As an alternative to overcome safety challenges associated with conventional drilling riser, freestanding drilling riser (FSDR) is disconnectable and the longer riser is left in freestanding mode. A comparative study of FSDR system from connected mode to freestanding mode to ensure the survival of drilling riser is established. Sensitivity analysis is carried out to establish an optimum installation depth of buoyancy can (BC) and threshold conditions of the mechanical behaviour of FSDR in the two modes have been established. Results of BC dimension analysis showed that the mechanical behaviour of the drilling riser improves when the BC is slender.
The frequent hurricane or typhoon in Gulf of Mexico, South China Sea and other susceptible deepwater environments disrupts drilling operations, and endangers personnel and offshore facilities. A FSDR system serves as a means to combat the challenges, such as tediousness and huge risk of untimely evacuation of personnel and facilities for safety, associated with the conventional practice of retrieving the whole length of drilling riser. A freestanding drilling riser (FSDR) system has principally additional two features, near surface disconnection package (NSDP) and buoyancy can (BC), in which the drilling riser is disconnectable at the former while the latter provides upthrust to support the drilling riser in freestanding mode.
Static and dynamic analysis of risers under various conditions, mathematical models, and numerical simulations have been put forward regarding the study of risers (Mao et al., 2020, Nguyen and Al-Safran, 2020, Zhu et al., 2020). As the offshore exploration of oil and gas gets deeper and deeper, the riser length increases which in turn poses greater challenges in understanding the mechanical behavior of riser as the riser becomes more susceptible to sea current and waves forces. Hence, the concept of freestanding risers was introduced to minimize waves and strong surface current effects on riser. The operational procedure for a near surface disconnectable drilling riser (Bye et al., 2000, Lim et al., 2008) and works on risers supported by artificial buoyant seabed (ABS) or BC (Norheim, 2003, Tellier and Thethi, 2009, Zhen et al., 2019) have been published. Kogure et al. (1998) presented a near surface disconnectable drilling riser concept for deepwater drilling operation. Fontaine et al. (2003) studied the hydro-elastic behavior of a freestanding riser using three different approaches. Nguyen et al. (2006) presented a feasibility study for a drilling riser in a freestanding mode. Qin et al. (2011) carried out the parametric sensitivity analysis and optimization of deepwater freestanding hybrid riser system. Kang et al. (2012) put forward a method for design and analysis of a buoyancy tank riser tensioner system with a goal to developing a buoyancy tank with minimal hydro force, maximal net lift, and no redundancy of compartments. Xu et al. (2013) addressed the challenges, such as typhoon, of deepwater drilling in the South China Sea and proffered potential solution to ensure safety in drilling. Yong et al. (2018) found that the effect of typhoon generated swell on freestanding drilling riser was significant and should be considered to have a reliable freestanding drilling riser mechanism. Kim et al. (2018) presented an optimal configuration of freestanding hybrid riser considering factors including span distance, BC up-lift force, and BC submerged location. Other researchers in the likes of Tan et al. (2018), Xu et al. (2019), Abimbola et al. (2020), Abimbola et al., (2021), Xu et al., (2021), Wang et al., (2021) respectively have investigated numerical calculation model response for connector assembly of a freestanding hybrid riser, carried out parametric study has to understand the FSDR response under typhoon condition, proposed an operational method for safe and quick retrieval of drilling risers in freestanding mode, presented a mathematical model to analyze the mechanical behavior of drilling riser freestanding mode (DRFM) and established threshold conditions for various BC positions, investigated the behaviour of a riser with a drill string based on the finite-element method, and developed a system to improve the adaptability of the drilling riser system under hang-off or transit operations.
