A New Approach to Completing a Previously Drilled Subsea Well

ABSTRACT Offshore operators tie successful subsea exploratory wells into field plans when possible and have used a variety of adaptation bases to complete the wells. The expense and complexity of these bases led to redesign of the subsea wellhead/- guidebase system to make completion less expensive and easier. Equipment described includes a universal 30" housing, retrievable drilling guide structures, and completion guide structures. Styles and procedures for diver/Remote Operated Vehicle-assist, diverless guideline, and diverless guidelineless installation are reviewed. In addition to achieving cost savings by reusing the retrievable drilling structures, two-stage guide structure systems described here allow custom design of the completion structure/ flowline assembly after the field development plan is finalized. INTRODUCTION (Figure 1) Subsea wellhead systems have been used for over 20 years for exploratory and development drilling. Two recent changes in operator's standard development programs have required design changes in the wellhead system to make exploratory well drilling equipment adaptable for subsea completion without large initial capital investment. The first change in common development programs has been to incorporate successful discovery wells into field development plans. In diver depths, these wells have been historically completed subsea with relatively simple adaptation bases landed over the existing guide base that include flowline connection and tree alignment provisions. Beyond diver depth, adaptation bases are complex and expensive, sometimes requiring a tubing spool adapter. This requires an additional wellhead connector and introduces a new set of seals with consequent leak path potential. The second change is to include satellite subsea completions in field development plans. When drilling a subsea well with completion a certainty, a flowline receptacle adaptation system can be incorporated in the drilling guide base. However, the well is usually drilled 6 to 12 months in advance of completion equipment manufacture. Close manufacturing tolerances and expensive test jigs are required to insure fit-up of the completion tree and flowline receptacle. Adaptation systems, whether for wells preplanned for completion, or discovery wells, also have the disadvantage of potential tree landing problems if excess cement or other debris has collected around the permanent guide structure during drilling. For these reasons, new retrievable drilling and completion guide bases were designed to simplify completing subsea wells, making the operation less expensive and more reliable. After drilling a well, the drilling base is retrieved, leaving the 30" housing ready to receive a completion base. The universal 30" housing provides a simple interface with known manufacturing tolerances. The completion base is manufactured as a part of the completion system, allowing complete fit-up testing at the factory and on the rig installing the completion equipment. Several types of retrievable drilling and completion bases have been designed allowing for diver/ROV assist and diverless guideline and guideline less drilling and completion.