Installation of flowlines and umbilicals in July-September 2000 in the Campos basin offshore Brazil set a depth record for rigid flowlines. The project for Petrobras connected three wells in the Roncador field back to the P-36 floating production platform.
Three pairs of rigid flowlines (4 and 6 in.) were installed in water depths to 1,870 m by the reel vessel DSND Skandi Navica. The purpose-built, flexible pipe installation vessel DSND Lochnagar installed flexible jumpers and risers to tie-in the rigid flowlines to the wellheads and platform, respectively. In addition, an electro-hydraulic umbilical for each of the wells was installed.
All subsea connections were performed by a vertical connection system that is widely used and specified by Petrobras for pipeline-end terminations (PLETs) and wellhead structures. Some of the flexible pipes and umbilicals used new designs of vertical connection systems.
The first of two articles described the pipeline system and methods used for its installation (OGJ, Sept. 24, 2001. p 70).
This concluding article evaluates the selected design of the pipeline system and the installation method used and comments on alternative and improved solutions that could be considered for future similar field developments.
Possible improved solutions
The initiation "dead man" anchor wires used on this project were 3,700 m long. Installing the anchors and laying out these very long wires were costly in both material and installation.
One alternative would be to perform a free-hanging pipe initiation with a small ROV-operated suction anchor or clump weight attached by a strop to the pipeline start-up head. The anchor would need to be sized and designed to resist the bottom tension during layaway of the pipeline, which was less than 12 tonnes for the 6-in. pipeline with PLET, using a top lay angle of 80°.
Depending on free access to the wellhead, an even more attractive solution would seem to be the use of a stab and hinge-over tool directly onto the wellhead.
Even if the initiation operation would need to include time for stroking and connecting the flowline to the wellhead, this method would also exclude the requirement for connection jumpers and pipeline PLETs and would represent a significant cost savings.
This method requires that the well be completed and ready for operation and have no drilling vessel over it restricting access.
For connecting the wellhead, a swivel, similar to the PLET swivels discussed in Part 1, could be built into the gooseneck of the vertical connection module (VCM) in order to eliminate rotational movement and misalignment of the VCM due to twisting of the flexibles.
Study should also look at increasing the length of the jumpers to allow more flexibility for the maneuvering and landing of the second connection, considering jumper characteristics.
The use of an ROV-operated horizontal connection system for the jumpers would also eliminate problems due to twisting of the flexible pipes. Such systems are available from several suppliers and have a good track record in North Sea projects.
The alternative of using rigid spools instead of flexible jumpers also seems attractive. The requirements for accurate survey and the consequences of a spool having incorrect length need to be considered for evaluation of this alternative.
Impact on design and cost of subsea equipment must be evaluated in detail for such alternative tie-in methods.
Pipe connections
More detailed soil information and more time for optimization calculations and design of the PLETs would lead to a reduced size for these structures. Reduced size and weight of the PLET would also require less buoyancy, which is very costly for use in 2,000 m of water.
Time and risk for the handling and deploying PLETs offshore could be significantly reduced with reduced sizes and weights. The PLETs were attached to rather slender pipeline structures with low torsional stiffness, and large buoyancy elements were used for uprighting. The effect of the swivels should be studied.
Depending on contractor experience and flexibility allowed by the subsea connection system, an alternative ROV tie-in system could allow the flexibles to be installed from the rigid-pipe lay vessel during start-up of pipelay.
An auxiliary reel with the flexible jumpers would be required from which they could be initiated into the water and hung-off ready for connection to the rigid pipe. The rigid-flexible connector could then be a standard flange connection.
The flexible jumper would be laid out straight in line with the rigid pipeline for later reconfiguration and tie-in by an ROV from the support vessel.
A cost comparison needs to be conducted considering:
- Reduced time on the rigid lay vessel due to easier handling of a flexible jumper compared to a PLET. No cost for PLETs and no installation time for the flexible lay vessel.
- Tie-in time for the support vessel. Cost for an auxiliary powered reel on the rigid lay vessel. The costs need to be identified for mobilization and hire of the ROV tie-in system and purchase of tie-in equipment. Cost and complexity for potential future jumper replacement.
With these factors for this project in mind, it would seem to be beneficial to study ROV based tie-in systems for future similar projects.
At the platform side, the rigid pipelines could be terminated with a standard flange and laid down with a blind flange with recovery rigging. The flexible lay vessel would come in at a later stage and recover the rigid pipeline to the stern hang-off table and connect the riser at surface using standard bolted flange. The lay of the risers would continue, and hand-over to the platform would be according to normal procedure.
It is envisioned that this operation would be less time consuming than use of standard VCMs. Also, there would be no need for PLETs at the platform side for this alternative, and hence this alternative would represent a clear cost saving for a future similar project development phase. It should be noted, however, that a solution with no PLETs will complicate future potential replacements of flexible pipe sections.
All-rigid solution; umbilical connections
Developing these ideas further would be to introduce an all-rigid solution for projects in which rigid risers (steel, titanium, etc.) are feasible as an alternative to flexibles. This would mean installation of steel catenary risers (SCRs) instead of flexible risers in combination with either rigid spools or a direct stab and hinge-over arrangement at the subsea end.
SCRs installed from reel ships seem to be winning more recognition, and several applications are already in service. DSND actually installed two partly reeled SCRs to the Petrobras P-36 platform just ahead of this project, and several other applications are in service in the US Gulf of Mexico.
As mentioned, installation of SCRs would require the platform topside be prepared for other top hang-off angles than is the case for flexible risers.
Moreover, it seems to be an obvious proposal to reduce offshore installation time and manufacturing costs by supplying umbilicals in a single continuous length with no need for offshore connections.
The average time spent on this project for each connection operation was in the order of 12 hr. In addition, a significant amount of time was taken up by failing electrical connectors leading to recovery of laid sections and offshore re-termination of these connectors.
A continuous-length umbilical would also provide a more reliable control system for the operation life of the field. Continuous-length umbilicals will put higher demands on manufacturers as well as installation vessel capacities and will also limit the flexibility to reposition wellheads.
