Rigless Intervention Case Studies in the North Sea and Equatorial Guinea
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 24065, ’Rigless-Intervention Case Studies: UK and Africa,’ by Oliver Willis and Phillip Bosworth, Helix Energy Solutions Group, prepared for the 2013 Offshore Technology Conference, Houston, 6-9 May. The paper has not been peer reviewed.
There are considerable benefits to conducting rigless subsea well intervention from specialized monohull vessels. Two case studies provide an overview of operations on subsea wells, from intervening through the production tree (XT) to XT change-out and well-plug-and-abandonment (P&A)/wellhead-removal operations.
Overview of Rigless and Riserless Light Well-Intervention (RLWI) Techniques
Rigless and RLWI activities have been carried out for more than 25 years across the North Sea. The purpose of the following case studies is to demonstrate that with effective project management (planning, developing, execution, and close-out), the use of RLWI is a flexible and extremely cost-effective method that operators can use for a single well or across a number (a campaign) of subsea wells, irrespective of subsea-tree type or close proximity.
In order to conduct in-hole subsea-well operations without the use of a traditional mobile offshore drilling unit, three pieces of critical equipment (along with a trained and competent crew) are required:
A dynamically positioned vessel of sufficient size and capability to accommodate the equipment and personnel required
A derrick or tower with heave compensation to deploy and recover equipment
A subsea-intervention lubricator (SIL) to facilitate well control and well access
SIL
The SIL provides access to the wellbore. It is interfaced with the well-specific re-entry interface (usually a tree-running tool) and is run from the vessel as a standalone unit on wire, latched onto the tree, tested, and then left free-standing on the well without riser back to surface. It is controlled by a single umbilical with full emergency-shutdown and emergency-quick-disconnect functionality and redundancy.
Wireline tools are run into the SIL through the water column, followed by the open-water latch system (OWLS) that then effects a seal around the wire upon latching into the top of the SIL. Once latched, the riser is flushed to the glycol mix being used, whereupon the well is opened, the toolcatcher is released, and the tools are run in hole in the normal manner with full compensation.
The SIL subsea system consists of a lower riser assembly housing the well-control blowout preventers (BOPs) and electrohydraulic control modules. This is connected by an emergency- disconnect package to the upper riser assembly (URA), which comprises the main umbilical interface, further control manifolds, the electric-line subsea grease-injection system, and contingency wireline BOPs. Integral to the URA is the lubricator (riser) section, which houses the downhole tooling before deployment, and the upper-upper riser assembly, which contains a wireline-cutting ball valve and the hydraulic latch for the OWLS.