Subsea Intervention Vessel Configuration and Equipment Handling

‍ By Grant Pierce, Intervention Performance Ltd.‍‍ ‍ ‍

‍ Add Your Comments and Additional Information

Introduction

Modern subsea intervention operations rely heavily on the configuration and capability of the intervention vessel itself. Over time, Light Well Intervention Vessels and Well Intervention Units have evolved from relatively simple wireline support vessels into highly integrated offshore intervention platforms capable of conducting riserless intervention, intervention riser operations, hydraulic stimulation, tubing retrieval, and portions of decommissioning campaigns. 

The effectiveness of these vessels depends not only on the subsea intervention systems deployed but also on their configuration for safe equipment handling, efficient deployment, and operational continuity. 

  1. Module Handling Towers and Vessel Layout 

Traditionally, the basis of a Light Well Intervention Vessel or Well Intervention Unit is the Module Handling Tower or Multi-Purpose Tower, equipped with a large-capacity Active Heave Compensated winch for deploying and recovering heavy subsea equipment. 

The MHT provides: 

  • Structural support for subsea deployment 

  • Controlled stack handling 

  • Stability during equipment assembly 

  • Active heave compensated lifting capability 

The tower can be likened to a drilling rig derrick, although specifically configured for subsea intervention operations. 

More recently, vessels have also been developed without conventional MHT arrangements, instead incorporating: 

  • Larger deck areas 

  • Larger moonpool hatches 

  • Improved XT handling capability 

  • Expanded subsea equipment movement areas 

These configurations improve handling of larger subsea architecture components.

2. Moonpool Operations 

Most LWIV and WIU assets incorporate a moonpool arrangement for deploying subsea equipment. 

These moonpools are designed to safely facilitate deployment of: 

  • Well Control Packages 

  • Xmas Trees 

  • Lubricator systems 

  • Intervention risers 

  • Subsea intervention assemblies 

The moonpool arrangement provides: 

  • Improved deployment control 

  • Reduced splash-zone exposure 

  • Better operational safety 

  • Improved equipment handling efficiency 

Within the Module Handling Tower, there are typically a number of auxiliary winches used for: 

  • Tool handling 

  • Guide-wire systems 

  • Umbilical routing 

  • Wireline sheave support 

  • Equipment stabilization 

A critical piece of equipment is the Active Heave Compensated crane or subsea winch system utilized to convey equipment to and from the seabed. 

3. Control Cabin and Remote Operations 

Typically, located near the Module Handling Tower is a centralized control cabin similar in philosophy to a drilling cabin. 

From this cabin, operators remotely control: 

  • Wireline systems 

  • Pumping systems 

  • Coiled tubing equipment 

  • Subsea intervention packages 

Operations are generally performed from ergonomic control chairs integrated with digital monitoring systems. 

Modern systems incorporate: 

  • CCTV monitoring 

  • Fiber optic communication 

  • Remote handheld controls 

  • Software-based data interrogation 

  • Centralized deck control stations 

Dynamic safety systems are integrated to protect the subsea stack during vessel drift-off conditions by controlling excessive loading through active tension management.

 

4. Deck Skidding Systems and Equipment Handling 

The main deck of a modern intervention vessel is commonly sectioned with guide rails that support hydraulic pallet-skidding systems. 

Handling pallets are secured onto these rails and used to transport subsea equipment around the vessel. 

Depending on operational requirements, pallets may be rated for: 

  • 50-ton loads 

  • 100-ton loads 

  • Heavier custom applications 

Hydraulic skid units allow subsea packages to be safely transferred between: 

  • Storage locations 

  • Testing areas 

  • Deployment positions 

Modern safety-focused vessel layouts increasingly incorporate: 

  • Walk-to-work systems 

  • Maintenance platforms 

  • Access gangways 

  • Automated handling systems 

These systems reduce: 

  • Rope access requirements 

  • Working at height 

  • Manual lifting 

  • Personnel exposure

5. Intervention Tension Frames (ITF) 

Some larger monohull vessels and Well Intervention Units incorporate Intervention Tension Frames. 

Examples include: 

  • Siem Helix 1 

  • Siem Helix 2 

  • Helix Q7000 

The ITF provides a safer environment for: 

  • Wireline operations 

  • Coiled tubing intervention 

  • Tool exchange activities 

  • Maintenance operations 

The ITF is generally attached directly to the Multi-Purpose Tower and incorporates multiple working levels accessible through telescopic gangways. 

One of the major operational advantages of the ITF is the ability to exchange tooling without disconnecting from the well. 

This improves: 

  • Operational continuity 

  • Vessel productivity 

  • Personnel access 

  • Campaign efficiency

6. Smart Tower Systems (STS) 

FTAI Offshore, together with OSBIT, developed the Smart Tower System for the MV Pride. 

The STS facilitates both: 

  • Riserless intervention 

  • Riser-based intervention operations 

in water depths ranging from approximately 80 m to 1,500 m. 

The system incorporates: 

  • Active Heave Compensated platforms 

  • Surface pressure-control handling capability 

  • Slickline and electric line functionality 

  • CWOR (Completion and Work Over Riser) deployment capability 

  • Adverse weather operational functionality 

The dual-level heave-compensated platform allows easier access to surface pressure-control equipment while reducing stresses transferred into the riser system. 

The ram-rig hoisting arrangement enables deployment and recovery of: 

  • 90-ft CWOR joints 

  • Subsea pressure-control equipment up to 250 tonnes 

7. RLWI Moonpool Deployment Philosophy 

Once RLWI has been selected as the preferred intervention methodology, the next consideration becomes deployment philosophy. 

The most common deployment method utilizes: 

  • A moonpool 

  • A Module Handling Tower 

  • Active Heave Compensated winches 

The RLWI package is typically stored near the MHT on a skid trolley in a pre-assembled and tested configuration, reducing offshore rig-up time. 

The tower generally incorporates: 

  • Active Heave Compensated subsea winches 

  • Pod and guide-wire winches 

  • Umbilical handling systems 

  • Wireline tie-off points 

  • Stabilization systems 

Many systems also incorporate cursor or tension-frame arrangements consisting of upper and lower support frames. 

These frames stabilize the Well Control Package during entry and exit through the moonpool until the package is landed onto a heavy-duty skid trolley and secured. 

During deployment: 

  1. The RLWI package is transferred above the moonpool. 

  2. The package is lowered through the splash zone. 

  3. Cursor frames stabilize movement. 

  4. Guide wires prevent unintended motion. 

  5. Active heave compensation controls landing loads. 

  6. The package is landed and latched subsea. 

8. Over-the-Side Deployment 

Over-the-side deployment is commonly used when operating from a Vessel of Opportunity, where a Module Handling Tower may not be available. 

In this method, an Active Heave Compensated crane deploys the Well Control Package over the side of the vessel. 

The deployment sequence typically involves: 

  • Deploying and latching the Well Control Package 

  • Suspending the wireline sheave from the crane fast line 

  • Suspending the lubricator section from the main line 

  • Making up wireline tools horizontally 

  • Pulling tools into the lubricator section for deployment 

Depending on the operational method, wireline tools may also be deployed through open water with ROV assistance. 

Once the operation is completed: 

  • The well is secured 

  • The lubricator section is disconnected 

  • Equipment is retrieved to the surface 

9. Fluid Handling and Pumping Systems 

Most intervention vessels incorporate fluid handling capability through: 

  • Below-deck storage tanks 

  • Deck-mounted tanks 

  • Chemical storage systems 

  • Transfer pumps 

  • High-pressure pumping spreads 

High-pressure pumping systems are mounted above deck with permanently plumbed lines routed to manifolds and downline reels. 

These systems support: 

  • Chemical pumping 

  • Stimulation operations 

  • Brine circulation 

  • Cementing activities 

10. Wireline and Coiled Tubing Deployment 

Most intervention vessels support both slickline and electric wireline operations. 

Dual-drum wireline units are particularly useful because one drum may contain slickline while the other contains electric line. 

Digital slickline systems further improve flexibility by consolidating functionality that previously required multiple units. 

Coiled tubing deployment philosophies vary depending on vessel configuration. 

For riserless operations: 

  • Flexible hose 

  • Coiled tubing downlines 

  • TCP systems 

may be deployed over the vessel side or through the moonpool. 

Emergency Quick Disconnect systems are incorporated into subsea jumpers to allow safe disconnect during drift-off conditions. 

On vessels equipped with Intervention Riser Systems, coiled tubing deployment may closely resemble conventional drillship or semisubmersible operations using a Coiled Tubing Lift Frame. 

Advancements are also being made in Riserless Coiled Tubing deployment utilizing: 

  • Surface injectors 

  • Subsea injectors 

  • Dual-tension management systems 

Successful RLCT deployments from LWIV assets continue expanding vessel-based intervention capability.