Seamless steel tube for use as a steel catenary riser in the touch down zone

Abstract

The present invention describes an upset SCR of novel low carbon chemical composition and microstructure as well as method of manufacturing the same, which achieves higher improvement in the fatigue life as it is integral with the riser pipe section at the Touch Down Zone. The low carbon upset SCR achieves its desired properties by the thermal treatment which it is subjected to. The novel low carbon chemical composition and microstructure comprises in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70, silicon 0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel 0.10-0.40, nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max, phosphorus 0.020 max, titanium 0.003-0.020, niobium 0.020-0.035, vanadium no more than 0.10, copper 0.20 max, tin 0.020 max, and carbon equivalent 0.43 max and PCM no more than 0.23 and having a yield strength of at least of 65000 psi, the ultimate tensile strength of at least 77000 psi and YS/UTS ratio below 0.89 in material representing the pipe body, the transition zone and the upset end.

1 . A seamless steel pipe for a steel catenary riser with upset ends comprising in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70, silicon 0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel 0.10-0.40, nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max, phosphorus 0.020 max, titanium 0.003-0.020, niobium 0.020-0.035, vanadium no more than 0.10, copper 0.20 max, tin 0.020 max, and carbon equivalent 0.43 max and PCM no more than 0.23 and having a yield strength of at least of 65000 psi, the ultimate tensile strength of at least 77000 psi and YS/UTS ratio below 0.89 in material representing the pipe body, the transition zone and the upset end. 
   
   
       2 . The seamless steel pipe according to  claim 1 , in which the microstructure of as-quench and temper material is homogeneous at midwall which is the most critical section and it is mainly bainite and a mixture of acicular and non polygonal ferrite independently of the section: pipe body, transition or upset zones. 
   
   
       3 . The seamless steel pipe according to  claim 1 , in which the prior austenitic grain size has an average size is at least 7 ASTM both in pipe body and in the upset pipe ends. 
   
   
       4 . The seamless steel pipe according to  claim 1 , which the material in the as quenched and tempered condition has a Hardness Vickers HV10 value below 250 both in the pipe body and the upset ends. 
   
   
       5 . The seamless steel pipe according to  claim 1 , in which the materials from pipe body and upset ends have an individual value of absorbed energy above 70 Joules, and 90 Joules average of three specimens and the transition temperature in the transverse direction was below −50° C. 
   
   
       6 . The seamless steel pipe according to  claim 1 , in which the materials from pipe body and upset ends exceed in at least 2 times the minimum individual value of 0.51 mm required of crack tip opening displacement test (CTOD test). 
   
   
       7 . The seamless steel pipe according to  claim 1 , which is weldable at the upset ends in a heat input range between 0.8 KJ/mm and 1.5 KJ/mm, where CTOD testing using SENB, Bx2B specimens undertaken from the heat affected zone run at −10° C. as per API RP2Z, gave CTOD values above 0.6 mm. 
   
   
       8 . The seamless steel pipe according to  claim 7 , which is weldable at the upset ends in a heat input between 0.8 KJ/mm and 3.0 KJ/mm and the hardness in the heat affected zone is less than 250 HV10. 
   
   
       9 . The seamless steel pipe according to  claim 7 , which is weldable at the upset ends in a heat input between 0.8 KJ/mm and 3.0 KJ/mm and the absorbed energy values evaluated at fusion line +1 mm in the heat affected zone are above 100 Joules. 
   
   
       10 . The seamless steel pipe according to  claim 1 , which is weldable at the upset ends in a heat input range between 0.8 KJ/mm and 1.5 KJ/mm, where CTOD testing using SENB, Bx2B specimens undertaken from the axis of the weld metal run at −10° C. gave CTOD values above 0.6 mm 
   
   
       11 . A method for manufacturing a seamless steel tube for steel catenary riser with upset ends having a yield strength at least of 65000 psi both in the pipe body, transition and the upset-zone comprising the steps of: (a) providing a steel tube comprising in weight per cent, carbon 0.04-0.10, manganese 0.40-0.70, silicon 0.15-0.35, chromium 0.40-0.70, molybdenum 0.40-0.70, nickel 0.10-0.40, nitrogen 0.008 max, aluminum 0.010-0.045, sulfur 0.005 max, phosphorus 0.020 max, titanium 0.003-0.020, niobium 0.020-0.035, vanadium no more than 0.10, copper 0.20 max, tin 0.020 max, and carbon equivalent 0.43 max and PCM no more than 0.23; (b) upsetting the tube ends in multiple steps with intermediate heating cycles in between to achieve the required thickness (c) quenching and tempering between 630-710° C.; (d) machining the upset ends. 
   
   
       12 . The method for manufacturing a seamless steel pipe according to  claim 11  wherein pipes were hot rolled using a recrystallization controlled rolling scheme, manufactured from round billets obtained by continuously cast (CC) process. 
   
   
       13 . The method for manufacturing a seamless steel pipe according to  claim 11  wherein the pipes were upsetted by reheating the pipe ends above the dissolution temperature of Nb (C, N) to provide adequate plastic flow during each upset operation whilst controlling austenite grain size by precipitation of fine TiN particles. 
   
   
       14 . A pipe string for use as steel catenary riser for non-sour service environment using the pipes according to  claim 1 , wherein pipes are welded on the upset ends. 
   
   
       15 . A pipe string for use as steel catenary riser for sour service environment using pipes according to  claim 1 , wherein pipes are welded on the upset ends.