Large scale commercial transportation of liquefied natural gas (LNG) started in the late 1960s and early 1970s, with a series of ships constructed to carry up to 70 000 cubic meters of LNG. As was normal at that time, ships of this size were powered by steam turbines connected via a reduction gearbox to a single propeller.
- Political and Commercial Perspectives
- Historical Considerations
- Issues Relating to Older LNG Carriers
- Cargo Containment Designs
- Type B (Moss and Alternatives)
- Cargo Handling Systems
- Alternative Propulsion Options
- Control Systems
- Compressed Natural Gas Shipping
Three kinds of tank and containment system were used, two of the supported membrane type and one of the self-supporting spherical type.
During the 1970s, the next generation of LNG ships was introduced, with their capacity almost doubled but using the same propulsion and containment systems. One of the major advantages of using steam propulsion is that boil-off gas, created by thermal conductivity through the tank walls and movement of the liquid in a seaway, can be readily burned in the ship’s boilers. This saves the consumption of fuel oil bunkers (which escalated in price during the 1970s) and avoids the need to consider installing a reliquefaction plant on board.
In the intervening 30 years a regular number of ships have been built most years for new projects, of almost exactly the same specification; none of which has taken advantage of the new means of propulsion which had become available. Now, in the early twenty first century, we find that an unprecedented number of new LNG ships are ordered or under construction – almost all with the same 1970s technology on board. Some of these ships will undoubtedly be employed in short term charters or spot market trading, where operating flexibility and day rates will play an important part in determining their economic viability.
The authors and their colleagues have been involved in LNG ship design, construction and operation for over 30 years, and are also studying options for carrying natural gas in a compressed form (CNG) for certain projects. They are presently participating in a significant number of the new LNG ships being constructed worldwide, including the development of the 200 000 m3 or larger ships expected to be ordered in the next couple of years.
This paper looks at the new developments likely to be adopted for LNG and CNG shipping, and also addresses key issues related to operating existing LNG carriers, some of which are now approaching 40 years of age.
Political and Commercial Perspectives
In predicting growth it is worthwhile looking back at the political driving forces that have led us within Europe and other OECD countries to the present and forecast levels of demand for the shipment of LNG.
Within Europe, during the 1980s, the UK found itself under an obligation to comply with the Large Combustion Plant Directive (LCPD), 1988. This entailed reducing 1980 levels of SO2 and NO2 emissions (from plant exceeding 500 MW) by 60 % and 30 % respectively, by the year 1998. The 1980 levels in the UK were 3 007 000 tonnes and 861 000 tonnes, respectively, and in 2001 had been reduced by approximately 75 % and 50 %, respectively. This reduction could only be achieved at coal and oil fired power stations through the fitting of desulphurisation equipment, at a capital cost of £75 million per generating set, so that, although this was done at Eggborough and Drax stations, alternative strategies also had to be adopted.
Additionally, in the UK, we are party to both the EU National Emissions Ceiling Directive (NECD), 2001, and the Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone, 1999. The latter requires that the UK reduces SO2 and NOX by 2010 from 1990 levels by 75 % and 50 %, respectively, as well as volatile organic compounds (VOC) and ammonia (NH3).
This has resulted in the following UK emission targets being set (in kilotonnes/year):
|UK Emissions in 2010|
|Pollutant||UK emissions in 1999||Projections based on current and agreed measures||Ceiling under NECD||Ceilings under Gothenburg Protocol|
|VOC||1 744||1 200||1 200||1 200|
|NOX||1 605||1 167||1 167||1 181|
For interest, the breakdown of sulphur dioxide emissions by source in 1999 was:
|Power Stations||776 (65 %)|
|Domestic||53 (4 %)|
|Commercial/public sector||22 (2 %)|
|Other industry||249 (21 %)|
|Road transport||12 (1 %)|
|Shipping||22 (2 %)|
|Others||53 (4 %)|
When power stations were state owned, strategies were adopted with regards to energy resources that took into account conditions such as global political stability and domestic trade unions. This meant that nuclear, oil and coal fired stations supplied the UK electrical base load. Both the OPEC price hikes of the 1970s and the miners’ strikes of 1972 had been instrumental in the development of these strategies.
However, privatisation of the electrical generating industry and the environmental policies outlined above have led to a switch from coal to gas as the prime fuel, for predominantly short term commercial and legislative reasons. Similar legislation in other parts of the world is leading to the same result.
Read also: Gas Tank Environmental Control
Many of the nuclear stations are now reaching the end of their working life and, therefore, our dependence on gas fired generating stations will increase further unless there is a change in political motivation and policy, or until history repeats itself. Also, whatever their pros and cons, the contribution of alternative energy sources is going to be of minimal contribution over the next 10 years. Therefore, as the demand for gas increases, and the output from indigenous resources falls, imports of gas (including LNG) will undoubtedly rise significantly during this period.
Until recently the number of owners and operators of LNG ships was limited, as was the number of shipyards capable of constructing them. Shipping was usually a minor part of the overall LNG project, which required a major investment in gas field production, liquefaction plant and loading terminal, plus one or more discharge terminals and regasification plants, to be undertaken before the first cargo could be moved. Therefore, the project management and finance was normally controlled by a combination of the oil and gas production company and the gas purchasers, who all wished to minimise the project risks throughout the gas delivery chain. Once the original LNG ship designs had proven themselves reliable, there was little benefit seen in changing the shipping technology. Far more was at stake through a failure to deliver the cargoes on schedule than could be gained by reducing fuel consumption or improving the shipping economics. The shipping companies involved often had little say in the overall project and contract strategies, and the shipyards had no need to offer innovative designs to reduce operating costs and improve transportation efficiency.
The shipyard numbers are still limited, although Spain has recently delivered LNGCs, and China is certain to become a producer https://sea-man.org/lng-future.htmlin the near future. However, the market situation has changed significantly, with far more players becoming interested in owning and operating LNG ships. Many of these will receive little or no part of the profits from the LNG production or gas sales, and will rely for their income on making money purely from the shipping part of the delivery chain.
The price attached to the consumption of boil-off gas (BOG) has always been a controversial issue that, in the early years, was assigned a value of zero – on the basis that a seagoing reliquefaction plant was not available, and the ship was effectively burning free gas which would otherwise be vented to the atmosphere and lost. Because, in these original contracts, the gas buyer was also part of the project consortium, the need to deliver as much LNG as possible was not as important as it may now be. Current projects should consider the value of the gas burnt onboard at the market price to the customer, and they are therefore able to assess the relative benefits of using BOG as fuel for propulsion or of reliquefying BOG for delivery at the discharge port.
Issues Relating to Older LNG Carriers
The extension of a ship’s trading life into the fourth decade or beyond introduces its own particular demands on ship owners and managers. Although present LNG ship new buildings are designed with a 40 year fatigue life, a ship is a ship whatever the cargo, and the marine environment is no respecter of ship type; with the result that vessels reaching 20 or 25 years need significant upgrading to manage such a life expectancy. The safety record of LNG shipping is one of which we are all proud, and none of us needs to be told the consequences that will befall the industry if there is a serious incident involving an older LNG carrier.
The cryogenic containment system tends to receive most of the limelight in any discussion of LNG carrier longevity. However, in reality, The Liquefied Gas Tanker typescargo tanks are only a part of the full picture, and it is equally necessary to concentrate on the remainder of the ship and its systems.
Ballast tanks will be a problem throughout any life extension if they are not coated to a standard that keeps them on a five-year inspection cycle. Annual surveys, as requested often by Class for older ships, do not fit in with biennial docking cycles and do not meet the availability criteria.
Boilers and ballast tanks are the fundamental drivers, both in capital cost and downtime, of all probable upgrades. In any longevity exercise, it should be realised that the major upgrades will need serious consideration. It should also be noted that failure of the gearbox or the main steam turbine could result in unavailability of the ship for six to 12 months, a fact that has not been lost on certain operators.
There has been much talk in the industry of removing spherical tanks from aging hulls and building new ships around them, thus maintaining the use of these expensive resources; however, to our knowledge, nobody has done this yet! The care shown by all present and past owners to the cargo tanks of LNG ships has also been applied to the hulls, thus permitting the life-extension exercises to be considered.
In any extension operation, criteria have to be set, and the obvious consideration is availability of the vessel. Much of this operation is clearly an exercise in cost/benefit analysis. With respect to steam ships, consideration should be given to allowing the flexibility of (say) a regular three-day down-time for maintenance at six-monthly intervals.
In order to extend the operational life of ships between 20 and 25 years of age, it is considered worthwhile to take them out of service for a period of approximately four months at that age and submit them to thorough repairs to the ballast tanks and boilers.
Cargo Containment Designs
During the 40 years that LNG has been transported by sea there have only been 5 or 6 cargo tank designs that have been put into international deep-sea service; of these, the Conch design was the first to be developed and constructed, but also the first to be abandoned. Only 6 ships were ever built with this system.
More than half of the LNG ships currently trading are of the Moss spherical tank design, with most of the others being of the membrane type (the GazTransport invar tank design being twice as prevalent as the Technigaz stainless steel design). There are also 2 ships that have been built by the IHI shipyard in Japan to their own self-supporting prismatic type B tank (SPB) design, recently licensed to Samsung.
Type B (Moss and Alternatives)
The spherical tank design dominated new construction until the end of the 1990s, but the new order boom in the past few years has seen a shift towards the GTT membrane systems. Reasons for the shift include limited air draught under bridges at import terminals and Suez Canal tonnage fees due to the inherent inefficient use of hull space.
There are a number of other systems which have been mooted at various times, almost all being self-supporting. Most, but not all, have been for refrigerated and non- pressurised tank designs. The prime drivers for new systems have to be for ways of increasing cargo capacity in a given hull size, of reducing capital cost of tanks and insulation, of reducing the overall construction time of ships, and of increasing the capability of the ships to trade deep sea with slack tanks but without incurring sloshing damage. The major constraint for all of the presently successful LNG shipbuilders is space in the shipyard. The Korean yards have all been forced to increase their berthing quay space in order to boost the number of LNG ships they can build each year, because the present designs need the ships to sit alongside a berth for many months while tank installation and outfitting are carried out.
The limiting factors for spherical tanks are those of increasing diameter. In order to produce higher capacities, sphere diameters must increase, or parallel inserts included at the equator, which provide a taller tank. To increase the diameter, thicker materials are required, and these are pushing the limits of shore side cranes in dedicated tank fabrication facilities. Controlled welding of thicker materials becomes ever more difficult. Just adding more spherical tanks to a ship design has the effect of making a longer thinner ship. As stability is already a concern for these ships carrying raised tanks, increasing the length to breadth ratio is not recommended.
Another option muted a few years ago involves egg shaped, or even less regular equivalent shape, tanks that fit into the hull form more readily and efficiently. While this might give a technically and aesthetically pleasing ship-shape solution, it does not make use of current production processes.
It is also noted that Kvaerner Masa were offering a long trunk deck covering the spherical tanks, thus removing deck stresses by taking the load higher up. Whether they succeed in getting dispensation from the Suez Canal Authorities remains to be seen. Clearly there are many advantages in this system, but serious consideration needs giving to the disadvantages too, e. g. tonnage, windage and gas dangerous zoning.
Other Cargo Containment Systems of LPG and LNGType B self supported tank systems, involving companies such as IHI and others, offer a technically superior solution, providing the flat deck of a membrane ship, no filling level restrictions and, as with the Moss tanks, prefabrication of the entire tank. This allows the shipyard to undertake construction of the hull and tanks in parallel. Unfortunately, so far, these designs have proven expensive to build, although developments are believed to be underway to reduce costs. Samsung’s development of the IHI system will be closely watched by the industry.
Since their merger, the GTT systems have been marketed as the invar/plywood box based GazTransport system, for shipyards with a high skills base, and the Technigaz prefabricated reinforced polyurethane/triplex and stainless steel corrugated system aimed at shipyards with a more rudimentary work force. The merger enabled the two systems to evolve to the hybrid Combined System, CS1, now being installed in Gaz de France’s newbuilds. This is envisaged as having all the advantages of the off- ship prefabrication, giving a reduction in outfitting time, with the benefit of faster automated welding for an invar membrane. Future developments are revolving around simplification and improved production rates.
It is noted that GTT are no longer the only contenders in the membrane market, with the combined Korean shipyards developing a new solution to the containment problem.
A major design consideration is a tank’s reaction to cargo sloshing. Type B tanks are not generally considered to be limited by sloshing considerations; however, the design and construction of membrane tanks does make them susceptible to sloshing damage under certain loading conditions. While the supporting steel bulkheads can have increased scantlings to withstand the cargo wave energy, the membrane insulation that lines the inside of the hold space is susceptible to high pressure crushing from localised impact pressures. If a cryogenic cargo were to breach the thin membranes and crush the insulation it would have devastating effects on the steel structure of the ship, potentially splitting the ship very quickly. It cannot be over-emphasised that extreme caution needs to be taken.
Following experience on early membrane ships, a barred filling range has been accepted as an industry norm for membrane tanks that is between 10 % of the tank length (expressed as a height) and 80 % of the tank height. However, with the advent of offshore discharge and improved insulation reinforcement, the upper level of this range has been reduced to 70 % of height, which allows entire-ship flexibility for a typical membrane ship with 4 tanks. That is, any partial cargo quantity can be accommodated within the ship by careful distribution among the membrane tanks, allowing stand-off during hurricanes in the Mexican Gulf in applications such as Energy bridge.
There has been much talk and investigation into removing the barred filling range altogether, but this has to be treated with extreme caution. It has been proven that significantly large impacts are possible that could jeopardise the integrity of the insulation system. Major sloshing impacts generally occur at resonance between tank fluid motion and ship movements. The authors understand that nobody has yet modelled the cargo sloshing to vessel roll (or pitch) couple, which would enable realistic predictions of resonance, or damping of roll and sloshing periods. It is not even clear whether anybody is capable of satisfactorily calculating such a coupling effect.
While some in the industry are discussing localised barred filling ranges, it is considered dangerous to allow one barred range for one set of waters and a reduced barred range for a different set of waters, as misunderstandings in their application could be catastrophic.
Furthermore, the pump tower must be significantly strengthened above current practice to withstand higher cargo velocities. It is noted that all three GTT designs have recently changed their pump tower base support to a much simpler conical arrangement, which should improve matters noticeably over the previous square top hat base guide.
Cargo Handling Systems
In common with many other forms of energy, the main gas consuming markets are rarely located in the same place as the gas producing fields, and it is therefore necessary to link the two together. Carrying LNG on ships has often been likened to a floating pipeline connecting the producer with the gas user and, as such, the primary purpose of the ship is to load its cargo in one port and then discharge it in another. In the early days of LNG shipping the boil-off gas from the cargo tanks was either burnt as fuel or vented, although the port of Tokyo banned venting early in the 1970s. Potential reliquefaction plant designs are now available for LNG ships but, apart from a MHI prototype, have not yet been built or fitted. This may change if the commercial economics change in favour of reducing fuel consumption and conserving LNG.
Submerged electric driven cargo pumps have been used on all LNG carriers to date, and have proven their reliability in service over many years. Gas compressors (either steam or electric driven) are used for returning boil-off gas to shore during loading operations, and for cooling, inerting, warming up and gas freeing the tanks. Gas fuel for the ship’s boilers is generated either by natural boil-off pressure or a low duty gas compressor. The cargo handling systems on all LNG ships are very similar and, therefore, there are few problems experienced or changes foreseen. The major challenge, other than incorporating reliquefaction plant, is expected to be the equipment capacity required to meet loading and discharge times for the larger ships presently being specified.
The present research and development work into both floating LNG production systems (FLNG) and floating storage and re-gasification units (FSRU) will certainly bring changes in the cargo handling systems for LNG shipping. Current ships are not suitable for loading from FLNG units, except in exceptionally benign environments, nor are they designed to berth at FSRUs. New cargo handling facilities may involve tandem shuttle tanker solutions, with pantograph loading arms, floating cryogenic hoses or floating jetties, or alongside cargo discharge or loading equipment. All options require the ability to berth LNG carriers easily and safely adjacent to the floating unit. Additionally, the Energy Bridge concept of offshore discharge into a high pressure subsea gas pipe, through an onboard regasification plant, is expected to see operational service within the next 12 months.
Alternative Propulsion Options
Since the METHANE PRINCESS and METHANE PROGRESS were built in 1964 all LNG ships have generated most of their power, for both propulsion and ship services, through steam boilers. The steam has driven both the main engines and the electrical generators as well as powering many auxiliaries (compressors, pumps, fans, etc.) and providing the heat source for fuel tanks, air conditioning, etc. The ships always have either one or two diesel generators, but these are only for backup when manoeuvring and in port, and for cold-starting purposes. A consequence of utilising steam power is the need to continue resourcing experienced crew, familiar with the technical challenges of maintaining a steam plant in good condition.
In the same period of time, we have seen the development of dual-fuelled (oil and gas) diesel electric power generation on many ship types, and the adoption of gas turbines as the power generation prime mover on many offshore oil and gas production facilities. These power generation solutions have also been taken up and developed by the world’s major naval fleets, to the exclusion of steam power. LNG ships have become notorious for their conservatism of design and reluctance to adopt and take advantage of the latest technology. They are now probably unique in retaining the use of steam as a main power source on new ships. Although this has mostly suited the existing LNG shipyards, they are now actively studying alternative propulsion systems.
The conventional single main shaft and propeller arrangement (normally coupled with a bow thruster to improve the handling of these high freeboard ships when manoeuvring in port) could equally well be driven by geared gas turbines, one or two slow speed diesel engines, geared medium speed diesel engines or electric motors. Although many of the present operators see no need to change the existing arrangement, even if the prime mover changes, there is an increasing likelihood that electric motor propulsion could be adopted, as this (like steam) allows a common power source of electricity to be used for all main and auxiliary services on the ship.
The first diesel electric powered LNG ships are now being constructed by the Alstom shipyard in France, which are also adopting the first application of the new GTT composite cargo tank system (CS1).
Any future propulsion system designs, where the electrical power may be generated by dual fuel gas turbines or diesel engines, will be able to take advantage of the advances in propulsion motors and thrusters, including pods, which have been applied in the offshore drill ship and, more recently, cruise ship industries. These designs are characterised by multiple propulsion drives, enhanced redundancy and improved manoeuvrability.
Control and automation systems have probably been the one area on LNG ships where continuous technical development has taken place, and where the latest technology is presently being used. It is also the one major area where retrofitting of new systems is taking place on existing ships in order to take advantage of the latest designs, although the main reason for this is the difficulty of finding spare parts and skilled maintenance technicians after 25 years in service.
The technology of automation, control and monitoring has moved from the semi-automatic and predominantly pneumatic controls of the 1960s and 70s, through a process of continuous development to the current integrated computer based systems of today. The recent development and application of integrated automation systems has the potential to link all systems on the vessel, which should bring with it the capability of combining all systems on a single workstation display. This in turn can allow control from one location and, equally, make it available everywhere on the ship.
The potential need for redundancy in both control systems and propulsion systems will result in the application of Failure Mode and Effects Analysis (FMEA) techniques to the design of these ships. FMEA is already being requested by owners and classification societies in their specifications for the integrated control system and its components, and it may well spread to the overall machinery systems on the vessel. As well as ensuring that unsafe conditions and situations are avoided, this type of analysis study also improves the availability of the ship at all times and reduces the downtime, and possible demurrage costs.
Dynamic positioning (DP) is another specialised control system that may be specified for the enhanced manoeuvrability required at FLNG and FSRU facilities. This technique has been around for nearly 40 years and in the last 20 years it has become well proven and established in the offshore, and now the cruise ship, industries. The main manufacturers of DP systems also deliver integrated control solutions, so that they can either be stand-alone or integrated.
Compressed Natural Gas Shipping
As a straight comparison of moving natural gas, CNG cannot possibly compete with LNG. Ships of a comparative size will not carry even 10 % of the cargo carried in liquid phase. However, for certain offshore oilfield projects it can make commercial sense. It is often not commercially viable to liquefy and ship stranded gas in oil fields due to the prohibitive cost of liquefaction systems. Therefore, natural gas is frequently injected back into the well, avoiding flaring but diluting the subsequent oil.
On the other hand, compressors are relatively cheap to install and to operate, and thus CNG becomes viable. The benefit at the oil well is sufficient to pay for the gas withdrawal, and furthermore, the gas can be sold on the open market. Receiving terminals require the minimum of equipment; typically a valve on the end of a pipe and a berth are all that is required. The ship opens the taps to shore, and the cargo self-discharges into the gas pipe transmission network, or local holding tanks where they exist. Smaller markets then become viable, such as towns and industries, out of reach of existing pipelines. It can also become commercially viable to transport gas from marginal fields without the expensive liquefaction and regasification facilities.
All CNG systems involve pipes and cylinders of various types. There are 5 main systems on the market as follows:
- CNG Solutions; one long 24 or 42 inch pipe wrapped around the ship’s sunken deck;
- EnerSea Transport (VOTRANS); vertically aligned bottles in refrigerated holds (presumed standard 42 inch);
- Knutsen PNG; vertically aligned short 42 inch pipes containing gas that is chilled;
- TransCanada Pipeline; longitudinally aligned short inch pipes;
- Williams Coselle; bobbin wound inch pipe.
Each system has its advantages and disadvantages.
Wavespec is responsible for obtaining Class Approval and introducing the TransCanada Gas Transport Module (GTM) system into the marine field. The TransCanada transmission pipeline hybrid has taken standard lengths of composite 42 inch diameter transmission pipe and added dome ends to form the GTMs. The GTMs are already ASME certified, which is a major benefit in getting a project into commercial operation. Typically a ship would carry hundreds of GTMs.
A major consideration is the use of higher pressure cargo vessels than the marine industry is familiar with, a challenge in itself for the suppliers.
From a naval architect’s point of view, the considerations include:
- Weight – most of the “deadweight” is taken up by the cargo containment, rather than by the cargo itself. At typical pressures of around 200 bar, the cargo has a specific gravity of only 0,16, not surprising for a gas. Hence, containment weight is a serious issue. When it is considered the draught changes very little from fully laden to unladen, ballast may not be necessary but docking is a major consideration. Designs will be limited to whether or not they can be dry-docked in existing facilities.
- Speed – unlike LNG, which has a boil-off time penalty, speed is not an issue for deterioration of the cargo itself; however, the more that gets through quickly, the more cargo is sold and the less ships are required.
- Alignment – we, at Wavespec, feel horizontal alignment works best due to greater flexibility of arrangement and scaling, whereas other proponents prefer vertical pipes. Effective use must also be made of fore and aft spaces. There may be ship stability issues associated with vertically mounted cylinders.
- Containment system survey requirements need to be addressed and accessibility of the system for inspection and repair must be considered.
Cargo control is also important as simply opening the valves could have disastrous effects, and thus needs regulating. Aside from pressure build up, and the pressure capability of receivers, the Joules-Thompson effect, where a pressure drop through a venturi neck or valve could chill the containment significantly, needs consideration
From a regulatory point of view, the International Gas Carrier Code for Ships Carrying Liquefied Gas (IGC) is not strictly applicable, although many of the principles can be used. Class has been quick to respond to the interest shown by developers, with some Societies producing Class Requirements, generally based on offshore requirements, or more typically risk based. As yet, there are no specific IMO requirements. Flag and Port State Authorities will have to be involved for each and every case.
Wavespec (a part of the Braemar Seascope group) has attained Approval in Principle from LR for TransCanada’s marinised GTM system. It is believed that Knutsen PNG has Approval in Principle from DNV and EnerSea from ABS.
With the recent orders by Gaz de France of diesel electric propelled ships, and the submission of tenders for bigger ships and/or alternative propulsion systems, the industry is finally seeing some movement in the application of recent advances in technology to the future LNG ship designs.
The increase in the number of LNG ship owners and operators will introduce more commercial priorities into the vessel specifications, and this will affect both the flexibility and the economic design of the ships. Inevitably, it will result in changes to the power generation and propulsion systems installed. The gas turbine and diesel engine manufacturers will eventually see their products installed in LNG ships.
Developments of the existing containment systems, and possibly alternative designs allowing cheaper and quicker building of self-supporting tanks, should allow the shipyards to reduce the overall building time and (possibly) cost. Because the tanks may be constructed off-site, this will also expand the number of shipyards able to construct LNG ships. Compared with the 28 shipyards which have built LNG ships since 1964, the large number of ships on order over the next 3 to 5 years will be constructed by less than 10 shipyards.
The technology in LNG ships will certainly catch up with the advances made in other ship types over the past 10 to 20 years, and will probably move ahead in specific areas of propulsion and cargo handling. It is also likely that more diverse designs of LNG carrier will evolve, particularly as floating production and reception facilities are developed. CNG is also moving, albeit slowly, towards a position where a choice of technology solutions will be available once the projects can be shown to be commercially attractive.