Background The present invention relates to a method for operating a plain suction dredging vessel, the vessel comprising a suction system with at least one lower suction pipe at a respective suction location.

Such a method is known from US 6499239 wherein is disclosed a method for extracting and grading or sizing sand, gravel or a mixture thereof, from a pit formed in untouched ground. Liquid is sprayed into the ground, resulting in a fluidized state in the ground, in which state sand or gravel with a relatively high relative density and/or relatively large dimensions moves downwards, and sand or gravel material with a relatively low relative density and/or relatively small dimensions moves upwards. Thus a pit is formed in the ground which contains fluidized sand or gravel, in such a manner that a layered state is obtained. The layer of sand or gravel with a specific relative density and/or size is sucked out, while the remaining fluidized material remains behind in order to stabilize the wall of the pit. The material which has been sucked out is fed to a grading device or sizing device.

EP 0078563 Al relates to a method for suctioning submerged bottom material and a system for carrying out said method. The suction tube is anchored in the bottom while the vessel is anchored by anchoring means such as a cable 33 running e.g. to an anchoring point onto the dry land and a cable 34 running from a winch 35 e.g. to an anchoring buoy.

US 4 445 290 A relates to an appliance for dredging the bottom of a body of water. The appliance is anchored by two pivoting dredging cables 22, 23 anchored at the bottom 16 of the body of water. The bucket wheel is not suitable for any anchoring function.

US 4 386 473 A relates to a method for the recovery of sediments from the bottom of the sea by means of a freely suspended suction pipe provided on one end with a suction mouth. The suction mouth is anchored at the bottom of the sea while the ship maintains directional alignment with respect to the vertical axis by using a drive means instead of the suction pipe which is freely suspended. JP 2006 104728 A relates to a sand suction mechanism for a sand removal facility. The suction mouth is anchored in the ground as seen in fig. 9. The suction pipe is coupled with the vessel by a flexible lifting device which is unsuitable for force transmission which is required for anchoring the vessel. Plain suction dredging refers in general to a method for suctioning submerged bottom material with the aid of at least a suction tube connected to a vessel, whereby said suction tube is lowered from said vessel in an inclined or vertical position, and an underpressure is created in said suction tube for suctioning a mixture of bottom material and water through the tube via one or more suction openings near the lower side of the suction tube, whereby eventually the suction openings are gradually lowered under the bottom surface such, that by suctioning the bottom material an excavation or trough is formed into the bottom.

A plain suction dredging vessel is usually a non-propelled vessel which is held in position by means of 4 to 6 anchors located at ample distance from the vessel. The mobilisation -and demobilisation time for a plain suction vessel is largely determined by the time required to place the anchors before commencing the work and the time to bring in the anchors when stopping the work. The long mobilisation -and

demobilisation time increases the down time of a plain suction vessel which is undesirable. Summary of the invention

The invention aims to solve at least partly a problem associated with known methods for operating a plain suction dredging vessel.

Another object of the invention is to improve a method for operating a plain suction dredging vessel such that at least the up time of the vessel is increased.

Yet another object of the invention is to provide an alternative method for operating a plain suction dredging vessel.

According to a first aspect of the invention this is realized with a method for operating a plain suction dredging vessel, the vessel comprising;

- a suction system having at least one lower suction pipe at a respective suction location,

the method comprising the step; - anchoring the vessel at an anchoring location proximate the suction location.

This enables the use of only one anchor while maintaining the suction pipe with the suction mouth at the right and controlled location. The time required to anchor the vessel is importantly reduced because only one anchor is required and in addition the anchor is in the proximity of the suction location.

In an embodiment of the method, the anchoring of the vessel is via the at least one lower suction pipe only. This even more simplifies the anchoring, and reduces part count. It will be clear from the wording "anchoring the vessel" that it is implied that any coupling means for coupling the vessel with the lower suction pipe as well as the suction pipe itself must be suitable to withstand anchoring forces for anchoring the vessel.

In an embodiment of the method, the suction location and the anchoring location are substantially vertically aligned. This enables use of the suction openings to facilitate the application of placing the anchor in the bottom by jet water through these suction openings. In addition, efficient use is made of pipe section material.

In an embodiment of the method, the longitudinal axis of the at least one lower suction pipe is aligned with the anchoring location.

In an embodiment of the method, the at least one lower pipe section comprises an anchoring portion, and the method comprises the step of driving the at least one lower pipe section for the anchoring portion at least partly extending in the bottom at the suction location. This enables improved control of the anchoring of the vessel during plain suction dredging.

In an embodiment of the method, the driving of the at least one lower pipe section comprises one or more of the following steps, hammering the lower suction pipe, and water jetting at the suction location and sucking. This even more enables improved control of the anchoring of the vessel during plain suction dredging. In an embodiment of the method, the method comprises a step of driving the vessel to take a position regarding wind and current conditions. This reduces possible loads to the anchor. This is important since a suction pipe line is normally not designed to anchor a vessel. It is noted that driving here means driving the vessel to take or maintain a position such that the load to the anchor is optimal.

In an embodiment of the method, the driving of the vessel comprises rotation of the vessel around the longitudinal axis of the lower suction pipe. This enables reducing the required anchoring force for the anchor portion of the lower suction pipe.

In an embodiment of the method, the vessel comprises a propulsion system for driving the vessel to take a position regarding wind and current conditions. This even more enables reducing the required anchoring force for the anchor portion of the lower suction pipe.

The invention further relates to a plain suction dredging vessel for performing the method according to the invention, the vessel comprising a;

- a suction system for pumping sludge from the bottom to the water surface to e.g. a barge moored alongside the vessel, the suction system having an at least one lower suction pipe section for winning sludge at a suction location at the bottom,

- an anchoring system for anchoring the vessel at a predetermined production location and wherein the anchoring system couples the vessel with the bottom proximate the suction location,

wherein the lower suction pipe forms part of the anchoring system and is suitable for use as an anchoring pipe, like e.g. a spud pile.

In an embodiment of the plain suction dredging vessel, the at least one lower pipe section comprises an anchoring portion.

In an embodiment of the plain suction dredging vessel the at least one lower pipe section comprises at least one suction opening for sucking sludge and the anchoring portion in use extends below the at least one suction opening. In an embodiment, the plain suction dredging vessel comprises coupling means for coupling the vessel with the lower suction pipe such that during dredging the vessel is able to rotate around the vertical axis.

In an embodiment, the plain suction dredging vessel comprises a propulsion system for enabling the vessel to take a position regarding wind and current conditions.

Usually, a stationary production platform, like a plain suction dredging vessel, does not have a propulsion system.

The invention further relates to a device comprising one or more of the

characterising features described in the description and/or shown in the attached drawings.

The invention further relates to a method comprising one or more of the

characterising features described in the description and/or shown in the attached drawings.

The various aspects discussed in this patent can be combined in order to provide additional advantageous advantages.

Description of the drawings

The invention will be further elucidated referring to embodiments shown in the schematic drawing wherein shown in:

Fig. 1 in side view a first embodiment of the invention;

fig. 2 a top view of the embodiment of fig. 1;

fig. 3 a side view of a detail of the embodiment of fig. 1;

Fig. 4 a top view of a detail of the embodiment of fig. 1.

Fig. 5 in side view a first embodiment of the invention;

fig. 6 a top view of the embodiment of fig. 1;

fig. 7 a lower pipe section of the embodiment Detailed description of embodiments

In the figures a first embodiment of a vessel according to the invention is described in the figures 1-4 and a second embodiment is described in the figures 5 and 6.

Fig. 1 shows a plain suction dredging vessel 1 for performing the method according to the invention. The vessel comprises a suction system for pumping sludge 7 from the bottom 11 to a barge moored alongside the vessel 1. The suction system has one lower suction pipe section 2 for winning sludge at a suction location 3 at the bottom. At the suction location 3 a trough or pit 22 is formed around the lower pipe section, see fig. 7.

The suction system comprises an elongate beam member 9 with a beam wall 10. The elongate beam member 9 couples the lower pipe section 2 with the vessel 1. Here, the lower pipe section 2 is coupled with the beam member 9 via a linkage mechanism 24 which gives the lower pipe section 2 freedom to move vertically relative to the elongate beam member 9. Here, lower pipe section 2, the sludge is transported through the elongate beam member 9. The elongate beam member 9 is coupled with the vessel 1 via a hinge mechanism 23 which is particularly useful for transferring anchoring forces.

The, in use, lower pipe section 2 is moveable, here hingeable, between an upper position a and a lower position c through an intermediate position b. In the upper position, the elongate beam member 9 extend substantially horizontal.

The vessel 1 comprises an anchoring system for anchoring the vessel 1 at a predetermined production location 3. The anchoring system couples the vessel 1 with the bottom 11 proximate the suction location 3.

The lower suction pipe 2 forms part of the anchoring system and is suitable for use as an anchoring pipe, like e.g. a spud pile. In particular, dimensions like pipe diameter and pipe wall thickness as well as choice of material do make the lower suction pipe 2 suitable as an anchoring pipe. The one lower pipe section 2 comprises an anchoring portion 6 with an outer wall 19 which in uses engages the bottom 11. The anchoring portion 6 has a leading edge 18 for facilitating penetrating into the ground.. The one lower pipe section 2 comprises a number of suction openings 13 for sucking sludge. During use, the suction openings 13 are positioned in the winning trough 22. The anchoring portion 6 in use extends below the suction openings 13.

The vessel 1 comprises coupling means (not shown) for coupling the vessel 1 with the lower suction pipe 2 such that during dredging the vessel is able to rotate around the vertical axis.

Now referring to fig. 5 and 6. In this embodiment a tension member like a cable 14 couples the lower pipe section 2 with the vessel 1. The sludge is transported through a flexible tube 15 having a tube wall 16.

The upper side of the lower pipe section 2 is provided with a hammering means 17 for hammering the anchoring portion 6 into the ground 11.

Now referring to fig. 7. During use of the vessel 1 the following steps are taken: Anchoring of the vessel 1 at an anchoring location 4 proximate the suction location. Here, anchoring the vessel is anchored via the one lower suction pipe 2 only. The suction location 3 and the anchoring location 4 are substantially vertically aligned.

The longitudinal axis 5 of the one lower suction pipe 2 is aligned with the anchoring location 4, see fig 7 stap a. Both the anchoring portion 6 and the suction portion 20 extend along the longitudinal axis 5 of the one lower suction pipe 2. The suction portion 20 and the anchoring portion 6 are preferably integrated and may form one part or in other words make up one pipe section. The suction portion wall 21 and the anchoring portion outer wall 19 may run continuously.

The at least one lower pipe section 2 is driven into the ground such that the anchoring portion 6 at least partly extends in the bottom 7 at the suction location 3, see fig 7 step b. Finally, the anchoring portion extend over a distance ha into the ground below a winning trough 22 with a height hp, see fig. 7 step c.

The driving of the one lower pipe section 2 comprises one or more of the following steps know per se; hammering the lower suction pipe, and water jetting at the suction location and sucking. The vessel is propelled, here self propelled for driving the vessel along the waterline 8 of a body of water 12 to take a position regarding wind and current conditions, so called wheathervaning. This driving involves rotation of the vessel 1 around the longitudinal axis 5 of the lower suction pipe 2.

It will also be obvious after the above description and drawings are included to illustrate some embodiments of the invention, and not to limit the scope of protection. Starting from this disclosure, many more embodiments will be evident to a skilled person which are within the scope of protection and the essence of this invention and which are obvious combinations of prior art techniques and the disclosure of this patent.