Comparison Of Mooring Schemes For Subsea Suspended Manifold And Mooring Analysis Under Different Working Conditions

The main function of subsea manifold is to provide gathering points and distribution points for crude oil transportation,gas injection,water injection,etc.It plays an important role in optimizing the layout of subsea pipelines and reducing the number of pipelines used.So it is an core component in subsea production system.The traditional subsea manifold is hard to be discarded and recovered,and the foundation of the manifold has certain requirements for the terrain smoothness of the seabed.Therefore,the concept of subsea suspended manifold is proposed.The general layout concept of the subsea suspended manifold is to create a deep-water full-space layout solution which is easy to be recycled.Compared with the traditional subsea manifold,the subsea suspended manifold replaces the foundation structure with buoyancy tank at the bottom of the manifold body.At the same time,the subsea manifold is moored by the mooring system.The scheme design and mooring analysis of mooring system is essential for the stability of subsea suspended manifold.In this paper,the shape and force distribution of the mooring chain are deduced through the catenary equation,and the mathematical model is established to solve the chain tension distribution and displacement of the underwater suspension manifold,which will provide a reference for the preliminary design of the mooring system.In order to study the motion characteristics of mooring chain more accurately,lumped mass method and finite element method based on small deformation linear elasticity theory are used to analyze the dynamic response of mooring chain.A preliminary overall design of the subsea suspended manifold mooring system is conducted under the guidance of mooring criteria such as DNV and API.On the basis of mooring schemes above,the motion response of the subsea suspended manifold and tension distribution of the mooring chains and the flexible jumpers in three mooring schemes are simulated respectively through Orca Flex.A steep wave layout of flexible riser is designed based on the arrangement characteristics of subsea suspended manifold.The overall shape of the flexible jumper is optimized by changing the position and diameter of the buoyancy block.In the end,the motion response of subsea suspended manifold in three different mooring methods are simulated under different conditions such as current velocities,current directions,transport medium and one mooring line breakage,and then a mooring scheme that are most suitable for subsea suspended manifold is selected from the analysis results.Subsea suspended manifold is a new design scheme for subsea manifold,which can better adapt to various complex seabed environment and realize the recovery and utilization of subsea manifold.Three mooring schemes of subsea suspended manifold are proposed in this paper,and the catenary mooring and vertical tension mooring are common mooring schemes.Especially the flexible jumper is adopted as mooring and oil transportation at the same time when the seabed environment is stable,which may shed light on the future mooring scheme for subsea structures.