This paper describes the DISPS Project design, development and prototype testing, in the dry, of three diverless tie-in systems for application to flowlines of 4 - 12 inches diameter in water depths of 350 - 750 metres. Full scale simulation of direct pull-in, lateral deflection and vertical stab techniques using two tools for flowline pull-in and connection was undertaken and the design limits established.
Within the DISPS development, diverless flowline tie-in concepts were recognised, as key flowline issues. A study was undertaken and this included the evaluation of possible flowline installation methods, pipeline materials and available connection hardware which would be suitable for diameters of 4 to 12 inches in water depths of 350 to 750 metres.
The main conclusion of this study was that diverless flowline tie-ins play a considerably greater role in dictating field development and installation strategies than flowline tie-ins in conventional diver depths. These diverless tie-in techniques warranted a design philosophy which favoured a highly adaptable system over a case-specific system.
The following engineering targets were established for the DISPS development of diverless flowline tie-ins.
The flowline tie-in bays within the template or satellite well structure should be capable of accommodating any of four types of tie-in:
Direct Pull-in for Steel Pipe (Figure 1)
Lateral Deflection for Steel Pipe (Figure 2)
Vertica1 Stab and Hinge-over for Flexible Pipe (Figure 3)
Slack Loop Pull-in for Flexible Pipe.
The tie-in system should be compatible with flowline installation by laying, reeling or towing.
The alignment mechanism should be by structural interaction between a passive guide and pull head without manipulation of the flowline.
All pull-ins should be achievable with a single pull wire acting through the pipe centreline.
Recoverable tools should be used for pull-in and make-up of the mechanical connector. Both alignment modules should be compatible with these tools.
The pull-in and make-up procedure should be performed in discrete steps and allow support vessel demobilisation after docking. This enables a smaller diving support vessel to be used for subsequent connector make-up.
The alignment components should be modularised such that finalisation of the template or satellite well configuration can be deferred prior to deployment. This allows field development plans to remain flexible while the seabed structures arebeing fabricated without compromising template/flowline interface areas.
The pullhead should allow pigging and hydrostatic testing of the flowline before the connection is made to inboard piping on the subsea structure. This allows the pipe-lay contractor to satisfy contractual obligations even if the final connection is deferred. In keeping with this objective, the flowline remains fixed after docking, with final stroking of the connector by movement of the inboard piping.