The King flowlines, located in the Mississippi Canyon area of the Gulf of Mexico, are dual Pipe-in-Pipe (PIP), approximately 17 miles (27 km) long, installed in 5330 feet (1625 m) maximum water depth.
The novel PIP arrangement circulates heat medium in the annular space between the 8" and 12" pipes to achieve active heating over the entire length. Insulation around the outer pipe retains system heat.
PIP active heating and insulation results in large thermal expansion forces, causing local lateral deflections (buckles) in the unburied flowlines. Without active buckle management, the resultant peak bending strains and cyclic stresses at buckle locations were unacceptable. This paper describes the sequence of events leading to the selection of vertical upset supports, called "sleepers", to initiate buckling at selected locations and control subsequent peak and cyclic buckling stresses. Results from installation and system start-up are presented.
The use of sleepers to control thermal expansion is a unique and original development, which has potential applications for any unburied pipeline where expansion forces are high enough to ultimately fail the pipeline if uncontrolled buckling occurs.
The King flowline system is located in the Gulf of Mexico approximately 65 miles east of the Mississippi River outlet and 100 miles due south of Mobile, Alabama. An artistic rendition of the system configuration is depicted in Figure 1.
The King flowline system was designed during period 1999-2001, and installed during 2nd half of 2001.
Figure 1 King Pipeline System Configuration(Available in full paper)
The King flowline system was initially envisioned  (early 1999) as a buried flowline system utilizing burial to restrain flowline thermal expansion and provide additional thermal insulation value, reducing the external insulation thickness on the outer 12" pipe. During the second half of 1999, design recognized that burial would be difficult for the very soft GoM (Gulf of Mexico) soils along the King flowlines routes, generally < 50 psf (2.4 kPa). Early design checks indicated that upheaval buckling would be a severe challenge to avoid or mitigate. However, it was suspected that burial might have significant cost benefit by reducing insulation requirements for the flowlines. Consequently, installation was awarded with burial as an option. During late 1999, and early 2000 the installation of King Flowlines was competitively bid and awarded to Allseas USA. Installation was based on S-lay installation with Allseas' vessel 'Solitaire'.
Further engineering and soils investigations (described in this paper) confirmed the upheaval buckling problem, and indicated that burial using excavated material backfill for the trench would require an extensive consolidation period (months to years!) to provide any effective overburden restraint to upheaval buckling. These facts strongly supported elimination of burial. Additionally, upon procuring insulation coating in 1st half of 2000, it became apparent that the incremental cost of additional coating, to avoid burial, was essentially the same cost as burial. This removed any incentive for burial of the King flowlines.