TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractUltra-deepwater drilling activity is at an all time high. Recently, there were only two rigs rated to, and actually working in, water depths greater than 6,000 ft. Soon there will be over twenty. This increase will involve new deepwater operators and bring a large number of new people to deepwater. This paper presents some of the problems that have occurred in deepwater operations under the assumption that an understanding of what can go wrong is the best way to avoid problems. It will identify these problems and discuss the critical topics of rig positioning, environmental considerations, BOP control, riser management, well control, wellbore construction, production problems, and personnel training and safety.

The advantages of a dual gradient mud column have been well documented. Significant work has been done on the riserless drilling systems by several different companies. This paper will propose two methods to achieve the same dual gradient.Nitrogen Injection: Building from proven air drilling procedures and underbalanced techniques nitrogen can be used to cut the mud weight back in the riser above the seafloor or the cut can be made deeper by combination with a concentric riser.Floating Mud Cap: A dual activity rig can use a casing riser linked to the adjacent marine drilling riser. A submersible pump in the casing riser regulates returns that will set the mud cap level in the drilling riser. A pressure sensor in the subsea BOP allows monitoring the effective hydrostatic pressure. This approach again combines field proven procedures and hardware in the deepwater environment. Both methods will be presented with emphasis on the hardware and operational procedures required to successfully implement them. Both of the methods employ existing equipment and procedures. Introduction One of the major challenges of drilling in deepwater in the Gulf of Mexico is due to low fracture gradients and shallow abnormal pressure. The margin between the fracture gradient and required mud is often less than 1 ppg.In young formations, fracture pressures are almost equal to the overburden1,2,3. Because much of the overburden is only due to the weight of the seawater the fracture pressures are only slightly greater than ‘normal pressure’. Unfortunately, there is not a corresponding drop in deepwater pore pressures due to the immaturity of the deposits typically encountered. The water confined in the rapidly deposited clay is in part pressurized by the weight of the overlying sediments trying to wring the water out, which creates a narrow margin between the pore pressures and fracture pressures. As a result the pore pressure and fracture gradients curves almost lie atop each other. A dual gradient drilling approach will result in the effective mud weight at the previous casing shoe being less than the effective mud weight at the drilling depth. The industry's effort for riserless drilling4 is designed to take advantage of this benefit. If casing points are not limited by the mud weight of the previous casing shoe, it is possible to eliminate casing points from the well program. Figure 1 shows a common goal of the dual gradient drilling. Previous efforts only brought the mud weight back to sea water equivalent at the seabed. Even better results can be achieved by cutting the mud to less than seawater at the seabed or at the shoe. By doing this it is possible to find a combination of the two mud weights that cause the heavier mud gradient to always lie between the pore pressure and fracture pressure lines. The pressures may be underbalanced inside the cased hole or riser but the open hole is overbalanced. This can be achieved by reducing the density in the upper mud section to less than seawater or by adjusting the cut point to below the seabed. This paper proposes to achieve the dual gradient by two methods. Both of which use existing equipment and employ known drilling techniques. The first proposal is nitrogen injection where normal gasification drilling methods are applied to the riser section. 1) Nitrogen injection option This paper will outline how nitrogen injected subsea can effectively create a dual gradient by gas lifting the mud in the riser. Another operator had considered a similar approach, but had proposed to sweep the entire 21" marine drilling riser with gas. This paper proposes to combine nitrogen injection with a high pressure concentric casing riser5. This reduces greatly the gas required to cut the mud. Figure 2 illustrates an example of the nitrogen being injected into a subsea BOP stack.

The startup of the dual-activity drilling vessel Discoverer Enterprise in early 1999 heralds a new age for deepwater development. For the first time a drilling vessel exists that can perform many of the aspects of deepwater field development cost effectively, including a variety of subsea production equipment installations and flowline/pipeline laying. This capability minimizes the barges and equipment required for field development and yields schedule flexibility for the installation tasks. Aspects of the dual-activity drillship related to non-drilling activities are described in the paper. The second rig on the drillship is only required part of the time to support drilling operations. The remainder of the time can be used for other field development activities. The subsea tree is run with the auxiliary rig while drilling with the BOP and riser with the primary rig. In addition, subsea manifolds, umbilical pods, flowline jumpers and supporting piles can be placed and secured with the auxiliary rig while drilling progresses with the primary rig. Concepts for subsea equipment that can be installed with the dual-activity drill rig are presented. Some flowline installation work can also be performed while drilling is in progress. The flowline installation technique, designated "discrete lay" and/or "reverse lay", results in sectional flowline placements using an anchor handling vessel and an ROV boat (for inspection and monitoring). This technique can also be used for the placement of "long jumpers". Flowlines, as well as export pipelines in deeper waters, can also be installed by the dual-activity drillship in pipelay mode before or after drilling is completed. Using both rigs, two lines can simultaneously be laid which is an advantage for flowline sets requiring pigging loops or parallel oil and gas export lines. Burial of the lines is also achievable with the dual-activity drillship. A conceptual timeline of how drilling and non-drilling operations can be achieved by the dual-activity drillship is provided. Introduction The concept of a dual-activity drillship has been in existence since the mid-1990's. A year-long study performed by Transocean Offshore Inc. concluded that deepwater drilling efficiencies could be enhanced by design and construction of a single drillship with dual drilling capabilities. Construction of the first dual-activity drillship, the "Discoverer Enterprise", began in 1997 and is scheduled for delivery in the spring of 1999. Two additional drillships, the "Discoverer Spirit" and the "Discoverer Deep Sea", are currently under construction and are expected to be delivered in the year 2000. One advantage identified in association with the dual-activity drillship is that it has the potential to enhance offshore operations in unconventional modes. Because the auxiliary rig is out of the critical path, it can be used for operations which would not be economic if the rig spread cost were applied to the operation. For example, the second rig is capable of being employed to drill a disposal well for flared gas or for produced water, or the second rig can be used to pull in pipelines, run production jumpers, run control lines or other subsea construction work. The extra rig is available for heavy subsea lifts and work as a 5,000 hp ROV.

Transocean Offshore has received a contract from Amoco Corporation to build the first new floating drilling rig since the orders placed in the mid 1980's. The Discoverer Enterprise will be the world's most advanced offshore drilling rig. Representing the next generation of ultra-deepwater rig, the Discoverer Enterprise resulted from an innovative approach which addressed the overall well construction process and the equipment required to significantly decrease the time required to drill a deepwater well. The Discoverer Enterprise will be a large, dynamically positioned drillship capable of drilling in up to 10,000' of water. Unique drilling features include hull size, a single tubular derrick equipped with two rotary tables, two drawworks and two topdrives which will result in an increase in well construction efficiency making the Discoverer Enterprise a cost effective rig for deep water. It is the first ultra-deepwater rig designed specifically for handling subsea completions and extended well tests. Introduction Improvements in deepwater economics as a result of advanced deepwater production technology, tax and royalty relief, and the establishment of deepwater infrastructure are evidenced by the growth in major field developments in the Gulf of Mexico, Brazil and West of Shetlands. The result has been a significant increase in demand for rigs capable of drilling in deep water, particularly in the Gulf of Mexico where over 700 blocks have been leased in greater than 5,000' of water. To date, only three rigs (two DP drillships and a moored semisubmersible) have drilled in greater than 5,000' of water in the Gulf. Increasing demand for deepwater rigs resulted first in an increase in day rates, and then in an increase in contract term. In response to improving economics, contractors began upgrading existing equipment and also converting non-drilling semisubmersible units to meet operator demand. As the capital cost of conversion increased rapidly, new construction became a viable alternative. The last new floating rig built capable of operations in 5,000' of water was the Transocean Richardson, an anchored semisubmersible, which was delivered in 1988. That vessel was built at a cost of $90 million. Building the same rig today is estimated to cost more than $250 million. This level of capital cost requires a dayrate in excess of $180,000 per day to provide an adequate return to the owner. In the case of a duplicate Richardson, the technology or drilling process would not be significantly improved versus the product delivered by the original; therefore, Transocean believed a more cost effective approach to drilling was needed to justify such a large dayrate. Transocean formed an in-house team of operations and engineering experts whose goal was to develop a process for reducing well construction cost by 40%. At this level of efficiency, an operator can justify the dayrate required for new construction as faster well construction will offset higher daily costs. The Enterprise development team analyzed the entire well construction process and determined the equipment required to improve drilling efficiency. Rig activity has been centered around a single rotary table; therefore, any activity that requires use of the rotary is in the "critical path" to complete the well. The operator must pay for the cost of that activity through the rig dayrate, as well as through additional spread costs such as ROV, MWD, personnel, boats, helicopters, shore base, rental tools, and communications, etc. By utilizing a dual derrick with two rotaries, two topdrives and two drawworks (the dual activity drilling system), the Enterprise concept will remove many typical tubular handling activities from the critical path. The result is greater flexibility, enhanced efficiency and lower cost to the operator. P. 719^

Use of a higher pressure rated riser run inside the marine drilling riser would provide floating rigs with some advantages enjoyed by fixed rigs; the potential to drill underbalanced or near balanced, improved gas handling capability, improved well testing capability, expanded kick control capabilities and increased mud weight rating. Drillers have found areas in ultra-deepwater where the fracture gradient is close to the pore pressure within a few thousand feet of the seafloor. These wells cannot be drilled with conventional equipment. Underbalanced or near balanced drilling which has been successfully used onshore may be the only method to drill such formations. The concentric riser would make it possible. The time and equipment required to test a deepwater well would be significantly reduced if a conventional surface type hook up could be used. However, few rigs will be able to run a concentric riser. The latest generation of drillships, such as the Discoverer Enterprise, will have the capability and equipment to do so. The paper will address the advantages, the operational procedures and hardware needed to utilize a concentric riser safely P. 53