Investigation On Fracture Assessment Methods For Reeled Offshore Pipelines

Reel-lay is an efficient and economic method to install offshore pipelines,but its main disadvantage is that pipeline material undergoes great plastic strain during installation,about 1%-3%.Due to seismic activities,in-service pipelines are also subjected to great plastic strain during operation.As a result of the pipelines being constructed by the girth welding method,flaws,such as cracks and lack of penetration,may be introduced into welds.Fracture assessment must be conducted to avoid fracture failure of girth welds during installation and operation of offshore pipelines.The main contents of the thesis focus on the estimation of crack driving force of circumferential cracks and measurement of resistance curve of weld metals.A modified reference strain method was proposed in the thesis.The method extends the application range of the strain-based failure assessment diagram method to fracture assessment for severe circumferential surface cracks in reeled pipes.In comparison with other factors,the crack depth,crack length and?₀/?_u ratio have a pronounced effect on the reference strain,so the reference strain ratio is defined as the function of the crack depth,crack length and?₀/?_u ratio.An empirical formulation for the reference strain is developed.Furthermore,it is verified that the empirical formulation is conservative to conduct ECA results for reeled pipes.To conduct fracture assessment for in-service pipelines subjected to large plastic strain,the limit load solution of Kastner for pipelines with circumferential surface cracks under tension was modified based on the von Mises yield criterion.Then,the limit load solution of Kastner was adapted to the fracture assessment for in-service pipelines subjected to large plastic strain.Furthermore,the optimised reference stress method was applied to improve the accuracy of the fracture assessment for in-service pipelines.Finally,the formula for the non-dimensional parameter,?,was developed.The results of three-dimensional finite element analyses indicate that directly applying the fracture assessment results for surface cracks to assess deeply-embedded cracks is unnecessarily conservative.We introduce a factor F,which serves as an adjustment factor to reduce the degree of conservatism.It is assumed that F is the function of the non-dimensional deeply-embedded crack height?2ae/t?,since F is markedly affected by 2ae/t.Furthermore,after introducing the factor F,the crack tip constraint of the deeply-embedded cracks is still lower than that of the surface crack,which further validates the use of F.At present,the growing demand for the subsea transport of the highly corrosive media has driven the oil and gas industries to use clad or lined pipes.In order to ensure the corrosion resistance performance of circumferential welds in clad or lined pipes,nickel based alloy consumables are routinely used,which indicates that the yield strength of the weld metal is lower than that of base metal.To measure the toughness of the undermatched weld metal,plastic eta factors for clamped-end and pin-loaded single-edge tension specimens with a wide range of configurations and different undermatched levels were obtained based on detailed plane strain finite element analysis.From this analysis,a set of equations for the plastic?factor is proposed.At last,detailed fracture assessment flowlines for circumferential surface and embedded cracks in pipelines subjected to large plastic strain are developed.