Recent development of Managed Pressure Drilling (MPD) Techniques for use from floating drilling rigs began in the late 1990's concurrent with the development of 5th generation offshore drilling vessels(1). During this period a number of industry efforts focused on developing "riserless" or dual gradient drilling systems employing subsea mud return pumping system. One of these efforts (2) culminated with the drilling of a test well in the Gulf of Mexico employing a subsea pump based Dual Gradient drilling method. Additionally, during this period another industry group employed aerated drilling fluid and a closed loop circulating system to drill a pressure depleted well section offshore Brazil from a floating drilling rig (3). While technically successful, these efforts did not result in commercially deployable drilling systems. Much of the initial industry focus has been on developing MPD methods for deepwater applications, many of the techniques are applicable to all offshore drilling operations and a number of recent applications of Constant Bottom Hole Pressure drilling methods have been employed from fixed installation in both the North Sea and US GOM (4, 5, 6). At the same time both Surface BOP and RiserCap™ equipment configurations (Fig 1) have been employed to implement Pressured Mud Cap Drilling from floating drilling rigs offshore South East Asia. Introduction A number of authors have presented studies of drilling non-productive time (NPT) suggesting that some 20 to 30% of total time spent drilling is non productive (7, 8). Further these studies indicate that as much as 50% of this NPT is well bore pressure related. By solving many of the technical challenges related to well bore pressure problems, MPD methods can significantly improve drilling efficiencies by minimizing the time spent monitoring and interpreting well conditions (flow checks, drilling fluid expansion, ballooning, influx and loss detection, etc). Further, having the ability to manage the well bore pressure profile may enable changes in well design minimizing the requirement for close tolerance casing programs, and contingencies such as drilling liners and expandable casing. In other instances these methods may enable wells to be drilled where application of conventional drilling practices would not be technically or economically feasible. As MPD methods gain broader application offshore, opportunities to combine various techniques will emerge enabling further optimization of well design and operations yielding significant improvements in well construction efficiency and reduction in cost (9). Background Most of the drilling practices now referred to as Managed Pressure Drilling have their origins in land based drilling operations. These drilling practices can be categorized as; over balanced or under balanced with respect to pore pressure depending on the pressure regime maintained in the wellbore while employing a particular method. Generally MPD refers to overbalanced or, at balance operations, and is defined (10) as: "... an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly"

As the drilling industry advances to drilling wells in deeper waters, technology must also advance so that we understand and manage the associated risks. We must be careful not to extrapolate existing conventional procedures. The rules that we apply to gas migration behaviour in shallow water drilling risers may not be adequate when the riser is significantly deeper and new factors start to dominate. Factors such as bubble fragmentation and suspension, which have little effect in a shallow water situation, begin to have a significant effect as the riser depth increases. Under certain conditions, the gas bubbles stop migrating. The gas is then held static by the yield stress of the mud, and must be circulated out of the riser. This paper reviews the theory on the behaviour of gas migration through water based drilling fluids and shows how this can affect operations in a deep waterwell. The operations following gas getting above the BOP stack and into the riser during a well control incident are considered. Computer model simulations are presented to define a range of conditions. Field test data is compared to simulator data to validate the predictions. If the amount of gas that gets above the BOP stack is limited by successful detection, the gas can be safely removed from the riser using controlled circulating procedures, without the need for much specific equipment. These proposed procedures are also presented. The use of advanced kick detection equipment is evaluated as a method of minimizing the risk of gas volumes in the riser. Introduction In a deepwater drilling situation, where the productive horizons tend to be relatively shallow below the mud line and where operators are using horizontal and extended-reach well profiles, there is a greater chance that an influx of gas will be above the subsea BOP stack before it is detected. Indeed, most of the volume of the circulating system is contained within the riser of many deep water wells. Much has been written about the potential problems that can arise from gas getting above a subsea BOP stack and into the riser. However, there is not much data quantifying the effect that this gas could have. There is a shortage of full-scale tests covering the broad range of operating conditions. One test that has been very useful to the industry was conducted jointly by Amoco, Shell, and Exxon in 1986 using the Discoverer Seven Seas in 3118 ft of water. The use of advanced well control simulators has become very beneficial inexpanding the industry's understanding of the complexity of gas migration. Over 300 simulations covering a broad range of operating depths and conditions wereconducted while preparing for this paper. Others, too, are using simulators to increase their understanding in this area. The industry will continue to learn and would benefit from more field tests. Advanced hardware is available to handle gas at the surface by allowing back pressure to be held on the riser. However, there are additional things to consider before using this equipment. There is a possibility of creating a pressure inversion, where bottom hole pressure is brought to surface, if the gas migrates and is not allowed to expand while the annular element is closed at the surface. Risers are usually not designed to handle this pressure, so could burst. Also, this additional equipment can be expensive.

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Recent application of Surface BOP from floating drilling rigs to drill offshore wells began in early 1996 in the relatively benign environments of Southeast Asia. The techniques and equipment required to use surface blowout preventers from floating rigs have evolved progressively from the initial application in relatively shallow water benign environments using moored semisubmersible rigs to recent application in very deepwater moderate environments using dynamically positioned drilling rigs (Fig 1). While much of the industry focus has been on developing floating surface bop deepwater applications, a significant number of wells have been drilled effectively in shallow water depths using Surface BOP drilling units. The appeal for utilizing floating drilling rig-based surface bop technology in these areas is that the technique can address a number of site-specific issues facing bottom-supported rigs. These include:Bottom-supported rig site assessment issues related to soil conditions.Risk mitigation associated with shallow geo-hazards.Improved operational efficiency compared with conventional subsea BOP or bottom-supported rig alternatives. Additionally availability of an economic alternative to bottom-supported units effectively expands the available rig fleet thereby mitigating possible rig availability project constraints. In addition to providing viable alternatives to bottom-supported drilling units in certain shallow water circumstances, floating drilling rig-based surface BOP is more readily adapted to other emerging drilling methods than a conventional subsea BOP configuration. This paper will:Review current practice with respect to surface BOP application from floating drilling units.Review alternative applications of surface BOP from floating drilling unitsIntroduce surface BOP from floating drilling units as an enabling technology for deploying emerging drilling methodologies and equipment.