AU690036B2 - Method and device for petroleum loading - Google Patents

Description

WO 97/07016 PCT/NO96/00205 Method and Device for Petroleum Loading The invention concerns a method and a device for use in loading petroleum from a subsea petroleum deposit or a subsea petroleum store on to a vessel on the surface, wherein there are employed one or more risers which lead from the petroleum deposit or store to the vessel.

For the exploitation of oilfields or for other special purposes where a subsea petroleum deposit has to be exploited or oil products have to be taken up from a store on the seabed, a production vessel is often used which is connected to the subsea deposit via risers. Such ships are normally specially designed and equipped with a so-called turret due to the fact that the ship will turn round during production.

When a ship is permanently located in deep water the ship may move around to a considerable extent, and this must be compensated for in the riser system and its connection to the ship. To a certain extent the same problems are encountered in connection with ships which have to bring up oil and petroleum from a store on the seabed.

The object of the present invention is to provide a method and a device which will simplify this kind of petroleum loading especially from a subsea petroleum deposit, thus making it possible to avoid the need for the expensive and complicated turret designs on special ships.

The main concept on which the present invention is based is to utilize the advantages of deep water in order to avoid such expensive designs. When the water becomes deep the ship can move to a considerable extent without causing any excessive local movement of the risers. This means that the need for swivels, chains, support and other rotating parts is essentially eliminated.

The object of the invention is achieved by a method and a device which are characterized by the features which are presented in the patent claims.

The method and the device according to the invention are especially suitable for use in connection with ships which employ dynamic positioning, thus avoiding the necessity of any anchoring, which results in additional cost savings. Even though dynamic positioning and to some extent also a good WO 97/07016 PCT/NO96/00205 2 mooring will be capable of securing the ship in a fixed position, movements can still occur, even though this rarely happens. This means that the system must also be equipped with a release system for such extraordinary situations.

In order to ensure a high degree of production regularity on board the ship the connection of risers must also be able to be performed quickly and simply. The invention will be able to be used with many different types of riser devices and is intended for use at depths of between 300 and 2,000 m.

However, the effect of currents on risers will represent a limitation at very great depths.

The invention will be able to be employed both in connection with older ships on which a device according to the invention is installed, as well as on new ships where the invention is incorporated in the ship's hull during building.

The invention will now be explained in more detail by means of an embodiment which is illustrated in the drawing, which illustrates in a purely schematic form: Fig. 1 is a side view and a front view of a section of a vessel on which a device according to the present invention has been installed, fig. 2 is a perspective view and a side view which illustrate the installation of the device according to the invention as a retrofit installation on a ship, and fig. 3 is a view corresponding to fig. 2 for incorporating the invention in a newly-built vessel.

In the invention a so-called balcony for suspension of risers is placed near the outside of the ship's hull in the midships area where the ship's movements due to waves are as small as possible. In the drawing the balcony is indicated by 2. The balcony's surface level lies on the ship's 1 main deck level and is an area for operation and maintenance of the system. On the balcony there are provided quick coupling/decoupling elements (QC/DC) 7 on the balcony's deck level. On the top of the balcony the upper parts of these elements are attached and is permanently connected to pipelines which go to the vessel's processing area. The coupling elements 7 are designed to take up to four flow lines per element. The elements 7 will be mechanically connected for simultaneous release or they can be released separately. There are further _ur I WO 97/07016 PCT/NO96/00205 3 provided winches 6 in the area of the balcony and wires from the winches pass through conductor casings in and under the balcony 2.

Under the balcony there is placed a buoyancy or floating element 3. All risers and umbilicals pass through the floating element 3 from the seaward side into the lower part of the coupling elements 7. At the top of the floating element 3 there are provided guide pins 4 for coupling purposes. The top of the guide pins is designed for connection with the wires from the balcony winches 6.

The floating element will have a freeboard which abuts against the underside of the balcony 2. The underbody of the floating element will have sufficient buoyancy to keep the floating element in position under the balcony. Ballast chambers 5 will be located in the lower parts of the floating element. The purpose of the ballast chambers is both to perform a rapid ballasting and to perform ballasting/deballasting in a controlled manner. Power supply for the ballast pumps will be taken from the ship itself. The power supply will be released when the floating element 3 is released. When the floating element 3 is connected to the ship, or the balcony, it is attached to the balcony's conductor casing at the top and the lower part of the floating element is connected to the ship by means of simple locking devices 9 which are illustrated in fig. 2 and are located at the front and after ends of the floating element. The floating element will be locked when the buoyancy for the element presses against the buoyancy element and locks it in place against the balcony. In order to protect both the floating element and the ship's side a fender device 10 is provided between the ship's side and the floating element as illustrated by the sketch on the left of fig. 1. This fender device may, e.g., be made of polyethylene. At the bottom of the floating element 3 the inlet part of the risers i 1 is protected with bending limiters 12. In order to protect the floating elements against movements in the water and particularly wave movements, a wave breaker 13 is located on the ship's side in front of the floating element. This wave breaker also serves as a vertical attachment point for the locking device 9. A corresponding, but inverted, projecting part is located in the after end of the floating element and similarly equipped with locking elements 9. Both these parts represent, a projection on the actual ship's hull and it will be possible to pass the floating element in between these and lock it securely in this fashion.

Fig. 3 illustrates how the same effect and positioning of the balcony and the floating element can be obtained on a newly-built ship. In this case an area i WO 97/07016 PCT/NO96/00205 4 14 will be inserted in the ship's side at the ship's central area and as illustrated purely schematically in fig. 3 the floating element 3 can be inserted and locked in this area, e.g. with an oblique lock as illustrated in fig.

2. In fig. 3 the balcony is not illustrated, and in the floating element the penetrations for the riser connection can be seen.

One of the essential features of the invention is that the connection to the risers can be released very rapidly if a critical situation arises. Thus if a shut down occurs, the vessel makes leeway or a lengthy stop occurs for other reasons either in the production or due to extreme weather conditions, it will be possible to undertake a rapid emergency release from the ship by means of the coupling elements 7. This will be necessary in order to prevent fracture of the risers, pollution or substantial mechanical destruction. The floating element 3 is ballasted after disconnection, thus causing the floating element to sink and thereby be released from the locking devices 9, with the result that the ends of the risers remain lying at rest in the water, while the ship can move independently. The wires from the winches 6 are only connected to the guide pins in the connecting phases. Only the buoyancy and to a certain extent the actual coupling cause the floating element to be locked in place in a normal operating position. This means, therefore, that the release will be very simply and rapidly implemented, since in principle it incorporates only a filling of the ballast tanks, thus causing the floating element to sink slightly further down and thereby be automatically released after the coupling has been disconnected.

When the weather situation or the conditions again exist for connection, a person, can be brought on board the floating element by suitable means, e.g. by means of an auxiliary boat or a helicopter. The ship will then be manoeuvred into position along the floating element, the winch's wires will be connected to the floating element's guide pins and the element guided into position between the wave breakers 13, whereupon the floating element is deballasted and thereby slides into position in a locked position under the balcony. The coupling elements 7 can then be connected.

The invention is illustrated with the use of a floating element with an oblong, rectangular shape. It should be obvious, however, that other shapes can also be used for the floating element and there will also be a certain degree of freedom of choice with regard to the type of coupling elements. In the above,

I

rr- -L I~ WO 97/07016 PCT/N096/00205 only risers are mentioned, but it is obvious that there is also freedom of choice with regard to the type of pipe connections which are established. The invention is especially intended for use in connection with production ships, but can also be used in other connections, e.g. for the transfer of cargo from a subsea petroleum store to a tanker. Many modifications will be possible within the scope of the invention.

I

Claims (9)

1. A method for loading petroleum from a subsea petroleum deposit or a subsea petroleum store on to a vessel on the surface, especially a production ship, wherein a floating element which is located in the water and carries risers which are connected to the petroleum deposit is brought to the vessel's side and secured to the vessel, and the risers are coupled to pipe couplings on the vessel for loading petroleum, characterized in that during loading the floating element is secured to the vessel by buoyancy forces which attempt to raise the floating element and press steering and locking devices on the floating element against and into engagement with corresponding steering and locking devices on the vessel.

2. A method according to claim 1, characterized in that the floating element is secured to the vessel by deballasting the floating element, thus causing it to rise and the steering and locking devices on the floating element engage with the steering and locking devices on the vessel.

3. A method according to one of the preceding claims, characterized in that after loading or in an emergency situation the floating element is released from the vessel by ballasting the floating element, whereby the steering and locking devices on the floating element are released from the steering and locking devices on the vessel.

4. A device for use in securing a floating element which carries risers to a vessel, characterized in that the floating element and a receiver section for the floating element on the vessel comprise corresponding steering and locking devices designed to secure the floating element by means of an upwardly directed movement of the floating element in relation to the vessel, and to release the floating element by means of a downwardly directed movement of the floating element in relation to the vessel.

5. A device according to claim 4, characterized in that the receiver section is composed of a section inserted in the vessel's side, between two vertical surfaces which form lateral surfaces in the insertion. WO 97/07016 PCT/NO96/00205 7

6. A device according to claim 4, characterized in that the receiver section is provided outside the vessel's side, between two vertical surfaces of bodies or plate elements which projects from the vessel and forms lateral surfaces for the receiver section and wave protectors for the floating element when it is located at the receiver section.

7. A device according to claim 5 or 6, characterized in that the steering and locking devices are located in the lateral surfaces.

8. A device according to one of the claims 4 to 7, characterized in that the steering and locking devices on the floating element and the receiver section are in the form of corresponding surfaces tilted slantingly upwards towards the vessel, with the result that when an upward movement occurs the floating element is pressed against the vessel's side.

9. A device according to one of the claims 4 to 8, characterized in that the floating element has ballast tanks for deballasting in order to produce an upwardly directed movement when secured against the receiver section, and for ballasting in order to produce a downwardly directed movement when released from the receiver section. A device according to one of the claims 4 to 9, characterized in that the floating element is given an oblong shape in the horizontal direction where the risers are arranged in a row, thus producing two long sides and two short sides, where one long side faces the receiver section when secured against the vessel. I-,