GB2273694A

GB2273694A - Offshore loading system.

Info

Publication number: GB2273694A
Application number: GB9226941A
Authority: GB
Inventors: R Uittenbogaard
Original assignee: Bluewater Terminal Systems NV
Current assignee: Bluewater Terminal Systems NV
Priority date: 1992-12-24
Filing date: 1992-12-24
Publication date: 1994-06-29
Anticipated expiration: 2012-12-24
Also published as: GB2273694B; GB9226941D0

Abstract

A flexible loading system for a dynamically positioned tanker (19) that does not require very accurate positioning of the tanker to be maintained. A positively buoyant flexible flowline (11) extends from a base (10) on the seabed (14) and is longer than the depth of the sea so a portion of the flowline (11) floats on the surface (17). The flowline (11) can be connected to a storage reel (12) on the tanker (19) and is reeled in or out depending on the position of the tanker (19) relative to the base (10). Preferably the connections are bellmouths to prevent strain on the couplings. <IMAGE>

Description

OFFSHORE LOADING SYSTEN The present invention relates to an offshore loading system for a dynamically positioned vessel.

Traditionally, tankers which require loading or unloading in less sheltered offshore area's, moor themselves to Single Point Mooring Terminals (SPM).

Such terminals allow the tanker to aligned with the main weather direction much like a weathervane.

Simultaneoulsy, the cargo is loaded or unloaded through a floating and submarine hose system.

The mooring hawser plays a key part in the above described situation, as this element effectively ties the tanker to the SPM while carrying all the environmental loads and allowing for all wave induced motions of the vessel. Further, for all intent and purposes, the tanker has, while it remains moored, become a "dead" ship.

Modern day technology has made it possible that many ships can stay on a specific offshore location by means of their propulsive systems. These propulsive systems have to meet certain requirements such as output thrust and power should be continuously variable in magnitude and direction. Also the propulsive system will have to be tied in to a data acquisition system which monitors and measures the magnitude and direction of wind, current and waves, as well as the actual position of the ship. The direction and magnitude of the propulsive thrust is adjusted on a continuous basis to counter the continuously varying environmental forces which act on the ship and which would normally tend to move it off location.

Ships fitted with such a propulsive system are referred to as Dynamically positioned (DP) ships.

In the offshore oil industry such ships are of great importance because they allow a quick turn-around time as no single point mooring operation needs to be performed. These operations are time consuming and sometimes, due to weather, even impossible.

The DP tanker, however still requires a flexible loading system for its cargo and it is the subject of the present invention to describe such a loading system.

Obviously the preferred layout of such a system should allow the hook-up and disconnection of the loading hose system to be simple, without unnecessary manipulation and handling of components, and be fail-safe once disconnected and left in the field after the tanker has departed.

The above can be met by providing a system which will, once disconnected, always return to a stable, pre-determined position and geometry in the sea, and by designing the actual connect-disconnect interface around a single wire only pull-in system.

Such systems are already known to the offshore oil industry and e.g. described in patent applications US 07/823,104 and GB 2 239 441 A.

The drawback of the above described systems is that, due to their geometrical layout, they impose severe stationkeeping requirements on the DP tankers which operate in conjunction with these systems. This is due to the fact that more energy is required, given a certain seastate, to maintain a vessel in a smaller operating area than in a larger operating area.

It is the object of the present invention to provide a flexible loading system which substantially lowers the stationkeeping requirements for the DP tankers in terms of total energy consumption and power and to reduce the risks following propulsion power failures.

Accordingly, the present invention provides a flexible loading system for a dynamically positioned vessel comprising a base firmly attached to the seabed and forming an interface between a seabed pipeline or well head and a flexible flowline and having a substantially vertical outlet to which the flexible flowline is connected, the flowline being positively buoyant such that it rises substantially vertically from the base to the surface of the sea, the flowline having a length substantially greater than the depth of the sea such that the upper portion floats on the sea surface, a storage reel rotatably mounted on the vessel to which the upper end of the flowline can be releasably connected such that the flowline may be reeled in or out depending on the position of the vessel relative to the base.

The invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows the flexible loading system of the present invention, and Figure 2 shows the seabed base in more detail.

Generally speaking, the flexible loading system is composed of the following elements: - a subsea base (10) - a flexible flowline (11) - a motorised storage reel on the tanker (12) The subsea base (10) is the interface between the seabed pipe line (18) or wellhead and the flexible flowline (11). This base (10) is firmly attached to the seafloor (14) by either gravity or anchor piles.

The base (10) is connected to the end of the seabed pipe line (13) and an internal spoolpiece (15) is fitted such that a more or less vertical outlet (16) is created on the base (10).

The flexible flowline (11) is attached to this outlet (16) and rises from this point more or less vertically to the sea surface (17). As the flexible flowline (11) is positively buoyant, even when filled with seawater, the flowline (11) will float on the surface (17) of the sea at a certain distance away from the base (10).

The flexible flowline (11) has a length of say, 100 to 300 meters plus the depth of the sea at the offshore site. The flexible flowline (11) may be composed of a single or multiple sections. The free end (18) of the flexible flowline (11) is fitted and spooled on a motorised storage reel (12) in the DP tanker (19).

The principle of operation is now based on the DP tanker (19) nominally maintaining position at the offshore site and heaving in or paying out the flexible flowline (11) from the tanker (19) by rotating the storage reel (12). In principle any length of flowline (11) can be accommodated and hence it is not important that certain short term weather conditions or e.g. propulsion failures move the tanker off location. This can easily be catered for by paying out more flexible flowline (11).

The flexible flowline (11) is typically fitted with a fluid swivel (not shown) to alleviate torsion in the flowline (11) when the tanker (19) weathervanes with respect to the seabed base (10). Furthermore, a self-sealing coupling and overload protection may be integrated in the flexible flowline.

The seabed base (10) and the storage reel (12) exit point are preferably fitted with a shaped bellmouth (20) such that local overstressing of the flexible flowline (11) is avoided. Also, local reinforcement may be applied to the flowline (11).

The system may be applied to both permanently or temporary on-site applications. In this last instance, means are provided to release the flexible flowline (11) from the storage reel and to retrieve this at some later point in time.

The storage reel (12) function may also be executed in other suitable fashions e.g. as a conveyor belt or similar. Depending on the DP tanker employed, a combination of a storage reel and a conveyor belt may be selected.

Furthermore, in applications where crude oil is loaded directly from the wellhead, the flexible flowline (11) may have plural conduits to allow control of the wellhead valves through hydraulic means. In such applications however, it would be more advantageous to install a direct electrical/acoustic link between the tanker and the wellhead.

The flexible flowline (11) may also be fitted with variable buoyancy elements (not shown) such that the flowline (11) could be sunk to the seabed (14) in periods when there is no tanker (19) at the offshore site. This would prevent damage to the flexible flowline (11) by ships passing through the area.

These buoyancy elements could be operated locally or remotely.

Claims

1. A flexible loading system for a dynamically positioned vessel comprising a base firmly attached to the sea bed and forming an interface between a seabed pipeline or wellhead and a flexible flowline and having a substantially vertical outlet to which the flexible flowline is connected, the flowline being positively buoyant such that it rises substantially vertically from the base to the surface of the sea, the flowline having a length substantially greater than the depth of the sea such that the upper portion floats on the sea surface, a storage reel rotatably mounted on the vessel to which the upper end of the flowline can be releasably connected such that the flowline may be reeled in or out depending on the position of the vessel relative to the base.

2. A flexible loading system as claimed in claim 1 wherein the upper end of the flowline is fitted with a fluid swivel.

3. A flexible loading system as claimed in claim 1 or claim 2 wherein the outlet from the base and the connection point of the storage reel are fitted with a shaped bellmouth to avoid local overstressing of the flowline.

4. A flexible loading system as claimed in any preceding claim wherein the flowline has a plurality of conduits therein.

5. A flexible loading system as claimed in any preceding claim wherein the flowline is attached to at least one variable buoyancy element such that the flowline may be sunk to the seabed when it is not connected to a vessel.

6. A system as claimed in any preceding claim wherein the hose is releasably connected to the base.

7. A flexible loading system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.