The Hidden Complexities of Scrubber Retrofits

The Hidden Complexities of Scrubber Retrofits 

The True Meaning of Available Space in a Funnel

Owing to the lack of availability and short window to make a meaningful decision, scrubber retrofits have become a predominantly commercial decision, with little, and in many cases, no attention being paid to the complexities associated with the retrofit engineering aspect of the project.

Indeed, in our experience, some shipowners have selected and ordered scrubber systems, understandably through haste with the 1st January 2020 looming, that, upon detailed inspection from a retrofit engineering perspective, require drastic alterations to the vessel simply to fit the scrubbers onboard.

However, looking beyond these unintentional oversights, in our experience, it is the hidden complexities of scrubber retrofits that are causing the most significant headaches for shipowners and operators. Complexities that are all too often undisclosed in their significance by the scrubber manufacturer, leaving the owner, via their engineering partner, to discover the issue well after contracts have been signed…

In this series of articles, we will examine a number of hidden, and often times, critical, aspects of scrubber retrofits, that owners and operators are wise to consider when determining suitability of either a vessel to be retrofitted, or a scrubber system itself.

This week we will examine the true meaning of available space in a funnel…


#1 – The True Meaning of Available Space in a Funnel

The Challenge of 2D Drawings

In most cases, the high level suitability of a scrubber for a particular vessel is undertaken directly between the shipowner and the scrubber manufacturer – using predominantly existing vessel drawings to determine the space availability for the scrubber and its various auxiliary equipment. The fast paced nature of the scrubber market means there is precious little time for the luxury of having a retrofit engineering partner involved prior to signing contracts – meaning most scrubbers have been ordered based on high level 2D conceptual arrangements only.

Whilst this approach is generally the norm for the earlier stages of any retrofit project, in our experience, it is almost impossible to determine definitively whether a scrubber system will fit in an existing funnel through existing drawings alone.

-Example 2D drawing of existing funnel

2D vessel drawings are notoriously unreliable for existing vessels – particularly for older vessels or those which have changed owners in the past. In some cases the original 2D drawings themselves may simply be reproductions of a sister vessel, and so were unlikely to be accurate to begin with, and in other cases significant alterations to the vessel may not have been reflected on its original 2D drawings.

In some recent projects we observed such significant differences between the vessel drawings on file, and the true as-built condition of the vessel (obtained from 3D scanning). Upon closer comparison of the 2D drawings used by the owner to order the scrubber equipment, and the 3D scan data we obtained during our vessel survey, we identified that an entire new deck had been added to the funnel and bridge 7 years prior but had not been updated on the existing 2D drawings. In this specific example, the project was very nearly derailed – with the scrubber having to be located one deck higher than the owner had expected, causing untold challenges with the air draught of the vessel. To make matters worse, this was a fleet of sister vessels where not all of the vessels, after detailed comparison of them all, had had the new deck added.

Key Takeaway – Reliance on existing 2D drawings as a basis for selecting and ordering a scrubber system is highly unreliable, and should be confirmed though a full 3D scan onboard.


Available Space is an Iterative Process

Fundamentally, the determination of available space in the funnel is, in itself, a complex and iterative process that can only be accurately determined using 3D scan data in a fully 3D environment. 2D drawings alone, even if they collectively offer the level of detail required, cannot provide the full picture. Existing pipework, mist boxes, access ladders, gratings, electrical equipment, silencers and supports, to name but a few, are all significant drivers in determining the available space in a funnel – and must all be considered in a 3D environment to ensure accuracy.

– Example feasibility study of two inline open loop scrubbers

However, beyond developing this fundamental spacial baseline – each individual scrubber system itself presents its own, and often significant, space requirements, making the entire determination of available space, an iterative process.

Take, for example, a comparison between two open loop, inline scrubbers from two prominent manufacturers, for the same vessel. Examining the scrubber towers themselves, the first and often most critical difference is the vertical location limitations – with each individual manufacturer requiring slightly different configurations of the exhaust pipework before and after the scrubber tower. On larger scrubber towers, these differences can cause a specific scrubber to sit several meters higher in the funnel than that from a different manufacturer.

Then there are the strict requirements governing connection points, access manholes and sensors, and the highly important relative rotation of each. These connections points vary significantly in frequency and location between different manufacturers, but are critical considerations in determining not only the location of a scrubber, but the free space required around it. Even a seemingly innocent aspect such as access manholes can significantly impact the final location, and hence available space, for the scrubber tower,

Key Takeaway – even in circumstances where an owner is confident of the space envelope within a funnel, each scrubber system itself will drive the suitability of that free space – it cannot be presumed that physical size is the only consideration.


Scrubber Manufacturer Assumptions

One of the most critical issues we have experienced across a variety of scrubber retrofit projects, are the limitations made and enforced, often times without declaration, by the scrubber manufacturer.

Often, again, a symptom of the haste of shipowners to secure scrubber build slots, critical limitations made by scrubber manufacturers are written in to contracts well in advance of the engineering partner becoming involved, which have a drastic impact on the complexity of the retrofit project. The most common of these limitations is the maximum height of the scrubber above the low waterline of the vessel.

In broad terms, the efficacy of a given scrubber system is a function of its wash water supply, and, as such, most manufacturers pay very close attention to the relative heights of the scrubber tower above baseline, and the wash water pumps they specify. When these heights are pre-determined by the manufacturer prior to an engineering partner becoming involved, they can present significant challenges to the project.

Take, for example, a recent venturi type scrubber retrofit we undertook for a bulk carrier. As with most venturi type scrubber retrofits, it was necessary to create an extension to the existing funnel to provide sufficient space for the scrubber tower itself, as well as the exhaust pipework, which is generally more complex on venturi scrubbers. Whilst we awaited the final technical specification from the scrubber manufacturer, we examined the practicalities of extending the existing funnel in a “backpack” type arrangement, settling eventually on the concept of extending the funnel aftwards on “A” deck, one deck above the aft deck, so as to keep the extension as low as possible. This approach would also allow the exhaust pipework to route back into the main funnel after the scrubber. All in all, it would be a fairly small and low “backpack” saving material and cost.

– Example of Scrubber Retrofit Model by Cleanship Solutions

Upon receipt of the technical specification from the scrubber manufacturer, however, it transpired that the scrubber tower itself could not be located any lower than “B” deck, due to the capability of the wash water pumps being provided as part of the scrubber package. The pumps had been selected during the contract phase, based on an arbitrary height of the scrubber above the low waterline, which, as became apparent, strictly limited the height of the scrubber tower.

In the end, the need to locate the scrubber one deck higher, caused the support structure for the “backpack” extension to be much more significant, which in turn impacted the existing mooring equipment on the aft deck and due to the limited height differential to the existing funnel’s watershed, meant it was not possible to route the scrubber exhaust pipework back into the existing funnel. This meant that the “backpack” had to extend the full height of the existing funnel – requiring significant additional material and cost. The arbitrary height requirements set by the manufacturer caused unnecessary complexity to the project.

Key Takeaway – Ensure that critical limitations such as pump capabilities and associated scrubber tower heights remain open until such time as the engineering partner is involved and can help determine the optimum configuration for the specific vessel.



Overall, the available space within a funnel is not something that can be simply determined, but is something that can intuitively be critical to the practicality, and indeed the entire success, of a scrubber retrofit project. The market forces driving the necessity of owners to move quickly to secure scrubber build slots, often without sufficient feasibility analysis of suitability, continues to create significant, and often times, unnecessary, complexity in many scrubber projects.

Where possible, it is advisable to engage a retrofit engineering partner such as Cleanship Solutions to help with the determination of the suitability of space, preferably prior to final contracts being signed with the relevant scrubber manufacturer.

In the next instalment of the hidden complexities of scrubber retrofits, we examine pumps, valves & instrumentation…