV-OSS-121

CLASSIFICATION BASED ONPERFORMANCE CRITERIA DETER-MINED FROM RISK ASSESSMENT

METHODOLOGYMAY 2001

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FOREWORDDET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life, prop-erty and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification and consultancyservices relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carries out researchin relation to these functions.DNV Offshore Codes consist of a three level hierarchy of documents: Offshore Service Specifications. Provide principles and procedures of DNV classification, certification, verification and con-

sultancy services. Offshore Standards. Provide technical provisions and acceptance criteria for general use by the offshore industry as well as

the technical basis for DNV offshore services. Recommended Practices. Provide proven technology and sound engineering practice as well as guidance for the higher level

Offshore Service Specifications and Offshore Standards.DNV Offshore Codes are offered within the following areas:A) Qualification, Quality and Safety MethodologyB) Materials TechnologyC) StructuresD) SystemsE) Special FacilitiesF) Pipelines and RisersG) Asset Operation

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Offshore Service Specification DNV-OSS-121, May 2001Contents Page 3

CONTENTS

Sec. 1 Introduction........................................................... 5A. General....................................................................................5A 100 Introduction....................................................................... 5A 200 Application........................................................................ 5A 300 Scope................................................................................. 5A 400 Class notation.................................................................... 5A 500 Terms and conditions........................................................ 5

B. Definitions .............................................................................. 5B 100 Terms ................................................................................ 5

Sec. 2 Design Principles and Risk AssessmentMethodology .......................................................... 7

A. General....................................................................................7A 100 Introduction....................................................................... 7

B. Design Principles....................................................................7B 100 General.............................................................................. 7

C. Risk Assessment ..................................................................... 7C 100 Hazard identification......................................................... 7C 200 Risk analysis ..................................................................... 7C 300 Risk assessment ................................................................ 7C 400 Acceptance criteria ........................................................... 8

D. Safety-Critical Elements.........................................................8D 100 General.............................................................................. 8

E. Performance Standards ...........................................................8E 100 General.............................................................................. 8

F. Risk Reduction........................................................................8F 100 General.............................................................................. 8

Sec. 3 Verification, Class Entry ..................................... 9

A. General....................................................................................9A 100 Introduction....................................................................... 9

B. New Installations ....................................................................9B 100 Verification scheme for design, construction and

commissioning.................................................................. 9

C. Existing Installations ..............................................................9C 100 General.............................................................................. 9

Sec. 4 Verification, Maintenance of Class................... 10

A. General..................................................................................10A 100 Introduction..................................................................... 10

B. In-Service Verification Scheme for Maintenance of Class ..10B 100 General............................................................................ 10B 200 Review of safety-critical elements.................................. 10B 300 Verification scheme ........................................................ 10

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Offshore Service Specification DNV-OSS-121, May 2001Page 4 Contents

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Offshore Service Specification DNV-OSS-121, May 2001Sec.1 Page 5

SECTION 1INTRODUCTION

A. GeneralA 100 Introduction101 This document presents the DNV service specificationfor classification of offshore installations based on risk assess-ment techniques. This means that the acceptability of designand the extent of verification are based on risk assessment rath-er than reliance upon prescriptive rules.102 The objective of this approach is to provide an alterna-tive route for classification of individual and novel designswhilst maintaining an acceptable level of safety and integrityequivalent to that achieved under more traditional classifica-tion rules. Although not mentioned specifically through thisservice specification, avoidance of adverse environmental con-sequences shall also be taken into account as far as covered bytraditional classification rules.

A 200 Application201 The requirements are applicable for fixed offshore in-stallations, including floating installations designed for use ata fixed location.202 The service is alternative or complementary to classifi-cation based on other DNV service specifications (DNV-OSSseries) for such installations. Where classification based onrisk assessment is applied only to a part of an installation, theremainder of the installation shall meet the requirements ofother applicable DNV rules.In such cases, it is important to ensure a safe interface betweenthe two classification methods i.e. that design based on risk as-sessment does not have a negative effect upon safety or integ-rity under classification rules and vice versa.

A 300 Scope301 Under this service specification, risk assessment meansthe systematic identification and evaluation of hazards to per-sonnel and plant (see 102).Risk assessment is the overall process of risk (hazard) identifi-cation, risk analysis and risk evaluation. The results of the as-sessment identify areas of most significant risk and enable riskreduction measures to be targeted where most effective.The use of risk assessment for classification shall include:

a) Hazard identification and evaluation of hazards and therisks they pose to safety and integrity on the installation.

b) Demonstrating that the risks are acceptable, i.e. meetagreed risk acceptance criteria and or target risk reduction.

c) Identifying the safety-critical elements and their requiredperformance standards that are required in order to meetthe acceptance criteria.

302 In order to assign classification, DNV will review andapprove the selection of safety-critical elements and perform-ance standards, and verify their design, construction, installa-tion and commissioning.DNV will establish an installation-specific verification schemefor initial class entry as well as maintenance of class. The DNVscope for approval and verification is detailed further in Sec.2to Sec.4.303 Maintenance of class shall be through ongoing verifica-tion that the safety-critical elements remain in adequate condi-tion and repair to foreseeably meet their performancestandards.

Whilst classification is maintained, DNV will retain the verifi-cation scheme under review based on findings of verificationactivities. DNV will issue an annual verification status report,which may be used as documentation with other regulatory re-view requirements.304 Classification in accordance with this specification indi-cates that the installation has been verified as attaining, andshould foreseeably continue to attain, an acceptable level ofsafety and integrity as specified by agreed risk criteria. Theowner or operator is responsible for ensuring compliance withrequirements of maritime administrations or conventions, andfor seeking concessions where any such requirements are notfulfilled.

A 400 Class notation401 Offshore installations classed in accordance with theprovisions of this service specification (DNV-OSS-121) willbe assigned the class notation RA.

A 500 Terms and conditions501 The general terms and conditions for classification shallbe as stated in the relevant OSS for the unit type, for exampleDNV-OSS-102 for floating production units.502 DNV will manage and document approval and verifica-tion activities for class entry and maintenance of class. DNVwill also interface with and direct the risk assessment activitiesas necessary to ensure acceptability for classification. Theowner or operator is expected to co-operate with DNV as nec-essary to establish and undertake the approval and verificationprocess.

B. DefinitionsB 100 Terms101 Consequences are the expected effects of an event oc-curring.102 Frequency is the number of occurrences of an event perunit time. In risk assessment, it is usually expressed as the fre-quency per year.103 Hazard is a source of potential harm or a situation witha potential to cause loss (any negative consequence).104 An inspection and maintenance plan or schedule is aprogramme of scheduled inspection and maintenance activitiesthat ensure the safety-critical elements continue to meet theidentified performance standards to maintain the safety and in-tegrity of the installation.105 Major hazards are hazards with a potential for causingmajor accidents, i.e. involving fatality due to fire or explosion,multiple fatalities, or severe damage to the installation. Majorpollution should be covered as indicated in A102.106 A performance standard is a statement, which can be ex-pressed, in qualitative or quantitative terms as appropriate, ofthe performance required of a safety-critical element in orderto ensure the safety and integrity of the installation.107 Risk is the combination of likelihood and consequenceof hazards being realised, i.e. the chance of a specific event oc-curring within a specific period of time.108 Risk analysis is the quantification of risks without mak-ing judgements about their significance. It involves identifyinghazards and estimating their frequencies and consequences, so

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Offshore Service Specification DNV-OSS-121, May 2001Page 6 Sec.1

that the results can be presented as risks. Risk analysis is some-times known as risk estimation or risk quantification.109 Risk assessment is a systematic analysis of the risks fromhazardous activities and making a rational evaluation of theirsignificance by comparison against predetermined standards,target risk levels or other risk acceptance criteria. Risk assess-ment is used to determine risk management priorities.110 Risk acceptance criteria are standards by which the re-sults of the risk assessment can be measured. The acceptancecriteria represent the acceptable level of safety and integrity ofthe installation. They relate quantitative risk estimates to qual-itative value judgements about the significance of the risks.111 Safety-critical elements are parts of the installation, orplant, which are essential to maintain the safety and integrityof the installation. This includes any items which:

if they failed, could cause or contribute substantially to amajor hazard affecting safety or integrity of the installa-tion; or

are intended to prevent or limit the effect of a major haz-ard.

Safety-critical elements include measures for prevention, de-tection, control, mitigation (including personnel protection) ofhazards.112 Verification is examination, testing, audit or review toconfirm that an activity, a product, or a service, is in accord-ance with specified requirements.For DNV classification, this normally includes verification ofthe design, manufacturing, construction, installation, commis-sioning and in-service condition of safety-critical elementssuch that they fulfil the requirements of the performance stand-ards.

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SECTION 2DESIGN PRINCIPLES AND RISK ASSESSMENT METHODOLOGY

A. GeneralA 100 Introduction101 This section states the design principles and require-ments for risk assessment that are to be applied for award ofDNV classification.102 Risks shall be assessed in accordance with recognisedmethods and shall be performed by qualified and competentpersons with the necessary understanding of risk, and the riskassessment process.103 The risk assessment methodology and tools, assump-tions, and system boundary limits shall be clearly documented.Sensitivity analysis to examine how the results may vary assignificant individual assumptions are changed shall be used todocument the uncertainties of the identified risks. This docu-mentation shall be subject to DNV approval.

B. Design PrinciplesB 100 General101 The following principles shall apply in addition to theidentified requirements from risk assessment:

1) The installation shall be designed and constructed withsufficient integrity to withstand operational and environ-mental loading throughout the installation lifecycle.

2) Systems and structures shall be designed with suitablefunctionality and survivability for prevention, detection,control and mitigation of foreseeable accident events af-fecting the installation.

3) Escalation to plant and areas, which are not affected by theinitiating event, shall be avoided.

4) Effective escape, shelter and evacuation facilities shall beprovided to safeguard all personnel, as far as practicable,at all times when the installation is manned.

C. Risk AssessmentC 100 Hazard identification101 Hazards with the potential to threaten safety of person-nel or integrity of the installation shall be identified. The haz-ard identification should include all normal expected states ofthe installation, such as operation, maintenance, and shutdown.

Guidance note:Typical hazard identification techniques include e.g. HAZOPs,FMEA, safety reviews, etc.

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102 A typical, but not necessarily exhaustive, list of hazardsincludes:

loss of well containment (blowout etc.) hydrocarbon releases with potential to result in fires, ex-

plosions, or toxic hazards release of other toxic or hazardous substances collisions helicopter crash structural and or foundation failure stability and buoyancy dropped objects

mooring, propulsion, and station keeping.

103 The results of the hazard identification and any relevantassumptions shall be documented.

C 200 Risk analysis201 The identified hazards can be ranked based on combina-tion of likely frequency and consequence. Insignificant risksmay be eliminated from further evaluation provided that rele-vant assumptions are documented.

Guidance note:Risks shall not be subdivided such that individual risk elementsappear trivial, whereas collectively still representing a substan-tial risk.


Consideration of frequency includes identification of initiatingevents, and combinations of events, which could lead to a haz-ard. The likelihood of occurrence of such events can be foundfrom historical or other appropriate data.The consequences of the hazards shall include analysis of theeffects of accidents or accidental events on the safety of per-sonnel and integrity of the installation.202 The availability and vulnerability of key prevention andprotection systems shall be assessed with respect to requiredfunctionality against each of the identified hazards. Any signif-icant findings shall be consistent with assumptions made inother parts of the risk analysis and assessment.203 The hazards remaining after the screening exercise aretermed significant major hazards. The selection of significantmajor hazards, including assumptions made as part of the rank-ing process, shall be documented.

Guidance note:Tools such as HAZOPs, fault trees and engineering judgementmay be effectively applied to screen out hazards that are trivial orof minor significance. This includes hazards and escalationswhich are extremely unlikely to occur (e.g. due to the effective-ness of prevention measures in place), or which will have minorconsequence to personnel or property.


Guidance note:Note that the screening exercise can also be useful for early opti-misation of prevention and protection measures against majorhazards prior to performing the risk assessment.


204 The basis for the risk ranking, including assumptions re-lated to functioning of safety systems and exclusion of low sig-nificance risks shall be subject to DNV approval.C 300 Risk assessment301 The risks from significant major hazards shall be as-sessed and considered together in order to show:

the annual safety risk for typical personnel groups on theinstallation (individual risk)

the annual loss of integrity risk for the installation the relative contribution of different hazards to the total

calculated risks.

302 The annual risks shall be assessed against predefinedrisk acceptance criteria (see 400). If necessary, risk reductionmeasures shall be applied in order to meet the acceptance cri-teria.

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303 The results of the risk analysis and evaluation shall bedocumented and subject to DNV approval.C 400 Acceptance criteria401 DNV Rules for Classification have been developed overmany years in order to give an acceptable level of safety andintegrity for design, construction and ongoing condition of theinstallation. Classification based on risk assessment tech-niques is expected to achieve at least the equivalent level ofsafety as class based on prescriptive DNV rules.In cases where deviations from the rules are justified on the ba-sis of risk assessment, this must be shown as resulting in an ac-ceptable level of safety on the installation. This shall bedemonstrated through fulfilment of agreed acceptance criteria.402 The owner or operator shall define the acceptance crite-ria before performing the risk analysis. The criteria should takeinto account both the probability and consequences of signifi-cant major accident events. Meeting the acceptance criteriawill establish the basis for identification of safety-critical ele-ments and selection of performance standards.403 Acceptance criteria shall be approved by DNV.

D. Safety-Critical ElementsD 100 General101 Once the acceptance criteria have been fulfilled, thesafety-critical elements and performance standards shall be re-corded for input to classification.102 Safety-critical elements are defined in Sec.1 B111, andinclude all components or systems upon which the safety of theinstallation depends. This includes all means for the preven-tion, detection, control and mitigation of risk from major haz-ards.103 Safety-critical elements should be identified from, andshall be fully consistent with, the major hazard scenarios con-sidered in the assessment. Identification of elements shouldalso include input from good engineering judgement.The selection of items as safety-critical shall be based on con-sequence of failure.104 The safety-critical elements shall be recorded and shallbe subject to DNV approval.

E. Performance StandardsE 100 General101 Performance standards shall be established for the safe-ty-critical elements. The performance standards shall be suchthat the safety-critical elements are suitable for fulfilment ofthe acceptance criteria as demonstrated in the assessment.102 The performance standards shall be suitably described,normally in quantified terms, so as to be verifiable for the safe-ty-critical elements.

103 The performance standards shall reflect any relevant li-fecycle requirements of the critical element.104 The performance standard shall also reflect any interac-tion or dependence between safety-critical elements for a par-ticular major accident scenario.105 Performance standards shall be documented and ap-proved by DNV.

F. Risk ReductionF 100 General101 The results of hazard identification and evaluationpresent a good opportunity for targeted risk reduction. In as-signing classification, DNV expects that such risk reductionwill be undertaken wherever significant and practicable oppor-tunities are identified.102 Hazard identification and evaluation is most effectivelyinitiated at concept design stage where risks can be avoided orreduced.For existing installations where this is not possible, hazardidentification and evaluation should be used to address hazardsand optimising the protection measures to manage them. In or-der of importance, this means measures to prevent, detect, con-trol and mitigate against hazards.

Guidance note:Identified hazards can be avoided through e.g.:- removal of the source of a hazard (without introducing new

sources of hazard)- breaking the sequence of events leading to realisation of a

hazard.Where hazards cannot be avoided, installation design and opera-tion should aim to reduce the likelihood of hazards occurringwhere practicable, e.g. by:- reduction in number of leak sources (flanges, instruments,

valves, etc.)- removal or relocation of ignition sources- simplifying operations, avoiding complex or illogical proce-

dures and inter-relationships between systems- selection of better materials- mechanical integrity/protection- reducing the probability of external initiating events, e.g. lift-

ing operations, etc.- reduction in inventory, pressure, temperature- use of less hazardous materials, process or technology.The consequences of hazards should be controlled and mitigatedwith the aim of reducing risk to personnel where practicable, e.g.through:- relocation of equipment, improved layout- provision of physical barriers, distance separation, fire walls,

etc.- provision of detection and protection systems- provision of means to escape and evacuate.


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SECTION 3VERIFICATION, CLASS ENTRY

A. GeneralA 100 Introduction101 In order to award classification, DNV will verify that theinstallation is designed and constructed to an acceptable levelof safety.102 This includes verification of the selection of all safety-critical elements and that they are specified, provided and in-stalled as suitable for the intended purpose. In this context,suitable means appropriate for the intended use and able to per-form as intended.

B. New InstallationsB 100 Verification scheme for design, construction andcommissioning101 The initial verification will include review and approvalof:

a) Selection of safety-critical elementsThe safety-critical elements shall be traceable back to ma-jor hazards threatening safety or integrity. The selectionshould be identified via the risk assessment but may alsoreflect good engineering judgement.

b) Specified performance standardsThe performance standards shall adequately define thenecessary functionality and availability. The performancelevels stated should be justified based on the performanceassumed or required in the risk assessment, including thatrequired before, during and immediately following majoraccidents.

c) Design and specification of safety-critical elementsThe safety-critical elements shall be designed as suitableto meet the approved performance standards.

102 In order to verify that the safety-critical elements areprovided to meet agreed performance requirements, they shallbe subject to final approval of:a) Manufacture or construction or assembly of safety-critical

elementsThe safety-critical elements shall be supplied in accord-ance with the approved design specification.

b) Installation or operation of the safety-critical elements

The safety-critical elements shall be installed and commis-sioned in order to function as required in an emergency.

These tasks can include a selective combination of examina-tion, testing, examination of certificates and records etc.103 The content of the DNV verification scheme depends onthe selection of safety-critical elements and their respectiveperformance standards.Once this information is identified, DNV will establish a struc-tured verification scheme to apply the review, approval andsurveying requirements to the safety-critical elements.The scheme will:

record the identified safety-critical elements for the instal-lation, and the required performance of such elements

set out the work plan and schedule for the verificationwork at each stage of the project

document the results of verification work.

The scheme will provide a direct link from verification activi-ties back to the risk assessment, and hence a documented basisfor safety and integrity of the installation.104 For new build installations, early DNV involvement isstrongly recommended to facilitate timely execution of theverification process.

C. Existing InstallationsC 100 General101 Existing installations without class or classed with a so-ciety other than DNV may apply for risk based classification.The initial verifications requirements will be determined on acase by case basis depending upon installation design, novelty,previous class (if any), age, history etc. As a general rule, therequirements will be similar to those for newbuildings but withthe following exceptions:

a) Limited design and construction verification dependingupon the extent of original documentation and certifica-tion available.

b) Examination of records pertaining to the operating historyof the installation.

c) Comprehensive condition surveys, potentially includingexamination and testing, in order to indicate the actualcondition of the installation and its acceptability for class.

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SECTION 4VERIFICATION, MAINTENANCE OF CLASS

A. GeneralA 100 Introduction101 In order to maintain valid classification in-service, DNVwill verify the suitability of the safety-critical elements. Thisincludes verification by examination, testing, audit and review,as necessary to give assurance that all safety-critical elementsare selected and that they will remain in good repair and con-dition suitable for the intended purpose.

B. In-Service Verification Scheme forMaintenance of Class

B 100 General101 DNV will establish a verification scheme for the ongo-ing review, approval and surveying of safety-critical elementsfor an installation in service.The scheme will include:

review of the selected safety-critical elements and per-formance standards, particularly with respect to:

changes in operational requirements modifications planned or performed knowledge accumulated during operation of the

scheme.

continuing and updated work plan and schedule for theverification work

documented results of verification work including anyidentified remedial actions or other need for change.

B 200 Review of safety-critical elements201 The review of safety-critical elements will continue onfrom the initial verification during design and construction,and will take account of:

changes in operational requirements modifications planned or performed accumulated knowledge during operation of the scheme.

B 300 Verification scheme301 The verification scheme will indicate all activities to beperformed for each safety-critical element. The scheme willtake credit for activities performed under the operator's inspec-tion and maintenance plan.The extent of verification work will be determined based on areview of the operators own maintenance arrangements, in-cluding review of:

maintenance and inspection objectives and management competency of personnel planning, scheduling and reporting tasks inspection intervals inspection and maintenance methods type, accuracy and condition of equipment used systems for planning and recording.

Once the level of maintenance and testing has been reviewed,the content of the verification scheme will be tailored as nec-essary to provide assurance that safety-critical elements meetthe performance standards. Activities under the scheme willconsist of an appropriate combination of:

physical examination testing of systems and or components audit of activities and procedures review of inspection records.

302 The content of the operators maintenance and inspec-tion plan is a key element in the verification scheme. There-fore, any revision to the maintenance and inspection plan shallbe notified to DNV for review and approval. DNV will updateor revise the verification scheme as necessary to reflect suchchanges to the maintenance and inspection plan.303 Verification activities shall be undertaken as and whenappropriate according to the verification scheme. This may beon a continuous basis dependent upon the actual safety-criticalelements, and the operator's own maintenance and inspectionplan.304 Based on indications and results, the verification schememay be modified to include greater or lesser level of activitiesas necessary to ensure the ongoing performance of safety-crit-ical elements.305 Results and status of verification tasks will be docu-mented within the scheme itself and through annual verifica-tion reports. The reports may in turn be used as documentationfor other obligatory or regulatory requirements.306 The operator is expected to co-operate with the applica-tion of the verification scheme, through timely provision of ad-equate information and access to all facilities as necessary tofulfil the verification tasks.307 Necessary remedial measures with associated condi-tions of class and time scales will be notified to the operator orowner and recorded within the verification scheme.308 Failure to effect remedial measures within the giventime scales or obstruction of execution of the verificationscheme may result in suspension or withdrawal of class.

DNV OS 121 - Classificatio Based on Performance Criteria Det

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