Tom Schmitz
Cooper Cameron
Two flexible risers have been successfully installed on Conoco's Heidrun tension-leg platform using a new connector system supplied by Cooper Oil Tool. The risers will transport crude oil from pipeline end manifolds (PLEM's) which terminate Heidrun's two 16-in. export links to a pair of turret moored tankers.
Each PLEM, located in about 1,100 ft water depth, has an upward-facing hub. A collet connector, with a 15 elbow, joints the riser's lower end to the hub. The flexible riser has a bend restrictor at its seafloor end and an array of buoyancy modules to configure the riser in a lazy-S on its path to a submerged turret buoy (STB). The STB docks with the tanker's turret bottom to provide moring and to allow flexible riser connection to the tanker's piping system.
Crude oil produced from the platform flows to one of the tankers. When a tanker is filled, its STB is released, and the tanker moves off to its unloading site. Since the system does not use an intermediate stage vessel, it is called direct shuttle loading (DSL).
ROV assist
After land assembly of a pipeline, during which the riser mandrel was welded to the PLEM's 90_ elbow, a pig launcher was locked and sealed to the mandrel. The platform end of the pipeline was also sealed. Pipeline and PLEM assemblies were then towed at controlled depth to final destination points where platform connections were made and each PLEM was pile- anchored subsea. The pig launcher and temporary connector were then retrieved. An installation tool was lowered down guidelines and latched to four lockdown alignment posts on the PLEM. Guide posts and guidelines were then removed.
The installation tool has guide surfaces for final positioning and orientation of a running tool, plus two sheaves through which pull-down wires were reaved.
On the construction vessel, the running tool was mounted onto the riser connector, and a clump weight was connected via cables to the running tool's pull-down cylinder rods. The flexible riser, with running tool and connector on its end, was reeled overboard through the moon pool as buoyancy devices were attached to the riser lower end.
When the clump weight reached the sea floor, pull-down loads were transferred from the weight to cables on the installation tool. Riser offloading continued as pull-down cables were reeled in, until the running tool reached the installation tool on the PLEM. Slack given the riser decoupled it from the construction vessel.
When the running tool reached the PLEM, pull-down cylinder rod ends engaged locking sockets in the installation tool, and pull-down cables were slacked. Here, the running tool and riser were two ft above their final positions, and the riser-connector axis was about 15 from vertical because of the matching riser-end elbow. Retracting the pull-down cylinders brought the riser into contact and alignment with the PLEM mandrel. The connection was then made and hydro-tested. The running tool and installation tool were released and lifted to the surface.
Connector
The connector, which locks the flexible riser to the PLEM mandrel as outlined, has several features:
- A secondary lock sets automatically upon connector lock, and releases with running tool action.
- Connection can be released via manual override.
- Hydraulic operation is by R0V supply through a subsea panel on the running tool; there is no need for a separate umbilical.
- The primary metal-to-metal seal can be tested after installation.
- There is an inhibitor-injection system for space around segments.
- All hydraulics are retrieved through the removable running tool system.
- Dual AX gasket prep is provided for use with primary and contingency AX gaskets.
Locking is achieved by action of segments, which clamp the connector body to the mandrel hub. Segments rotate inward as the actuator ring is forced down by the running tool. Downward movement of the actuator ring stops when ring resistance equals the tool's hydraulic locking force. Because of the tapered segments and hub geometry, a pre-load sufficient to prevent hub face separation under worst-case conditions is created.
The taper, combined with segment and locking ring coatings, makes the system self-locking. To prevent inadvertent loss of pre-load or unlocking, a secondary lock is also provided.
Connector testing
The connector has two seal test ports to allow testing with nitrogen stored in a bottle on the running tool. Seal testing for the connection between the upper flange and lower riser joint is done in the annular space between BX gasket and an outer O-ring face seal. This test is normally made on the surface after is attached to its connector flange.
The AX gasket seal, between the riser connector and mandrel, is tested through a port in the connector body, which communicates with the annular space between the AX gasket and an outer 0-ring test seal. Since the seal is normally energized by riser internal bore pressure, the external test pressure need only be enough to verify existence of an initial seal. The self-energizing gasket will seal for rated internal pressure. A contingency gasket is also provided, to be used if the hub's primary gasket surface is damaged.
Tom Schmitz is project manager for Cooper Oil Tool, a Cooper Cameron Corporation company.
Copyright 1995 Offshore. All Rights Reserved.
