Special Guide Bases for the Gulf of Alaska
Authors: Schuh Frank J. 
Published: December 2007

This paper discusses the designs of two medal guide bases developed for part of the Gulf of Alaska. Required support capacity of a guide base is part of the Gulf of Alaska. Required support capacity of a guide base is compared with predictions of the bearing capacities of Gulf of Alaska soils using published soil bearing capacity relationships and soil shear-strength data. Field measurements during installation of one base indicate satisfactory agreement between predictions and performance.

Introduction

One of the most important steps when drilling a well from a floating rig is the installation of the 30-in. conductor and permanent guide base. This string must be set essentially vertical and at a depth that will support the overhead and blowout preventer stack. In areas outside the Gulf of Alaska, the 30-in. conductor always has been installed using one of two commonly used methods. Usually, the process involves setting a temporary guide base on the process involves setting a temporary guide base on the sea floor, drilling a 36-in. hole, landing the 30-in. conductor with the permanent guide base on the temporary guide base, and cementing the conductor back to the sea floor. In areas where the soils are too weak to support the temporary guide base, the 30-in. conductor could be jetted to the required penetration. Unfortunately, neither method can be used in a significant portion of the 1976 Gulf of Alaska OCS (outer continental shelf) sale area.In part of this area there is an unconformity at or near the sea floor that separates a hard overconsolidated silty clay from a very soft marine clay. The soft clay varies in thickness from a few feet to about 100 ft in the areas of interest. The soft clay is too weak to support a conventional temporary guide base. The 30-in. conductor cannot be jetted to the required penetration because the silty clay is too difficult to get through and the soft soil is too thin to provide the required penetration. provide the required penetration. Special Guide Bases for the Gulf of Alaska

Two special temporary guide bases have been developed to permit drilling in areas that cannot be drilled conventionally. Figs. 1A, 1B, 2A, and 2B are conceptual drawings that also show the bases during installation and after drilling the 36-in. hole and installing the 30-in. conductor and permanent guide base.The floating base is shown in Figs. 1A and 1B. The base is a 15- x 15- x 10-ft-high water tight box that derives much of its support by floating in the soft soils at the soil-water interface. When filled with water, the base weighs 84 lb/cu ft, which is halfway between the 104+ lb/cu ft minimum density of the soft soils and 64 lb/cu ft sea water. The 15- x 15- x 10-ft two-piece construction is a good compromise between transportability and maximum size that can be handled on the spider deck of large semisubmersible drilling units.The floating base is attached to a 40-in. conductor that is used to case off most of the soft soils. This base and 40-in. conductor are designed for use in areas where the soft soils range from 10 or 20 ft to 200 ft or more, where the 30-in. conductor could be jetted to sufficient depth without using a temporary guide base. The 40-in. conductor and floating base are run as a unit and are installed by jetting the 40-in. conductor. The 30-in. conductor and permanent guide base are installed by drilling a 36-in. permanent guide base are installed by drilling a 36-in. hole below the 40-in. conductor, landing the 30-in. conductor and permanent guide structure on the floating base, and cementing the 30-in. conductor back to the sea floor.

JPT

P. 1635