Riserless Light Well Intervention (RLWI) Deployment Methods

‍ By Grant Pierce, Intervention Performance Ltd.‍‍ ‍ ‍

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Riserless Light Well Intervention (RLWI) is a specialized offshore well-intervention method used primarily on subsea wells to perform maintenance, diagnostics, stimulation, logging, and light remedial operations without the need for a conventional drilling rig or a marine riser system. Instead of mobilizing a large and expensive drilling rig, RLWI operations are conducted from purpose-built intervention vessels or suitably equipped offshore vessels using compact intervention systems and subsea well control equipment. 

RLWI is commonly used to deploy and retrieve equipment such as wireline tools, lubricator sections, perforating systems, downhole gauges, plugs, shifting tools, and other light-intervention assemblies into subsea wells. Depending on the scope of work, coiled tubing systems may also be deployed using specialized work towers. 

One of the key advantages of RLWI is that it significantly reduces operational cost and mobilization time compared to conventional rig-based intervention methods. RLWI vessels also provide improved flexibility, reduced fuel consumption, lower environmental footprint, and the ability to perform interventions efficiently in fields where frequent well access is required. Because the operation is riserless, subsea intervention packages and well control systems are deployed directly onto the subsea tree, eliminating the need for a full marine riser system. 

The deployment methods most commonly utilized in RLWI operations (in no particular order) are: 

1. Deployment through a moonpool using a Modular Handling Tower (MHT) equipped with an Active Heave Compensated (AHC) winch  

2. Deployment over the side using an Active Heave Compensated (AHC) crane  

With the first method mentioned, Modular Handling Towers (MHTs) are typically found on specialized Light Well Intervention Vessels (LWIVs), although smaller versions may also be installed on some Subsea Equipment Support Vessels (SESVs) and Construction Vessels (CSVs). The MHT is positioned above the moonpool area, which serves as the primary deployment zone for most subsea intervention equipment. These towers allow handling of larger and more complex equipment spreads because components can be stacked and assembled vertically prior to deployment. In many ways, the MHT serves a role similar to the derrick on a drilling rig. 

The RLWI package is normally stored close to the MHT on a skid trolley in a parked position, already assembled and tested prior to offshore operations. This arrangement helps minimize rig-up time once offshore. These towers are typically equipped with an AHC subsea winch together with several auxiliary winches mounted at upper elevations. These may include pod winches, guide-wire winches used to stabilize equipment movement, umbilical handling winches and sheaves, as well as tie-off points for wireline sheaves and associated intervention equipment. 

Some systems also incorporate a cursor or tension-frame arrangement consisting of upper and lower support frames. These frames support and stabilize the Well Control Package (WCP) while it enters and exits the moonpool and until it is landed onto a heavy-duty skid trolley, often rated around 150 tons, where it can be secured safely. 

From the skid trolley position, the RLWI package is transferred above the moonpool for deployment. The assembly is then lifted and lowered through the splash zone using two cursor frames. Where required, four guide wires may be used to prevent unintended movement and improve positional control during subsea deployment. Active heave compensation enables the winch system to compensate for vessel motion, allowing controlled lowering, accurate landing, secure latching, and safe disconnection of the subsea equipment. 

The second common method is “over-the-side” deployment, in which an AHC crane is used to deploy the Well Control Package over the vessel side. This method is typically selected when using a Vessel of Opportunity (VOO) where a permanent MHT may not be installed. In such cases, the vessel may or may not have a moonpool available for subsea deployment operations. 

Using this method, the Well Control Package is deployed, landed, and latched onto the subsea tree. The Wireline Sheave is commonly suspended from the fast line of the AHC crane, while the Lubricator Section (LS) is suspended from the crane’s main line. Wireline tools are generally assembled horizontally on deck and then pulled into the lubricator section prior to deployment. Depending on the intervention method being used, tools may also be deployed directly through open water with assistance from a Remotely Operated Vehicle (ROV). Once the intervention work is completed, the well is secured, the lubricator section is unlatched from the WCP, and the equipment is retrieved back to the surface. 

If a moonpool is available on the Vessel of Opportunity, an A-Frame or Mini Tower may be utilized instead of a conventional over-the-side arrangement. In such cases, the operational sequence remains largely the same, except that the Wireline Sheave is suspended from the A-Frame or Mini Tower rather than directly from the crane. 

In situations where the selected vessel does not have a permanently installed MHT, several third-party modular solutions are available depending on the planned scope of work. These range from lighter wireline intervention towers to larger and heavier coiled tubing work towers capable of supporting more demanding intervention operations.