Shashi Bhushan Kumar

ECA of Pipeline With Girth Weld Strength Mis-Matching Subjected to Large Strain

In recent years, the strain based design for pipeline has been widely accepted by the industry, but the definition of a rational flaw acceptance criteria for girth welds subjected to axial strain within the context of the existing codified fracture mechanics based assessment procedures is problematic since these are essentially stress based. To extend the FAD method to the large strain conditions, several challenges i.e. weld strength mismatching, fracture toughness, and welding residual stresses have to be understood. With appropriate modifications as per DNV-RP-F108 [1], the assessments procedure detailed in BS7910 document for stress based situations have been used successfully for several projects to develop acceptance criteria for pipeline installation involving plastic straining. But only weld metal strength over-match comparing with base metal is considered in DNV-RP-F108 [1]. High strength line pipes are required to reduce the transmission cost of natural gas in long distance and internal clad with corrosion resistant alloy (CRA) is used for transportation of sour gas. Steel manufactures have developed such line pipes to develop new oil and gas field. The inconel filler metal was selected as weld consumable for the production girth weld in the lay budge. From the all weld tensile tests, it was found that the yield strength of the weld is under-match comparing the base metal, and the pipeline maybe subjected to a strain level up to 1.0% due to the lateral buckling. In this research the effect of weld strength mismatching on the structural integrity of the pipeline subjected to large strain was studied. The Engineering Critical Assessment (ECA) was performed to derive the critical flaw acceptance criteria for the AUT system. The segment tests and numerical analysis were performed to validate the assessment procedure, and the finite element analyses of the pipeline girth weld with surface crack in the weld centre were carried out to investigate the effect of bi-axial loading on the ECA results.Copyright

Dynamic Positioning Reliability Index (DP-RI) and Offline Forecasting of DP-RI During Complex Marine Operations

The Dynamic Positioning (DP) System is a complex system with significant levels of integration between many sub-systems to perform diverse control functions. The extent of information managed by each sub-system is enormous. The complex level of integration between sub-systems creates more possible failure scenarios. A systematic analysis of all failure scenarios is tedious and for an operator to handle any such catastrophic situation is breath taking. There are many accidents where a failure in a DP system has resulted in fatalities and environmental pollution. Therefore, reliability assessment of a DP system is critical for safe and efficient operation of marine and offshore vessels. Traditionally, the reliability of a DP system is assessed during the design stage by methodologies such as Failure Mode Effects and Analysis (FMEA), Proving Trials, Hardware In-the Loop (HIL) testing, Site-Specific Risk Analysis, DP capability Analysis and during operation by annual trials to verify functionality. All these methods are time consuming, involving a lot of human effort and notably no analysis of previous accidents are indicated in the reliability assessment. This imposes in-built uncertainty and risk in DP system during operation. In this paper, a new concept of Dynamic Positioning Reliability Index (DP-RI) is introduced and a state-of-the-art advisory decision making tool is proposed. This tool is developed based on information from various sources including Offshore Reliability Data (OREDA), International Marine Contractors Association (IMCA) Accident database, DP vendor equipment failure databases, DP System supplier’s manuals, previous system level FMEA and HIL testing results, Site specific risk analysis documents, Project design specification and Operator’s operational experiences. Thus, DP-RI addresses the pitfalls of existing reliability assessment methods and will be an efficient tool in reducing the number of DP-related accidents.

Fitness-For-Service Assessment of the Pipelines with Localized Geometric Imperfections such as Buckles and Wrinkles

In this paper, FFS assessment procedure for the buckle damaged pipeline with cracks in the girth welds is presented. For FFS assessment the tensile and J R-curve data from a pre-strained pipeline material, API 5L X65 were obtained in the laboratory to study the influences of the large plastic strain on the material properties and the fracture resistance of the pipeline girth welds. Tensile and single edge notch bend specimens in as-received, 10% pre-strained and 20% prestrained conditions were tested. The test results show significant increase in yield and tensile strength in the pre-strained specimens. Generally, the elongation and fracture resistance decreased after pre-straining. In FFS material specific failure assessment diagrams (FADs) generated based on the stress-strain curves obtained from testing were used. The critical flaw sizes of the pipeline girth welds were calculated, and the influence of the large plastic strain on the FFS results was discussed.