Influence of Thermal Gradients in Design Verification of Uninsulated HPHT Bolted Connections

Abstract

Temperature effects on closure bolting play an important role when assessing high-pressure, high-temperature (HPHT) subsea equipment. This paper discusses the evaluation of uninsulated bolted connections, which are internally exposed to high pressure and temperature and externally exposed to cold seawater, via finite element analysis (FEA). These thermal gradients cause nonuniform thermal expansions, contractions, or both. The paper then compares this to identical insulated connections, which have a uniform temperature distribution across the bolted connection. Two pressure-containing bodies featuring nonstandard API four-bolt connections of different sizes were analyzed using FEA at 20,000-psi bore pressure. The larger body featured an API BX-169 gasket housing a 5-1/8-in internal bore at 300 degF, and the smaller body featured an API BX-151 gasket housing a ½-in internal bore at 150 degF. Both bodies were analyzed at high and low seawater convection rates with a temperature heat sink of 35 degF. This was then compared to identical insulated connections assuming a uniform temperature distribution. All verification was performed per API Technical Report 17TR8 utilizing American Society of Mechanical Enginers (ASME) Boiler and Pressure Vessel Code (BPVC), Section VIII, Div. 3 elastic-plastic analysis methods focused only on the serviceability check. The results of this design verification showed an increase in bolt tensile stress in the un-insulated cases when compared with the same internal pressure for insulated cases. This stress increase exceeded the allowable tensile stress as specified by API Spec 17D even at an internal temperature of 150 degF, which is below the design threshold where thermal effects must be accounted for as required by API Spec 6A. This required either a reduction in the installation preload, an increase in bolt size, a reduction in the design load-carrying capacity, or a combination thereof. For this study, a reduction in the installation preload to reduce the tensile stress below the API Spec 17D allowable limit was selected as the corrective measure due to preexisting bolt size constraints and operational requirements. Gasket contact pressure was reevaluated at the reduced preload and was found to be sufficient to maintain proper joint sealability. FEA provides a more comprehensive understanding of thermal gradient effects on subsea bolted connections and identifies loading effects not normally observed using standard analysis methods. It is also a recommendation to evaluate both insulated and un-insulated cases to ensure that the bolted connection will perform its intended function. This study showed bolt stress differences between thermally insulated and un-insulated connections that can result in bolt stresses beyond acceptance criteria for one while the other is satisfied. This highlights the importance of evaluating bolted connections to ensure acceptability for all operational cases. This study also shows the benefits of using advanced analysis techniques to gain a more comprehensive understanding of the behavior of insulated and un-insulated subsea bolted connections.