Riser with composite choke/kill lines ready for Gulf of Mexico trials
Stewart and Stevenson and Lincoln Composites plan to introduce a hybrid drilling riser before the end of this year. Replacing the steel auxiliary lines in riser joints with composite lines can reduce the weight of the riser string by almost 20%. - The Stewart and Stevenson riser system with composite choke and kill lines reduces riser weight by almost 20%.
One of the limiting factors in taking a mobile offshore drilling unit (MODU) out into deepwater and ultra-deepwater is the deckload capacity of the vessel. Mooring lines, riser joints, hook, and variable loads all contribute to this weight. A variety of other limitations require third generation semisubmersibles to undergo major upgrades before they can operate in water depths as great as 6,000 ft.
While there are a number of composite riser projects underway, this is the only one that focuses on replacing the auxiliary line tubulars (including the choke, kill, booster, and hydraulic lines) with composite materials. By leaving the main riser pipe body and flange joint intact, Douglas Johnson, Business Development Manager for Lincoln Composites, said these riser joints could be substituted one-for-one with steel riser joints in a deepwater riser system. The brackets and pin and box arrangement are the same; even the buoyancy modules will be interchangeable.
Stewart and Stevenson first approached Lincoln at a con ference in 1998. Johnson said the two companies are well suited to collaborate on this project. Lincoln Composites has extensive experience in the design and manufacture of similar products for the oil and gas industry, as well as industrial, defense, and aerospace applications. Stewart and Stevenson has experience in the development and manufacture of risers, blowout preventers (BOP), valves, and other components and equipment for the offshore industry.
Previous products
Lincoln Composites has been manufacturing composite structures using the filament-winding process since 1963. Johnson said Lincoln is completing the development of a composite production riser in partnership with Shell, Conoco, and BP Amoco under an NIST ATP grant.
Separately, Lincoln is under contract to Norske Conoco and Kvaerner Oilfield Products to develop a composite drilling riser pipe. Statoil, Saga Petroleum, Norsk Hydro, Shell USA, Petrobras, and Chevron are sponsoring this project, along with European community support under the EU Thermie Program. A field or sea-trial is scheduled for later this year on Statoil's Heidrun tension leg platform, following approvals by the Norwegian Petroleum Directorate and operations management. "It's taken us about a year to go through the design, development, and qualification testing of composite choke and kill lines," Johnson said.
Once testing is complete, Stewart and Stevenson will have a composite auxiliary line that can be retrofitted onto existing riser joints. This not only lightens the deckload, hookload, and craning requirements for the drilling unit, but reduces the volume of syntactic foam buoyancy needed to support the riser joints under water. This means foam modules with a smaller outside diameter can be used. Because many of the vessels operating on the fringe of deepwater still have 49.5-in. rotary tables, this is a critical ingredient. If the foam-encased joints exceed the diameter of the rotary table, they cannot be run.
Louis Slaughter, Marketing Manager for the Petroleum Division of Stewart and Stevenson, said this is a very specific market focus. The new generation of ultra-deepwater drillships does not need to worry about deckload. They have capacity to spare and large, 60-in. rotary tables.
The drilling and production units being targeted by this project are those on the edge of deepwater that want to be able to be able to drill in 6,000 ft water depths without undergoing major upgrades.
Testing program
The key to any composite riser system would appear to be the interface between the composite material and the steel of the end fittings. To test the design of the metal composite interface (MCI), Slaughter said the composite tubulars were subjected to a burst test at Stress Engineering's labs in Houston. He said the MCI surpassed the API standards for flexible choke and kill lines.
From this success, the design underwent a series of fatigue tests. The tubular was pressured up to the API test pressure for the design, then back to zero. Then it was cycled between 15,000 psi and zero pressure for the equivalent of 20 years life cycle.
A third test is now underway to determine how the materials will perform when exposed to a variety of compounds. A sample of the composite material will be exposed to hydrogen sulfide, carbon dioxide, diesel oil, and a variety of drilling muds under pressure. Once these lab tests are concluded, Slaughter said Stewart and Stevenson would like to field-test the design, mainly to test for erosion resistance within the lines.
Assuming all goes well, Slaughter said the product will be on the market this year. Slaughter said the design will be incorporated into the middle joints of a riser string so that at the top of the riser would be the telescoping joint and a bare joint of riser. Below would be the joints with composite auxiliary lines. These would be encased in buoyancy modules, which not only provide support, but also protect the composite lines in the same way it would protect steel lines.
Below this would be a number of bare joints used to give the string the proper balance. Because the design allows for less syntactic foam, it not only allows the joints to pass freely through the rotary table, but gives the string a narrower profile in the water. Slaughter said this narrower profile means the riser string is less susceptible to the forces of sea currents.
The lighter riser string would mean a given rig could step out into deeper water without the need for additional riser tensioners. Slaughter said he estimates the value of the weight savings on a deepwater vessel, when all the factors are considered, is about $7-10/lb.
