Chapter 2...........................................................................................-......"---"----"-- 2
2.2 Extension of service life for ageing platforms
In Malaysia, about 51% of jacket platforms have exceeded 25 years of operation life.
Most of these jacket platforms were designed for 20 to 25 years service life. Table 2.1 shows the age distribution for jacket platforms in Malaysia. Due to strong production and conservative estimation of reservoir capacity, many of these platforms are still producing oil and gas beyond their intended service life. It is a challenge for the engineer to maintain and operate a huge fleet of ageing platforms. Replacement of the structure will not be a feasible option by management because of financial limitations. Many operators opt to extend the service life of their existing platforms.
The life extension of jacket platforms however depends on many requirements such
as:
I. Component strength for each element must satisfy unity check in static/linear analysis.
II. The current capability of the jacket platform to resist environmental load
must be sufficient.
III. Integrity of the structure must be intact.
IV. Feasibility of the reservoir and production of the well must be above the minimum financial requirement for profitable operation.
V. The structure must pass the minimum reliability requirement for safe operation.
VI. Air gap of superstructure must be more than 1.5 m.
VII. Re-assessment of ageing structure must give an RSR value greater than 1 with respect to 100-year wave, wind and current conditions.
VIII. Equipment, pipeline, piping and valve on the superstmcture must be certified
as fit for service in accordance with API 579 Fitness for Service.
After all the above requirements are addressed properly, the decision to extend the structure service life can be made. For existing jacket platforms, it is necessary to perform more detailed analysis, new inspections, tests, and measurements rather than redesigning and increasing the steel weight and reconfiguring the structural parts with new parts or structures. In an overview, re-assessment for ageing platform is entirely different from structural design process before installation. Kallaby (1994), Moan and Vardal (2001) discussed the differences between design criteria and re-assessment
criteria as tabulated in Table 2.2.
Table 2.1. Age distribution for jacket platforms in Malaysia
Less than 10 years
Between 10 to
20 years
20 to 25
years
25 to 30 years
More than 30 years
Peninsular
Malaysia
16(42%) 5 (13%) 9 (24%) 7(18%) 1 (3%)
Sabah 5(16%) 1 (3%) 7 (23%) 6(19%) 12 (39%)
Sarawak 3 (3%) 30(28%) 9 (8%) 20(19%) 44 (42%)
Table 2.2. Differences between design and re-assessment criteria Design criteria Re-assessment criteria
Environment Forecast from existing data
collection.
Same criteria as for design but include
recent data collection and use of current
state of the art review, experience from adjacent field and additional data from actual field sea states. Also includes
possibility for Tsunami and typhoon.
Loading Possibly conservative
evaluation from proposed use of
structure.
Use of recent survey for marine growth, appurtenances, modifications, topside weight control and wind areas.
Foundation Forecast from site investigation and laboratory testing of soil.
Includes subsidence information, maintenance, and experience from adjacent field, post-drive analysis, scour survey and maintenance.
Structural model
Topology and dimensions may be changed. No service inspection available.
Conservative modeling using global percentages to cover not- finalized details and simple geometric assumptions.
Structural configurations are fixed and known. In-service inspections maybe applied. Actual characteristic strength of
steel based on actual material certificates
may be used. Structural performance may have been measured and used to update structural analysis.
Stress
analysis
Strict compliance with code of practice and regulatory
documents.
Removing of unnecessary conservatism, redundancy study to determine ultimate strength of structure and foundation, sensitivity studies on various parameters to improve confidence level.
Results Structare has members and
joints with acceptable utilization (UC).
Structure may have some stress up to yield, but some assessment standards allow for some yielding if the structure has proven strength and redundancy.
Figure 2.1 shows the flow chart in accordance with ISO 19902 (2004) of the procedure for assessing ageing platform's capability for life extension and fitness for purpose. Level 1 is a linear analysis and component check. Level 2 is also linear analysis with component checks but with refined actions and resistances. Level 3 includes linear elastic redundancy checks. Level 4 is a non-linear analysis on system level including component checks as an integrated part of the non-linear system analysis. The final level 5 is a check by using structural reliability analysis.
Assemble structure data and history
Identify exposure level
Isassessmentiniti ator tri ggered?
Yes
Review condition of structure
Review action
Compare with similar structure
Implement prevention and mitigation measures
Implement prevention
"and mitigation measures
Implement prevention and mitigation measures
Not acceptable Determine actions
*i Level 1 assessment
Refine action and resistance
Level 2 assessment
Fail
Level 3 assessment
Fail
No
Acceptabk
•
Pass
Pass
Pass
Pass
Level 4 assessment ) ».
Fail
Level 5 assessment
Fail
Structure not fitfor purpose
Pass
Assessment not necessary
Assessment not necessary
Structure fit for purpose
Structure fit for purpose
Structure fit for purpose
Structure fit for purpose
Structure fit
for purpose
Figure 2.1. Procedure for ageing structure assessment
2.3 Codes and standards for re-assessment
In engineering design and assessment, the application of codes and standards as the best guidelines and practices, shall be used in conducting structural analysis of ageing platforms. In assessing reliability for any ageing stmcture, the engineer may refer to various codes and standards, depending on the region of applicability of each standard. For all the analyses used in this study, the relevant codes and standards referred to are given below:
I. API RP 2A - American Petroleum Institute "Recommended Practice for
Planning, Designing and Construction of Fixed Offshore Platform" for both WSD and LRFD, latest editions.
II. AISC - American Institution of Steel Constmction "Specification for Structural Steel Building", latest edition.
III. AWS Dl.l - American Welding Society "Structural Welding Code", latest
edition
IV. BSI PD 6493 - British Standard Institute section PD6493 "Guidance on
Some Methods for Derivation of Acceptance Level for Defects in Fusion Welded Joints", latest edition.
V. PTS - PETRONAS Technical Standard, latest edition
VI. DIM - Design Instruction Manual "Section 1-1, Common Requirements", ESSO Production Malaysia Inc, latest edition.
For Malaysia's operational regions, the substructures of existing platforms were
designed in accordance with API 20th edition or earlier editions of the same API
standards. This makes all the fixed jacket platforms in Malaysia following API RP 2A. Since Malaysia does not have its own regulatory codes and standards for jacket platforms, API RP 2A became widely accepted. API RP 2A along with company/organization in-house technical standard is used in the design level analysis and ultimate strength analysis for fixed jacket platforms. PTS is mainly for PETRONAS usage and DIM is for ESSO internal usage. These codes however do not
differmuch from API, as both are derived from API RP 2A WSD and AISC forjacket
platforms.
2.4 Risk definition for jacket platform
In assessing the risk for jacket platforms, risk based inspection (RBI) concept is a
well-accepted approach in the industry. PRSS RBI Manual (2000) states that RBI concept is a method used to assess the risk of platform structures and their components in relation to a level that is acceptable, where inspection and repair used to ensure the level of risk are well below the acceptance limit. Combined with Structural Integrity Management System (SIMS) program for Malaysia's region that was launched in 2008, the estimation of risk for jacket platforms has become more accurate and accepted by oil and gas operators. Risk is defined as the product of probability of failure (POF) and consequence of failure (COF).Probability of failure (POF) is an estimated value based on degradation mechanisms operating in the component within the global structural system. The degradation mechanisms such as scour, damaged member, missing member, fatigue, corrosion and fire can reduce the strength and stiffness of the global stmctural system over time. POF is calculated as the area overlap in the distributions of the degradation rate. Each degradation mechanism is based on uncertainties of the resistance for each component to failure. The degradation rate for stmctural deterioration is represented on the left side in probability density function. The resistance of the component to failure is represented by the right side in probability density function. This overlap of resistance and degradation is illustrated in the Figure 2.2.
ijJLAV^TTTYMW^j
I '• Bz£i aihtionpclf
\ Resistance s^
^Probability offailure
Figure 2.2. POF in resistance and degradation relationship
Consequence of failure (COF) can be derived from the combination of consequences when the platform experiences total failure or collapse. Generally the COF can be divided into three categories as presented by Goh (1999):
I. Economy - Consequence to economy when the jacket platform collapses.
Cost of replacement, production deferment, stmctural savage cost, insurance and compensation paid are Included in this category.
II. Environment - Evaluation of environmental consequence is a complex issue and involves organizational reputation, short term and long term effects in global and local perspective in oil and gas industry.
III. Safety - It is usually estimated for failures that lead to stmctural collapse of platforms that include loss of lives of personnel on board.
Table 2.3 shows the overall consequence of platforms in Malaysia in accordance with PRSS RBI Manual (2000). Depending on the company or organization's definition of POF and COF, the definition can vary. A complete definition of POF and COF is
mandatory before any risk calculation or estimation can be done. Table 2.3 shows ttie
definition of COF for Malaysian regions according to PETRONAS.Table 2.3. COF definition according to PETRONAS (1999) Cr.F Ranhmg Qua! it CiVf Cuantitatwe
(Abstract Value)
Dest-Jiptiun
- -'. "" j '. : : t - •• icr
very high cost.
D High US$ 75 Million -
US$ 100 Million
The consequence of failure represents high cost.
C Medium US5 45Mil!ion - US$ 75 Million
The consequence of failure represents
medium cost.
B Low US$ 6 Million -
US$45 Million
The consequence of failure represents low
cost.
A Very Low < US S6 Million The consequence of failure represents very low cost.
Risk is usually presented in the form of risk matrix. Risk matrix is presented as 3x3 or 3x5 or 5x5 matrixes. Figure 2.3 shows a typical 5x5 matrix used in SIMS.
B
Figure 2.3. 5x5 risk matrix
The risk zones can be divided into five zones according to PETRONAS RBI Manual (2000):
I. Very high exposure (Red Zone) II. High risk exposure (Orange Zone) III. Medium risk exposure (Brown Zone) IV. Low risk exposure (Yellow Zone)
V. Very low risk exposure (Green Zone)
All fixed jacket platforms in Malaysia are categorized in this manner. The decision to replace, strengthen, inspect and/or operate depends on the risk status of the particular jacket platform. The categorization is dependent on POF rather than COF since COF is defined by corporate and is financially driven. Engineers can reduce the structural POF by increasing the structural resistance. This will reduce the overlap area of resistance and degradation in the probability density function shown in Figure
2.2.
One popular technique to assess the safety performance of ageing jacket platform is by taking the ultimate limit strength instead of the yield strength and using plastic theory and non-linear analysis. By this method, the platform can resist more loads beyond the elastic limit and meet the global minimum requirement prescribed by the
codes and standards.
2.5 Reliability assessment in Malaysia
According to Guideline for Offshore Stmctural Reliability Analysis issued by DNV (1995), level of reliability is not a property of the stmcture. Unlike linear analysis, where it calculates the ratio between applied stress to allowable stress, reliability is heavily dependent on the information about the uncertainty of a parameter that will change through time.In reliability calculations, the engineers and analysts often refer to certain reference period, usually one year or the design life. In the re-assessment of ageing platform, it is recommended to use one year reference period in all cases.
Reference period, or called return period in some literature, is inversely proportional
to the global stmctural probability of failure (Pf) given by expression below:
T= j-f (2.0)
API RP 2A (2000) and PETRONAS Carigali Inspection Maintenance Guideline (CIMG 2010) clearly define the level of reliability application in operation. This definition however is unique according to the organization/company and maybe different worldwide. Three categorization of reliability known as Level I, II and III are defined to ensure safe operation in jacket platforms.Level-I provides a practical design method where suitable safety margins are provided usually on a stmctural element basis by specifying a number of partial safety factors related to some predefined characteristic values of the basic variables in the system.
In the strength model, these values usually correspond with the nominal values specified in the design such as minimum yield. No precise reliability calculation involving probability of failures and consequence of failures are undertaken and the
level of risk for different platforms, are essentially unknown. Design methods involving a number of partial safety factors are practically better compared with level II and III approaches. In day-to-day practice, level-I reliability method is good enough for the screening and overview of stmctural element safety performance.
Level-II method utilizes the properties of load and strength distributions for
component and stmctural system in terms of a reliability safety index. This
corresponds to an estimated probability of failure or reliability for each failure mode
or limits state during the life of that platform. Appropriate partial safety factors are then derived for the particular design condition. These safety checks are made only at selective points on the failure boundary as defined by the appropriate limit state rather than as a continuous process as level III. This method will not make any attempt to find the region of basic variable or state space which has the highest probability of failure density. This is important for the level II methods and provides basis for calculating partial safety factors for level III. In practice, level II reliability method is used when there is deep interest toknow the safety status of a particular structure in a precise and accurate approach compared to level I.
Level III provides the most accurate and exact probabilistic analysis for the whole stmctural system involving the convolution integral. This method is theoretically straightforward and accurate, but in practice, difficult to formulate and to solve. In addition, it cannot directly be used in design process, for specified reliability level.
This is the only approach which can satisfactorily incorporate all the modes of failure when estimating the total reliability. Clearly, these methods are not suitable for normal application in design purpose.
Level III techniques can be used for checking the validity and accuracy of the simplified level II method by analysis of specified stmctural systems. There are also numbers of reliability analysis techniques available according to different fields in engineering industry. In jacket platforms, the main reliability techniques used to assess the safety performances are:
I. Quantitative Risk Analysis (QRA)
This method is concerned with the estimation and calculation of overall risk
to ensure safe operation and minimize risk to human, environment and
economy.
II. Stmctural Reliability Analysis (SRA)
This is the most common reliability technique that addresses the stmctural probability and consequence of failure, by including estimation of reliability calculation to get reliability index or fj index. It consists of several steps and procedures with mathematical models.
III. Organizational Reliability Analysis (ORA)
In this approach, the focus of methodology is on the organizational procedures. It calculates and determines how a particular strategy and procedure gives impact to the safety, reliability of the system and quality of the products. Swain and Guttman (1983) include ORA into the field of Human Reliability (HR) engineering.
IV. Simplified Stmctural Reliability Analysis (SSRA)
This method is by far the quickest way to calculate reliability index using pre-determined value for covariance and biases from selected SRA calculations for specific collections of stmcture within same region. SSRA is based on SRA of Further Stmctural Integrity Assessment (FSIA) conducted by Petronas Research and Scientific Services (PRSS) and Det Norske Veritas (DNV) in 1997 for thirteen jacket platforms in Baram and Balingian field
Sarawak.
For this study, reliability (P) index is calculated using SSRA methodology since it is the quickest way and the most widely used in Malaysia for calculating P index for fixed jacket platforms. For a very load-sensitive and unique platform (5 legged, semisubmersible, monopod), SSRA usage is still limited. SRA needs to be performed for such platform to calculate its p index. Table 2.4 shows the differences between SSRA and SRA methodology (for Malaysia).
Table 2.4. Differences between SSRA and SRA
SRA SSRA
Time for analysis 8-12 months 3-6 months
Cost (minimum) RM 1.5 Millions RM150K
Local expertise Limited Few
Complexity High Medium
Accuracy for predicting reliability (P) index
High Medium
Basis for calculating Individual limit state function Depends on the pre reliability(P) index per structure determined COV and biases
from SRA for the particular
structure within the same
operation area
Identification of stochastic Performed in this analysis Performed in this analysis variables and parameters in
limit state functions
Specification of probability Performed in this analysis Based on SRA for particular
distributions for each structure within the same
variables and parameters for operation area
limit state functions
Interface with QRA and Possible Limited interface
ORA
Analysis pre-requisite Non-linear push over and Finite Element Analysis
Non-linear push over
Engineer's level required High (Custodian & Principal Medium (Staff & Senior Engineer level) Engineer level)
Assess whether the estimated Modification of requirement No modification for
reliability is sufficient and and limit state function is requirement and limit state able to modify the calculation possible function
concept necessary.
Covariance and biases Unique and individually Based on SRA pre
calculated for each members determined value for and failure mechanisms particular structure within the
same operation area
Software requirement Highly probabilistic such as Microsoft Excel with SSRA ProbCalc and ATENA formula embedded
Probability simulation model Performed in this analysis Not available
Monte Carlo Simulation Performed in this analysis Not available
Bayesian Probability Performed in this analysis Not available
Frequency Probability Performed in this analysis Performed in this analysis for Inspection Reference Plan (IRP) in Risk Based Inspection approach
For routine operation, due to cost restraint by operation management, SRA is not as practical as SSRA. The low cost and short period of time for execution made SSRA a popular choice by organization to assess safety performance of their assets. Figure 2.4 shows the procedure in performing SSRA calculation for reliability (P) index.
Reserve strength ratio
Biases for resistance
Non-linear push over analysis
Identified mechanism of failure
Calculation /solver
Maximum base shear resisted before total collapsed
•'
Biases for dead and environmental load
probability period
1 i.
Cava ri a nee f or tota 1 environmental load
'r
Check against minimum requirement by code and standards or company requirement Calculation of annual
reliability index 1
Covariance for dead
load Ai
T
* Operating decision:
1} Operating as usual 2} Operation limitation 3) Inspection reference Cova ri a nee for tota 1
resistance
Calculation of mean strength to mean load ratio
pla 4} Str
m c n
engthening
>dification ancrepar
Figure 2.4. SSRA procedure