Optimal Treatment Systems for LNG Operators.
LNG owners and operators are following the media closely, waiting for the 35% threshold to be achieved and the IMO Ballast Water Convention to come into force. They are facing up to and coming to conclusions about which ballast water treatment system option (BWTS) to select, deciding which of the many systems available best suits their needs.
Coldharbour Marine has a reputation for providing practical engineering solutions to technical problems and offers over 35 years’ experience of working with operators of very large vessels, including crude carriers and LNG tankers.
At the forefront of thinking about BWTS, Mark Wells, Coldharbour Marine’s Chief Technical Officer, believes that rather than viewing BWTS as just another system to worry about, LNG operators can, by adopting a ‘canny’ approach with a focus on maximising vessel efficiency, come to the most relevant, cost effective and appropriate solution for their vessels. Natural gas is a challenging cargo for crews, so the easier a new system can be integrated with existing on-board systems, the better and safer the choice.
LNG carriers typically operate with long ballast legs, frequently running 2 or 3 week journeys between terminals. This means that the BWTS on an LNG carrier has to overcome a problem that others running shorter legs of 5 or 6 days simply do not meet, specifically, organisms regrowth in ballast tanks.
BWTS can be separated into 3 types, being broadly based on electrochemical, ultra violet or ultrasonic/ physical technologies. Two of these (UV + EC) treat ballast water as it is pumped on board at terminal. For LNG operators this is when crews are up to their eyes with work and when pressures and risk to the vessel are maximum.
This is risky time, as John Lang, Chief Inspector of the UK’s Marine Accidents Investigation Branch, explains, “Safety breaks down for a number of reasons, when people don’t know what to do, take a short-cut, are badly distracted, too tired or when they are trying to do too much at once.”
“At terminal” treatment systems can have inherent difficulties, starting with the initial filtration which targets 50 microns or less and is thus vulnerable to blocking particularly at those terminals where uptake water has high levels of suspended solids. This can happen when crew and machinery are already working at maximum and problems or downtime triggers unwanted delay and significant costs.
Electro chlorination introduces an additional risk; the production of Hydrogen gas as an unwelcome by-product in an environment where a spark at the wrong time will have disastrous consequences. Add to this question marks over long term impacts to tank coatings leading to increased corrosion and it is apparent why some operators are nervous.
UV systems, which already struggle when faced with turbid water, target microbial DNA, making organisms unviable by being unable to reproduce.
The current view of USCG is that the organisms are still alive. Unless agreement can be reached with USCG and EPA on the use of Most Probable Number (MPN) testing protocols then many UV vendors will be forced to redesign equipment that already carries IMO type approval. The new equipment is also likely to demand far more power as manufacturers seek to deliver a lethal dose of UV to organisms rather than simply damaging DNA.
Aware of these issues, some LNG operators are opting for an in-tank in-voyage BWTS system which makes clever use of a low oxygen inert gas generator (IGG) at the core of the treatment process. IGGs are familiar technology for LNG operators and, in this latest application can, if properly configured, be used to deliver inert gas for both the bulk inerting and the ballast water treatment duties on board an LNG carrier. This saves a considerable amount of space on the vessel and further reduces the risk of breakdown or disruption during terminal operations. In this BWTS application, inert gas de-oxygenates the ballast water, killing organisms by hypoxia (lack of O2 ) and by hypercapnia, (as CO2. dissolves, the water pH drops). Gas Lift Diffusion (GLD) assemblies mounted inside the ballast tanks are used during the voyage (rather than at terminal) to stir the tank contents and diffuse the gas, whilst gas powered ultrasonic units and micro bubble generators inside the GLDs ensure high bacterial kill rates. The absence of moving parts inside the GLDs means that the units are maintenance free, “fit and forget” as far as the crew is concerned.
For LNG operators this new BWTS option brings numerous additional benefits:-
being based on well understood technologies, crew are already familiar with its use.
The system operates after the vessel leaves the terminal and when work pressures on crews are reduced.
The systems can be configured as duel fuel – Using either MDO/MGO or LNG boil off gas.
By using IGG equipment and by siting the ultrasonic generators inside the tanks the onboard footprint of these systems is small.
The use of the IGG for two functions means that the equipment is used more regularly, thereby enhancing long its term reliability.
Once ballast water has been treated it should meet IMO/USCG requirements. However no treatment is 100% effective and all BWTS suffer from some degree of microbial regrowth. Scientific data shows that organism regrowth during extended ballast voyages is a reality that cannot be ignored. Official testing for regrowth under the IMO certification is 5 days, whilst USCG certification requires only 1 day! LNG tankers can typically be in ballast for up to 20 days by which time regrowth in the ballast tanks following treatment at ballast uptake could be severe. The longer the ballast voyage, the greater the risk of microbial counts approaching and even exceeding pre treatment levels. Since both the IMO and USCG regulations set ballast water DISCHARGE standards rather than simple TREATMENT standards, operators of LNG carriers do have a potential problem.
An in-tank, in- journey BWTS strategy allows treatment to be timed so that regrowth is completely avoided and operators can be confident that the ballast water will pass Port State Control tests prior to discharge.
In summary, the demands on crews during terminal operations coupled with the frequency of long ballast legs and the high risk of regrowth when in ballast mean that the operating requirements for a BWTS on an LNG carrier are quite different from and specific when compared to those of other types of vessel. For operators already familiar with the use of inert gas generators the option of a dual duty, dual fuel IGG linked to an in-tank in-voyage BWTS could prove to be the perfect answer to the ballast water treatment conundrum.