Subsea Intervention Systems and Operational Planning

‍ By Grant Pierce, Intervention Performance Ltd.‍‍ ‍ ‍

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Introduction

Over the last three decades, subsea intervention technology has advanced considerably. In a relatively short period of time, the industry has moved from water depths of less than 100 m to operations now surpassing 3,000 m, with the ability to perform a large percentage of subsea intervention and abandonment work using lighter assets and advanced technologies. 

Light Well Intervention Vessels (LWIV), Well Intervention Units (WIU), Intervention Riser Systems (IRS), and Riserless Light Well Intervention (RLWI) systems have become standard methodology throughout the North Sea and Gulf of Mexico, while also gaining increasing acceptance across Australia, Brazil, and West Africa. The primary driver behind this evolution has been the industry's need to improve operational efficiency while reducing dependency on conventional drilling rigs. 

Understanding the differences between RLWI and IRS methodologies is fundamental when planning subsea well operations. The selected intervention philosophy influences everything from vessel selection and equipment spread to well control strategy, operational limitations, and overall campaign economics. 

  1. Evolution of Subsea Intervention

It is remarkable how far the industry has come in the last three decades. In that relatively short time, subsea operations have progressed from shallow-water intervention activities to deepwater and ultra-deepwater campaigns using sophisticated vessel-based systems. 

In the mid-1970s, Flopetrol designed the first subsea well-intervention lubricator and conducted initial trials at a depth of approximately 20 m in the Zakum Field offshore Abu Dhabi. Then, in 1984, a joint venture between BP and Camco further developed the system, which was subsequently deployed throughout the North Sea. During the same year, Otis Engineering filed a patent for its own subsea intervention lubricator. 

Since that time, Light Well Intervention Vessels and Well Intervention Units utilizing both Intervention Riser Systems and Riserless Intervention Systems have become increasingly common across the offshore industry. 

There has also been a general increase in operator acceptance, which has driven global expansion of services associated with LWIVs, WIUs, and, more recently, purpose-built P&A and decommissioning rigs. 

Today, there is an established list of experienced contractors providing integrated subsea well solutions, some with vessels included as part of their offering and others focused primarily on intervention systems and services. 

These contractors include: 

  • AKOFS Offshore (Aker Solutions) 

  • Baker Hughes 

  • C-Innovation 

  • Enovate Systems Ltd (Aker Solutions) 

  • Expro Group 

  • FTAI Ocean 

  • Halliburton 

  • Helix Energy Solutions 

  • Oceaneering 

  • Optime Subsea 

  • Sapura Energy Well Services 

  • TechnipFMC (TIOS Group) 

  • Trendsetter Engineering 

  • Worldwide Oilfield Machine 

  • Well Safe Solutions 

Examples of active subsea intervention and decommissioning assets include: 

  • C-Innovation – Island Performer, Island Intervention, Island Venture 

  • FTAI Ocean – Pride 

  • Helix Energy Solutions – Q4000, Q5000, Q7000, Seawell, Siem Helix 1, Siem Helix 2, Well Enhancer 

  • Sapura Energy Well Services – Constructor 

  • TechnipFMC – Island Constructor, Island Frontier, Island Wellserver 

  • Well Safe Solutions – Guardian 

    2. Vessel Classification Philosophy

So, what options are available and what is required for a specific well operation? Which equipment spread or methodology provides the best combination of safety, productivity, and efficiency? 

To answer that, it is important to first define the vessel classifications commonly used within subsea intervention and decommissioning. 

Operators loosely define these classifications as follows. 

2.1 Category A

Category A vessels are light intervention vessels capable of performing: 

  • Wireline operations 

  • Subsea inspection 

  • Repair activities 

  • Riserless intervention work 

These vessels are commonly associated with RLWI operations and are traditionally utilized for slickline and electric wireline campaigns. 

2.2 Category B

Category B vessels are capable of performing: 

  • Workover operations 

  • Tubing retrieval 

  • Intervention riser operations 

  • Coiled tubing intervention 

These vessels utilize a high-pressure small-bore intervention riser extending from the vessel to the subsea Xmas Tree, allowing hydrocarbon returns to be handled safely at the surface. 

Category B units provide some of the same capabilities as a MODU, although in a lighter configuration. 

2.3 Category C

Category C assets are conventional drilling rigs and MODUs equipped with: 

  • Marine risers 

  • Subsea BOP systems 

  • Heavy intervention equipment 

  • Full drilling and workover capability 

These units are capable of performing all well operations, including full plug-and-abandonment campaigns.

3. RLWI vs Intervention Riser Systems

Category A and Category B vessel-based approaches provide significant cost savings compared to utilizing conventional drilling rigs. 

MODUs or Category C units traditionally use low-pressure marine risers for drilling and completion operations and are equipped with workover systems capable of performing heavy intervention and full abandonment. 

Category B vessels provide some of the same functionality, although with a lighter setup. These vessels use high-pressure intervention risers and are generally necessary for heavier intervention activities such as coiled tubing. 

Category A vessels are commonly referred to as Riserless Light Well Intervention vessels and have traditionally been associated with wireline operations. 

These vessels are generally: 

  • Cheaper to mobilize 

  • Faster to rig up 

  • More flexible operationally 

  • Lower in overall footprint 

RLWI systems are highly effective for: 

  • Logging 

  • Slickline work 

  • Electric wireline intervention 

  • Production diagnostics 

  • Mechanical intervention 

  • Plug setting 

  • Perforating 

  • Scale removal 

However, once large-volume circulation or heavy coiled tubing becomes necessary, an Intervention Riser System may become the preferred option. 

An IRS provides: 

  • A pressure-containing conduit between the vessel and the well 

  • Controlled hydrocarbon returns 

  • Improved circulation capability 

  • Enhanced CT functionality 

Although lighter and easier to deploy than conventional marine risers, IRS spreads still require substantially larger equipment footprints than RLWI systems.

4. Dynamic Positioning Philosophy

As for the basics of an LWIV or WIU, the vessel itself is dynamic positioning-compliant, typically DP2 or DP3-classified, because station-keeping while connected to a subsea well is paramount. 

DP2 and DP3 compliant vessels are designed such that a single failure should not result in loss of vessel position. DP3 systems incorporate additional redundancy. 

Maintaining position while attached to a subsea well is essential for: 

  • Protecting the subsea stack 

  • Preventing excessive riser loading 

  • Maintaining well integrity 

  • Supporting emergency disconnect functionality 

5. Subsea Intervention Planning

Once the vessel classifications and intervention methodologies are understood, planning considerations become the next major focus. 

Several important engineering and operational questions must be addressed before selecting either RLWI or IRS. 

5.1 Scope of Work

The first question is always: 

  • What is the actual scope of work? 

  • Is it simply a wireline intervention? 

  • Is coiled tubing required? 

  • Are pumping operations planned? 

  • Will large circulation volumes be necessary? 

The operational scope often determines whether RLWI remains suitable or whether a riser-based system is necessary. 

5.2 Riser vs Riserless Decision Logic

The next consideration becomes the operational risks and benefits associated with each approach. 

Questions include: 

  • Can the work scope be completed riserless? 

  • Are large fluid circulations required? 

  • Are hydrocarbon returns expected? 

  • Does the operation require a riser for safe execution? 

Large circulation requirements generally indicate the need for a riser-based system. 

5.3 Deck Space and Equipment Spread

Equipment spread requirements vary significantly depending on intervention complexity. 

Planning, therefore, requires consideration of: 

  • Available deck space 

  • Equipment layout 

  • Structural deck loading 

  • Weight distribution 

  • Sea fastening requirements 

This becomes particularly important when utilizing smaller vessels or Vessels of Opportunity. 

5.4 Crane Capacity and Deployment Depth

Subsea intervention equipment packages can become extremely heavy, particularly in deepwater applications. 

Operational planning, therefore, requires evaluation of: 

  • Maximum deployment weight 

  • Water depth 

  • Crane capacity 

  • Active Heave Compensation capability 

  • Dynamic deployment loading 

The primary deployment crane should typically be Active Heave Compensated. 

5.5 Moonpool vs Over-the-Side Deployment

Deployment methodology is another key consideration. 

Questions include: 

  • Is a moonpool available? 

  • Can equipment be safely deployed over the side? 

  • Does the scope benefit from a Module Handling Tower? 

  • Are sea-state limitations a concern? 

Certain operations are better suited to moonpool deployment, while others may be safely executed over the side. 

5.6 Regional Availability and Logistics

Regional vessel availability also influences operational planning. 

Important considerations include: 

  • Is a fully integrated LWIV available locally? 

  • Is a WIU available within the region? 

  • Is a Vessel of Opportunity more practical? 

5.7 Fluid Storage and Pumping Requirements

Where hydraulic intervention or stimulation activities are planned, fluid handling becomes an important factor. 

Planning must account for: 

  • Total fluid volumes 

  • Chemical storage requirements 

  • Tank placement 

  • Deck loading 

  • Pumping spread footprint 

  • Transfer and piping systems 

5. Operational Selection Philosophy

Once these questions have been answered, the preferred intervention methodology usually becomes much clearer. 

RLWI systems remain highly effective for: 

  • Wireline campaigns 

  • Production diagnostics 

  • Mechanical intervention 

  • Light stimulation activities 

Intervention Riser Systems become more suitable when: 

  • Large-volume circulation is required 

  • Continuous hydrocarbon returns must be handled 

  • Heavy coiled tubing work is planned 

  • Greater intervention flexibility becomes necessary 

As subsea intervention technologies continue to evolve, both RLWI and IRS capabilities continue to expand, enabling increasingly complex intervention and abandonment work scopes to be performed from lighter, more agile vessel-based systems.