GB2483630A

GB2483630A - Floating mooring platform for tubular offshore structures

Info

Publication number: GB2483630A
Application number: GB1010912.2A
Authority: GB
Inventors: Philip William George Wiggs
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-06-28
Filing date: 2010-06-28
Publication date: 2012-03-21
Also published as: GB201010912D0

Abstract

The floating mooring platform provides a method of greatly improving the safe access of personnel onto wind farm turbine structures, or any similar structure, such as the leg of an oil rig installation, or indeed a large version could be built that totally surrounds an offshore oil or gas installation. The platform comprises a buoyant ring member that, in use, surrounds an offshore tubular member and floats up and down with the tide, to provide a platform for landing service personnel. Motion of the platform may be damped and smooth movement of the platform along the tubular member may be facilitated by roller means.

Description

Floating Mooring Platform (FMP).

Description:

I. Present government plans, internationally, propose thousands of offshore wind farm installations. Maintenance access will be vital and repetitive; and there could be a useful application of the device outlined herein in the maintenance of these installations.

2. Access to offshore wind farm turbines is presently difficult, especially in areas such as around the UK coast, where there is a significant tidal rise and fall and potentially rough sea conditions.

3. This means that for a workboat to have a constantly available soft landing onto a well fendered facility, a specific device needs to be provided.

4. At present, most turbine columns are fitted with vertical fenders, spaced at intervals around the circumference of the turbine column. They are fitted as appropriate to the change of water level height as incurred by the rise and fall of tide. These are a basic but not entirely user friendly answer and they do not provide a platform to transfer spares, tools and equipment and nowhere for personnel to use safety as an access or departure point from the turbine.

S. The basic concept of the Floating Mooring Platform (FMP) can be simply explained by imagining a rubber tyre fitted around a vertical column in tidal water. That tyre could float up and down the column with the tide. However, in applying that principle to the column of an offshore wind turbine column, there would be problems with such an arrangement, even if a tyre could be made that were big enough. The "tyre" can be considered as the shape of a floating mooring platform (FlIP) that encircles the column.

6. Firstly, most turbine columns have a shape that tapers toward the top, so that, in order for the "tyre," or FMP to maintain a constant fit, a vertical sleeve section could need to be fitted around the column, in the area that the FMP moves up and down as a result of tidal rise and fall and sea and swell wave activity.

7. Next, if the device is not of sufficient mass, the excessive motion of the FMP as a result of sea conditions needs to be damped so that it is not moving violently.

This motion can be countered with appropriate weights. A sample arrangement of this basic concept is shown in Drawing I, which shows the vertical sleeve around the column and the counterweights on the inside of the column. The sheaves, ropes and weights could be as shown in opposing pairs, but there could be any number of such pairings. Two are shown for simplicity.

8. Instead of an extremely large "tyre," the FMP could be constructed as in Drawing 2, which shows a number of individual buoyancy compartments or tanks, such that if one part is damaged the other buoyancy tanks would maintain the buoyancy of the FlIP until the damaged tank is repaired. Or the FlIP could be of timber construction, so that it would float naturally, but its size would give it mass necessary for dampened movement from the sea conditions.

9. The outer circumference would be heavily protected with rubber, or other fender device, forming an effective circular fender around the FMP.

10. It is possible that something could be swept against the underside of the FMP and potentially jam the bearing between FMP and column; so a rubber gasket could be fitted around the circumference of the underside of the FMP covering the gap between the FMP and the sleeve.

I I. If a smaller version of the FlIP did jam against the column, not by fouling as prevented above but by distortion or wear, it should be capable of release by pulling on a freeing rope, perhaps with a boathook. When freed, the FMP would only drop back slowly because of the counterweights. The freeing rope would be attached at both ends so that it would not foul workboat propellers and be kept under tension to prevent it lying slack in the water at low tide. (This is not shown in the sketches for the sake of clarity.) 12. For access to the ladder for the maintenance door of the turbine column, the vertical sleeve could also have slots cut into it to act as steps up to the ladder to the usual access to the turbine column. This would stop ladder rungs fouling the FlIP as it rises and falls with the tide.

13. In addition to a circular shape, the FlIP could be constructed in a horseshoe shape as shown in Drawing 3. This shape would naturally tend to turn the round, or closed part, of the horseshoe shape toward the most environmentally strong effect and allow a workboat to approach the inner pan of the horseshoe from the lee side as shown. In addition to the shape, which should itself turn toward sea, side drogues could be added to assist the weathervane effect, as could the wind vanes shown.

14. This horseshoe type, if acting with counterweights as described above, would need the vertical sleeve and the counterweight system to rotate with the FMP.

Thus, it would also require a space between the outer sleeve and the turbine column for the counterweight system to work inside the sleeve. It would therefore be necessary to support the sleeve on upper and lower bearings, wide enough to allow space for the sheaves and counterweights, with the counterweight system fitted in the space between them. In addition, an access platform would need to be added as shown in Drawing 3, as the sleeve would not always be in line with the main ladder access to the turbine door.

IS. If however it was fitted with the pressured roller system, as described in paragraph 17 below, it might not need the complication of counterweights, but the rollers would need to work both in the vertical and horizontal planes to cope with movement in those directions.

16. As a further modification, a "surfer boat" mooring device could be fitted as used by Total Oil Company off West Africa. This uses a "1" shaped docking facility shaped to fit the bow of the workboat and a purpose built ladder access from the foredeck to the FF11', but that device is already in use so not included in this proposal.

17. Further variants on the FMP would be possible for larger turbine columns. This is shown in the sketch "B" of Drawing I and in the lower sketch plan titled Drawing 2 and also on the sketches titled Drawing 4. Instead of using a vertical sleeve to compensate for the taper of the column, rubber rollers are held against the column by hydraulic pressure or springs of whatever kind. Counterweights are not likely to be needed as the mass of the larger FMP should be sufficient to dampen the vertical motion caused by sea and swell; and its own mass should prevent it from jamming on the column as described in paragraph 10. In a worst case analogy, a jam should be capable of being freed when the tide refloats the FMP.

lB. Given the huge amount of wind turbines about to be constructed around the globe and the thousands anticipated for the UK coast alone, the market potential for the FMP is significantly high.

19. it would be of relatively simple construction, using materials that would be familiar to, and easily workable for, wind turbine component manufacturers. It would not be a high unit cost in relation to the rest of the turbine installation.

20. It would provide a simple, virtually maintenance free mooring platform that will ease access, loading and unloading; and aid safer personnel transfer, especially if injured or sick personnel are being dealt with; when at present it is difficult, even in only slightly demanding environmental conditions.

21. The same concept can also be applied to oil and gas installations, whether they are in tidal areas or not A simple structure could be fabricated around one leg of an installation; or a larger structure could be built that totally surrounds an installation, providing protection from damage by supply and maintenance vessels.

22. One of the more difficult manoeuvres for offshore supply vessels is when holding a vessel in such a position that it does not drive in and damage the installation's legs or other structures. The FMP would provide protection to the main structure and an alternative area to transfer stores and equipment to and from.

23. Lastly, I hope my sketches are sufficient to help explain the concepts above.

Philip Wiggs MNI (Capt MN Retd.). 19 June 2010.