Surface conductors containing external water-expandable elastomeric elements increase conductor bearing capacity in deep and ultradeep waters where water-saturated soils may lead to wellhead sinking, tilting, and other instabilities. Researchers at China University of Petroleum, Beijing, evaluated several swellable material options to produce a conductor that swells in deepwater environments upon exposure to water but with sufficient delay to transport the conductor to the seabed and jet in before significant swelling occurs.
The swelling surface conductor casing was tested in the South China Sea by China National Offshore Oil Corp. (CNOOC) in 1,500 m of water. Results showed that the conductor had 20% more vertical bearing capacity than traditional conductors.
Swellable conductor design
Subsea surface conductors support the subsea wellhead, inner casings, lower-marine-riser-package blowout preventer (LMRP-BOP), low-pressure housings, and high-pressure housings during completion and production, producing hundreds of tonnes of load (Fig. 1). The conductor must not sink or tilt under such loads.
Offshore surface conductor jetting without cementing has become the installation method of choice in the Gulf of Mexico, North Sea, offshore West Africa, and in the South China Sea. During jetting, the soil around the surface conductor is disturbed by the water, and the bearing capacity of the surface conductor initially decreases but gradually recovers as the soil fills in.
The axial bearing capacity of the subsea wellhead derives from vertical friction between the outside surface of the conductor and surrounding soil, and resistance of the conductor’s bottom against the soil (usually negligible due to the small surface area). With increasing water depth comes increasing water content in shallow soils which reduces friction and therefore resistance of the conductor to loads. Measures to counteract this decreasing friction include using larger and longer conductors or waiting for the soil to collapse and partially dehydrate around the conductor. These approaches add material cost, time, and complexity to conductor setting.
Hydrophilic groups and rubber additives vulcanized into the rubber matrix provide high strength and water swelling. An adjustable permeable coating on top of the rubber delays swelling until the conductor is jetted into place. Average initial-state rubber density is 2.5 g/cu cm, the elastic modulus of the material is 2.8 gigaPascal (GPa), Poisson’s ratio is 0.3, and yield strength is about 50 megaPascal (MPa).
A set of experiments proved out the expansion process under subsea pressures and temperatures. Temperature had a large impact in initial expansion due to the temperature-insensitive permeability of an isolation coating, but only a 5% difference in expansion at later times. For example, at 5° C. and 5 MPa, expansion was 5% after 20 hr and 250% after 140 hr. At 20° C., initial expansion was 20% but after 140 hr expansion was 260% which was only slightly higher than expansion at 5° C.
Installing the expandable conductor follows the same procedure as standard conductors. The conductor is located near the mudline to site the final target position. The conductor is jetted in place until it reaches target depth. Expansion starts once the conductor is exposed to water, delayed by the coating, and final expansion occurs after the conductor is in place. Expansion curves show that minimal expansion occurs in the first 20 hr of exposure, after which expansion occurs rapidly.
Material tests show that the friction coefficient of the expansion material per unit area was about 1.5x that of the conductor body. The strength of the expansion material is higher than that of shallow soil and will not deform, resulting in end resistance calculations identical to those for surface conductor pipe, but with altered dimensions to account for expanded elastomer.
Bearing capacity of the swellable conductor mainly depends on expansion material length, expansion efficiency based on subsurface temperature and pressure, and the segmented arrangement of expansion material. Expansion calculations based on subsea conditions determine segment lengths and numbers.
The swellable surface conductor was installed by CNOOC in the Ling-Shui area of the South China Sea in 1,500 m of water. Up to eight 11-m surface conductors, connected for a total length of 88 m for each well, were planned for the project. Drillbit OD was 660.4 mm and unexpanded OD of the surface conductor was 914.4 mm. Conductors were jetted to a planned 80-m depth. Undrained soil shear strength ranged from 3.0 kPa at initial jetting to an average 94.0 kPa at 82.5 m.