Well conductors in gravity base structures (GBS) are subjected to a variety of consideration of the static axial wellhead and loads that are not present in conventional fixed steel platforms. Important GBS conductor design considerations are defined and a method is described which can be used to realistically analyze conductor/casing behavior. The method employs a widely beam-column computer program modified to account for thermal strains induced by changes in the wellstring temperature at the onset of oil production. External loads and restraints, resistance may be considered. An example problem highlighting the pertinent design parameters is presented. For situations in which predicted conductor/casing stresses exceed yield, an alternative design method is discussed.
Wellstrings used in offshore oil and gas drilling have two distinct components, the conductor and a series of inner well casings. For conventional fixed steel platforms, the conductor is simply a steel pipe pile installed by driving. After conductor installation, drilling is initiated and well casings are grouted in the pre-drilled hole. Several inner casings of decreasing diameter are normally required to reach target penetration.
A major function of the conductor is to provide a stable hole near the mudline so that excessive hydraulic fracturing is minimized during drilling and installation of the first casing. The conductor is also used to support the weight of the inner casing prior to grout set-up and to protect the inner casing from the environment.
Selection of the conductor penetration and wall thickness for fixed steel platforms is typically based on drilling experience and wellstring loads. Design of conductors and casing strings for use in gravity base structures (GBS) is more complex due to the presence of additional loads. Lateral loads are applied to the conductor by both he available structure and lateral soil movement beneath the base during wave loading. Axial loads include soil downdrag resulting from consolidation of soil beneath the GBS and thermal loads produced by temperature changes in the soil downdrag loads, and nonlinear soil well casing during oil or gas production.
In the following sections, the pertinent GBS conductor/casing design considerations are described, a realistic stress analysis method and example problem are presented, and an alternative design approach is discussed for cases in which the predicted stresses are beyond yield.
Major design considerations for conductor casings used in both conventional fixed steel structures and gravity base structures are illustrated in Fig. 1. Conductor/casing design for GBS platforms must account for important variables that cannot be reliably based only on experience and installation wellhead loads. Installation techniques, lateral loads and axial loads must all be considered to establish a reliable design.
Since GBS platforms are best suited to locations having strong surficial soils, driveability of the conductor is an important consideration. Historically, GBS conductors have been installed by "drill-and- drive" techniques where the internal soil plug is removed and a pilot hole drilled ahead of the conductor at or near refusal.