BWTS Retrofit: Drydock vs Riding-Crew Installation

What Are the Three Installation Methods?

A retrofit can be executed in drydock during a scheduled docking, afloat alongside at a repair or lay-up berth, or incrementally by a riding crew of technicians who work while the ship continues to trade. Each method delivers the same installed system but distributes the work, the off-hire, and the risk very differently.

Drydock is the traditional method because it gives dry access to the hull and sea chests and allows the yard to concentrate labour. Alongside installation keeps the ship in the water but still at a berth, suitable when no underwater work is needed. Riding-crew installation is the most operationally flexible, doing pre-fabrication and cabling during voyages and completing tie-ins at convenient port calls or a short stop.

In practice many projects blend methods: pre-fabricate pipe spools and pull cables with a riding crew, then complete the sea-chest tie-in and final connection during a short docking or an alongside stop. Blending shortens the critical dry time while keeping the highest-risk work under controlled conditions.

When Is Drydock the Right Choice?

Drydock is the clear choice when the retrofit requires opening or modifying sea chests, new hull penetrations, or extensive work below the waterline. With the ship out of the water this work is straightforward and safe, whereas afloat it would need divers or blanking arrangements.

It is also efficient to bundle the retrofit into an already-planned special or intermediate survey docking, sharing mobilisation, staging, and off-hire with the rest of the docking scope. This is usually the lowest-cost route because the off-hire would have been incurred anyway.

The downside is competition for time. A BWTS installation adds to an already-full docking list, and if it becomes critical path it can extend the docking. Careful sequencing and pre-fabricated spools keep it off the critical path.

When Does Riding-Crew Installation Make Sense?

Riding-crew installation shines when off-hire is expensive and the ship has a busy trading schedule with no convenient docking window. Technicians travel with the ship, doing foundation work, cable runs, and skid mounting between ports, then complete tie-ins during cargo operations or a short stop.

Because the tie-in into the live ballast system is the one job that cannot be done under way, planning centres on isolating that work into a controlled window — a short alongside stop or a brief yard visit — while everything else is spread across the voyage. This can reduce dedicated off-hire to just a few days.

The trade-off is complexity: managing a riding team, accommodation, spares logistics to multiple ports, and above all hot-work and enclosed-space safety on a working ship. On tankers this demands rigorous gas-freeing and permit control, which limits how much can safely be done under way.

How Does Vessel Type Affect the Choice?

On tankers and gas carriers the dominant factor is safety. Hot work in or near cargo and pump-room spaces requires gas-freeing and explosion protection, so riding-crew hot work is tightly constrained and much of the risky work is pushed into a gas-free yard period. This nudges tanker retrofits toward drydock or a dedicated yard stop.

Dry bulk carriers and container ships have more latitude for riding-crew work in the engine room, where hot-work control is more manageable. Bulk carriers that gravity-ballast may need integration work with existing valves and flow paths, which is engineering rather than a method constraint.

For all types, the amount of below-waterline and sea-chest work is the deciding technical question. If little or none is needed, afloat and riding-crew methods become viable; if substantial sea-chest modification is required, drydock is hard to avoid.

How Do the Methods Compare on Cost and Off-Hire?

Bundled into an existing docking, drydock installation often has the lowest incremental cost and off-hire because the ship was going to dock anyway. A standalone dedicated docking purely for the BWTS carries its own mobilisation and full off-hire, making it the most expensive route unless no alternative exists.

Riding-crew installation minimises dedicated off-hire — sometimes to just the tie-in window — but adds riding-team, travel, accommodation, and multi-port logistics cost, and carries schedule risk if a planned tie-in port falls through. The net economics depend heavily on the vessel's charter rate and how well the schedule cooperates.

Alongside installation sits between the two: no docking cost but the ship is off-hire at a berth for the duration. It suits vessels with a natural lay-up or waiting period and no underwater work.

How Do You Decide and Prepare?

Decide by weighing three questions: how much sea-chest and hull work the design requires, how expensive off-hire is against the charter, and how much hot work the vessel type safely permits under way. The answers usually point clearly to one primary method with elements borrowed from another.

Whatever the method, the preparation is the same: an accurate 3D scan, class-approved drawings, pre-fabricated pipe spools, and confirmed electrical margin. Pre-fabrication is the single biggest lever for shortening ship time regardless of installation method.

Sea Clean AS can support installation planning and attendance, and works to the same engineering discipline whichever method is chosen. In Norway and the North Sea region attendance is available 0–24 hours; elsewhere by prior arrangement. As an authorized Headway agent it supports OceanGuard directly and provides independent, brand-agnostic support for other systems.

Frequently asked questions

Can a BWTS be installed without drydocking the ship?

Yes, if the design needs no sea-chest or below-waterline work. Alongside and riding-crew installations complete the piping, cabling, and skid mounting afloat, with the ballast-system tie-in done during a short controlled stop. Drydock is only unavoidable when substantial hull or sea-chest work is required.

How much off-hire does riding-crew installation actually save?

It can reduce dedicated off-hire to just the tie-in window — often a few days — because the rest of the work is spread across trading voyages. The saving depends on the schedule cooperating and on the added riding-team and logistics cost, so the net benefit is vessel- and charter-specific.

Is riding-crew installation safe on a tanker?

It is possible but tightly constrained. Hot work near cargo and pump-room spaces requires gas-freeing and explosion protection, so on tankers most high-risk work is pushed into a gas-free yard period rather than done under way. Non-hot-work tasks can still be spread across voyages.

Which method is cheapest?

Bundling the retrofit into an already-planned drydock is usually cheapest because the off-hire and mobilisation are shared with other docking work. A standalone dedicated docking is the most expensive. Riding-crew installation minimises off-hire but adds logistics cost, so the cheapest route is ship-specific.

Sources

• IMO Ballast Water Management Convention
• DNV — Ballast water management

Related articles

• BWTS Retrofit Guide for Existing Vessels
• BWTS Retrofit Timeline Planning and Yard Selection
• BWTS Retrofit Space Constraints: Engine-Room Solutions
• How Much Does a BWTS Retrofit Cost in 2026?
• BWTS Crew Training After Retrofit: What Is Required?
• What Is a Ballast Water Treatment System? Explained

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