WO1998056651A1

WO1998056651A1 - Device for offshore lng production

Info

Publication number: WO1998056651A1
Application number: PCT/NO1998/000157
Authority: WO
Inventors: Anders Wadahl; Kåre Rygg JOHNSEN
Original assignee: Kvaerner Maritime As
Priority date: 1997-05-30
Filing date: 1998-05-26
Publication date: 1998-12-17
Also published as: AU7678498A; NO972497D0; NO972497L

Abstract

A vessel comprising spherical tanks (2) and a production plant for production of gas and possibly oil at sea, wherein the spherical tanks (2) are arranged in a row in the vessel's longitudinal direction and arranged for storage of liquefied natural gas, and the section (1) of the vessel's deck which is located at the spherical tanks (2) extends under the top thereof. In the vessel according to the invention the deck section (1) extends closer to the spherical tank's (2) top sections than to the spherical tank's (2) centres.

Description

DEVICE FOR OFFSHORE LNG PRODUCTION.

The invention concerns a vessel as indicated in the introduction to claim 1.

The development of offshore gas fields at great depths requires new production methods. On account of the depth of the ocean the production equipment has to be located on a floating device. The production equipment consists of a processing plant, a storage plant and unloading equipment. The object of the processing plant is to stabilise the well flow in a separation process. The gas flow has to be dried and purified of undesirable components. The gas then has to be condensed into liquid natural gas (LNG) and stored in special tanks. Oil and condensate are transported to storage tanks after processing. Gas and oil/condensate are collected by shuttle tankers which transport the products to onshore plants.

Concepts for production in such marginal fields have been proposed in the literature, as, for example, in a publication by B. Borgaas and D. Eimer "Offshore production of LNG from associated gas" and W. Bach et al.,

"Offshore LNG production with LIN cold-process design and evaluation of coiled and plate fin heat exchanger", both of these publications being presented at the "9th International Conference on Natural Gas", in Nice in 1989.

The first publication describes a basic idea which is based on the concept that the processing plant, storage devices and unloading equipment are located on a single ship. The storage plant is based on known spherical tanks. However, a floating offshore installation for production of LNG and oil/condensate at great depths has not so far been planned and put into production. In known structures which are based on spherical tanks for LNG, processing equipment is located on the vessel's main deck outside the tank area. It is desirable, however, that the plant should be as compact as possible in order to give an acceptable hull size. The object of the invention is to provide as compact a plant as possible. The characteristics of the invention are presented in the characteristic features indicated in the claims.

The invention will now be described in more detail with reference to the drawing which illustrates an embodiment of the invention. Fig. 1 is a schematic side view of a vessel according to the invention.

Fig. 2 shows a schematic cross section along line I - 1 in fig. 1.

The invention is in three parts.

1. Raising of the processing deck. The principle of the design is illustrated in fig. 1. The processing equipment is installed on a processing deck 1 , which extends between the spherical tanks 2, and which is located above the spherical tanks' equatorial height. By raising the processing deck 1 above normal deck height in this manner, the result is achieved that the spherical tanks 2 occupy less space on the deck.

2. Increasing the width of the processing deck.

The principle is illustrated in fig. 2. The processing deck 1 is extended, with the result that sections 3 of the deck are located outside the vessel's side.

3. Increasing the distance between the spherical tanks. Greater distance between the spherical tanks 2 provides more room on the processing deck 1.

These measures offer the following possibilities:

The space which is available for the processing equipment can be increased.

By increasing the distance between the spherical tanks 2, the space which is defined by the spherical tanks, the vessel's hull and the processing deck can if necessary be used for oil and condensation tanks 4. These tanks may be adapted to the relatively complicated shape of the space between the spherical tanks.

Between the domes at the upper section of the tanks there extends a pipe rack. Since the height of this pipe rack can be reduced in the invention, a less complicated foundation can be obtained for the pipe rack.

Since the openings for the tanks in the deck are smaller, the longitudinal strength of the vessel can be increased. This entails a reduction in the movement of the pipe racks between the processing modules and between the domes.

Claims

PATENT CLAIMS

1. A vessel comprising spherical tanks, hereinafter called spherical tanks (2), which are arranged for storing liquefied natural gas (LNG), and whose centres are arranged in a row in the vessel's longitudinal direction and on the same level, which spherical tanks (2) extend through openings in a deck section (1) of the vessel, characterized in that the deck section (1) extends closer to the level of the spherical tanks' (2) top sections than to the level of the spherical tanks' (2) centres.

2. A vessel according to claim 1 , characterized in that the deck section (1) extends at a distance of approximately 20% of the spherical tank diameter from the top of the spherical tanks (2).

3. A vessel according to claim 1 or 2, characterized in that on the deck section (1) there is located processing equipment of a production plant.