Terra Nova system designed for quick release, iceberg scouring

April 1, 1998
The Terra Nova Field will host 24 wells drilled through seven subsea templates from a separate conventional drilling unit. [48,379 bytes] Terra Nova's turret system will be the most sophisticated and complicated turret system ever constructed, according to Sofec's Ron Mack. [18,040 bytes]

FPSO spider, glory holes, trenches are unique

William Furlow
Technology Editor
The Grand Banks area in the Western Atlantic Ocean off Newfoundland features some of the roughest weather in the world, including sub-zero temperatures and glacial activities. Considered a frontier theater, the area does not have the necessary infrastructure to support extensive offshore development. These factors all contributed to the technology choices made by the Terra Nova Alliance.

The Terra Nova Field, 350 km off St John's, Newfoundland, lies in 100 meters of water. The field is made up of three fault blocks: Graben, East Flank, and Far East. The reservoir sands are located at a depth of about 3,200 meters subsea. Five delineation wells were drilled on the field and identified five major and two minor oil sands. Total reserve estimates for the field run close to 1 billion bbl of oil. Recoverable reserves are 300-400 million bbl. The productive life of the field is 15-18 years. Total project cost has been put at $4.5 billion.

Development plan

The field will host about 24 wells which will be drilled through seven subsea templates from a separate conventional drilling unit. The semisubmersible Transocean Explorer has been contracted by Petro-Canada, operator of the field, for a two-year period with three one-year options. Drilling will begin this spring. The semi is undergoing a C$58-74 million refit and upgrade prior to commencing drilling operations.

FMC has begun design and engineering work on the subsea completion system and FPSO turret mooring and manifold system for the project. The first phase of development involved six subsea wells and manifolds, in addition to the FPSO.

The 950-ft, double-hulled FPSO will be designed with a maximum capacity of 125,000 b/d and average annual peak production of 100,000 b/d. The vessel will be capable of injecting up to 270,000 b/d of sea water and compress 300 MMcf/d of gas for gas lift and pressure maintenance.

Coflexip Stena Offshore (CSO) will engineer, manufacture, and install more than 40 km of flexible flowlines, control umbilicals, and dynamic risers as well as managing the subsea construction and installation. The flexible flowlines, risers, and control umbilicals will be installed by one of CSO's subsea construction and installation vessels. A second vessel will be deployed to perform diverless tie-ins and flowline burial. This installation is scheduled for mid-2000.

The oil will flow from these subsea production trees through flexible flowlines and risers to the floating production, storage, and offloading (FPSO) vessel. The flowlines will be trenched into the seafloor to protect them from passing icebergs. The wells will be protected from iceberg scour by four uncased glory holes dredged into the seafloor.

Floating production is one of the technologies that make frontier fields economically viable. Without the pipeline and refinery infrastructure of a more mature market, Canada's eastern offshore theater would be facing prohibitively high front-end start-up costs.

Likewise, the existence of the giant Hibernia Field, located near Terra Nova, ensures the presence of support facilities and skilled workers in Newfoundland who will be available for this project. By taking advantage of the facilities and workers already in place, the development of Terra Nova becomes increasingly economical.

Design challenges

The Terra Nova field is located in some of the most hostile environmental conditions of the world. Wave heights often run up to 52 ft and winds blow at up to 77 knots. Surface currents average 1.7 knots. The water depth at the site is only 312 ft.

According to Sofec's Alliance Manager Ron Mack, with the combination of severe weather, shallow water, a large vessel, a large number of risers, and the requirement to disconnect for icebergs, Sofec's turret system faces unique design challenges.

This relatively shallow water, while commonly considered an asset to fixed structures, is a definite challenge for a single point mooring with flexible risers. Typically, the vessel in deepwater can move further off station without damage to the risers and flowlines, but in such a shallow environment, the risers are shorter and the changes in station more pronounced.

The combined second order drift motion and the wave frequency vessel motions had to be kept within the tolerances of the flexible risers. In this case, close coordination between alliance partners - Sofec, Brown & Root, Coflexip - determined the optimum solution for the mooring system, the vessel's thruster system, and the constraints defined by the flexible risers.

Further, despite having the ability to disconnect, Petro-Canada required that the vessel remain on station through a 100-year storm condition. This resulted in a fairly heavy mooring system, consisting of nine anchor legs using 5.75-in. studless chain.

Despite the subsea manifolds used in this development, there are still 17 risers to contend with, including production and injection risers, and control umbilicals. The turret is actually designed for 19 risers. The two spare riser slots will be used later in the development, when the Far East reservoir is tied into the FPSO.

The combined requirements of the heavy anchor legs, the many risers, and the mandate required to disconnect both the moorings and the risers simultaneously, results in a unique disconnect system.

Disconnect criteria

Mack said that Sofec originally developed the turret with the idea that they would have one disconnect per year due to iceberg conditions. He said that Petro-Canada put the figure at closer to one disconnect every three years.

Regardless of the frequency, Petro-Canada requires a planned disconnect for the approach of any iceberg in excess of 100,000 tons. The vessel will be ice classed and can withstand impacts from smaller icebergs. Further, the disconnect must be accomplished in a one-year ice season storm and must be completed in about four hours from the decision to disconnect.

This time would include the shut-in of the wells, flushing of the risers and flowlines, blowdown of the gas lines, and the mechanical disconnection of the turret. Once the lines are flushed, the quick-disconnect valves at the bottom of the turret would be closed. The space between the valves at the coupling would be purged and the couplings separated.

The disconnect of the FPSO from the mooring occurs when a large, specially designed mechanical connector releases a large spider buoy from the bottom of the turret. The moorings and risers are connected to the turret through this spider buoy so, as the spider buoy releases, so do the moorings and risers. The actual disconnect of the mooring is nearly instantaneous.

An important design feature is that the entire disconnect sequence is reversible, up until the final moment of disconnect. Mack stressed that this is important in the event the iceberg changes course and is no longer a threat and disconnection is no longer necessary.

Another feature is that all critical mechanical components are located in a dry environment and the disconnect procedure can be accomplished with only minimal operator assistance. All operations take place remotely from the vessel's bridge.

In addition to the planned disconnect, Petro-Canada requires the ability to conduct an emergency disconnect in only 15 minutes. A disconnect under these circumstances would only occur if an iceberg were to approach the FPSO undetected in the fog and pose an immediate threat to the safety of the vessel.

Sophisticated systems will monitor the movements of icebergs in the area as they enter the Grand Banks, but it is possible to lose track of the icebergs in heavy fog. Sofec's design allows the emergency disconnect to take place in a fashion similar to the planned disconnect, except it will not be possible to flush the flowlines and risers.

Once disconnected, the FPSO will sail out of harms way under its own power. The mooring system and risers will drop down under water to a mid-depth equilibrium position and will remain supported by the spider buoy. Mack said that in most cases the iceberg will pass safely over the top of the buoy with no damage or environmental impact. It is expected that only under an extreme condition with a large bottom scouring iceberg might there be damage to the flowlines, risers or moorings. In any case, even a scouring iceberg will not reach the wellheads because of the glory holes.

Reconnect procedures

Once the danger has passed, the FPSO will return to station. The turret system is designed to reconnect in a matter hours using only vessels that would normally be on site. A floating retrieval line will be recovered from the spider buoy and will be brought up onto the FPSO. The line will then be rigged through the bottom of the turret and connected to a winch inside the turret.

The retrieval line is then pulled in while the FPSO vessel maneuvers on its thruster system to maintain station immediately above the spider buoy. As the retrieval lines pull the buoy closer to the turret, a pull-in chain is drawn from the centerwell of the buoy and rigged through a high-capacity chain jack inside the turret.

The pull-in chain would then be jacked in, bringing the buoy into the bottom of the turret where it is properly aligned. Once the buoy is seated in the turret, the connector is engaged. At this point, the connector is preloaded, using a novel Sofec tensioning system. The preload is locked in place and stored in the connector.

This preload exceeds the maximum design axial load experienced by the connector in a 100-year storm condition. The purpose of the preload is to eliminate nearly all fatigue loading in the connector and ensure that the connector never experiences any axial loads greater than the preload.

Mack explained that this preload capability is an important aspect of the Terra Nova design and is unique to the Sofec turret system. Other disconnectable turrets operating in the South China Sea and Australia are designed to disconnect in typhoon conditions and never experience these high design loads.

"I can say with confidence that this is going to be the most sophisticated and complicated turret system ever constructed," Mack said.

Glory holes not new, but rare

As icebergs move across the Grand Banks, their keels may scour the seafloor. This is a definite hazard for wellheads and subsea templates. To protect these systems, Seacore will excavate four glory holes, each 11.5 meters deep. The base of each hole will differ in size, ranging from 16 meters by 16 meters to 56 meters by 16 meters.

Seacore Managing Director Bob Goodden estimated the total volume of the four glory hole excavations will be in the neighborhood of 150,000 cubic meters. As a dredging operation, this is not an extraordinary volume, but the holes are being excavated in water depths over 100 meters and in relatively high sea states.

Using a cutting and vacuuming scheme which Seacore's Gooden labeled as "sort of a drilling operation." A 5.6-meter-diameter cutting head, similar to a flat drilling disk with cutters on it, will be used to excavate. Ports on the head will suck up seawater and cuttings, which are then pumped to the surface through a 20-in. drillstring, and then along a floating pipeline to a disposal pile about 300 meters away.

The cutting head is lowered on the drillstring to the seafloor where it cuts a circular plug from the floor about a meter deep. Afterwards, the cutting head is raised and moved about four meters to cut another circular disk. These cutting areas overlap so that the individual glory hole is gradually cut one meter layer at a time. Each layer is a bit narrower than the one above it. "Eventually we should end up with something looking like a grandstand with seats on the edge," he said.

The Seacore-manufactured cutting system is similar to those used off South Africa in the excavation of diamonds from the seabed. These operations are commonly undertaken in depths of up to 350 meters, much deeper than the 100 meter depths of the Terra Nova glory holes. Seacore had to manufacture a new system because all the existing units are occupied.

The excavator should be completed in June and is scheduled to go on site in July. Excavation should begin in August. Goodden predicts all four holes will be excavated three to six months after drilling begins. The seabed is made of glacial till (rock debris from the ice age), clay, cobbles, gravel, sand, and pebbles. "It's nasty stuff really, and some of the sands are cemented sands. It could be quite tough going," he explained.

The unit will be mounted on the Cal Dive service vessel Sea Sorceress which has a moonpool through which the system will be lowered, and attached to a heave compensator and drill tower.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.