Overcoming the challenges of a Floating Storage and Regasification Unit conversion

Jose Navarro, Global Gas Technology Specialist, takes a detailed look at the issues involved in giving a carrier a new lease of life as a Floating Storage and Regasification Unit.

Repurposing a liquefied natural gas (LNG) carrier into a Floating Storage and Regasification Unit (FSRU) can be a highly effective way to maximise the value of that asset. Converting a vessel can be a complex process however, one that requires operators to overcome a range of technical challenges. 

Offering a comparatively rapid and cost-effective regasification solution for LNG, FSRU projects have gathered a great deal of momentum over the past decade; around 60 programmes of this kind are either in development or already operational around the globe today.

While many FSRUs are purpose-built vessels, operators have been quick to grasp the value of LNG carrier conversions, too. Vessels approaching the end of their originally intended operational lives are prime candidates for transformation, though operators must have a firm understanding of a wide range of factors when considering a conversion of this kind.

Let’s explore some of the most prominent issues that have an impact when considering the conversion of an LNG Carrier to operate as a FSRU.

Propulsion systems

Since transit is no longer its primary purpose, a converted carrier’s propulsion systems can usually be repurposed to provide an energy source for the regasification systems. Steam and dual fuel diesel electric (DFDE) vessels are best suited to be considered for a conversion of this kind.

Though substantial modifications are likely to be required to render them suitable for the task, it is possible to use the steam generation capabilities of steam ships as a source of heating for a regas system, and the power electrical generation capacity of DFDE ships as a source of power for the large pumps needed for the regas system.

Containment systems

The key risk here is the potential for aggressive sea conditions, depending on the intended FSRU operational location. This is because of the potential sloshing effects to the containment when cargo tanks are partially loaded. While spherical containment tanks in the Moss Rosenberg style can provide inherent protection against sloshing, subject to design re-appraisal, membrane containment systems can struggle to deal effectively with this when the FSRU is located in an unprotected location.

As a vessel’s containment systems cannot be modified, any conversion will need to consider the existing design of the LNG Carrier and verify the solution design against the meteocean data for the intended FSRU operation location.

It is anticipated that membrane containment systems should be acceptable for well protected locations, although for certain unusual periodical sea weather conditions – such as seasonal storms – additional operational steps will need to be taken to ensure that sloshing problems do not arise.

These operational steps may result in rearranging the cargo loading within the tanks or, in a worst-case scenario, ensure that the ship’s propulsion system remains active and well maintained, and that a sufficiently sized crew remains on hand in order to move the ship to a safe location should that prove to be necessary.

Similarly, a membrane type LNG carrier forced to depart unexpectedly from an FSRU during a period of volatile weather is also likely to require clear guidance on how to minimise the risk of sloshing damage based on the particular stage of the unloading operation process at which it disembarks.

Ship-to-ship transfer considerations

At a basic operational level, a converted FSRU needs to replicate the functionality of an unloading LNG terminal on shore.

One key consideration here is that when a transfer is made from the LNG carrier to the FSRU, additional boil-off gas (BOG) vapour will be generated as a result. The primary objective here is to ensure that this is neither released nor wasted. This means ensuring that the FSRU has the capability to, first, send vapour return to the LNG carrier to compensate for the cargo unloading, and second, manage the BOG generated within its own tanks.

This second objective can be achieved via accumulation in the cargo tanks, sending it for the regas trains, using it as fuel within the power generation systems, or re-condensing it as LNG inside the tanks.

BOG tank accumulation is the most practical way of accomplishing BOG control, and this will need to be done during the conversion by resetting the cargo tank pressures from a traditional 0.25 bar g to 0.7. This, of course, means understanding whether the vessel can accommodate this change in pressure, including a re-evaluation of a tank’s lifespan based on these new parameters, and structural analysis. Most well-maintained LNG vessels currently in operation are likely to pass the threshold for this transformation, however.

One area of particular concern here – for older ships – is corrosion. Temperature differentiations and condensation in certain areas such as the trunk deck void space (in membrane containment type ships) can cause extensive corrosion, which may in turn require “as it is” structural analysis in order to generate a clear appreciation of the maximum pressures that can be applied.

Reducing the environmental footprint for converted FRSU

The deployment of a FSRU in a new location requires a detailed evaluation of its environmental impact to the area, not only due to the cooling effect on the sea water around the FSRU but also the additional CO2 and heat emissions generated during the LNG regasification process. When designing the conversion, the financial and environmental efficiency of the technologies being introduced should be a primary concern for any operator.

Optimisation of regas system designs can make a big difference not only on the operational cost efficiency, but also the overall environmental impact.

One of the recent developments in the industry has been the introduction of an intermediate heat exchanger using Organic Rankine Cycle (ORC) systems. This allows the recovery of much of the power used during regasification, helping to reduce both CO2 emissions and the overall cost of the process. While ORC systems have long been used in onshore LNG terminals, they are now beginning to make their way on to FSRUs too, and should be a major consideration for any operator planning a conversion.

Manning

As mentioned above, the minimum crew requirements for a FSRU can be an important concern – both from a cost perspective and that of being able to operate the vessel effectively when circumstances change.

While the specific staffing requirements of an FSRU will be mandated by the flag under which it operates, LR has extensive experience in helping operators to balance adequate operation with financial effectiveness.

Ultimately, LNG carrier conversions offer significant opportunities for owner/operators to extend the return on investment of their asset for up to a couple of decades or more – all while addressing a pressing storage need. It can also, of course, be a technical and sometimes complex process, and LR experts are here to help you change course with support at every stage of the FSRU conversation journey.