Single mobilization completes Hoover Diana's pilings, manifolds, trees
Subsea installation work, including two custom foundation piles, two manifolds, and five horizontal christmas trees, was recently completed on Exxon's Hoover/Diana deepwater field in the Gulf of Mexico. The work was undertaken by Cal Dive International (CDI), which completed the project ahead of Exxon's schedule and under budget, primarily a result of extensive prior engineering work.
Wayne Bywater, Business Development Manager for CDI, said the company received the contract from Exxon in April of 1999, and spent three months going over the detailed engineering involved in completing the work. The installation was accomplished using the CDI vessel, Uncle John.
The Uncle John is a semisubmersible construction vessel able to handle the type of drillstring installation work usually reserved for a mobile offshore drilling unit (MODU), but can afford to operate for less than the cost of contracting for a drilling rig. The Uncle John was able to accomplish the foundation pile, manifold, and christmas tree installations in one mobilization, while undertaking other work off the critical path.
Semi work platform
For several of the key installations, the semisubmersible platform is advantageous because of its extreme stability, especially in the direction of heave. It is critical that heave is kept to a minimum when stabbing the manifolds into the foundation pilings.
Often on subsea installations, where a semisubmersible is not available, it is necessary to design a special stabbing connection to cushion the mating and compensate for heave. This was not necessary for Hoover/Diana, which simplified the design and reduced cost.
Bywater said the vessel was mobilized for about 25 days working on East Breaks Blocks 945 and 946. The foundation piles were installed for the two drill-center manifolds installed about three miles apart. To properly support the manifolds it is necessary that the piles be within 2% of vertical. John Sokol, Project Manager of CDI said both piles were installed using the suction technique to an accuracy of within 1% of vertical. He said the stability of the semisubmersible hull helped with accuracy in 4,600-ft water depth.
The manifolds measured 24 ft square and weighed 61 tons and 92 tons. The larger manifold required an unusual installation process, because it was too heavy for the offshore cranes used to sling haul the first manifold.
Installation challenges
Cal Dive maneuvered one of the Hoover/Diana manifolds under the Uncle John prior to installation.
The smaller of the two manifolds was lifted off of a barge, transferred to the Uncle John, and lowered to stab into the foundation piling. Load transferring involves attach ing crane lines to the manifold, which are brought along side the installation vessel on a barge. A sling from the drillstring is also attached to the manifold.
The barge carrying the manifold is removed, and the manifold lowered by the crane until it is far enough under water that its weight transfers to the slings on the drillstring. Once the manifold is under the control of the drillstring, the crane cable is removed by ROV and the drillstring lowers the manifold to the seafloor. This installation procedure is necessary because the dimensions of the manifolds are too great to clear the vessel's moonpool.
The larger manifold exceeded the capacity of the offshore cranes and thus required an alternative approach for attachment to the drillstring. The manifold was loaded out on a small barge, guided by a tug. Once alongside the Uncle John, the barge was slowly maneuvered into place by the tug and stabilizing winches connected to custom installed pad eyes on the semisubmersible.
The barge could not pass through the end of the vessel because access was blocked by a crossmember. The Uncle John had to be ballasted down so that the barge could clear the vessel's pontoon and enter from the side. This was a tight fit and the subject of extensive motion studies.
The barge had to be guided between two columns of the hull to be centered under the moonpool. At one point, there was as little as five ft clearance for the manifold and barge as it moved under the semisubmersible. Once in place, the manifold was attached to the drillstring, lifted and lowered after removal of the barge.
Float-under method
CDI came up with an innovative solution, said Tim Arthur, Exxon Upstream Develop-ment Company Installation Manager, when they realized the Central Drill Center manifold would be too heavy for the cranes. The original plan was to use the cranes and lift each manifold. The change in weight was due to changes during the evolution of the design. The float-under method is a relatively new concept that Exxon had not used before.
This was a weather sensitive process, Arthur explained. CDI took advantage of the fact that the christmas trees could be installed when the seas were too rough to handle the manifolds. During the calm weather, CDI could set the manifolds. If the seas came up they could concentrate on the piles and trees.
While performing the installation work, CDI was able to take spool piece acoustic metrology (measurements for the jumpers). Bywater said this work was accomplished off the critical path and was another key to the success of the project.
This multitasking required about six months of up-front engineering analysis and systems integration and testing to ensure that the installation and additional work could be accomplished quickly and with only one mobilization. Sokol said there were some very in-depth motion studies conducted between the Uncle John and the barge prior to the manifold installation. In addition, remotely operated vehicles (ROV) played a critical role in this project.
ROV support
The Hoover Diana installation used two 100 hp ROVs rated to 2,000 m water depth. In addition to the Triton XL built into the Uncle John, Canyon Offshore provided a second Triton. This allowed one ROV to work while the other was being serviced or outfitted for a specific function. Sokol said this was another example of how prior planning paid off. Working in advance with Canyon, as well as Aker, who designed the foundation piles and the ROV-mounted suction skid, CDI was able to coordinate all the ROV work so that the two vehicles were able to operate in tandem around the clock. The jumper measurement work, usually requiring a separate ROV support vessel, was an example of how this coordination saved time and expense.
CDI will return to the Hoover/Diana site later this year to install three of four jumpers at the Northern Drill Center. Once the flow lines are installed by the Saipem S7000, the Uncle John will install the remaining three well jumpers and four flowline jumpers. This installation should occur next spring.
In addition to Aker and Canyon, Sokol said CDI worked with several key subcontractors on the Hoover/Diana project. MPC performed the rigging analysis. Zentech, Inc. conducted the motions analysis and relative motions analysis between the barge and the Uncle John for the critical hook up of the heavier manifold. OPE, Inc conducted the rigging analysis and finite element analysis for the design of the padeye installed on the Uncle John to help maneuver the manifold barge. Eagle Maritime Consult-ants performed the load-out analysis and tie-down design for the piles and manifolds on the material barge.
