Deepwater drilling riser is the key technology equipment in deepwater drilling, and its safe and reliable operation is related to the success and economy of deepwater drilling operations. Under certain operating conditions, the riser will be in hang-off mode. Ensuring the safety of suspended riser is an important content in the mechanical analysis and optimization design of riser. In this paper, considering the damping of seawater, the distribution of buoyancy block and the motion of floating drilling equipment, the mechanical analysis model and governing equation of riser in both soft and hard hang-off mode are established, and the natural frequency, vibration displacement amplitude and vibration force distribution of riser under suspension are analyzed. Besides, the influences of water depth, wave height and wave period on the mechanical characteristics of suspended riser are also discussed. The results show that the natural frequency of riser decreases with the increase of water depth in hard hang-off mode. Both the vibration displacement amplitude and the vibration force of the riser increase from bottom to top along the riser. The vibration displacement amplitude and the vibration force of riser in soft hang-off mode are decreased compared with that in hard hang-off mode. It is significant to control the heave motion of the platform reasonably for the safe operation of the suspended riser. The research results of this paper have reference value for the design and control of deepwater drilling riser.
The riser system between the wellhead and the drilling platform is the weak link in the whole offshore drilling system. With the development of offshore drilling to deepwater and ultra-deepwater areas, risers are exposed to a variety of complex external loads, which make the risers under complex load conditions, and greatly affect the normal operation of the risers. During drilling and completion, riser systems are required to perform a series of key operations, including riser down/recovery operations, suspension operations, and emergency disconnection operations. In these operations, the riser is not connected to the subsea wellhead, and the suspended riser is in relatively harsh environmental conditions. The control of the riser system is complicated, and the riser faces increasing risks. Therefore, the research on the mechanical properties of suspended riser can reveal the mechanical properties and main control factors of riser in hang-off mode, which is a scientific basis for the safety of riser operations.
