Method for operating an LNG fuelled marine vessel and a corresponding marine vessel

A method for operating an LNG fuelled marine vessel having a propulsion system, a power plant and an LNG storage tank, whereby LNG is supplied as a gas fuel from the LNG storage tank to the power plant. The marine vessel deploys a fixed LNG storage tank and a removable LNG storage tank on board the marine vessel. In order to facilitate bunkering, the removable LNG storage tank is removed and replaced by another removable LNG storage tank at a bunkering facility.

TECHNICAL FIELD

This invention relates to a method for operating an LNG fuelled marine vessel, which marine vessel comprises a propulsion system, a power plant and an LNG storage tank, in which method LNG is supplied as a gas fuel from the LNG storage tank to the power plant, and in which method the marine vessel is refuelled at a bunkering facility according to the preamble of claim1. This invention also relates to a corresponding marine vessel.

BACKGROUND ART

Marine vessels can be fuelled by LNG (Liquified Natural Gas). Clearly, as also with LNG, marine vessels need to be refuelled or bunkered at certain intervals. The bunkering operation usually takes place in port, but may also take place at other locations, e.g. at a floating bunkering facility out at sea.

The bunkering operation of an LNG fuelled marine vessel usually takes a long time. The reason for this is the temperature difference between LNG (normally about −162° C.) and the bunkering line (normally in ambient temperature, around +20° C.). This temperature difference causes the LNG to boil in the bunkering line, which leads to a two-phase flow of gas and liquid. The two-phase flow causes control problems and pressure pulses, which are harmful for the supply procedure and for the piping arrangements of the bunkering line. Consequently, in order to arrive at a functioning bunkering operation, the LNG flow rate has to be kept very low in the beginning of the operation in order to minimize, or at least in order to attempt to keep the pressure pulses at an acceptable level. After the bunkering line starts to cool down, the flow rate may slowly be increased. The bunkering operation consequently takes a very long time.

A typical practice and trend in marine vessel operation of today is shortened port times and high operating speeds, the latter with high energy consumption, which would necessitate a transfer of large amounts of fuel to the marine vessel in a short time. Such fast transfer of fuel is not available in the prior art.

SUMMARY OF INVENTION

An object of the present invention is to avoid the above mentioned problems and to achieve method for operating an LNG fuelled marine vessel providing a fast turn-around time at the bunkering facility. This object is attained by a method and a marine vessel according to the claimed invention.

The basic idea of the invention is to eliminate time consuming fuel transfer procedures, whilst maintaining an operative supply of LNG for a power plant on board the marine vessel. To this effect, the marine vessel deploys a fixed LNG storage tank and a removable LNG storage tank on board the marine vessel, whereby the removable LNG storage tank is removed and replaced by another removable LNG storage tank at the bunkering facility.

The removable LNG storage tank is connected to the fixed LNG storage tank when the removable LNG storage tank is deployed on the marine vessel, whereby LNG can be transferred from the removable LNG storage tank to the fixed LNG storage tank out at sea, and advantageously at a given rate during the operation of the marine vessel.

In order to allow a fast replacement of removable LNG storage tanks at a bunkering facility, the removable LNG storage tank that is deployed on the marine vessel is emptied before being disconnected from the fixed LNG storage tank, whereby the emptied removable LNG storage tank is removed from the marine vessel so that another removable LNG storage tank provided with LNG can be deployed on the marine vessel and connected to the fixed LNG storage tank.

The advantages of the invention are particularly relevant when the marine vessel is a marine vessel such as e.g. an offshore support vessel, a tug, a short sea ferry, or the like, whereby the very short bunkering time allows for keeping the marine vessel available for operation almost without any noticeable intervals.

DETAILED DESCRIPTION

In the figures a marine vessel is generally indicated by reference numeral1. The marine vessel comprises a propulsion system2, a power plant3and a fixed LNG storage tank4on board the marine vessel1. The fixed LNG storage tank4is connected to the power plant3by means of a fuel line41. The power plant3is connected to the propulsion system2for providing driving power to the propulsion system.

The marine vessel1further comprises a removable LNG storage tank5aor5b, which is detachably connected to the fixed LNG storage tank4by means of a releasable connection line51. The fixed LNG storage tank can be located above or below the main deck, whereby the removable LNG storage tank advantageously is arranged to be located on top of the fixed LNG storage tank. Clearly, the removable LNG storage tank can be arranged on the side of the fixed LNG storage tank or at another location on board the marine vessel as well.

The operation of the marine vessel1can shortly be described as follows.

When the marine vessel1is out at sea, as illustrated byFIG. 1, the power plant3, or propulsion machinery, of the marine vessel1is driven by vaporized NG (Natural Gas) supplied from the fixed LNG storage tank4. Normally LNG is supplied by way of a discharge conduit through a heat exchanger, where the LNG is vaporised, so that it is delivered in gas mode (NG) to a fuel feed line of the propulsion machinery. At the same time, when the marine vessel is operated out at sea, LNG is transferred from the deployed removable LNG storage tank5ato the fixed LNG storage tank4at a given rate through the connection line51. Both the fixed LNG storage tank4and the deployed removable LNG storage tank5acontain LNG as indicated by referenceLNG.

FIG. 2shows the marine vessel1arriving at a bunkering facility, e.g. its port of destination. In addition to possible unloading and/or loading operations, it needs to be refuelled. Prior to the marine vessel1arriving at the bunkering facility, the removable LNG tank5ais emptied (as indicated by referenceE) before it is disconnected from the fixed LNG storage tank4on the marine vessel1. In order to facilitate and speed up detachment, all connections, including the connection line51are heated.

LNG can still be supplied to the power plant3from the fixed LNG storage tank4in a constant manner, thus not disrupting the operation of the marine vessel. The amount of LNG left in the fixed LNG storage tank4should thus be sufficient to allow full operation, until another removable LNG tank5b(provided with LNG) is appropriately connected to the fixed LNG storage tank4, as discussed in connection withFIG. 3in the following.

FIG. 3shows the marine vessel1at the bunkering facility. After the deployed removable LNG tank5ahas been emptied (as indicated by referenceE) and the connections detached, the emptied removable LNG tank5aon board the marine vessel1is replaced by another removable LNG tank5bprovided with LNG (as indicated byLNG). The replacement procedure is illustrated by block arrows. The marine vessel1can leave the bunkering facility, preferably after the removable LNG storage tank5b, which is provided with LNG, has been connected to the fixed LNG storage tank4by means of the connecting line51.

As shown inFIG. 4, the marine vessel1can continue its operations out at sea, while, as described above in connection withFIG. 1, LNG is transferred from the deployed removable LNG storage tank5bto the fixed LNG storage tank4at a given rate through the connection line51.

Consequently, the arrangement with a fixed LNG storage tank4on board the marine vessel1and a removable LNG storage for refuelling provides for a fast bunkering operation and a short port time. This is particularly advantageous when the marine vessel is a marine vessel such as e.g. an offshore support vessel, a tug, a short sea ferry, or the like, which preferably is in operational mode in a more or less constant fashion.

The LNG storage tanks and the fuel supply arrangement, including components like pumps, heat exchangers, compressors, connection and fuel supply lines, as well as valve means, are as such considered to be known by a person skilled in the art and are therefore not described in more detail in this connection.

The different stages of operation described above can be suitable controlled by appropriate automation systems. This is considered to lie within the knowledge of a skilled person in the art and is therefore not discussed in detail in this connection.

The description and the thereto related drawings are only intended to clarify the basic idea of the invention. The invention may vary in detail, such as to the number of fixed or removable LNG storage tanks, fuel lines, or other components, within the scope of the ensuing claims.