A Second Generation Of Subsea Maintenance Tools
Authors: Chitwood James E. 
Published: December 2007

Subsea tools and workover riser systems are presently evolving into their second generation. Significant work causing this evolution is not only design and feature improvements but analytical evaluation of component options. This paper examines the requirements and past solutions for workover systems. It then develops a new recommended system configuration and reviews each component of the system in detail.

Introduction

A second generation of deepwater Subsea Wellhead Installation and Maintenance tools has been under development. This hardware is ideally suited for completing a well including the installation of the tubing hanger in the wellhead, running the tree, and performing workover jobs.

These new tree handling tools have been engineered, developed, built, and land tested over a 2-year period culminating in a sea test in the Fall of 1978. These tools consist of two major items:the handling tool and its interface to the tree and control pod, andthe riser system for connecting the tree to the surface.

The riser system is a small composite multi-bore riser specially designed for its unique jobs. The most remarkable feature of the riser is the multibore flexure column. Many flexure concepts were evaluated before selection and development of this design. Stresses in the riser were evaluated using surface supports that are typical for the northern North Sea. One hundred year storm conditions superimposed upon a static offset of the vessel revealed the riser is limited not by stress, but by operational considerations of men working around the system.

The handling tool is guidelineless utilizing the funnel down concept for reentry. Hydraulic cylinders keep the connection from mating until successful orientation has been obtained. This handling tool provides for the replacement of the control pod if the pod is attached to the tree cap.

By locating the tree control system on the tree cap, components such as shuttle valves and remote connectors are minimized. Thus anytime the well is reentered the control system is recovered for inspection and testing while river control of the tree is obtained through a 'riser pod'. The handling tool features the ability of being run on a wireline for pod replacement, as remote orientation of the tool to the tree cap may be achieved with a ratchet mechanism on the tool.

This hardware is readily adaptable to trees of any manufacturer. A clean interface flange provides for the adaptation of the equipment to any tree.

This series of equipment brings to the industry a second generation of test-proven hardware adaptable for use with a broad range of subsea equipment.

System Requirements

Subsea wellhead installation and maintenance systems are riser units configured for three primary tasks related to the completion and production of subsea wells. These are:1Running and tying back tubing hangers during the completion operation.To run trees and other subsea equipment and tie it back during initial installation.To service the wellhead. This includes wireline maintenance, tree recovery, and component replacement such as control pods or other related and recoverable equipment on the subsea installation.