Model Tests on the Vortex Induced Motions of the Air Can of a Free Standing Riser System in Current

Free standing and hybrid risers are increasingly finding applications in deepwater fields all over the world. This is particularly true in West Africa and Brazil, where free standing risers are now often the basis for deep and ultra deep water developments. Deepwater developments often require riser systems that provide robust solutions with additional ‘features’, such as thermal insulation to improve the flow assurance performance, or an artificial lift capability in the form of a riser base gas lift.
A free standing riser generically consists of a vertical arrangement of a steel pipe, connected to a seabed foundation. The steel section is supported by a submerged air can which provides buoyancy and tension to the system. A flexible jumper connects the top of the riser to the vessel, using a gooseneck assembly. The top of the air can is positioned approximately 50 m below the mean water level to minimise the effects of waves and surface currents, whilst maintaining diver access. As a result of the decoupling from the vessel motion, the riser requires only a simple vessel interface, with low payload on the floater.
The Vortex Induced Motions (VIM) response of the air can in current has not well been investigated so far. It is generally believed that the fatigue is not critical due to the low current speeds and the long periods of the principal mode of response. However, uncertainties remain regarding the participation of higher modes, the interaction with the riser VIV and the possible other types of instabilities such as galloping and wake induced motions. Especially clashing can be a concern for fields with multiple riser configurations.
An exploratory research model test campaign was undertaken to study the vortex induced motions for a range of current speeds between 0.16 and 3.3 m/s. The model was constructed at scale 1:68.75, with the vertical riser truncated at 375 m water depth. Two different weights of the vertical riser were considered. The tests were carried out in a large towing tank (8 m deep, 18 m wide and 240 m long), ensuring uniform flow conditions. Mode 1 response was observed for small current speeds, with a normal ”figure-of-eight” type trajectory of the air can. At higher 1 speeds, the response became more complex with participation of higher modes and irregular wake-induced motions.