Contents:
SHIPYARD SELECTION
PROJECT ROUTE
MATERIAL SELECTION
COST RATIO
PORT FACILITIES AND DESIGN
SAFETY REQUIREMENTS
VOLUME REQUIREMENTS
SERVICE SPEED
TANK CONTAINMENT SYSTEMS
CARGO HANDLING EQUIPMENT
INTERCHANGEABILITY BETWEEN PROJECTS
IMO - IMDG
CLASSIFICATION SOCIETY
UNLIMITED LIABILITY
SIZE OF VESSELS
PROJECT RELIABILITY
BOIL OFF RATES
DESIGN RELIABILITY
RELIQUEFACTION PLANTS
CONVENTIONAL SHIP DESIGN
TANK SYSTEMS
FACTORS AFFECTING SHIP AND TANK CONTAINMENT SYSTEM
SELECTION FOR AN LNG PROJECT
LNG ships carry their cargo at a temperature of -1650 Celsius. This particularity determines the most important characteristics of LNG ships (and projects).
LNG ships are usually tailor-built ships to the needs of a specific project. Different project requirements and surrounding factors require different ships and hence one ship built for a specific project might be totally unfit for another project.
However the factors affecting the ship and tank containment system choice are more or less the same for every project although the different possibilities in responding to these factors produce totally different ship requirements.
Here are the most important and influential factors affecting the ship and tank containment system selection for an LNG project.
Ship design and tank containment system choice depend greatly upon shipyards’ technology. Several shipyards are dedicated to specific solutions because:
1. they have patents over specific technological innovations,
2. they are committed to their historical choices,
3. they are politically forced to adopt certain technologies,
4. they are environmentally forced to pursue certain standards obtained only by specific designs.
Ship design must be selected upon the capital (initial), operating and repair costs expected.
Furthermore, the shipyard chosen may have a full order book and hence alternative shipyards committed to other designs and systems should be used.
The shipyard chosen to built the ship must be labour efficient and labour prices must be within world levels so as to produce an economic result.
Building ship for an LNG project in a country far from the place where the project will be in force might emerge the problem of spare parts availability. Severe delays might be caused if spare parts have to travel long distances to reach the ships. Furthermore, availability also concerns the shipyard even if it in the close proximity of the project, i.e. the shipyard must be able to produce and/or construct the required spare parts within sensible time limits or have stocks from the beginning of the project.
The tank containment system must be chosen with respect to the shipyard’s willingness to guarantee the system. Low initial cost, safely designed and easily operated systems might be rejected if the shipyard cannot guarantee its technology since high innovative systems might present unimaginable problems.
If the mother shipyard is distant from the area of the project then dry-dock facilities with the necessary skills required for the maintenance of the ship must be available in the area of the project. If they are not available one must consider adopting widely used technology in the ship design and tank systems or buying and transferring the technology to a convenient shipyard.
It is a matter of utmost importance in an LNG project to obtain guarantees from the shipyard. Normal yard guarantees given for all vessels (speed and time penalties) are insufficient for LNG ships. In these case, the shipyard must be able to guarantee the long term integrity of the design, the efficiency of the insulation (boil off rate agreed) and the secondary barrier (methane ship PVC plugs). If these guarantees cannot be given, alternative designs, systems or even shipyards should be evaluated.
When time is of importance the ease of construction and expected time of delivery are crucial in deciding which design to use which tank system to adopt and which shipyard to trust.
Since LNG ship are tailor-built for specific projects, it is important to secure the shipyard’s ability to undertake immediate repairs. If a ship break down, the shipyard must be able to accept it immediately because no replacement ships can be found in these cases and usually no other shipyard can perform certain operations since LNG ships are based on patented technology which only the mother shipyard has.
Project route(s) play a significant role in ship design. If adverse weather, strong winds and ice are likely to be encountered, designs with increased stability standards or ice-strengthened hulls must be considered as well as the possibility of increased service speed to offset time losses.
Since several unusual (for ships) materials must be used in the construction of an LNG tank containment system, the prices of these materials at the shipyard must be considered. These affect greatly the total initial cost of the ship and hence low availability or high prices for certain materials at a specific shipyard might encourage project operators to look for alternative systems, materials or even shipyards.
Reliability is also the pre-requisite which imposes the use of proven materials of construction such as: aluminium, invar, S.S. Balsa, PVC, glass wool, plywood, oak plywood, perlite, etc.
It is very important to keep the project cost/ship cost ratio to a certain level. Since LNG projects require increased investments in land facilities, the initial cost of the ship(s) must not exceed a certain level because each project sector (ships, land facilities) is intended for specific profit levels and this balance can not be disturbed.
Port facilities are crucial upon ship design selection. Cargo handling equipment must be compatible (ship/shore) and either shore facilities should be designed upon ship requirements or inversely. Independent shore and ship designs would create delays and additional cost in adopting the necessary equipment interfaces. Finally, on-board pump arrangements must have adequate capacity to meet the standards set by the project in order to have small port turn around times.
There are some operational limitations with respect to the size and design of the ship(s). Maximum berth length at the loading and discharging terminals, maximum allowable draught at the same ports, size of and ease of approach to the port entry must be considered before deciding the maximum vessel dimensions.
Safety requirements are of utmost importance since LNG ships are very sensitive designs. Apart from the conventional stability, structural failure, fire, collision and grounding considerations (typical for all ships), LNG ships are subjected to increased dangers due to the particularities of the tank containment system used. Safety standards are usually set by the classification societies but in innovative designs they are set by the shipyard. Cargo handling equipment failure, tank failure and potential operational faults must be carefully considered and evaluated in the final designs bearing in mind that increased safety means additional cost.
Both ship design and tank containment system selection are strongly influenced by the volume needed. Certain tank containment systems are designed with low space utilisation factors (offset by other factors such as cost) and hence these systems must be chosen after having carefully evaluated the average volume of cargo to be transported.
Hull size restrictions affect tank choice due to space utilisation. Hence if systems requiring large spaces for installation are to be used, the vessel size should be increased to obtain the same cargo volume.
Once the ship design and tank containment system have been chosen, service speed required is determined by the following factors:
1. boil off rate - the higher the higher the speed must be,
2. tank volume - the lower the higher the speed,
3. spare capacity needed - the higher the higher the speed.
Thus, after service speed has been determined, engine installation and design must be considered as to provide the required results. Ship speed is primarily related to capacity, since low capacities require higher speeds for obtaining the same deliverability levels. However the are power limits once a single screw vessel design has been chosen.
Furthermore, one must consider:
1. the shipyard’s capability in producing high speed engines,
2. the required service speed,
3. adverse weather conditions likely to be encountered,
4. the expected speed fall off due to fouling and ageing of the machinery,
5. the expected dry docking intervals,
6. hull form with respect to vibration and pounding.
Tank containment system technology usually imposes several restraints as to the final ship design, characteristics and cost. Single or semi membrane, independent designs, insulation materials, once chosen, they determine several design aspects of the ship such as its hull form and volume, the cargo gear and ballast spaces and adjoining spaces.
In certain cases where reliability 100% is required, proven operating experience of a design or a tank system should be considered in order to avoid the risk of failure of a new innovative design or tank system.
Tank designs should have maximum resistance to damage from accidents. If such damage occurs it is likely that the ship would remain out of service too long and the cost or repairing it would be too high for the project to withstand (e.g. damage to membrane).
Tank systems should also be assessed upon the simplicity of the system and ease of operations. Complicated systems should be avoided because:
1. sea conditions are not optimal for coping with complicated problems,
2. they would require higher costs to maintain them,
3. higher skills and training for operating the system would be required and these are hard to find on seamen,
4. they have increased sensitivity to system malfunction and difficult to repair.
Tank containment systems should also be assessed on the basis of their:
1. construction reliability as long term integrity of the system is crucial in LNG projects (to avoid delays and high costs for maintenance),
2. ease of access to tanks and insulation which are needed for inspection,
maintenance and repair purposes,
3. cubic efficiency as low utilisation factors might require larger or faster ships which means higher initial costs
4. material cost (as analysed before),
5. material content since:
5.1. density and weight play an important role in the ship’s final DWT,
5.2. the chemical contents of the material might be sensitive to sea water, or moisture which are usual in sea conditions,
5.3. certain materials present welding difficulties (aluminium),
5.4. thermal expansion coefficient differ from material to material and are
not allowed to exceed certain safety limits,
5.5. different materials have different physical and chemical properties
which might make them unsuitable for certain applications.
6. prefab potential because construction time is minimised, initial costs decrease and because repairs can be undertaken immediately is the specific prefab part is available on stock,
7. cool down time since if long cool down periods are required then turn around times are increased and consequently the ship’s speed might have to be increased to produce the same deliverability levels,
8. test potential since it is important for the project operators to have guarantees for the system operational characteristics (integrity, boil off rate, reliability, cost, sensitivity to damage and flooding), before deciding whether it is appropriate for the specific project or not,
9. independent tank construction since this will reduce the risk of total failure of the ship,
10. simplicity of construction design (as analysed before),
11. insulation friction which must be minimum in order not to impose excessive stresses on the cargo tanks,
12. stability, since several tank system designs contribute negatively to the ship’s overall stability,
13. full time ballast, which if needed will produce more intensive corrosive effects to the ship’s hull and therefore will require higher maintenance time and cost,
14. pipe and pump structure which require lot of space to be installed in and
hence will reduce space utilisation if their requirements are high, and furthermore complex pump and piping systems require skills, time and cost to be efficiently maintained,
15. stress from hull which must be as low as possible because they may affect the operational efficiency of the system and might cause severe disformation in certain cases,
16. visibility which is decreased as tank systems usually rise higher than the upper deck and hence bridge operations become more difficult if extremely high tank designs are selected and installed on the ship,
17. IG system which must be as simple and as effective as possible since safety depends greatly upon the preciseness of the IG system in order to avoid
explosions and fires on board and furthermore it must have small operational cycles as to avoid effecting port turn around times,
18. liability to flooding damage which must be minimum since vessels are
continuously subjected to water penetration to the hull from sea, rain and collision cases,
19. insulation costs which greatly affect the initial cost of the ship and the project as a whole and must be kept down to a minimum always with respect to the boil off rate they sustain,
20. boil off rate (as analysed next).
21. tanks must also be able to withstand external overpressure and water leakage from side ballast tanks.
22. sensitivity to ship’s motion (rolling, heaving pitching, slamming) and to ship’s vibrations. These must have minimum effect on the system since any on the above might damage, disform or distort the tank system or its insulation. Hence, tank support systems must provide the following characteristics:
22.1. freedom to expand and contract freely,
22.2 ability to keep tank in position when the ship is rolling, pitching,
heaving, etc.
22.3 absorb any hull deflection due to her motion or wave motion,
22.4 absorb temperature differences occurring during filling and emptying the tank.
Cargo handling equipment technology in conjunction with the tank containment system chosen imposes several design features on the final vessel. Spaces were cargo gear will be installed, piping arrangements through the cargo holds, thermocouple fittings, gas sampling points, heating coils for vaporising umpumpable cargo, means to monitor temperature levels in the tank structure and the hull structure of the ship, and means to measure liquid levels in the tanks must be considered on the primary designs since they require space for their installation and this means lower space for cargo.
Different types of pumps have emerged the last years such as the blow-case pump, the fully submerged electrical pump, the gaseous piston pump and the deepwell pump. Pump choice depends on tank system choice, initial cost and maintenance requirements. Bearing quality, insulation efficiency, cabling sensitivity and safety and sealing techniques must also be considered as part of the pump quality.
Cargo handling design parameters dictate space utilisation efficiency and operational safety. Some of these parameters are as follows:
1. cool down and warm up procedures must be designed to avoid unacceptable temperature gradients.
2. cargo discharge equipment must have great reliability. There should be no bottom or other below deck connections to the tanks.
3. there must be means to monitor temperature levels in the tank structure and the hull of the ship.
4. there must be means to measure liquid levels in the tanks.
5. there must be means to monitor for tank leakage.
6. there must be means to control and measure vapour pressure in both tanks and deck piping systems under all service conditions.
7. There must be provisions for expansion and contraction of piping caused by thermal changes and ship structural movements.
8. there must be means to prevent or counteract cooling of hull structure in the event of local insulation failure.
9. there must be means to safely dispose of or jettison cargo from the tanks
themselves or the spaces around them in the event of a major tank/insulation failure.
10. there must means to safely collect and burn in the ship’s main machinery installation the cargo boil off.
11. there must be means to detect and dispose of water leakage from adjacent ballast spaces into LNG containment spaces.
12. there must be means to prevent venting methane vapour to atmosphere under any circumstances.
Ship-shore transfer of cargo poses certain problems since hoses have failed as cargo gear interfaces. Swivel joints using teflon seals and articulated pipe designs must be taken into account in the preliminary ship/tank system design.
Deck piping and valves must be made by stainless steel which presents better results against vibration, corrosion, temperature changes and can be welded more easily than aluminium. It is however very important to find a suitable stainless steel alloy to match the specifications imposed by the specific tank system chosen originally for the vessel.
Cargo operations control must be able to detect system malfunctions and damages and should be installed in the bridge so as the deck officers could remotely monitor the situation with respect to the ship’s navigational status.
INTERCHANGEABILITY BETWEEN PROJECTS
If the vessel is to be used in two or more projects simultaneously or consequently, versatility and flexibility must be considered. Ship design and tank containment systems must be chosen so as to fit in all project requirements with respect to volume, speed, safety, spare capacity, profitability, deliverability and efficiency factors.
If the project life is limited then one must consider the possibility of using the ship in another project subsequently or restructuring it as to be used for other purposes or to build her as cheap as possible as to be able to cover its initial cost and still have profits.
Projects require a number of ships. When considering what ships to use, it is important to look into the market of existing ships since existing ships might be bought at lower cost. These might not be the ships required by the project characteristics but even if re-construction is required the initial cost might still be lower.
For small projects (i.e. small volumes of cargo or short term needs), ship conversions or old ship restructuring must be considered since these might prove to be the most economical solutions.
The IMO - IMDG code for LNG carriers must be followed faithfully both in design and in operational aspects. Cargo gear, tank containment systems and hull form and strength are determined in those regs and must be accordingly adopted.
Tank containment systems, cargo handling equipment and operational procedures must comply with the IMCO Gas Code Regulations and these have to be taken into account in the early designing days (Code for the construction and equipment of ships carrying liquid gases in bulk and Code for existing ships carrying liquid gases in bulk).
Classification society rules must also be followed so as to gain approval of design in order to obtain low insurance premiums covering the total initial cost.
If the ship(s) will be engaged in voyages between countries where the legal status imposes unlimited liability for causing environmental problems, then a great deal of consideration must be given in ensuring the highest available safety standards as to minimise the possibilities of structural failure or operational faults.
The theoretical optimum size for an LNG ship depends:
1. on port facilities and their ability to discharge/receive the total cargo within sensible time limits (rate of discharge),
2. on capital and operating costs since increased vessel size means increased costs and in relation to the project’s expected profitability vessel sizes above a certain level would be uneconomical,
3. on the design service speed since long distances with slow speeds and high boil off rates would make a vessel unfit for the project.
The maximum size of the project’s vessels depends also upon the shipyard’s
capability i.e. the berth or dock size available at the shipyard.
Project reliability determines:
1. whether the ship(s) should have spare capacity,
2. additional speed to offset down time,
3. the number of ships required for the project since low reliability demands more ships for emergency reasons.
The shipyard must be able to produce ship design and tank containment systems with efficient boil off rates and guarantee them since higher boil off rates need larger or faster ships in order to have the same deliverability levels.
The boil off rate is crucial. The normal boil off is .25% per day. If this can be reduced by half the annual cost saving might be $400,000 which in turn can amount to as much as $3.6m per ship reduction in capital cost. Hence, new systems or designs achieving this aim should be pursued having in mind that the shipyard must also guarantee the efficiency of the product.
Operational, material, system and design reliability affect the annual repair days anticipated. The lower the reliability the more repair days are required and hence, running and operational costs and loss of profits must be incorporated in the initial cost budgets.
Reliquefaction plants should also be considered. The cost of the equipment could well be offset by the increase in deliverability should gas prices rise ahead of fuel oil prices. This means however that extra bunker capacity has to be provided possibly with an increase in ship size.
Conventional parts of the ship should not be ignored long lasting designs should be preferred as both dry-docking costs and days would be reduced.
Some of the available patented tank system designs are as follows:
i. Constock/Conch system
ii. Lorentzen’s spherical designs (aluminium spheres)
iii. Shell’s cylindrical system with Balsa insulation
iv. Burness/Corlett’s cylindrical system with multi horizontal or vertical
cylinders
v. Chantiers du Dunkerque multi lobe or polycylindrical designs.