Free-standing riser features robust but familiar technology

Two production systems show benefits
of disconnected motions for drilling

Leonard Le BlancEditor

PART III: This is the third in a four-part series on riser solutions for drilling in water depths exceeding 7,500 ft. Part I in December featured riserless drilling. Part II in January featured the development of composite risers.

Drillers face a formidible hurdle beyond water depths of 7,500-8,000 ft. Robust mobile drilling units may be needed to support equally robust conventional riser strings, or alternatively, new drilling riser technology could be required.

Riser options for ultra-deepwater include doing away with the conventional riser string entirely, lightening the riser string with composite materials, reducing the weight of fluid contents in the riser, drilling a smaller diameter borehole, or deploying a riser that is weight neutral or buoyant.

The latter solution, a weight neutral or buoyant drilling riser string, may be nearer to deployment on an actual well in ultra-deepwater depths than versions of the other solutions, if only because the technology is more familiar to the industry.

Free-standing production risers have already been deployed on two US Gulf of Mexico fields - one in 1,525 ft ( Placid Oil - Garden Banks 29) and one in 2,100 ft (Enserch - Garden Banks 388).

In the mid-1980s, when Placid Oil set out to produce a field in Garden Banks 29, producers were trying to understand the complex riser-vessel forces and motions and wanted to avoid the problems of disconnecting 30-50 joints of riser each time a tropical storm entered the Gulf of Mexico. Rather than over-designing a surface vessel and riser to withstand storm forces, Placid opted for a solution that incorporated a self-supported free-standing production riser. The drilling vessel did not need to retrieve or disconnect from the production riser during rough weather.

In effect, the surface production vessel motions were partially de-coupled from the riser. However successful the producing design proved to be, the geological failure of the field after two years of operation cast an undeserved negative shadow over the technology.

Pursuing the same minimum engagement solution in 1995, Enserch opted for a free-standing riser design when the Garden Banks 388 field was developed with a semisubmersible floating production system. In fact, the manufacturer, Cameron (Cooper) refurbished most of the Placid development riser components, including the bottom titanium flex joint, for the GB388 development.

Today, Cameron, the manufacturer of both earlier free-standing riser systems, is offering a similar riser for drilling in water depths up to 10,000 ft.

Major benefits

In addition to employing technology familiar to design engineers and de-coupling the motions of the surface vessel, the free-standing drilling riser will have other advantages:

Since the free-standing riser's top termination is about 300 ft below the drilling unit, the rig can disconnect the drillstring, suspend it in the buoyant riser, and drive off the site, all relatively quickly. Shear rams at the top of the riser can shear the drillpipe, when neccessary.
The retrieveable riser section above the free-standing segment can be pulled in 2-3 hours, minimizing the weather downtime period and increasing drilling time.
All of the equipment below the riser sections with air tanks is conventional. More robust flanges have been designed for tensioning loads and pressures associated with depths up to 10,000 ft.
The riser can be disconnected from the wellhead intact and parked elsewhere, while other functions take place at the wellhead.
Drill floor and tensioner maintainence can be conducted without pulling the free-standing riser segment.
Servicing the BOP stack and installing subsea trees can be undertaken by parking the riser over a dummy wellhead and using the drill pipe to pull and run equipment.
The riser can be converted into a production riser, if needed.

Running time

The most important feature for the free-standing riser is not having to pull and re-run the riser for any number of wellhead and maintenance functions, or when severe weather enters the area. When drilling water depths were under 3,000 ft, the task was troublesome, but not unduly time-consuming.

Today, for a well in 7,500 ft water depths, the running time (installing or removal) for a conventional top-tensioned riser is 3-8 days. Frequent riser tripping, especially in areas with frequent and serious storms, sharply diminishes drilling time and increases the risk of damage and maintenance downtime.

An important dictum in the drilling business is to avoid tampering with or breaking seals on equipment or connections that are functioning successfully, unless wear is evident. Further, tripping the drillstring and riser string enhances the risk of developing a problem in wellhead or BOP connections or loss of well control. Many riser trips on a 60-90 day well can be a costly process during a development drilling campaign. This was the impetus behind the development of Cameron's free-standing drilling riser.

Surface tensioning

Arguably, the second most important feature of the free-standing riser is the impact on the drilling rig, or lack thereof. The extra cost and problems associated with the installation and maintainence of air tanks on the riser and air compression equipment aboard the drilling unit is repaid by the large reduction in rig deckload and tensioning equipment.

Light tensioning equipment is maintained for 300 ft of retrieveable riser, but 50-100 tons of heavy tenioners can be eliminated. The added deckload capacity can be used in two ways:

(1) To expand consumeables capacity during a long drilling campaign (2) In water depths beyond 6,000 ft, the mobile rig requirement can be reduced by one generation, meaning reduced day rates and probably enhanced rig availability.

A deepwater mobile rig would still have to have space and deckload capacity to stack free-standing riser joints in between trips, unless a barge was available alongside the rig. The top riser joints that are surrounded by air cans will be larger than conventional riser joints and require more space.

Riser parking

The third benefit is the ability to park the drilling riser off the wellhead. This is especially the case when the drilling phase ends and completion begins, but it has a special attraction when template drilling is underway or explorationists anticipate closely packed exploration wells.

The drilling riser, including the upper and lower marine riser packages, is released from the BOP stack intact and positioned on a dummy wellhead a short distance away. Then, using drillpipe, a number of intervention tasks can be conducted on the BOP stack, on the wellhead, or downhole.

Riser parking is an asset where BOP intervention is a frequent requirement, or where high pressures and temperatures are encountered, or where rubber or polymer seals must be changed often. Also, riser parking can be an asset when switching to a temporary production mode. The BOP stack can be replaced by a production tree and flowline connections made. If the riser is retained, the drillstring can be replaced by production tubing.

On template drilling, after each well is drilled, the riser is parked until the well is completed and brought onstream, shaving days or weeks off the production schedule. The larger the number of wells on the template, the greater the savings in time.

10,000 ft riser

In principle, the drilling riser destined for 10,000 ft depths varies little from the riser equipment deployed on the Placid and Enserch production programs. Enhancements were made to the 21-in. flanges connecting the riser joints to accommodate the 3.5 million lb of tension load, compared with the 2 million lb for conventional designs.

The number of air cans in the Cameron design is increased according to the drilling depth desired. The free standing drilling riser has been designed to support the abandoned riser, drill string, and a riser full of 17 lb/gallon drilling fluid with the appropriate number of air tanks for the water depth and environmental conditions.

In water depths of 6,500 ft, the number of air cans needed would approximate 12. At a depth of 10,000 ft, the number of air cans needed rises to about 20.