Ballast Water Management – Sister Vessels – The True Cost of Retrofitting

Malin Group Ballast Blog, Malin Blog

Just how crucial is similarity between sister vessel ballast systems? Just how big an impact do small changes have on a retrofit design? Many retrofit engineering companies insist on repeating surveys, 3D laser scans or, in some cases, the full retrofit design package, for each sister vessel. In most cases, this can be entirely avoided and is unnecessary expense on the owners account.

Sister vessels can be enigmatic, particularly when the term itself is somewhat equivocal. What really constitutes a “sister vessel?” The Classification Society American Bureau of Shipping (ABS) describes a sister vessel as:

A vessel that as a whole is identical in design to another vessel. The vessel is normally ordered by the same owner and is one of a series of vessels. (

But is this an accurate description?

Broadly speaking, most would agree that it is. Sister vessels are fundamentally identical vessels, and if/where design tweaks between builds resulted in minor dissimilarities between sisters, to most, these are somewhat irrelevant.

However, to the budding retrofit engineering partner, tasked with engineering and installing BWMS across a number of sister vessels, minor changes could have a significant impact on the retrofit design. This is especially true on retrofit projects involving the pre-fabrication of pipework or structure, where minor differences in key dimensions or layouts between sister vessels could essentially result in different drawing packages for each vessel.

As a result of this potential uncertainty, many owners, upon advice from their engineering partner, have agreed to the costs of additional surveys, 3D laser scanning, and in some cases full retrofit engineering packages, for each sister vessel in their fleet.

But in many cases, the differences between sister vessels may not impact the BWMS retrofit at all. And in cases where it does, the impacts may be so minor that commissioning a full 3D laser scan to discover them seems entirely wasted effort and cost.

So why are these additional services (and the associated costs) insisted upon by many engineering partners?

Repeating these services on sister vessel should be the last resort, not the first. Any reputable engineering partner should be exhausting all other, minimal cost, methods of checking the physical differences prior to suggesting additional services.

There are numerous ways to do this – numerous methods that could be employed to ascertain any sister vessel differences, at a minimal cost to the engineering partner, and, most certainly, provided at absolutely no cost to the owner. All that is required is a little engineering innovation and dedication to minimising the cost to the owner.

Unfortunately, not all engineering partners have the same focus.

In our opinion, for a fleet of sister vessels, the sole focus of an engineering partner should be on engineering a single retrofit design for the whole fleet, taking into account as many idiosyncrasies of each sister vessel as possible. One single cost to the owner, providing a package that can simply be re-used across the fleet of sisters.

That is true dedication to minimising the cost to the owner.

Let’s make this about the owners, not about the bottom line.

Chris McMenemy is General Manager at Cleanship Solutions, which is part of Malin Group – a collection of companies offering a comprehensive range of services to the marine industry. To keep up to date please follow us on LinkedInFacebook or on our Instagram, for a steady stream of eclectic and interesting engineering images.