Late last year, the world's tallest freestanding structure, the Texaco Petronius compliant tower, was installed in Viosca Knoll Block 786 in the Gulf of Mexico off the coast of Louisiana in 1,754 ft water depth. As part of the installation, a unique remote tower leveling operation was carried out.
Using an ROV and four large hydraulic cylinder jacking assemblies and controls, located at the base of the tower, the program ensured that the tower bottom section (TBS) was perfectly level prior to installing the top tower section.
The 1,754-ft Petronius compliant tower consists of three major components: the pre-installed foundation piling, the tower bottom section, and the tower top section. A module support frame supports the platform topsides. The support frame is installed on the tower top section.
The tower bottom section measures 110 ft by 110 ft in width (cross section), 1,030 ft in length, and weighs 29,000 tons. The top section tapers from 110 ft by 110 ft at the mid-tower connection point, to 60 ft by ll0 ft (cross section) at the surface. The top section is 734 ft in length and weighs 7,500 tons. The compliant characteristics of the tower are provided by 12 flexpiles, which extend upward for the first 800 ft of the bottom section.
Installation parameters
The tower installation scheme called for the bottom section to be lowered onto leveling piles. During the final lowering stage, the feet of the four hydraulic cylinders would land on top of the leveling piles, with each assembly subsequently being activated as required to react against its corresponding leveling pile, thus providing level adjustments to the tower. Upon completion of the leveling procedure, the flex pile/foundation pile connection would be permanently grouted.
The primary role of the hydraulic leveling jacks would be to provide the ability to raise or lower any one of the four legs of the bottom section up to 8 in. to achieve tower levelness. Each assembly would be extended or retracted as required to obtain a tower alignment of within 0.l° of vertical. The jacks would also be required to accommodate 10 in. of axial misalignment and 2.3 in. of angular misalignment between the foot of the jacking cylinder assembly and the top of the leveling pile.
Each jack would be equipped with a lockout collar attached to each rod. The collars would hold the rods extended at 32 in. of stroke. The collars would serve as a contingency against premature cylinder retraction, which could allow the tower to tilt. The collars would also provide a robust means for beaming loads into the tower structure rather than relying on hydraulics alone in the event of a hard landing. The collars would have to be removed before the cylinders were retracted.
Another crucial requirement was for the hydraulic leveling jacks and controls to have nearly zero leakage and be completely self-contained. In the event a storm occurred during tower installation, the ROV would have to be withdrawn and the site abandoned. If that became necessary, the hydraulic leveling jacks would have to hold the tower load in position without moving for an extended period of time. Similarly, the jacks would have to hold the tower still during the grouting operation.
The final performance requirement for the hydraulic leveling jacks was to retract 24 in. free and clear of the top of the shim piles. This retraction would enable the leveling jacks to be physically decoupled from the permanent tower- to-seafloor connection created by the 12 flex piles prior to the installation of the tower top section.
Leveling system
Bardex Corp. provided McDermott Engineer ing Houston LLC, the compliant tower fabricator's engineering contractor, with the leveling system that satisfied requirements. The system components included four hydraulic cylinder jacks (one for each of the bottom section legs), individual control panels attached to each leveling jack, a primary control panel serving all four leveling jacks, and a unique power unit carried under the belly of one of the Oceaneering Magnum ROVs used to remotely operate the system.
The leveling jacks and panels were installed on the bottom section at McDermott's fabrication yard in Morgan City, Louisiana, and were designed to survive the tow-out and launch at the tower installation site.
The required lift and hold capacity of the leveling jacks was such that a high-pressure hydraulic system design was selected to minimize the already grand scale. Each jack was rated for 1,000-ton lift capacity at 5,000 psi. Holding capacity was 1,500 tons. Piston seal leakage was less than 20 drops per minute. This equates to less than 0.1 in. of movement per day.
To provide visual feedback to the remotely operated vehicle (ROV) operator on how high or low the tower leg had been moved, a stroke indicator bar was connected to the cylinder rod of each assembly. A permanent lockout feature of the cylinder rod in the retracted position was accomplished by means of a large pawl connected to the cylinder, which could be used to capture a wagon wheel-like structure connected to the rod.
To achieve maximum reliability, the design of the control panels was kept as simple as possible. There were no relief valves or load holding valves. The primary panel was equipped with an API 17D hot stab fluid receptacle, four directional control valves, and four pressure gauges. Each local control panel was equipped with a hot stab and three shear seal valves (one for directional control and two for isolation from the primary control panel). The panels were designed and fabricated by Oceaneering Intervention Engineering and used a Scana rotator shear seal valve.
A key element of the hydraulic leveling system was the ROV equipped with the hydraulic power unit. Because of the large volume of fluid required to extend each cylinder (40 gal. for 8 in. of extension) a normal ROV power unit would have been grossly undersized. The hydraulic ROV-carried skid by Oceaneering provided an alternative to unwieldy hoses.
System implementation
During the November 9-17, 1998 period, the tower bottom section was launched, lowered, and set down onto the leveling piles. The Oceaneering ROV with on-board hydraulic power pack (ROV 1) was positioned at the leveling system's primary control panel. A second Oceaneering ROV (ROV 2) was stationed to observe at the leveling jack for tower leg A1.
At the pilot's command, ROV 1 plugged into the hot stab receptacle on the primary control panel, energized the power pack, and shifted the A1 control valve to the extend position. The jack was allowed to extend for one minute to achieve an inch of jack extension. The inch was required to facilitate removal of the lockout collar.
With the first jack extended, ROV 2 flew to the next jack and the operation was repeated. When the jack for tower leg B3 was activated, a leak was detected at the primary control panel and the system was shut down. ROV 1 then flew to leg B3's local control panel, shifted the isolation valve for local control and extended the jack one inch. Prior to loadout, it was determined through survey work that leg A3 was approximately one inch in elevation above the other three legs, so the jack for leg A3 was extended an additional inch.
The initial leveling operation was completed within four hours. This included an ROV trip to the surface to replace damaged hot stab seals. Survey work soon confirmed that the tower bottom section was resting at 0.03° from true vertical. This was well within the target of 0.1° and, as a result, the leveling operation was completed and grouting was begun.
Bottom section verticality was monitored throughout the grouting operation with no changes detected. On November 23, the grout was cured and the okay was given to retract the jacks as there was no observable change in stroke on the four jacks over the six days of load holding.
At that point, the ROVs were deployed and the lockout collar cables were cut and the collars were removed. Each of the four jacks was then retracted until the foot on the jacking assembly was visibly clear of the leveling pile. The jacks were then completely retracted and then extended 2 inches to engage the lockout pawl.
The Petronius tower leveling project demonstrated that subsea loads can be moved in a short period of time using hydraulic actuators. Further, the 0.03° from perfectly vertical reading exceeded the 0.1° verticality target and demonstrates the precision that such actuators can deliver. Lastly, it has been shown that hydraulic actuators will hold such loads in position without any observable creep.
