DRILLING & PRODUCTION

Leonard LeBlanc
Houston

HP/HT challenges pipeline
transport of crude production

A bending unit shown schematically at the rear of the Allseas Lorelay's firing line. [49240 bytes]

More oil and gas fields featuring high pressure and high temperature conditions are being developed today as producers overcome the associated problems downhole and lower the cost. One persistent problem, particularly in flat seabed areas, for which there are mostly expensive solutions is pipeline axial expansion.

As pipeline length increases with high product temperatures, it encounters soil resistance along its length. At the weakest points (curves, imperfections, soil weaknesses) the pipeline begins to deform into curves (snaking). Because of the weight of internal product and external coating, the first expansion on a flat seabed is lateral. However, lateral constraints of the trench forces the snaking pipeline upward (upheaval buckling).

Unless the weight of the pipeline overburden is great, usually enhanced by rockdumping, the curving pipeline will emerge eventually from the trench. Exposed pipelines become a navigation hazard in shallow water or fishing hazard in moderate water depths. In the North Sea, trawl boards penetrate up to 0.5 meters of seabed. Even cutting away and replacing the emerged section is not a predictable solution, since the problem could re-occur when flow is re-started.

In extreme uplift cases, the line can develop significant ovality or actually buckle, which could create slugging, pigging, and inspection problems. Producers have a choice of four solutions when transporting HP/HT production:

Overburden restraint: Depending on the needed height of a berm over the trench and water depth, rockdumping can cost as much as $1 million per mile.
Deep burial: Again, the cost of deep burial is high since a mechanical trencher is usually needed with additional time required to assure sufficient depth. Even so, there is no guarantee the pipeline would not emerge in time unless a rock berm is added.
Lay in horizontal curves: The lay barge can install the line in large curves that could accommodate axial expansion laterally. While the additional cost is minor, the restraints here are existing facilities and infrastructure, and sometimes, the short length of the production line.
Bending the line: S-shaped bends can be induced into the line periodically as the line is being laid to create points of expansion. This involves either preparation of the bent pipe joints ashore or investment in laybarge bending equipment.

Pipe bending ashore has been done previously in Mobile Bay (Alabama, USA) and the bent sections inserted into the pipeline at the end of the firing line, but the cost is not insignificant. Alternatively, bending equipment can be installed at the end of the firing line to bend the coated pipe as desired. Allseas and Advanced Mecha-nics and Engin-eering have developed and patented a bending machine for Allseas lay vessels to accommodate 200-400 mm bending requirements.

References:

Vermeulen, H., "Theory and Practice of Installing Pipelines by the Pre-Snaking Method," technical proceedings, ISOPE, The Hague, June, 1995.

Soevik, S., Levold, E., "High Temperature Snaking Behaviour of Pipelines," technical proceedings, ISOPE, The Hague, June, 1995.

Reversed ESP unit taps
production flow for electric power

A schematic of a reversed power generation unit hooked to a production flow regime for platforms that are unmanned or isolated. [42587 bytes]

The range of electrical power options for isolated unmanned platforms continues to grow. Photovoltaics and wind turbines can be used to power communications and minor control equipment, but power needs for production valving and process equipment require heavier impose heavier electrical loads. Generally, diesel generator sets or subsea power cables meet those needs, unless the platform is remote or the cost of generator sets and maintenance becomes too great.

Mobil, Hayward Tyler Fluid Dynamics, and British Gas are testing a device that uses production flow to generator power for use aboard North Sea platforms that may be de-manned in the future.

The unit is most like a multistage electrical submersible pump (ESP) that has been reversed and the drive motor turned into an induction generator. Development began with a gas expander, but has evolved into a device capable of taking various flow phases. The unit is designed to act as if the flow regime is not compressible. There is no gearbox and the unit can handle fast phase changes.

The generator is placed in parallel with a control valve that can be used to trim the differential pressure across the turbine. The electrical output can be in any form desired. In addition, power from the generator can be used to pre-heat the production stream full time, or in the event ice, hydrates, or waxes begin to form. Long-term tests, including sand erosion tests of the blades, are underway.

Reference:

Webster, G., Brazil, J., "A Novel Form of Offshore and Subsea Power Generation," technical proceedings, Offshore Mediterr-anean Conference, Ravenna, Italy, March, 1995.

Chinese chase reserves with S-shaped well

China's CNOOC believes it has drilled a record S-shaped well in one of its Bohai oilfields. The well turns east at an angle of 72 at a depth of 2,617 meters. The well was drilled 219 meters before turning to the west at an angle of 38 and a depth of 2,936 meters, and reached an oil-bearing structure soon afterward. The inclination reached as high as 3.3 per meter.