Design study of ship LNG storage and engine feeding system

Gerasimos TheotokatosNAOME, University of Strathclyde

Introduction

This study focuses on the design of the LNG storage and feeding system of a vessel and the machinery required to maintain the LNG in very low temperature (-163oC) into the appropriate conditions to be consumed by the vessel engines.

The study has based on regulations from national and international classification societies (HRS, GL, DNV) regarding the entire LNG system installations.

The above rules have been employed in order to provide, for the specific case study, the appropriate design of the piping and storage system (materials, dimensions, insulation, routes and supports).

The particular ship type (i.e. Ro-Ro passenger ship) and ship route was mainly considered.

The basics of the LNG supply system are:• LNG tanks • Distribution network • Evaporation skid and heating system• Gas valve units• Engine room machinery arrangement

Note that the design procedure is a “back & forward” process until the finalization of all the aspects of the ship’s studies (structural issues, stability, safety systems, modifications, etc)

Ano Hora II - Ro-Ro passenger ship

The LNG system arrangement

LNG tank size calculations

• During the journey, to and from the island, the ship uses each time 2 of the 4 its engines Cat 3512B corresponding to the direction of the journey. It, also, uses one of the two auxiliary engine, but its load doesn’t exceed the 50% of load.

• During on port the vessel uses one of its two auxiliary engines at 85%.

• Tank size calculations are based on a 1 week wanted autonomy plus a 10% of tank capacity

• The required volume of LNG is calculated to 95m3 for 7 days autonomy.

LNG tanks Type

In the project the option of removable cassette LNG fuel tank containers was selected instead of permanent fixed tanks.

Non-existent shore-based LNG distribution network.

These containers can be transported by truck or by ship to the nearest LNG terminal for refilling, and then loaded onboard the ship.

1. No fuel bunkering station necessity (limited space in open deck).

2. No bunkering procedures required at the port.

LNG tanks Location

The three cassettes can be located in open-deck or in enclosed space

• Open deck scenario:

Tanks should be located at least B/5 from the ship’s side

Tanks must be located in such a way that sufficient natural ventilation is provided

Fitted with drip trays below the tank which should be of sufficient capacity to containthe volume which could escape in the event of a pipe connection failure

• Confined space scenario:

The tank and all associated equipment (valves, piping etc) should be confined in a space(Tank room) designed to act as a second barrier

Storage tank(s) should be placed as close as possible to the centre line of the ship:

• minimum, the lesser of B/5 and 11,5 m from the ship’s side

• minimum, the lesser of B/15 and 2 m from the bottom plating

• not less than 760 mm from the shell plating in all cases

Tank room should not be located near Category A machinery spaces or near other high

risk spaces. If it is adjacent to machinery space, the two spaces are to be separated by

means of a cofferdam at least 900 mm thick

Cassettes LNG tanks

• Location inside the lower garage was rejected due to very limited available space and due to safety distances limitations.

• The three LNG fuel tank containers are intended to be located on the open deck at the after of the vessel.

• An appropriate platform will be fitted at the after of the ship.

Natural ventilation is ensured at all times.

The minimum safety distance from ship’s sides is satisfied.

Cassettes LNG tanks

• The inner and outer tanks are manufactured in steel of grade A304L.

• Each LNG fuel tank container is fitted with the pressure build-up system (PBU) for maintaining an operational pressure of approximately 5 bar in the tank.

• The LNG fuel tank containers are connected to the on board gas system in a docking station.

• The docking station consists of the connections between the LNG fuel tank container and the fuel gas handling system and a connection to the water spraying system built onto the LNG fuel tank container.

The forecastle of the vessel

The metallic platform

• The LNG tank containers, the docking station and the evaporator skid will be placed on a specially designed metallic platform that will be constructed next to the after castle.

• The three LNG fuel tank cassettes will be loaded by the port derrick and fastened and secured by twistlocks on the metallic platform.

• Drip rays will be installed under the evaporator skid and also under the hose connections the docking station.

• Things to be further assessed: Structural integrity (total weight of 115 tones approx.) and effect on ship’s stability.

Cassettes LNG tanks

• The container consists of an IMO ”type C” inner tank, an outer tank secondary barrier, and vacuum insulation for minimizing heat transfer.

• The inner and outer vessels are manufactured in steel of grade A304L.

• To modeling the LNG tank a computational analysis is performed.

• The tank dimensions, the distance of the interbarrier, the stiffering rings and the supports have been studied based on heat transfer and static structural calculations.

Heating the LNG

• The purpose of the evaporator skid is to turn the stored in the tank liquefied natural gas into a form that is consumable by the vessel engines (20⁰C-25⁰C).

• The calculations for the LNG evaporator are based on the total power of the 2 Cat 3512B engines and the auxiliary engine at 100% load.

Heating the LNG

• The heating system transfers the required heat to LNG by a heating mean that flows in a secondary circuit is the glycol-water skid that is located in the after engine room.

• The heating media is the 50% ethylene-glycol & water solution that enters the evaporator skid at 50⁰C and exits at -15⁰C. This solution has low viscosity, lowest freezing point and high safety on board and is the most used heat transfer intermediate fluid in marine applications.

• The glycol-water skid uses engine cooling water at 91⁰C in order for the glycol-water to regain its temperature.

Heating the LNG

• To modeling the heat exchanger a computational analysis is performed.

• Shell-and-tube heat exchanger of stainless steel 304L.

• The form of the heat exchanger involves 10 tubes and 7 passes.

• Overall dimensions of the outer shell 1.7m x 1.22m x 1.04m (LxWxH).

• Position on the metallic platform.

Machinery room type

ESD-protected machinery roomEmergency shut-down-ESD type requires that in the event of a gas hazard, all non-safe equipment (ignition sources) and machinery is to be automatically shutted-down (single wall gas piping accepted).

Gas safe machinery room Arrangements in machinery spaces are such that the spaces are considered gas safe under all conditions (double wall gas piping is required).

GVU arrangement and NG pipelines at the after and fore engine room of the lower deck

The gas valve unit

• It is a containerized compact unit of stainless steel that includes two quick closing valves and a ventilation valve in-between, filter, pressure transmitters, a gas temperature transmitter and control cabinet.

• The gas valve unit regulates the gas pressure to the engine and ensuring a fast and reliable shut down of the gas supply.

Four GVUs in enclosure – one for each engine, should be considered They should be installed in a vertical position inside the two engine rooms

The piping system

• Pipeline of LNG tanks to the evaporator skid

Consists of a double pipe of steel A304L with vacuum insulation in the interbarrier space for thermal insulation.

• Pipeline of NG from evaporator skid to GVU & main engines

All pipelines inside each engine room are of double-wall type (from the boundaries of theroom until the boundaries of each GVU and also from each GVU to the corresponding gasfueled engine)

These inner and outer pipes are made of steel 304L alloy while the space between theconcentric pipes will contain ventilated air under pressure, having a capacity of at least 30air changes per hour.

The vessel

Implementation of LNG fuel in Ano Hora II

Safety issues

Environmental protection

Performance

Convenience for passengers and personnel

THANK YOU !!!