Life Extension of Mobile Offshore Units Operation of classified aging units Sigmund Røine DNV Mobile Offshore Units in Operation
Presentation content MOU integrity during operation Survey Principles for DNV classed units Inspection arrangements Aging control by using Fatigue Utilization Index (FUI) Life extension and class requirement Systematization of information - plans and results Summary 2
MOU integrity during operation Objective: Class activities during operation are aimed at verifying that the required rule standard are maintained at all times The challenge: How to maintain required safety for the unit when in operation? Who is responsible for what? What is needed? How to document? 3
Survey Principles - General The class survey system is a periodical system with scope of survey depending on: - type of unit, class notations, age of the unit and consequence of failure. Periodical class surveys are carried out on an annual basis within a 5-year cycle. The basic survey arrangement is calendar based with visual inspections, NDT and opening up equipment to check status. 4
Class Surveys traditional way Machinery, safety -, and drilling system and other equipment etc. are inspected and opened up as prescribed (see DNV-OSS- 101), and owners inspections and maintenance are controlled. Hull structure survey as prescribed by the rule and prepared by DNV. Both class and statutory (MODU Code) requirements are included. Owner s requirements may be included upon special agreement. The class certificate is assigned for 5 years - provided that the inspections are performed according to the rule requirements, DNV- OSS-101, and that possible conditions have been satisfactory handled. 5
Survey Principles - Class Survey Schedule 1st Annual 2nd Annual 3rd Annual 4th Annual Renewal survey Year 0 Year 1 Year 2 Year 3 Year 4 Year 5 Intermediate Renewal Commencement Completion Commencement Completion Survey window of +/- 3 months from due date Minimum frequency of inspection is set by IACS unified requirements As an alternative the survey extent can be based on a quantitative risk-based approach. 6
Survey Principle - risk based approaches Modified Survey approach: Surveys for floating offshore units may alternatively be based on plans for: - Risk-Based Inspection (RBI) hull structure - Reliability Centered Maintenance (RCM) - machinery Hull structure - RBI Risk Based Inspection: Condition based approach Inspection history and experience included. Inspection quality (POD) Importance of individual items (consequence of failure) Probability of failure (Pf) - calculated fatigue life. Machinery and Systems - RCM - Reliability Centered Maintenance (RCM): RCM is a reliability based survey arrangement that in principle will allow the owner to perform surveys on all class related systems as part of the planned maintenance system. The system is broken down into a hierarchy of functions and analyzed. 7
Inspection arrangements - Systems - 1, 2 and 3 - systems for follow up based on calendar inspections - 4 - system for condition based maintenance - 5 - Planned Maintenance System (PMS) based on Reliability Centered Maintenance (RCM) provided as a new survey arrangement 5 Risk based PMS using RCM 4 Condition Monitoring (CM) 3 Machinery Planned Maintenance System (PMS) 2 Machinery Continuous 1 Machinery Renewal 8
Inspection arrangements - Hull structure Two categories of degradation mechanisms : - The visible (corrosion, wear and tear, damages etc.) - The invisible (fatigue) The survey plan is based on the barriers: Inspection, leakage detection and redundancy 9
The visible degradation The visible degradation - can be observed by visual inspection. Accept criteria for allowable corrosion is given in DNV-OSS-101 Ch.3 Sec.4: Corrosion allowance is not taken into consideration at the design. Corrosion in the most fatigue sensitive areas is critical The condition on anodes and surface protection is to be acceptable for another 5 year period at Renewal Survey. Corrosion control is highly dependent of an appropriate functioning corrosion system. 10
The invisible degradation Invisible degradation can normally not be found by visual inspection before a possible progressive effect occurs (e.g. visible crack by use of light coating or leakage). Analyses might be applied in order to calculate the possibility for a crack to occur. As a control parameter an important input will be the used fatigue life, not age. Therefore DNV applies the parameter Fatigue Utilization Index (FUI) = 1 as a definition for considering a unit as aging. 11
Aging control by using FUI FUI control parameter for accumulated use vs actual design life. - Fatigue Utilization Index is defined as the ratio between the effective operational time and the fatigue design life i.e. an overall parameter which indicates the actual consumed fatigue capacity vs. design life. - As the factor accounts for the actual use of the unit, it is adjusted for: - Laid-up time or yard stay; factor 0. - Benign environment; factor 0.33 - Harsh environment North sea, North Atlantic, Canada; factor 1.0 - An example: - 30 years benign + 10 years harsh + 10 year laid-up = 30x0.33 + 10x1.0 +10x0 = FUI = 1.0 (act. Age 50 years - design life 20 years) More than 50% of the DNV classed Mobile Offshore Units is above 20 years of age. 12
FUI as guide for inspection planning When FUI < 1.0 Standard 5-year cycle inspection program can be applied. When FUI > 1.0 The Rules give two (2) options: - Alt.1: Increased number of inspections - Alt. 2: Detailed analyses for control of the safety level. Systematic thickness measurements shall be performed at renewal surveys (RS) for planned locations. Both - FUI < 1.0 and FUI > 1.0 The corrosion protection system has to be inspected and the efficiency of the system for the forthcoming 5-year period shall be confirmed. 13
Assessment of aging units Options to maintain necessary safety levels: Inspection Improvement of critical details Replacement of steel/details Failure probability 1,0E+00 1,0E-01 1,0E-02 1,0E-03 Fatigue life = 20 years No Inspection Target Probability Level Perform detailed analyses Fatigue and RBI analysis. Probability of failure, P F 10-2 10-3 10-4 Failure probability 1,0E-04 End of service life 0 5 10 15 20 25 30 time (years) 1,0E+00 1,0E-01 1,0E-02 1,0E-03 14 Fatigue design life = 20 years MPI every 5 year T Target probability 3 Level 1,0E-04 0 5 10 15 20 25 30 time (years) 0 10 Target Probability Level (1.0 10-3 ) T 1 T 2 20 30 40 Years
Lifetime extension and class requirement Alt. 1. 1) Increased inspections shorter inspection interval. - Inspection scope at intermediate surveys corresponding to the extent required for renewal surveys has to be performed. This means that the NDT inspections planned every 5 th. year have to be performed at intermediate survey. - Class minimum. Table A2/A3/A4 i OSS-101/102. - In case of serious crack finding, the requirement for NDT inspection might be further increased. 15
Lifetime extension and class requirement Alt. 2. 2) Perform a quantitative risk based inspection planning (RBI). - Update the design analyses by using the to-days most acknowledged analysis methods.. - Detailed model state-of-the-art shell model - Mapping of the most fatigue utilized areas - ranking of details - Determine required safety level dependent on consequence and access. - Inspection history findings or no findings, evaluated considering the inspection quality, POD. - Uncertainty modeling quality on the basis applied for the RBI analysis. - The uncertainty is reduced and the quality of the RBI analysis is increased. - Optimized inspection intervals are determined. - Inspection plan generated based on condition of the structure - Necessary, required safety is maintained throughout the service life. 16
Hull Integrity In-Service System Basis for inspection Calculation. Evaluation of inspection results & repair history Structural Risk Based Inspection planning - reliability approach Inspection results. Findings and repairs Planning of Surveys and updates Inspection packages HIM - database 17
Systematization of information - plans and results Existing system for inspection planning - papir based New version; electronically database NPS/IIP 18
DNV Exchange Client 19
Fleet Overview Hull Integrity Manager - HIM 20
Unit overview - Hull Integrity Manager 21
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Graphical visualization of Vessel Condition Color codes give the first indication of the overall hull condition for the tanks. - Easy detection and follow up of findings - Efficient knowledge transfer for new employees 23
Summary Life extension of MOU units - 1 To alternatives for using the units beyond their initial intended service life: - ALT1: Perform NDT inspection program every 2 nd. year similar to the program planned performed at class renewal; every 5 th. year. - ALT2: Perform detailed compensating measures, e.g. detailed analyses of the most critical areas with continuous updating of the inspection plan based on inspection results. 24
Summary Life extension of MOU units - 2 Proper functioning of the cathodic protection system has to be assured. Leakage detection system to be checked and tested regularly. We encourage use of Risk based approaches for survey planning and maintenance system DNV can help you keeping control of the condition: - HIM (Hull Integrity Maintenance System) can be used by the owner. 25
Safeguarding life, property and the environment www.dnv.com 26