Deepwater swellable conductors increase load capacity

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.

Conventional vs. Swellable Conductor (Fig. 2).

New elastomeric materials are proposed to increase friction between the surface conductor and soil in deep and ultra-deepwater environments. The elastomer adheres externally to the pipe and expands in place (Fig. 2). The material’s properties include delayed expansion, high compressive strength, high expansion ratio, and high toughness.

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.

Swellable-Element Expansion, 5 degrees C. (Fig. 3).

Pressure had significant impact on the expansion process. Understanding pressure sensitivity is critical because the surrounding soil will gradually fill around the conductor after it is installed and increase pressure against the rubber. Pressure tests at 5° C. showed that early-time expansion under 30 MPa and 5 MPa was largely the same due to rate-limiting effects of the permeable coating. After 140 hr, however, expansion under 30 MPa was about 110% compared with 280% at 5 MPa (Fig. 3). The amount of expansion under high pressure should still be sufficient for application given typical jetting conditions.

Application

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.

Conductor Load-Bearing Elements. (Fig. 4).

After expansion, the thickness of the expansion material can be more than 5 cm. It not only increases friction through a larger outer diameter of the conductor (more exposed area), but it also increases friction between the expansion material and soil (higher coefficient of friction). The volume of the expansion material is known when the conductor is fully expanded, and Equation 1 calculates conductor load-bearing capacity based on parameters shown in Fig. 4. This method assumes that the surface of the expansion material is uniform and expressed as the average coefficient of friction, the expansion radius is consistent, and the subsurface temperature and pressure are unchanged during the jetting process.

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.

Field application

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.

Conventional-Conductor Axial Load Resistance (Fig. 5).

Fig. 5 shows the axial loads, accounting for axial soil resistance, exerted on the 80-m x 914.4-mm conductor during well construction (Table 1). Highest axial load on the conductor occurs at the end of the sequence upon cementing the 9 5/8-in. casing (Step 13). The axial load is 9,775 kN at this point, above the axial resistance of 8,662 kN available from the conductor. Step 3 also exhibits an axial load above the conductor. An 80-m conductor is therefore not suitable for this well plan, and the traditional approach to well redesign would be to extend conductor length. A 90-m conductor would adequately support all loads during the drilling sequence, though below the API recommended safety factor of 2.0 at Steps 3 and 13.

Fig. 6 shows axial loads produced by an 80-m conductor with expandable elements. Time to install the conductor was estimated at 2.25 hr compared with 2.75 hr for the 90-m conductor, and the safety factor improved. The conductor contained 4 x 1-m expandable elements spaced 1 m apart per 11-m conductor. Eight conductors are required for 80-m jetting, and four conductors containing elastomers were assembled at the second, fourth, sixth, and eighth conductor from the subsea wellhead. Initial thickness of the expansion material was 20 mm, with 250% expansion efficiency at 4° C. and 15 MPa subsurface pressure.

Swellable-Conductor Axial Load Resistance (Fig. 6).

Waterproof plastic protected the conductors against sea spray, rain, and humidity in transit and at the platform. Based on the slow initial swelling rates, timing for swelling started after the conductor was installed at the designed depth in the soil after jetting. Drillstring lift force balanced conductor and drillstring weight during expansion and setting. Lifting force tests of the top drillstring, while not displacing the surface conductor from the soil, measured setting strength at 50-, 100-, 200-, 500-, 1,000-, 1,500-, 2000-, 2,500-, and 3,000-min intervals. Fig. 7 compares theoretical calculations of conductor vertical bearing capacity based on swelling test data, measured axial force from field tests, and theoretical results of traditional, non-swellable conductors.

Field results matched theoretical calculations and showed that the swellable conductor added 20% vertical bearing capacity of the subsea wellhead over traditional conductors. The end-surface area of the swelling material increased vertical bearing capacity by only 2% above standard conductors, but side expansion added 18%.