Verification Analysis and Validation Testing of Subsea Connectors

Abstract

Abstract There are many challenges faced in the oil and gas industry in subsea applications, especially when operating in high pressure and high temperature (HPHT) environments. Operating subsea systems in deeper waters and harsher environments is associated with higher risks. Subsea connectors, a crucial part of the drilling and production systems, present challenges to the industry that include higher structural static capacity requirements and preservation of material properties in harsh environments. Furthermore, subsea connectors are subject to high cyclic loads generated from vessel motions, ocean currents, pressure cycles, thermal cycles, and other internal and external sources of fatigue damage. Structural capacities are well understood when proper analysis methodologies validated with testing are used [1], and these requirements are now outlined for the first time in industry standards API 17TR7 and 16A [2, 3]. Fatigue however is a more complex and challenging subject. Determining the fatigue performance of subsea systems and connectors involves a lengthy and comprehensive process due to the large number of input variables required, such as water depths, vessel types, environments, metocean conditions, soils, static and dynamic loads, and the various operations in which the equipment is used during its service life. Various analysis methods are considered acceptable in the industry today for the fatigue performance evaluation of subsea equipment. Stress-life, strain-life, and fracture mechanics are some of the most common methods. These methods vary significantly when it comes to the fatigue verification analysis methodology, safety factors, and analysis inputs required, leading to varying estimates of fatigue life of equipment. The general perception is that, if properly used, these methodologies often produce conservative results. This paper focuses on subsea connectors in general and provides a verification analysis methodology that has been validated with physical testing for both static structural capacities and fatigue. Details of the verification analysis methodology and validation testing program are presented. Besides following the API 17TR7 and API 16A requirements for analysis and testing, this paper goes further as it addresses fatigue analysis in more detail, but most importantly for the first time presents results of the fatigue testing of full-scale wellhead/BOP connector and wellhead as a system. Analysis and testing of large diameter casing and conductor connectors is also discussed and test results for both structural capacity and fatigue are presented. This work highlights the importance of subsea connector validation test results for structural and fatigue performance evaluation and calibration of finite element analysis inputs and verification analysis methodologies. The validation test identifies any inaccuracies of inputs and assumptions that may exist in analysis methodologies, and allows for proper adjustments to be made. Validation testing provides valuable knowledge and a better understanding of complex designs and insights for design improvements which may not be obvious otherwise. The end result is safe and reliable equipment fully validated and suitable for safe drilling and production operations in various challenging environments. Some of the testing performed has been perceived as infeasible until now, and this work should provide guidance to the industry on how to properly evaluate this type of equipment.