Increasingly, large-scale deepwater develop-ments are being based around a Spar, TLP, or FPSO linked to 30-100 km of insulated flowlines. A big concern for the operator is the potential impact of unscheduled prod-uction shutdowns. In these cases, the flow assurance system must provide sufficient time for the operator to react while preventing formation of hydrates or wax.
Typically, there will be a "no touch" time of around two to six hours during which production can be resumed without further action on the pipelines. Then follows a longer period (10-20 hrs) in which hydrate-prone fluids are ushered out of the flowlines and replaced by inert fluids. In either case, the brevity of the no-touch period rarely allows replacement equipment to be drafted in, so the production facilities must essentially be autonomous, with back-up mandatory for all-important items.
ITP InTerPipe in Louveciennes, France, and Houston, and J. Ray McDermott have devised an alternative solution that could extend the cool-down time substantially, leading in turn to capex savings through reduced back-up requirements. McPipe is an insulated flowline system that employs a phase change material (PCM) that surrounds a carrier pipe and is insulated by ITP's proprietary microporous insulation material, Izoflex. The latter has been selected for the Bonga development offshore Nigeria in 1,100 m of water, having previously proven its worth in the Tchibeli pipe-in-pipe system offshore Congo.
In normal operation, McPipe's PCM stores a large amount of heat that is released slowly when production is halted. The system can be installed by numerous existing barges in S or J-lay mode. Aside from heat-traced systems, it is claimed to be the only one that can provide cool-down periods beyond 48 hrs, with the added advantage of being entirely passive.
The extended cool-down capability also makes it attractive for ultra-long step-outs and for hurricane-induced shutdowns, as the system could maintain production right up to the hurricane's approach.
In the Gulf of Mexico, where hurricane scenarios require a safe shutdown of three to five days, no specific action would be required on the flowlines. The designers say that the production fluids could be safely left in the flowlines for up to a week, with none of the production losses traditionally associated with shut-down/start-up operations. On brownfield developments, the McPipe's flexibility would also allow sequential circulation of new and existing flowlines, with reduced modification requirements to the main production facilities.
The McPipe system comprises three concentric pipes. Of these, the inner pipe is thick-walled and designed for containing the wellstream pressure. The intermediate pipe is thin-walled, and welded to the inner pipe at its extremities (this encapsulates the PCM). Finally, the outer pipe is designed for mechanical performance for installation and operational activities.
Offshore, flowlines are assembled by welding the inner pipe of each double or quad joint in the S-lay ramp or J-lay tower, followed by installation of an ITP-designed, thermally insulated sleeve to secure both thermal insulation and mechanical strength at this location. Among the advantages of this system for deepwater developments are that integrity of the insulation material is maintained and lay tension can be accurately determined, whatever the installation method.
Choice of the phase-change material is dictated by the well stream's maximum temperature and the critical temperature at which flow assurance can no longer be guaranteed. The latter is generally 19-22° C. The PCM should have a melting point slightly above the critical temperature. Mineral-based PCMs are recommended. Those containing hydrated salts will release 600-700 MJ/cu m for a temperature variation of 20-240° C, compared to just 300-450 MJ/cu m for hydrocarbon-based PCMs.
From McDermott/ITP's viewpoint, the preferred PCM is one used in brine by the drilling industry, so its aging properties are widely understood. Tests on this product confirm that encapsulation of the PCM between the flowline and the intermediate liner pipe limit interactions substantially, and that thermal performance remains unaffected after more than 4,000 cycles. This would exceed requirements for flowlines subjected to cool-down of one week or more. Over a field lifespan of 20-25 years, a maximum of 500 cycles is anticipated.
Izoflex was viewed as the most cost-effective insulation material for the outer diameter pipe as it combines strong thermal performance with good mechanical behavior. Izoflex is designed to reduce gaseous conduction by preventing captured gas molecules from colliding.
This result is achieved through the material's cell sizes being smaller than the mean free path of the gas molecules, causing the molecules to rebound inside the pipe walls rather than interacting. In turn, this leads to thermal insulation up to a factor of 10 greater than yielded by conventional syntactic foams. Thermal tests during the Tchibeli development demonstrated a U-value of 0.4 W/(m2.K) with only 15 mm of Izoflex around a 10-in. pipeline.
Typically, a McPipe would be insulated to 0.7 W/(m2K) or better. Minimum practical thickness of the PCM would be around 10 mm, leading to a cool-down time of four to five days. For a deepwater application, the outer pipe would be around 4-5-in. bigger than the flowline, i.e., an 8-in./12-in., 10-in./16-in., or 14-in./ 18-in. pipe arrangement. This configuration would provide cool-down times of one week or better, plus a U-value of 0.5 W/(m2K).
McPipe, by allowing optimization of cool-down time by up to 10-15 days, should be compatible with safe operation of 100-160-km flowlines with typical transit times of one to two days (six in degraded mode).
McPipe is currently being evaluated under a joint industry project for properties such as distribution of stress on the swaged ends, passage of the pipe in the sag bend area, maximum allowable bending moment, and maximum horizontal free span once installed. McDermott and ITP have also built and tested a full-scale McPipe joint. The mainline section was tested with an empty inner pipe to represent the most severe cool-down condition, as there would be no added heat capacity from the transported fluids.
Measured U-value was 0.45 W/(m2.K). Average external temperature during the test was 7.4° C, meaning that if the test had been performed in a 4° C environment, cool-down time would have been 12-13 days. The pipe was also cycled thermally three times, and the repeat experiments all exhibited a similar performance.
McPipe has been proposed for the Rosa/Lirio subsea development in Angolan deepwater block 17. ;
Cut-away of Prosernat's Split Flow separator.
