A System for Tieback of Subsea Wells to Fixed Platforms

Abstract

Summary Tieback systems allow wells drilled with floating drilling vessels and subsea drilling equipment to be tied back later to a platform installed over the subsea wells. The system includes templates and equipment for drilling, platform location and guidance, and tieback. Introduction Over the past few years, a different approach to developing offshore oil fields with platforms has proved to be economical, practical, and reliable. This approach allows development drilling and platform construction to occur concurrently. First, a subsea template is installed. The desired number of wells is drilled through the template using a floating vessel, while the platform is constructed at a separate location. The drilling vessel is moved, and the fixed platform is installed over the subsea template. Then, the subsea wells can be tied back to the platform deck. The tubing is run, surface production trees are installed, and the wells are produced in the conventional manner (Fig. 1). This method provides quicker production from an offshore field, with resulting accelerated return on the investment.Several of these tieback systems currently are producing to platforms, and numerous other offshore operators have installed templates and presently are drilling subsea wells through these templates.This paper describes the equipment used with this system for providing early production of oil fields as well as the experience gained to date. The equipment includes subsea templates, subsea drilling equipment, platform location and guidance equipment, and tieback equipment. Subsea Templates The subsea template (1) provides the base through which the subsea wells are drilled and (2) spaces and aligns the subsea wellhead equipment. Three basic types of subsea templates are unitized, modular, and spacer. Each template type has advantages in a particular application.The unitized template (Fig. 2) is recommended for water depths greater than 300 ft (90 m) and for use with more than six wells. Fabricated from 30-in. (762-mm) or larger tubular members, it incorporates receptacles for each well and for a three- or four-point leveling system. It normally will include receptacles for the installation of the platform location and guidance equipment.The template is fabricated at a dockside facility, Depending on the desired method of installation, the template either will be fabricated as a "passive" template or will incorporate built-in buoyancy control. After nondestructive and functional testing, the template is transported by barge to the drilling location. The unitized template is lowered to the ocean floor using one of the following methods.The passive unitized template simply is lowered to the ocean floor by a crane barge. The crane barge lifts the template from the work barge. Once it is in the water, the template load is transferred to lines connected to one or more deck winches for lowering to the ocean floor.Another method of installation requires incorporation of provisions for controlled buoyancy in the template. Such a template is initially buoyant. The cargo barge is submerged partially, and the template is floated off. Lines are attached from the template to the drawworks of a drilling vessel. As the template is ballasted, a work boat assists in keelhauling the template under the drilling vessel. JPT P. 188^