Buoyant drilling riser concept uses steel inner and outer hulls

Double hull provides buoyancy, control redundancy

Hans van der Poel
Adviesbureau
H. van der Poel

• Horizontal and vertical views of a dual-hull air buoyancy drilling riser section. [109,787 bytes]
• The running sequence for a drilling riser string with air buoyancy hulls. [65,731 bytes]

In deepwater, drilling vessels are connected to the bottom by the aid of a riser, which encloses drilling and workover operations. On the bottom of the riser, a lower marine riser package (LMRP) functions as a large valve that can be operated from the drilling vessels by pressure piping.

The pressure piping is installed on the outside of the riser. A riser pipe will be installed by lowering or running it from the drilling vessel in sections up to 75 ft, which are connected by couplings or flange connections.

Conventional riser pipes are made from a steel pipe about 19-in. in diameter. To reduce the load on the vessel caused by riser configuration and weight, buoyancy blocks are installed on the outside. These blocks are simply plastic or plastic-like material filled with air.

Exterior to the riser pipe, four or five pipe strings with diameters ranging between 2-in. and 4-in. assist in the operation of the LMRP. These pipes fit in the buoyancy blocks. The maximum water depth possible with this conventional riser configuration is about 2,200 meters. The maximum allowable diameter of couplings plus buoyancy is 45-in. The vessels' riser-running limitation is generally 46-in.

The disadvantages of conventional risers are as follows:

• Due to water pressure acting on buoyancy material, the buoyancy blocks need to be very strong, which means the deeper the water, the less effective the buoyancy.
• When running the riser, drag/wave forces create contact between the riser assembly and the hull of the drilling vessel.

Contact between the two results in damage and loss off buoyancy blocks and damage to the pressure piping. This means less buoyancy, increasing the hoist load required by drilling vessel, which is limited. In practice, this means a 10% reduction of possible water depth.

When pressure piping is damaged, the riser string has to be pulled up to replace the damaged riser section, otherwise the LMRP cannot be manipulated as required for attachment and detachment. Downtime for drilling vessels as a result of riser-vessel collisions cannot be insured.

There is a huge repair cost as a result of such collisions. Repairs sometimes have to be performed onshore. Because of the long periods required for delivery, it is necessary to maintain a large stock of buoyancy blocks.

Also, waves and drag forces at the surface and in the water column produce damage to the buoyancy blocks. Insurance companies therefore require an annual inspection of the total riser configuration.

New riser concept:

More effective buoyancy can be achieved by using the maximum allowable riser configuration diameter which can be filled completely by air. Such a configuration provides the following:

• Pressure piping for the LMRP is well protected.
• When clashes with the hull structure occur, damage to the buoyancy and pressure piping is reduced. Downtime is reduced.
• Connections of riser pipes can then be made by standard couplings.
• The riser string can be repaired on the drilling vessel's deck by a general maintenance crew.
• The cost of this type of riser is similar to that of conventional configurations.
• There is no difference in handling with respect to drilling tools.
• The stock of replacement riser sections is minimal.

By installing a strong/tough outer riser hull at maximum configuration diameter (made from weldable material), riser engineers can meet these desired requirements. With such a design, the outer riser pipe is connected to inner pipe, and the pressure piping runs between the outer and inner pipes.

Existing steel products, such as Weldox with a strength of 160 ksi, can be used for such a riser configuration. The material is strong, tough, and weldable. The space betwen the two hulls is precharged by compressed air. The result is that the maximum water depth of the new design can be extended beyond conventional configurations.

Calculations show that with the new riser concept drilling at water depths of about 3,500 meters are possible (based on the actual maximum hoist load of drilling vessels). A study shows that drilling companies expect in the next few years to explore oil wells at greater depths than conventional risers allow.

The double hull configutration concept allows the inner hull to take up some of the functions of the outer hull if failure occurs. As required riser outer diameters continue to increase with depth, this concept provides the additional buoyancy needed. The drag load on the larger diameter will increase, but that drag load increase is linear as buoyancy goes up by the power of two. This allows the maximum water depth application to move to 3,500 meters.

At the moment, Swedish Steel and DnV are doing material tests on the concept. Noble Drilling, Kværner, and Hydril have all expressed an interest in the concept.

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