Wells – Submerged well – Connection or disconnection of submerged members remotely...
Reexamination Certificate
1998-12-11
2002-03-05
Bagnell, David (Department: 3673)
Wells
Submerged well
Connection or disconnection of submerged members remotely...
C166S339000, C166S345000, C166S359000, C175S007000
Reexamination Certificate
active
06352114
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods and systems capable of efficiently drilling offshore wells in extremely deep water using a smaller, more economical floating vessel, along with smaller, and less expensive, drilling equipment (such as hoisting equipment, riser tensioners, mud systems, etc.) than heretofore possible. This is possible because the system is able to perform all requisite tasks and functions using a reduced diameter marine riser that dramatically reduces variable deck load and space requirements for the vessel.
In recent years, the search for oil and gas deposits has taken oil companies into ever deeper offshore waters. Floating rigs of only a few years ago were generally limited to perhaps 1,500 feet of water depth, but it is now commonplace to conduct offshore drilling operations in water depths up to 5,000 feet, and several rigs are under construction which are theoretically capable of conducting drilling operations in 10,000 feet of water or more. For extreme water depths, dynamic positioning, which is not sensitive to water depth, is commonly used for vessel station keeping.
The basic deep water drilling system is unchanged from that designed more than twenty years ago. The system employed to actually drill a well in deep water is basically an extension of that for drilling in shallower water. Typically, this system employs subsea components consisting of an 18¾″ subsea blowout preventer (BOP) stack installed at the ocean floor and coupled to a floating drilling rig at the ocean surface by a 21″ diameter marine riser system. This arrangement allows the driller to utilize the riser to convey to, and install, the typical 18¾″ API subsea BOP stack on the wellhead, and supports a well program typically including 30″, 20″, 13⅜″, 9⅝″, and 7″ casing. Occasionally, additional strings of casing and/or liner may be employed.
The major adaptation of the riser system for deeper water has been to lengthen it. Lengthening the riser requires greater material strength, thicker walls, additional and larger service lines, more exotic riser connectors and tensioner system, and thicker and denser floatation. Unfortunately, lengthening the marine riser gives rise to significant consequential rig related issues as well, which, as will be shortly disclosed, tend to dominate deep water rig design, particularly semisubmersible rig design.
All marine risers must be maintained in tension whenever they are deployed; the minimum tension requirement is that the riser not be in compression at the top of the subsea BOPs. The weight of riser which the tensioning system must support is comprised of two main elements. The first is the steel weight of the riser tubing, joining connectors, auxiliary conduits, and control lines. Syntactic foam buoyancy modules are strapped around the riser to compensate for part of the riser steel weight when the riser is in the water, but these modules add to the weight in air and increase the overall diameter of the riser to around 56″. By way of example, the weight in air of 10,000 feet of a 21″ marine riser with buoyancy modules is approximately 3,600 tons.
In addition to the steel weight, the tensioning system must provide sufficient axial tension at the top of the riser to control the stresses and displacement of the riser while the floating drilling vessel moves horizontally and vertically in response to wind, waves and current. The tension requirements increase with increasing drilling mud weights and riser offsets. This means that even after considering the buoyancy, the riser tensioners for 10,000 feet of water have a total tensioning capacity of about 1,550 tons. In addition, while the actual drilling operation only requires about 500 tons of hoisting capacity, this must be increased to 750 to 1,000 tons to handle the riser and BOPs in deep water.
The riser required for 1,500 feet of water weighed only about 150 tons in air, did not normally require much buoyancy, and could be stowed in about 1,200 square feet of deck space. The marine riser for 10,000 feet of water weighs about 3,600 tons in air and requires a storage area of about 10,500 square feet.
The marine riser is subjected to lateral forces due to ocean currents, and these forces are proportional to the riser diameter. The lateral forces are transmitted to the vessel at the surface, and ultimately must be resisted by the vessel's station keeping system. Current flow around the riser also results in vortices, which, when shed, “pluck” the riser and induce low frequency oscillations in the riser, causing stress and fatigue. The riser for 1,500 feet of water had an effective diameter of about 36″, while that for 10,000 feet of water has an effective diameter of about 56″ due mainly to the use of syntactic foam buoyancy modules. Consequently, a deep water riser is subjected to greater lateral forces and stresses than a riser designed for use in shallower water.
It is sometimes required to disconnect the riser from the blowout preventers during the course of a well to effect repairs to subsea components, or in an emergency occasioned by a station keeping failure. Prior to any planned riser disconnect, the mud in the riser is displaced with seawater with the mud being returned to the mud pits on the vessel. The mud to be displaced, and stored on the vessel, that is contained in the marine riser in 1,500 feet of water, is about 600 bbls and weighs about 200 tons. Conversely, 10,000 feet of riser contains about 3,600 bbls of mud weighing nearly 1,200 tons.
In deeper sections of an offshore well where the hole-drilling diameter is small, the rate of mud circulated through the bit is reduced proportionately. For these sections, the annular velocity of the mud returns in the 21″ marine riser is quite low, and while this is not much of a problem with shorter risers, in deeper water it is insufficient in the riser to carry drilled cutting solids to the surface, and an additional mud pump is required to circulate or “boost” the marine riser.
The overall cost of a deep water drilling unit is proportional to its displacement size, variable load requirements, and equipment capacity. By way of example, a conventional design for a shallow water drilling unit and a deep water drilling unit may have the following capabilities and costs:
ITEM
1,500′ WATER
10,000′ WATER
Vessel Variable Deck Load
2000
tons
10,000
tons
Hoisting Capacity
500
tons
1000
tons
Mud Pit Capacity
1500
bbls
5000
bbls
Mud Pump Capacity
3000
hp
6000
hp
Free Deck Space Required
7500
sq. ft.
17,500
sq. ft.
Hull Steel Weight
10,000
LT
16,000
LT
Total Building Cost
$180
million
$350
million
The increased size and cost of a deep water drilling unit are directly related to the increased length of the riser. It is postulated that the size and cost of a deep water rig will, within certain limits, be approximately proportional to the square of the riser diameter, and that if the riser diameter could be reduced to about ⅔ of its present diameter, the size and cost of a rig might be reduced by 40 percent or more.
The present invention is directed to a fully capable and functional drilling system capable of drilling, and/or, working over wells presently requiring the use of a 21″ marine riser while utilizing a reduced diameter riser. By way of example, the present invention may use a riser having a nominal diameter of about 15″. Consequently, use of the present invention will reduce the variable deck load, space requirements, hoisting, mud pit and pump capacities and, hence, the cost of a deep water floating drilling vessel.
SUMMARY OF THE INVENTION
The present invention is directed to a deep ocean drilling system for drilling an offshore well in deep water using a reduced diameter drilling riser. The reduced diameter drilling riser extends from a floating drilling vessel, such as a drill ship or a semisubmersible drilling rig, to a lower marine riser package. The lower marine riser package
Hunter William A.
Mowell Roger W.
Toalson David C.
Bagnell David
Howrey Simon Arnold & White , LLP
Kreck John
Ocean Drilling Technology, L.L.C.
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