Rapid growth in deepwater drilling activity has led to a large number of commercial discoveries. This success has spurred further exploration activity. However, the demand for high-spec deep water rigs is already undermining this success, by driving up drilling costs and slowing down development of new fields. Due to low supply side confidence among rig contractors, this situation is unlikely to correct itself in the short or medium term.
In 1996, Enterprise Oil was faced with a stark choice when planning its own deepwater drilling activity in the Adriatic Sea - either pay the very high cost of a rig upgrade, or alternatively, the inflated day rates (and mobilization costs) for a 4th generation semisubmersible.
Conventional deepwater configuration (left) and slim riser configuration (right).
A large percentage of the rig upgrade costs related to the need to increase riser tensioning capacity, mud storage requirements, and deckload capacity. These requirements arose from riser weight and diameter. Following studies of a reduced diameter riser, Enterprise concluded that this could be applied with minimal impact on both well design and rig equipment.
The Slim Riser concept provides a means of upgrading cost effectively 2nd and 3rd generation rigs (of which there is a relative surplus) so that they can work in deepwater. This concept could help overcome the supply problem which has resulted in high day rates and delayed project schedules. Enterprise's concept employs proven equipment to adapt low-spec rigs for deepwater operations with minimal delay.
In 1996, Saga Petroleum established that deepwater drilling costs were trapped in a weight-cost spiral. Deeper water wells, with increasingly heavy risers, also demanded larger, heavy duty rigs. This pushed up the cost of newbuild and upgraded rigs. Saga presented its own concept for reducing deepwater drilling expenditure through use of slimmer risers and smaller drilling rigs. Implementation, however, would have entailed a complete re-fit of key rig components, including a new small bore riser and BOP stack. Thereafter, the rig would no longer be capable of drilling conventional wells. This represented a major commitment, not easily reversed, which probably explains why the concept remains grounded.
Enterprise's solution involves replacing the 21-in. riser with a 16-in. riser (or smaller), while retaining the rig's existing 18 3/4-in. BOP stack. Other key features are:
- Rig retains the 21-in. upper riser assemblies (diverter, telescopic joint, and so on)
- Wellhead sleeve is pre-installed to reduce effective wellhead bore to 13 3/8-in. (this is proven technology used historically with dual stack rigs).
By reducing the riser size to 16-in. OD, a substantial increase can be achieved in the rig's water depth capability, for these main reasons:
- Reduced tensioner loads
- Lower variable deckloads, due to weight of stored mud and riser tensions
- Reduced pit space (riser mud inventory halved)
- Lessened riser storage requirements.
Other benefits include:
- Cost of the 16-in. riser is lower than for the conventional 21-in. version, and the buoyancy needed to support it is also considerably lower
- Reduced mooring loads due to the lower environmental forces on smaller rigs
- Increased riser fluid velocities, which may eliminate the need for booster lines
- Reduced circulation times
- Riser joints are smaller and easier to handle
- Logistical support needs lessened owing to the substantial reduction in the active mud volume (especially important when drilling in remote areas).
Generally, only minor upgrades would be required for rig equipment such as BOP controls, guidelines, and cameras. For radically deeper water operations, upgrades in tensioner capacity and pit volumes may prove mandatory in addition to use of enhanced control systems. However, these modifications would still be moderate compared with those imposed by a conventional upgrade.
The slim riser concept dovetails neatly with the well established "wire insert method" of increasing the attainable mooring depth. Wire inserts are a simple, cost-effective way of upgrading a mooring system, with minimal lead time and generally minimal impact on the rig upgrade needs. The method offers major capital savings compared with a standard mooring upgrade, although it does increase operating costs slightly through upping mooring time for each well by around 1-2 days. The method is particularly suited to upgrades of relatively small rigs, especially for short duration projects where full-scale winch upgrades would be costly or impractical. It also presents an effective approach for drilling templates of well clusters where moorings only need to be established once.
A downside to the use of relatively small, low spec rigs would be a tendency towards inferior motion response. To offset this, initial work by Enterprise suggests that rig positioning tolerance will increase with the lighter, more flexible riser. More importantly, the low cost and flexibility of the slim riser concept would improve prospects of the optimum weather window being selected for deepwater operations.
Well design impact
Use of a slimmer riser leads to the loss of one hole size (17 1/2-in.) through the BOP stack. In the majority of wells, this loss could be offset by:
- Drilling the 17 11/2-in. section riserless
- Using one less casing string
- Downsizing an 8 11/2-in. section to around 6 3/4-in.
Certain projects could be completed more cost-effectively by using a top hole drilling vessel. For example, the riserless well section (to 17 1/2-in. TD) could be drilled with a low spec shallow water rig. This rig would only need to handle pipe and casings strings in open water. The higher cost/higher spec rig, fitted with a deepwater BOP, could then complete the drilling and testing/completion operations. This concept could be stretched further with specialist rigs for various sections in a batch drilled development.
The restricted internal diameter of 16-in. risers will impose some constraints on the choice of tools, such as subsurface test trees and tubing hangers. A 21-in. riser has long been the standard, and thus, little design effort has been focused on reducing tool diameters. However, given a lead from operators, such tools should soon become available. The impact on drilling costs will vary considerably between projects. Estimated savings on overall well construction costs would stem largely from lower rig rates.
As a result, considerable savings should follow in terms of reduced mobilization and upgrade expenditure. Also, operators should derive significant economic benefits from their ability to shrink project cycle times. Larger operators (or consortia of small operators) could stock slim riser pipe (which would be the key capital and lead-time item for new projects). The riser could then be allocated to rigs as appropriate, allowing the operator to take full advantage of the market and the locally available fleet.
Daily drilling costs would also come down due to the reduction in active mud volumes, the reduction in logistical support and lower fuel costs incurred by smaller rigs. If the entire well is effectively slimmed down, savings can also be achieved in drilling fluids, waste re-cycling, casing, cementing, and other materials.
As the proposed concept does not necessitate change-out of the rig's BOP stack to a smaller system, a rig using this concept could switch readily from Slim Riser deepwater mode to shallow water conventional mode (with a full bore riser). This would increase flexibility in marketing the rig. For example, a rig in northern Europe could operate in the deepwater plays during the summer weather window and revert to shallow water North Sea operations in winter. This would be a very attractive option for operators working in the Atlantic Margin who have been forced in the past to operate in hostile winter conditions due to the shortage of deepwater rigs.
The slim riser concept would suit most of the major deepwater provinces around the world including West Africa and Brazil. Ideal projects are those involving:
- Water depths of 2,000-6,000 ft
- Shallow (drilled depth)/normally pressured wells
- Short programs with high upgrade or mobilization costs
- Long programs where high day rates push up project costs.
Wells where the 17 1/2-in. hole could be drilled riserless are the optimum candidates. Many wells would likely require little or no design changes to accommodate the slim riser.
Various phases in deepwater drilling operations require fundamentally different rig attributes and capabilities. The approach taken by most rig contractors to meet deepwater demand has been to build very large, high spec vessels capable of performing large numbers of operations. This approach has proven sound for one-off wells in remote locations. However, it might be worth opting for low or lower spec rigs to complete a project rather than just one, high spec vessel. This approach could even suit wells with demanding casing programs, such as occurs commonly in the Gulf of Mexico.
The principle of matching rig specs to operational phases has been recognized by several operators. BP Amoco has examined "installation-in-advance," a concept whereby the installation of taut leg moorings and free-standing risers and the drilling of top hole sections are all completed by low cost vessels. A high spec rig then takes over to drill the lower sections. Enterprise's Slim Riser concept could employ established technology to achieve a similar result.
This approach may incur higher mobilization charges and mooring costs, but on multi-well projects, the differential in day rates should more than offset this. The concept may even be stretched to exploration drilling, particularly if DP drillships could be used for drilling top hole sections, to minimize both mooring and mobilization outlay. It could provide an ideal DP application, since it would minimize concerns about stationkeeping, as the DP vessel does not penetrate the reservoir. Also, additional fuel costs would be offset easily through reduced mooring costs in a short operation.
Proceedings, Offshore Mediterranean Conference, Ravenna, Italy, March 1999.