Flexible Risers are composed of multiple spiral laid materials as shown in Figure 4, the flexible riser demonstrates excellent bending capacity while still possessing exceptional strength, making it extremely versatile with regards to application. Originally designed for flowline applications between vessels, the flexible riser has gained popularity in deep water due to its ability to withstand significant dynamic environments making it an excellent choice for harsh sea conditions.
Source: https://hub.globalccsinstitute.com/sites/default/files/publications/24452/advanced/fig-020.jpg
Installation methods are also a positive aspect of flexible flowlines in that normal installation campaigns can typically be completed much more quickly than other risers due to the high spooling capacity and offload speeds that can average up to 500 meters per hour. The ability to be wet stored (material choice permitting) prior to installation also proves a benefit of the flexible pipe in that it allows for schedule flexibility during an offshore installation campaign.
Throughout the design phase of flexible pipe, careful consideration must be made with regards to the hydrostatic pressure that will be observed at full depth. As depth increases, the pressure differential becomes greater and supporting layers, such as the pressure sheath and armor layers, must be designed with a thickness that is capable of withstanding the associated forces of deep water. If one of these supporting layers were to not be adequately designed to withstand these forces, the pipe would be crushed as it was lowered to the sea floor. Sufficient designs, however, have proven the flexible riser to be applicable to depths of 10,000 feet and operating pressures of 10,000 psi at 300oF.
A design characteristic that may deter the use of flexible pipe in some applications is the need for bend stiffeners or Bell mouths. Utilized to ensure that the stress throughout the joint is kept at a minimum, these stiffening methods should always be considered during the design process though topside space requirements may cause complications. A method of venting the annular space of the pipe layers should also be implemented to ensure any gas migration through the pressure sheath can be vented at the topside.
Source: http://subseaworldnews.com/wp-content/uploads/2012/08/4Subsea-to-Monitor-Statoils-Flexible-Risers-Norway.jpg
Source:
- http://www.gateinc.com/gatekeeper/gat2004-gkp-2015-02
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