INTRODUCTION
American Petroleum Institute (API) standard 16C, Choke and Kill System, requires that peripheral lines (P-lines) meet the hardness requirements of NACE(1) MR0175/ISO15156-2. This means a maximum hardness of 22HRC for low alloy steels. Further, NACE MR0175/ISO15156-2 allows downhole casing, tubing and tubular components made of AISI 41xx steels, quenched and tempered in tubular form, to have a hardness of 26HRC, but each heat should be qualified by Sulfide Stress Cracking (SSC) testing in accordance with NACE MR0175/ISO15156-2 Clause B.1 using the tensile test. During production of AISI 4130 Q&T drilling riser choke and kill lines weld cap hardness over 237HB/22HRC was experienced. This article describes Capcis(2) HSE(3) OTI 95-635 full pipe ring sour corrosion tests on high hardness AISI 4130 Q&T P-line pipe and weldments. In the full pipe ring tests, welded pipes were loaded by squeezing up to a Hoop stress of 552MPa (SMYS) in the outer surface. Test solution was the NACE TM0177 solution A, with H2S partial pressure of 0.01, 0.1 and 1.0 bar (1, 10 and 100 kPa), respectively. Pipes and welds (weld cap and heat affected zone) with hardness that ranged from 230HV10 to 357HV10 did not show any signs of hydrogen induced sulfide stress cracking after the 30 days exposure. Test procedure and an overview of actual pipe and HAZ hardness variations are presented.
A drilling riser connects the drilling rig with the well head on the sea bottom. It consists of a 21 inches (533 mm) diameter central pipe through which the drilling activities are performed and peripheral (P-) lines for hydraulics, booster and choke and kill activities. Peripheral lines fall under the governance of API standard 16C, which refer for the maximum allowable hardness in choke and kill lines to NACE MR0175/ISO15156-2, which states 22 HRC.
