The linear elastic fracture mechanics (LEFM) based remnant fatigue life (RFL) assessment of offshore pipeline is used to determine the inspection interval frequency of the aforementioned asset. One of the vital factors of the LEFM... more
The linear elastic fracture mechanics (LEFM) based remnant fatigue life (RFL) assessment of offshore pipeline is used to determine the inspection interval frequency of the aforementioned asset. One of the vital factors of the LEFM approach that determines the accuracy of the RFL estimate (and in turn of the inspection interval frequency) is the Stress Intensity Factor (SIF), which must be evaluated as accurately as possible. For simple crack geometries numerous closed-form equations available in various handbooks and industrial standards provide accurate SIF results. However, it is a common industry practice to utilize finite element method (FEM) for evaluating the SIF for the intricate crack geometries and the complex loading conditions. Although, FEM is known for its accurate SIF calculation, but due to its high computational expense and time-consumption, cycle-by-cycle SIF evaluation (required for the LEFM based RFL assessment) makes the aforementioned method quite laborious. Furthermore, using FEM to evaluate SIF for thousands of pipeline location (undergoing fatigue degradation) on an offshore platform seems to be impractical. Thus, in this manuscript authors have proposed a computationally inexpensive adaptive Gaussian process regression model (AGPRM) which may be utilized as an alternative to FEM for prediction of SIF to assess fatigue degradation in offshore pipeline. The training and testing data for AGPRM consists of 105 and 50 data points (load (L), crack depth (a), half-crack length (c) and SIF values), respectively. Latin Hypercube Sampling (LHS) is used to generate (L, a and c) values while SIF values are evaluated using FEM by carefully accounting for the discretization error emanating due to the finite mesh size in the FEM simulation. After the GPRM has been adaptively trained, it is used to predict the response of the 50 data points. On comparing the values of the SIF (obtained by AGPRM) with the SIF values obtained from FEM, the average residual percentage between the two is found to be 1.76%, thus indicating a good agreement between the AGPRM and FEM model. Furthermore, the time required to predict the SIF of 50 test points is reduced from 50 min (for FEM) to 12 s with the help of the proposed AGPRM, thus making RFL assessment less laborious and time consuming.
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Topside piping is the major source of hydrocarbon release (HCR) on offshore oil and gas(OOG) platforms in the North Sea region. Since 21% of piping failures are caused by vibration induced fatigue (VIF), an accurate remnant fatigue life... more
Topside piping is the major source of hydrocarbon release (HCR) on offshore oil and gas(OOG) platforms in the North Sea region. Since 21% of piping failures are caused by vibration induced fatigue (VIF), an accurate remnant fatigue life (RFL) assessment has the potential to minimize the chances of HCR from an operating piping system. BS-7910 gives two possible approaches for performing a RFL assessment: the S–N curve approach and the fracture mechanics (FM) approach. Since there are large number of uncertainties (such as uncertainty due to the crack growth model, future loading, material and geometric properties,etc.) involved in the RFL calculation process, therefore it is vital to consider the aforementioned sources of uncertainty in order to arrive at an accurate RFL estimate. Nevertheless,BS-7910 provides limited guidance on how to handle uncertainty in RFL assessment. The most common way of dealing with the aforementioned uncertainty is to evaluate RFL probabilistically. This manuscript thus explains the procedure of the probabilistic RFL assessment of offshore topside piping, with an emphasis on uncertainty quantification, propagation and management. Uncertainty quantification handles the identification and characterization of the different sources of uncertainty that may influence the future behavior of the piping component and, in turn, the RFL estimate. Thereafter, uncertainty propagation employs the formerly quantified uncertainties and utilizes the aforementioned information to esti-mate the RFL. Finally, uncertainty management deals with performing sensitivity analysis to find the individual contributors to uncertainty in the estimated RFL. A numerical case study illustrating the deterministic and probabilistic RFL assessment of topside piping is presented. Afterwards, probabilistically predicted RFL is used to demonstrate the calculation of an inspection interval. Finally, the implications of probabilistically estimated RFL on HCR from process piping is discussed.
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Vibration velocity measurement technique (VVMT) coupled with the vibration assessment criteria (VAC), employed to decide the level of fatigue damage on the piping system is afflicted by two main shortcomings. First being that the data... more
Vibration velocity measurement technique (VVMT) coupled with the vibration assessment criteria (VAC), employed to decide the level of fatigue damage on the piping system is afflicted by two main shortcomings. First being that the data measured by VVMT is not a direct indicator of the remnant fatigue life (RFL) of a piping system. The second is that the distinction between different regions of the VAC used to identify the level of fatigue damage on the piping system is too rigid. The paper thus proposes a fuzzy logic based approach to overcome the aforementioned shortcomings. An illustrative case study is presented within which the Mamdani procedure is used to establish the relationship between input variables (vibration velocity and RFL) and the output variable (fatigue failure vulnerability index (FFVI)). Thereby, value of FFVI decides both the level of fatigue damage and the corrective action necessary to minimize fatigue damage on offshore pipework.
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Setting an optimal inspection plan for fatigue critical offshore piping relies on accurately estimating its remnant fatigue life (RFL). Several modeling approaches, such as knowledge-based, model-based, data-driven, fusion techniques... more
Setting an optimal inspection plan for fatigue critical offshore piping relies on accurately estimating its remnant fatigue life (RFL). Several modeling approaches, such as knowledge-based, model-based, data-driven, fusion techniques etc., have been used to build RFL models in the past. The aim of this paper is to review these approaches and thereby recommend the most favorable approach for building a probabilistic RFL model for offshore piping. Firstly, a brief discussion about the aforementioned approaches is presented. Thereafter, a comparison is made between these approaches. For instance, there is uncertainty in model-based approaches, due to the assumptions of the underlying physical model, which poses substantial limitations on this approach. Conversely, a data-driven approach exploits the monitored operational data associated with the condition of the piping system. Fusion technique combines the features of the former two approaches and is recommended to build a model for estimating the RFL of offshore piping.
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A typical procedure for a remnant fatigue life (RFL) assessment is stated in the BS-7910 standard. The aforementioned standard provides two different methodologies for estimating RFL; these are: the S-N curve approach and the crack growth... more
A typical procedure for a remnant fatigue life (RFL) assessment is stated in the BS-7910 standard. The aforementioned standard provides two different methodologies for estimating RFL; these are: the S-N curve approach and the crack growth laws (i.e. using Linear Elastic Fracture Mechanics (LEFM) principles) approach. Due to its higher accuracy, the latter approach is more commonly used for RFL assessment in the offshore industry. Nevertheless, accurate prediction of RFL using the deterministic LEFM approach (stated in BS-7910) is a challenging task, as RFL prediction is afflicted with a high number of uncertainties. Furthermore, BS-7910 does not provide any recommendation in regard to handling the uncertainty in the deterministic RFL assessment process. The most common way of dealing with the aforementioned uncertainty is to employ Probabilistic Crack Growth (PCG) models for estimating the RFL. This manuscript explains the procedure for addressing the uncertainty in the RFL assessment of process piping with the help of a numerical example. The numerically obtained RFL estimate is used to demonstrate a calculation of inspection interval.
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This paper acts as a supplement to the Fuzzy Analytical Hierarchy Process (FAHP) based methodology proposed by the authors to prioritize inspection of Potential Fatigue Critical Locations (PFCLs) on topside piping. In order to account for... more
This paper acts as a supplement to the Fuzzy Analytical Hierarchy Process (FAHP) based methodology proposed by the authors to prioritize inspection of Potential Fatigue Critical Locations (PFCLs) on topside piping. In order to account for the uncertainties in the AHP based approach (proposed by the authors), arising from the crisp scale, authors have proposed an FAHP based methodology. The aforementioned approach uses a pre-defined fuzzy set scale and a certain degree of fuzziness (δ). Generally, the degree of fuzziness is determined by the sensitivity analysis. However, such an approach is not suitable for the FAHP methodology proposed by the authors. Hence, in this manuscript the authors have proposed an alternative approach to select the value of δ based on the experts' background knowledge regarding the technical reality of the piping system. In real-world situations the aforementioned depends upon his/her past on-site experience, accessibility to the technical reports and other technical documents. Thus, the value of δ based on the expert's background knowledge reflects his/her confidence regarding the technical reality/condition of the system. A case study is performed to illustrate the applicability of the proposed approach.
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Remnant Fatigue Life estimate and inspection cost are two vital factors for inspection planning of topside piping. Even though, it is possible to estimate RFL by numerous approaches; yet, the model-driven and data-driven approaches, have... more
Remnant Fatigue Life estimate and inspection cost are two vital factors for inspection planning of topside piping. Even though, it is possible to estimate RFL by numerous approaches; yet, the model-driven and data-driven approaches, have emerged as the main competitors. However, irrespective of which approach is selected to build the RFL model, the accuracy of the RFL estimate depends upon the uncertainty quantification and the propagation process. In this manuscript, the numerical example is presented to perform the aforementioned activities. Finally, once the RFL is estimated, discussion regarding the inspection planning and maintenance scheduling of the offshore pipework is presented.
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Topside piping is the single largest source of the hydrocarbon releases (HCRs) on the offshore oil and gas (OOG) platforms in the North Sea region. Consequently, if the leaked hydrocarbons from the process pipework are ignited, it may... more
Topside piping is the single largest source of the hydrocarbon releases (HCRs) on the offshore oil and gas (OOG) platforms in the North Sea region. Consequently, if the leaked hydrocarbons from the process pipework are ignited, it may lead to a catastrophic event, thereby causing significant economic losses, environmental damage, and posing serious threat to the safety of the onboard personnel. In order to avert such a fateful event and to enhance process safety, it is vital to maintain the technical integrity of the topside piping. In regard to this, risk based inspection (RBI) plays a vital role, as the inspection locations and frequency are decided based on the risk of potential failure. However, international standards such as API 570, API 581 and DNV RP-G101 provide limited guidance in regard to inspection of the fatigue degradation of the offshore topside piping. Due to the aforementioned,selection of the fatigue critical piping locations for inspection, is currently done either on the ad-hoc basis or using the three staged Risk Assessment Process (RAP) mentioned in the Energy Institute (EI) guidelines. Nevertheless, it has been revealed that the methodology for stage 1 of the RAP is laborious and time consuming. Thus, to reduce the toil of the practicing inspection engineer and with the aim of mitigating the dearth of RBI methodologies for topside piping fatigue, this manuscript proposes a Fuzzy-Analytical Hierarchy Process (FAHP) centered approach for selecting the fatigue critical piping locations for inspection and repair. The usability of the proposed approach is demonstrated by an illustrative case study.
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RLWI (Riserless Light Well Intervention) technology has the advantage of utilizing a special subsea lubricator to perform intervention activities in water depths of up to 1,200 m without the need for the marine risers. Utilizing the... more
RLWI (Riserless Light Well Intervention) technology has the advantage of utilizing a special subsea lubricator to perform intervention activities in water depths of up to 1,200 m without the need for the marine risers. Utilizing the technology, oil companies have been able to save up to 50% on the intervention costs. However, in the last five years, it has seen up to 25% downtime due to waiting on weather (wow). Thus, in this manuscript, it is attempted to identify the critical elements of the module deployment system and analyze their significance in the objective of raising the operational weather limit. Critical failure modes were found to be failure of crane wire due to excess loading, failure of the lower cursor system due to the impact loading and clashing of the module with the moonpool walls. Analysis of the module deployment system against these failure modes was ensued by using Orcaflex. The results showed the moonpool sea state to be the defining parameter. Although, changing moonpool dimensions affect hydrodynamics positively, however it's significance is small due to dependency on the vessel's breadth. Based on these results and the available data for the analysis, a recommended system particular was tested. Significance improvement, in lowering the risk of failure was observed.
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It is a common practice in the oil and gas industry to employ various Non-Destructive Evaluation (NDE) methods during in-service inspections to detect the cracks in the pipeline. The capability of NDE method is expressed by the metric... more
It is a common practice in the oil and gas industry to employ various Non-Destructive Evaluation (NDE)
methods during in-service inspections to detect the cracks in the pipeline. The capability of NDE method
is expressed by the metric called as the Probability of Detection (POD), which is stated as a function of crack size through the POD curve. This manuscript thus deals with various aspects of POD. Firstly, a brief discussion about NDE reliability and POD is presented. Thereafter, based on the literature review the history and development of POD curves is provided. Finally, a brief discussion about the future of POD i.e. model assisted POD is presented.
methods during in-service inspections to detect the cracks in the pipeline. The capability of NDE method
is expressed by the metric called as the Probability of Detection (POD), which is stated as a function of crack size through the POD curve. This manuscript thus deals with various aspects of POD. Firstly, a brief discussion about NDE reliability and POD is presented. Thereafter, based on the literature review the history and development of POD curves is provided. Finally, a brief discussion about the future of POD i.e. model assisted POD is presented.
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The paper proposes a Fuzzy Analytical Hierarchy Process (FAHP) based methodology to prioritize inspection of Potential Fatigue Critical Locations (PFCLs) on topside piping. In order to account for the uncertainties in the AHP based... more
The paper proposes a Fuzzy Analytical Hierarchy Process (FAHP) based methodology to prioritize inspection of Potential Fatigue Critical Locations (PFCLs) on topside piping. In order to account for the uncertainties in the AHP based approach, arising due to crisp scale, the FAHP uses pre-defined fuzzy set scale and certain degree of fuzziness. Based on the experts' background knowledge regarding the technical reality of the piping system, he/she selects the value that reflects his/her confidence regarding the technical reality of the system. Thereafter, the Composite Priority Weights (CPWs) of piping locations in different units are obtained from modified synthetic extent analysis. Thereby, the locations with highest CPWs serves as PFCL, and is prioritized for further assessment during stage 2 of the risk assessment process (RAP). A case study is performed to illustrate the applicability of the proposed methodology.
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Currently, a three-staged, risk-based approach given in the Energy Institute (EI) guidelines is employed to identify fatigue critical locations on topside piping. EI guidelines provide guidance for various stages of the risk assessment.... more
Currently, a three-staged, risk-based approach given in the Energy Institute (EI) guidelines is employed to identify fatigue critical locations on topside piping. EI guidelines provide guidance for various stages of the risk assessment. However, authors feel that the qualitative methodology employed during stage 1 for identifying fatigue critical system is cumbersome. The paper thus proposes an alternative methodology to prioritize inspection of Potential Fatigue Critical Locations (PFCLs) on topside piping based on the Composite Priority Weights (CPWs) obtained from Analytic Hierarchy Process (AHP). An illustrative case study is performed, within which the hierarchy tree structure is developed. Thereafter, the CPWs of piping locations in different units are obtained. Thereby, the locations with high CPWs are prioritized for the quantitative assessment during the stage 2 of the risk assessment.
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A significant number of offshore structures and mechanical items installed in production systems on the Norwegian Continental Shelf (NCS) are either approaching or have exceeded their intended design life. However, with the help of the... more
A significant number of offshore structures and mechanical items installed in production systems on the Norwegian Continental Shelf (NCS) are either approaching or have exceeded their intended design life. However, with the help of the advancement of technology and analysis approaches, most of the offshore production facilities are being considered for life extension. This requires regular inspection, fitness for service (FFS) assessment, remnant life assessment, maintenance and repair (or modification). In this context, fatigue and fracture related degradation play a vital role. Hence, this paper discusses the state of the art as well as two major methodologies used for fatigue life prediction of structures and mechanical items. The first (S-N approach) is based on experimentally derived S-N curves and linear damage rule (LDR). Since LDR does not take sequence effect of loading into account the S-N approach often leads to overestimation / underestimation of fatigue life. Hence, this paper also takes into simultaneous consideration the second approach, which relies on the principles of fracture mechanics (FM) and crack growth analysis. Furthermore, the paper discusses damage tolerance analysis (DTA) and the role of Risk Based Inspection (RBI) to detect cracks before they grow to a critical level and cause catastrophic failure of the component. Thereafter, the paper discusses the reliability of Non-Destructive Evaluation (NDE) methods quantified in terms of Probability of Detection (PoD), to identify the flaw size and location. Finally, probabilistic crack growth (PCG) models used for remaining useful life estimation (RULE) and for planning inspection regimes of structural and mechanical items are discussed briefly.
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Various Non-Destructive Evaluation (NDE) methods are used in offshore industry during in-service inspections to detect the cracks in the structures and mechanical items. The metric generally used to quantify the capability of NDE method... more
Various Non-Destructive Evaluation (NDE) methods are used in offshore industry during in-service inspections to detect the cracks in the structures and mechanical items. The metric generally used to quantify the capability of NDE method is Probability of Detection (POD), which is expressed as a function of crack size through POD curve. Human skills play a major role in determining overall reliability of NDE method used for crack detection. Thus, a brief discussion related to human factors in NDE reliability is presented. Thereafter, based on literature review the history and development of POD curves is provided. Likewise, the statistics of the POD curve, along with the statistical models of POD curve for different NDE methods is also presented. Furthermore, the limitations of POD curves are discussed in the paper. Finally, a brief discussion about model assisted POD is presented.
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With oil and gas reserves moving into deeper waters, Floating Production Units (FPUs) have been widely used for production purposes. Selection of a FPU for deepwater field development is mainly governed by factors such as water depth,... more
With oil and gas reserves moving into deeper waters, Floating Production Units (FPUs) have been widely used for production purposes. Selection of a FPU for deepwater field development is mainly governed by factors such as water depth, location of field, environmental conditions, deck space requirements, storage requirements and offloading requirements etc. Amongst all the available FPU alternatives, ship shaped FPSO has dominated the concept selection and is generally used in marginal and remote fields lacking pipeline infrastructure. Selecting riser concept for FPSO stationed in deepwater has posed challenges due to high hydrostatic pressure and large vessel payload. The most common riser concepts for the aforementioned are free hanging flexible riser, Steel Catenary Riser (SCR) and Hybrid Riser Tower (HRT). The condition is worsened if besides deepwater, FPSO is also stationed in harsh environmental conditions. This is due to large offsets and dynamics of FPSO which are directly transferred to the riser's base. To cope up with this situation offshore industry has focused on concepts like Steel Lazy Wave Riser (SLWR), Single Hybrid Riser (SHR) and Buoyancy Supported Riser (BSR).
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Splash zone crossing of structures with large horizontal surface (e.g. manifolds) and structures having large weight variation in the water and air ( e.g. suction anchors) are of particular concern. This is due to the large... more
Splash zone crossing of structures with large horizontal surface (e.g. manifolds) and structures having large weight variation in the water and air ( e.g. suction anchors) are of particular concern. This is due to the large slamming forces and added mass of the structures, which results in high dynamic loads on the crane . The solution to this could be attaching a Passive Heave Compensator (PHC) between the crane hook and the payload. This not only reduces the dynamic peak loads on the crane, but also mitigates the risk of slack wire situation during splash zone crossing. This work analyzes the deployment of a subsea manifold with and without a PHC in the North Sea, at a water depth of 370m. A detailed dynamic analyses is done for a sea-state of 3m Hs, over a range of Tz periods varying from 3s to 13s. it is concluded that by using a PHC for offshore lifting, reduction in maximum crane wire tension, in slack wires, in landing velocities and in crane tip velocity is achieved. Furthermore,the use of a PHC helps in increasing the weather window, as lifting can be performed in lower Tz periods. However, the use of PHC is recommended for short to medium wave periods, as its efficiency decreases at lower wave period.