An Operational Subsea Wireline System
Authors: Dines Chris , Cowan Pete , Headworth Colin 
Published: December 2007

This paper describes an operational subsea wireline system that is self-contained and flexible and offers a safe, economical, and proven method for riserless re-entry into any subsea well.

Introduction

Approximately 400 subsea wells currently exist worldwide, and most predictions indicate a doubling of this number within the next decade. Being time-consuming and costly, conventional methods of reentering subsea wells with wireline are not consistent with the low-cost profile required by many subsea developments. This factor has predominantly driven the development of cost-effective subsea wireline systems. The system discussed here was developed over a number of years and has been used to carry out a comprehensive range of wireline tasks from floating vessels in the North Sea. Although generally deployed from a diving support vessel (DSV), the system has been successfully used from a drilling rig to conduct wireline operations in a well, simultaneously with workover operations in an adjacent well, on a subsea template. It is important to realize that any function that can be achieved by conventional means can also be carried out by subsea wireline. Subsea wireline has been used to pull and run surface-controlled subsurface safety valves (SCSSV's), to set and pull plugs, to pull and run gas-lift valves in side-pocket mandrels, and to run production logging tools and electronic memory recorders.

Subsea Wireline Operating System - General Description

A subsea wireline system is characterized by the use of a subsea lubricator stack connected directly to a subsea tree and controlled from the support vessel. As Fig. 1 illustrates, the lubricator stack comprises top and bottom lubricator assemblies. The components in each of these assemblies are connected throughout with quick unions and API flanges, allowing flexibility of the stack makeup to handle the variety of tasks and toolstrings required. The tool-strings are deployed and retrieved while safely contained within the top lubricator assembly. The lubricator stack can be deployed either through a moonpool or over the side of a DSV by use of the vessel crane and a road-transportable, dedicated handling system, which provides the flexibility to use a wide range of vessels. The tree and lubricator control umbilicals are deployed by means of a clump weight. The wireline unit is situated on the DSV, and the wireline is reeved, along with the lift line, through an active heave-compensation system. An emergency shutdown system allows the completely safe and pollution-free severing of all connections between the well and the wireline vessel within a few seconds. This subsea wireline system can be deployed entirely without divers by a work-class remotely operated vehicle (ROV). In practice, water depth and subsea wellhead configuration determine the mode of intervention (diver or diverless). Divers are supported by an eyeball ROV that has its own launch vehicle and control cabin. The video monitor is connected to the central control van and wireline cab to enable monitoring during lifting, latching, and wireline- and pressure-testing operations. All equipment can he quickly and easily installed on, and removed from, a vessel, the majority being skid-mounted or containerized. For a standard wireline intervention, such as pulling and replacing a wireline-retrievable safety valve, a typical duration from mobilization to demobilization therefore would be 4 or 5 days, depending on sailing time. The most economical use of this system would be in a situation where an operator requires work to be carried out on various subsea wells in the same field or where the operator is contracting a DSV for other reasons, such as the annual inspection, repair, and maintenance program. Alternatively, it can be economically deployed from a drilling rig at the same time as the rig derrick is being used for other tasks on a subsea template. A dynamically positioning DSV has a particular benefit in that it does not require anchors; therefore, anchor-handling vessels are not required, any danger of snagging subsea pipelines, cables, or other subsea equipment is avoided, and the DSV can be located on-station very quickly.

Subsea Lubricator Stack Loading Analysis

The structural integrity of the subsea lubricator stack and the bending loads applied to the subsea wellhead are fundamental considerations for safe operation. When assembled on a wellhead, the lubricator stack is subjected to a complex combination of environmental loads from current and wave action, point loads induced by wireline and guidewires connected to the lubricator, and internal pressure. The magnitude of these applied loads varies with water depth, wireline tension, guidewire tension, vessel position, lubricator-stack configuration, and well pressure. Evaluating the influence on the stack loading of each varying parameter, acting independently or in any combination, by hand calculation would be extremely time-consuming. Therefore, computer software developed in-house is used to model the lubricator stack with standard engineering theory for loading and stress analysis. In addition to the global analysis of the lubricator stack, a detailed stress analysis of individual discrete components of complex geometry was carried out by finite-element mesh modeling techniques. The lubricator stack was also analyzed to determine its failure mode in a bending overload. The failure mode of the lubricator stack is designed to protect the subsea Christmas tree from overloading, thus reducing the potential of pollution. A stress joint fails in bending before any other component in the stack, Christmas tree, or conductor; it is sized to resist the environmental and operational load for a particular well location and to fail in bending below the design-tree bending load. This feature ensures adequate protection of the operator's wellhead in the unlikely event of a bending overload. Cylindrical objects in moving water are subject to vortex shedding, which can set up resonant oscillation and cause undesirable forces to be induced. Because of the ranges of possible water-flow velocities in which the system may be used, the stack is designed to prevent the problem from arising, by helical attachment of the hydraulic control bundle, which acts as a vortex spoiler.

System Operability

Computer analysis is conducted on the complete range of lubricator-stack configurations to determine vessel excursion envelopes for both normal and emergency conditions at each well location. These excursion envelopes are footprints of the vessel position at which the tensioned guide wires or wireline will load the stack to factors of safety on yield of 2.25 and 1.0. Fig. 2 is an example of a DSV excursion envelope for the slickline operating mode at a water depth of 128 m [420 ft] with 6-m [20-ft] waves and a 0.5-m/s [1-knot] current. The geometric limit (GL) shows the vessel position at which it is possible for a guidewire to foul the top of the stack because of the geometry of the stack design.

JPT

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