Method and apparatus for connecting an underwater flexible riser to a structure on the surface

A method for connecting an underwater flexible riser to a surface structure disposed at a water surface, using an apparatus comprising a guide device connected to a lower end of a hollow rigid member extending downwardly from the surface structure, said guide device having a cylindrical upper portion and a flared lower portion shaped as a frustum of a cone;

• a coupling device encircling and detachably fastened with shearing pins to a connection end of said underwater flexible riser, said coupling device having a cylindrical upper portion and a lower portion shaped as a frustum of a cone;
• a locking structure comprising a plurality of locking pawls for locking said guide device to said coupling device, said locking pawls being pivotable with respect to said guide device and each having a claw at a lower end thereof;
• a terminal connector fastened to the connection end of said underwater flexible riser;
• a traction head connected to an upper portion of said terminal connector;
• and at least one pulling cable extending from in the surface structure and having a lower end secured to said traction head, said at least one pulling cable passing through said hollow rigid member and said guide device and being adapted to raise said underwater flexible riser from a sea bed to a final destination located on the surface structure, wherein the method comprises the following steps:
• applying traction to said at least one pulling cable so as to raise an assembly formed by said traction head, said terminal connector, said coupling device and said connection end of said underwater flexible riser towards the surface structure;
• and
• continuing said raising movement so that the flared lower portion of said guide device acts as a guide for entry of said cylindrical upper portion of said coupling device into said guide device;
• and wherein said frustoconical lower portion of said coupling device touches each claw of said plurality of locking pawls thereby to push said claw outwardly causing a pivotal movement in a first direction of a respective locking pawl of said plurality of locking pawls, thereby enabling said frustoconical lower portion of said coupling device to continue its raising movement;
• wherein as soon as a lowest portion of said frustoconical lower portion of said coupling device is raised beyond each said claw, said respective locking pawl pivots in a second direction opposite to said first direction, causing each claw to move inwardly and thereby engage said lowest portion of said frustoconical lower portion of said coupling device, whereby said flared lower portion of said guide device acts as a bumper to said frustoconical lower portion of said coupling device and said coupling device becomes securely engaged and locked into said guide device;
• and
• wherein as said traction continues to be applied to said at least one pulling cable, each of said shearing pins is sheared, thereby enabling said assembly to continue its raising movement towards the surface structure.

A method for connecting an underwater flexible riser to a surface structure disposed at a water surface using an apparatus comprising a guide device connected to a lower end of a hollow rigid member extending downwardly from the surface structure, said guide device having a cylindrical upper portion and a flared lower portion shaped as a frustum of a cone;

• a coupling device encircling and being detachably fastened with shearing pins to a connection end of said underwater flexible riser, said coupling device having a cylindrical upper portion and a lower portion shaped as a frustum of a cone;
• a locking structure comprising a plurality of locking pawls for locking said guide device to said coupling device, said locking pawls being pivotable with resect to said guide device, and each having a claw at a lower end thereof;
• a plurality of movement-limiting assemblies, each linked to a respective locking pawl for limiting pivotable movement thereof;
• a terminal connector fastened to a connection end of said underwater flexible riser;
• a traction head connected to an upper portion of said terminal connector;
• and at least one pulling cable extending from the surface structure and having a lower end secured to said traction head, said at least one pulling cable passing through said guide device and being adapted to raise said underwater flexible riser from a sea bed to a final destination located on the surface structure, wherein the method comprises the following steps:
• applying traction to said at least one pulling cable so as to raise an assembly formed by said traction head, said terminal connector, said coupling device and said connection end of said underwater flexible riser towards the surface structure;
• continuing said raising movement so that the flared lower portion of said guide device initially acts as a guide for the entrance of the upper cylindrical portion of said coupling device into said guide device;
• wherein during said raising movement said frustoconical lower portion of said coupling device touches each claw of said plurality of locking pawls, pushing each said claw outwardly causing a pivotal movement in a first direction of a respective locking pawl, thereby enabling said frustoconical lower portion of said coupling device to continue its raising movement;
• wherein as said raising movement of said coupling device progresses, each said movement-limiting assembly impedes the continuation of said pivotal movement in said first direction, and thereafter a contact-force component between said lower portion of said coupling device and said each claw causes each locking pawl to move in an oblique direction with respect to an axis of symmetry of said guide device, thereby allowing said coupling device to continue its raising movement;
• wherein as soon as a lowest portion of said frustoconical lower portion of said coupling device raises beyond each said claw, said locking pawls elastically pivot in a second direction opposite to said first direction, causing each claw to move inwardly, thereby engaging said lowest portion of said frustoconical lower portion of said coupling device, and wherein said flared lower portion of said guide device acts as a bumper to said frustoconical lower portion of said coupling device, whereby said coupling device becomes securely engaged and locked into said guide device;
• and
• as said traction continues to be applied to said at least one pulling cable, each shearing pin fastening said coupling device to said underwater flexible riser is sheared, thereby enabling said assembly to continue its raising movement towards the surface structure.