For offshore petroleum production it is well known to load shuttle tankers from a subsurface or surface buoy connected with a vertical riser to a riser base on the sea bed.

The vertical risers are secured to the sea bed by anchoring means, such as chains, extending in catenary pattern around the riser, being anchored in the sea bed on or within a circle around the riser.

As petroleum production is performed for continuously larger depths, the need for such anchoring means is increasing. Such anchoring means consequently will be longer and heavier. Due to the weight of the anchoring means the subsurface buoy and the vertical riser must contain means for buoyancy. Furthermore the anchoring means are secured to the seabed at a rather long distance from the riser base, occupying relative large sea bed areas, which will influence for example fishing.

With the riser according to the present invention, the above limitations and disadvantages with prior art embodiments are avoided. This is overcome with the riser according to the present invention as defined by the features stated in the claims.

The drawing discloses in figure 1 schematically securing of a first riser section to a riser base at the sea bed, figure 2 discloses securing of a riser section to already installed riser sections and figure 3 discloses the riser according to the present invention, connected with a subsurface buoy during loading of a shuttle tanker.

The vertical riser for deep water oil loading comprises a number of riser sections, each of which having a length being suitable for transportation to the oil field, the entire length of the section thereby floating on the surface during towing. Furthermore the lengths of the riser sections also are adjusted to be suitable for handling in connection with installation as disclosed in figures 1, 2 and 3.

The first riser section 1 is pulled down to a riser base 2 at the sea bed by a wire 3 from a supply boat 4, the wire 3 is thereby secured to the riser base 2 through a pulley. A remote control vehicle 5 (ROV) is monitoring the operation of securing of the lower connection 6 of the riser section 1 to the riser base 2. The section 1 also may be provided with weight at the lower end, such as by a chain clump weight. After landing the riser section 1 correctly on the riser base 2, the riser section 1 is locked to the riser base 2 by the ROV 5. Locking may be executed hydraulically or mechanically.

The remote control vehicle 5 may be one single or two different vehicles. In the latter case one vehicle would be an observation ROV controlling a monitoring the connection operations and the other ROV being a working ROV adapted to perform the connection work and being correspondingly equipped.

Each riser section 1 comprises an upper connection 7 also containing buoyancy means. A second supply boat 8 is used for lowering the riser section 1 by means of a wire 9.

Further riser sections 1 are lowered from the supply boat 8 by means of the wire 3 being secured to the upper connection 7 of the uppermost riser section 1. The installation operation is monitored by the ROV 5.

Finally the last riser section 1 is locked to the previously uppermost riser section 1 also monitored by the ROV 5. The uppermost riser section 1 comprises a subsurface buoy 11 and swivel for connection of a catenary riser 12 to be connected to a shuttle tanker 10 during the loading operation.

The riser system according to the present invention provides a solution where a riser is anchored in the sea bed through a riser base, extending upwards substantially vertically, depending on the total buoyancy forces from the riser and the subsurface or surface buoy.