Offshore fatigue crack repair by grinding and wet welding

doi: 10.1111/j.1460-2695.2010.01541.x Offshore fatigue crack repair by grinding and wet welding J. E. RODR ´ IGUEZ-S ´ ANCHEZ 1 , A. RODR ´ IGUEZ-CASTELLANOS 2 , F. P ´ EREZ-GUERRERO 3 , M. F. CARBAJAL-ROMERO 4 and S. LIU 5 1 Instituto Mexicano del Petr´ oleo, Coordinaci´ on de Aguas Profundas, Eje Central L´ azaro C ´ ardenas Norte 152, 07730 M´ exico Distrito Federal, Mexico, 2 Instituto Mexicano del Petr´ oleo, Direcci´ on de Ingenier´ıa, Eje Central L´ azaro C ´ ardenas Norte 152, 07730 M´ exico Distrito Federal, Mexico, 3 Instituto Mexicano del Petr´ oleo, Coordinaci´ on de Aguas Profundas, Eje Central L´ azaro C ´ ardenas Norte 152, 07730 M´ exico Distrito Federal, Mexico, 4 Instituto Polit´ ecnico Nacional, Secci´ on de Estudios de Posgrado, Avenida de las Granjas 682, 02550 M´ exico Distrito Federal, Mexico, 5 Colorado School of Mines, Metallurgical & Materials Engineering, Hill Hall 254 Golden, Colorado 80401, USA Received in final form 25 Oct 2010 ABSTRACT A fatigue crack repair option based on crack removal by grinding and subsequent filling of the resultant groove by wet welding is presented. Author’s experimental and fracture mechanics analysis experience on fatigue crack removal by grinding has demonstrated that a fatigue life extension factor of 2 to 3 is feasible for crack removal done before crack penetration has reached 30% of the plate thickness. However, it has also been demonstrated when groove is left empty, that fatigue life extension after crack removal is mainly due to the stage of fatigue initiation because crack propagation after removal has a higher rate than in as-welded condition. To overcome this situation, this work proposes for an underwater application like in the offshore industry, that filling the grinded groove by means of wet welding would produce a crack repair with a longer fatigue life than the empty groove case. Keywords crack repair; fatigue life; grind; groove; notch; offshore repair; wet weld. NOMENCLATURE a = crack depth C = material constant da/dN = crack growth rate m = material constant Y = modification factor (Y factor) K = stress intensity range K max – K min σ = surface stress range of uncracked welded joint at the crack site INTRODUCTION Fatigue cracks in offshore structures are frequently found during inspection campaigns, and their effect on the struc- tural integrity can become quite relevant depending on the redundancy of the structure. For the case of jacket structures it is widely known that hot spot stress locations at main tubular welded joints are prone to fatigue crack- ing, fortunately jackets are quite redundant structures that can safely continue operating with some cracked joints considering that they are regularly inspected to moni- tor crack growth depth and length. This practice of en- grossing NDT inspection plans to sustain integrity when fatigue cracks are found is mainly due to the fact that Correspondence: Jos ´ e Efra´ ın Rodr´ ıguez-S´ anchez. E-mail: [email protected] underwater fatigue crack repairs are not an easy task to perform, otherwise a repair would partially reinstate the as-welded condition and general visual inspection would be enough to sustain integrity after a fatigue crack repair is performed. Present practice for fatigue crack repair of jackets tubular joints basically relies on mechanical clamps rather than on crack removal. However, clamping solutions require considerable economical resources for the fabrication and installation of the clamps and frequent stress monitoring of the studs during the operating life of the jacket. There- fore, the fatigue crack repair option presented in this pa- per is a feasible option to reduce the economical resources required for fatigue crack repair of offshore jackets. c 2011 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 34, 487–497 487 Fatigue & Fracture of Engineering Materials & Structures