API TR 21TR2

New API-backed research on the fatigue life of bolts used in underwater natural gas and oil infrastructure-including critical blowout prevention equipment-resulted in the publication of this technical report, a key advancement in industry’s continued efforts to protect workers and the environment from potential problems associated with bolting fatigue in subsea structures.

The test program detailed in this technical report was developed to obtain bolting material fatigue data required to perform design verification analysis of fatigue-sensitive bolts to assure accurate design life estimation. The bolting fatigue testing program provided S-N fatigue curves for three alternating stress ranges in air and saltwater with cathodic protection (CP) and for bolt sizes of 1 in., 2 in. (Grade L7), and 3 in. (Grade L43). S-N curves are used to determine the number of cycles (N) required to test a product to failure at a given stress (S).

The results of these S-N fatigue tests allow the bolting design to be assessed for this type of fatigue through structural analysis using the nominal root area stresses in the bolt, avoiding the need to define stress and load concentrations in the bolt root radius of engaged threads.

The design guidelines and the fatigue data provided in the document are intended to be used for bolting with unified national thread with root radius (UNR) specifications of ASME B1.1 class 2A/2B and is important information for safe design and operation of offshore bolting.