While the technology is proven and has been in use for decades, the use of drillships for deepwater operations has become the method of choice for operators. Currently, there are 26 deepwater drillships capable of operating in water depths of 3,500 ft or more. Eighteen of these are newbuild or converted drillships that have entered operation within the past year or are under construction.
The shallowest water depth rating for which these drillships are outfitted is about 4,000 ft, and this is mostly older vessels. Most of the newbuilds and conversions are rated for 10,000 ft water depth capability.
The type of vessel to use for the deepwater has been one of the longest ongoing debates in the industry. In the past, there has been a split between favoring the use of semisubmersibles or drillships. Each system holds definite advantages over the other, especially in terms of stationkeeping:
- Drillships offer maneuverability in terms of being able to move from location to location under their own power. Non-self-propelled semi submersibles need to be towed to location.
- Semisubmersibles are inherently more stable and offer better motion characteristics than drillships. But, drillships also offer a much higher deck load.
This long-standing debate has evolved into changes in design that attempt to bring the positive characteristics of both vessels together in one optimal solution. This has led to larger semisubmersibles with high deck loads and drillships equipped with better motion characteristics.
DP for drillships
On the drillship side, Transocean, for example, is building three massive vessels - Discoverer Enterprise, Discoverer Spirit, and Discoverer Deep Seas. "You get a lot more variable capacity for the amount of steel you have to use with a ship, compared to a semisubmersible, explains Dennis Heagney, President and COO of Transocean. "Trying to dynamically position a semisubmersible is a huge problem. When we have a hurricane, we can run with a ship; you have a tough time doing that with a semisubmersible. These drillships are so massive, they have excellent motion characteristics."
These new designs in the market have almost completely bridged the gap between the two technologies on the drillship side. The largest drillships ever built are under construction and offer motion characteristics and almost all the advantages of a semisubmersibles in a ship-shape form.
Size matters
The largest drillship design under construction is Transocean's new Enterprise-class design of drillship. The vessels, which include the Discoverer Enterprise, Discoverer Spirit, and Discoverer Deep Seas, measure 835 ft in length by 125 ft in breadth. Just behind this is Smedvig's West Navion drillship, which is being converted from a multi-purpose shuttle tanker. The West Navion measures 830 ft by 138 ft. The smallest newbuilds are the Pride vessels, Pride Africa and Pride Angola. These vessels measure 689 ft by 98.5 ft.
Compared to the existing fleet of drillships, this new breed of design will dwarf the fleet by more than 100 ft in length and about 20 ft in breadth. The largest purpose-built drillship that has been in operation is the Noble Roger Eason, at 541 ft by 80 ft, followed by Diamond Offshore's Ocean Clipper, at 528 ft by 109 ft. Both were constructed originally in 1977 and both are conversions. The smallest operating drillship is the Schahin Cury Lancer which measures 450 ft by 77 ft, also originally constructed in 1977.
The size of a drillship is crucial to its operating ability. The length, breadth, and draft of the vessels must be calculated precisely to be able to withstand the stresses of the drilling equipment, improve motion characteristics, accommodate the necessary drilling equipment, and meet requirements for dynamic positioning.
Crude oil storage
One aspect of necessary equipment that adds to the size of the vessel is the trend toward crude oil storage. A number of newbuilds will have the capability of storing 100,000-150,000 bbl of crude in the hull. This requires all equipment that would normally be placed within the hull to be stored topside in modules, leaving a relatively empty hull. Consequently, the drillship needs to be designed on a larger scale to accommodate this added equipment on the topside of the vessel as well as the added weight of the storage tanks and associated equipment.
"The main reason for the size is you need the infrastructure to support the dual activity operation," Heagney explains. "We wanted the ability to assemble subsea trees without interfering with the drilling operations, the ability to store crude oil in the event of a long-term well test, and make it easier to develop loading operations during that well test."
A good example of the difference the added production capability makes on the design of the hull is found in two new designs on the market from Gusto Engineering: the Gusto 10000 and the Gusto P10000. The Gusto 10000 is purpose built for drilling only and is the design used on the Pride Africa and Pride Angola drillships. The Gusto P10000 is an almost identical design except that it offers extended well testing and early production with a storage capacity of 125,000 bbl. This added capability extends the length of the design from 204.5 meters, with the Gusto 10000, to 224.4 meters, with the Gusto P10000, while the breadth remains comparable.
Gusto Engineering, one of the leading design firms in the industry, has been a specialist in the design of drillships for more than twenty years. Gusto designed the original Pelican class of drillship, of which 12 were built and operated. Bob Rietveldt, Marketing Manager, Exploration Vessels for Gusto Engineering said that the optimal dimensions for a drillship are 200 meters long by 32 meters wide, with a hull depth of around 18 meters to accommodate the required hull strength and stability properties. He added that the draft of the vessel should be a maximum of about 10 meters to give it sufficient freeboard to allow working in high sea states and keep personnel safe.
Motion characteristics
The dimensions of the vessel have a direct impact on its motion characteristics:
- Length is the primary aspect to consider when trying to optimize the motion of the vessel. The length of the vessel determines the vessel's reaction to heave and pitch.
- Width of the vessel determines its roll motion and associated stability.
"The vessel cannot be too short because it will have problems in the heave and the pitch motion," Rietveldt said. "Again, you would not try to create a very wide vessel, because it becomes too stiff, and the roll period shifts down to the lower sea state. Whereas, you want to get a roll motion period above twenty seconds to be safe from any exciting waves."
Mark Dreith, Global Marine's Sr. Manager of Marine Projects and Project Manager for Hull 456 adds further: "Your stability has a lot to do with what the beam of the vessel is, and how much cargo and how much drilling equipment are above the main deck. The wider the vessel, the more stable it is, and the meta-centric height is increased. From basic naval architecture you start with the center of buoyancy and you calculate the center of gravity based on vessel loading. The distance between the center of gravity to the meta-center has to be positive. If the center of gravity ever gets above the meta-center, the ship will possibly flip over."
However, these new vessels are not expected to have much trouble with these motion problems. With the massive size and displacement of the vessels, the waves have very little impact. The waves that will be encountered during safe operating time will not have the energy necessary to excite the amount of mass the vessel represents to cause any of these motion problems.
This offers the drillship a close approximation to semisubmersible characteristics. The only sea state that would be able to affect the vessels being built today would not be safe for operation, whether drillship or semisubmersible.
DP makes a difference
Another key factor that bridges the performance gap with semisubmersibles in terms of motion is the use of dynamic positioning. With the thruster power of the vessels combined with the mass, ships can be pointed into the waves and absorb energy across the vessel with little effect on heave, roll, or pitch.
"The old adage that if you have a drillship, you have bad motions, and if you have a semisubmersible, you have good motions, is a misstatement when you are talking about a DP drillship," Dreith said. "If we can keep the nose of the ship into the weather, we are going to get motions that are very similar to a semi submersible. We are not going to sit stationary and take the beam wind and wave that will make the vessel roll like the old drillships sitting on an 8-point mooring."
"Longitudinal pitch is going to be less just because of the length of the vessel. It depends on the period of the wave. We can take the maximum operating design wave on the bow and the vessel is not going to heave excessively or roll in the trough."
Shatto Sanity Check
In order to handle this operation, the vessels must have the thruster power to compensate for it. One way to establish how the vessel will maintain station is the revered "Howard Shatto Sanity Check." Howard Shatto, a retired Shell engineer who guided the development of dynamic positioning and other subsea systems, established a reality check based on actual vessel reactions when hit beamside during a sudden squall. The check equates to putting a 60-knot wind on the beam of the vessel and establishing whether the vessel has enough power to take the beam wind and maintain station.
An interesting design to improve the motion characteristics of drillships is being incorporated by Navis and R&B Falcon on the Navis Explorer I. The vessel is being built with a unique hull design from LMG Marin that has three moonpools. The moonpools have a total length of over 70 meters and add stability in that they reduce the waterline area of the hull.
Each moonpool is equipped with a box structure around the bottom and a box structure on the outside of the hull to add negative mass, therefore dampening the heave and roll motions along with the reduced water line. Navis says this provides motions more similar to that of a semisubmersible.
Block coefficient
An additional factor in the new drillship designs is the shape of the hull and sleekness. This relates to how well the vessel moves through the water and resistance to forward motion. The lower the resistance, the less energy it takes to keep it there. To assess the sleekness and the resistance on a vessel, a number called a block coefficient is used.
Dreith described the block coefficient: "If you take a basic rectangular shaped ship with a blunt bow and stern, it will have a block coefficient of 1.00. As you start carving away the pieces of that rectangular block and create the shape of the ship's hull which will allow water to flow smoothly around it, you lessen that basic block and thereby reduce the block coefficient."
Thus, the higher the block coefficient, the greater the resistance, and the more thruster power needed to keep it on station., "The optimal block coefficient is around 0.8," Rietveldt points out. "Usually that is a compromise between displacement, because you want to carry that much with you and the sailing speed. If you go beyond 0.8 together, with the effect of the thruster pods and the moonpool, it will be very difficult to maintain an efficient speed. Then, more power has to be installed."
Conversion philosophy
Many of the vessels that are being constructed as drillships are conversions from tankers. In terms of the block coefficient, a tanker is estimated to have a coefficient of around 0.90. However, this is not the only concern with a tanker hull. The conversion of a tanker hull has a greater chance to add additional cost to the project.
A tanker is designed to transport oil, not to drill. When different loads are applied to the hull that was designed for transporting oil, further engineering must be done to ensure the integrity of the vessel. Costs can escalate. One such example is the West Navion drillship that now has costs in the $500 million range.
"A tanker design has a lot of deck space and it can hold crude oil. That is where the conversion philosophy originated," Dreith said. "That design has to be modified from a longitudinal strength point of view. You are taking a tanker and cutting a hole in the middle. Tankers were meant to be tankers, and they had continuous hull form throughout. There is certain amount of "beefing up" that has to be done to the hull. A tanker hull is convertible, but there are structural requirements that have to be looked at when you cut the big moonpool out of the center."
Thomas Duhen, Technical Director of Pride International, also added that "other designs are in the market because they are conversions from tankers, but they are not initially designed for drilling load curves."
Rietveldt added that the "the tanker conversion projects we have seen, from a concept phase, look attractive. But, you have to go through basic design and a large big part of the detailed design to make certain the costs you get are the right costs, and you must make certain you can trust those costs when you are going into a shipyard contract."
Regardless, for some, the tanker conversions have been a true success story. R&B Falcon's Pathfinder design, for example, is a tanker conversion. Of the four that were ordered, three have already been delivered on time and under-budget or on-budget. The Deepwater Pathfinder and Deepwater Millennium also are operating, and unlike some of the other newbuilds, have not had to return to dock for early break-in problems.
Dennis Heagney summed up drillship design: "You need to think of a drillship or any drilling rig as a symphony. It must be well balanced. You have to have the right quarters, the right material handling equipment, the right deck space, correct motion characteristics, good stability, the ability to load a lot of equipment, and the vessel still has to have the right properties for flotation and safety. It is a vicious circle, and you have to get the proportions right."
Regardless of how the design looks on paper, the only true way of proving its worth is in operation. In the next two years, these designs will be put to the test. For the most part, they seem to have achieved two primary goals: to capture some of the main advantages of semisubmersibles, and move the operator into greater water depths.
