Buried in the Sand

Trinity House crews are always happy to share their experience which is both exceptionally useful and interesting. Here, Chief Officer Adam, explains all about how conditions can impact on mooring systems and how the process for recovery to deck can be adapted to compensate for this.

“When we say a Sinker is ‘Sanded’ or ‘prone to sanding’, this refers to what happens down on the seabed for the 12 months between annual inspections. In areas where the seabed is sand/mud, or areas close to sandbanks (i.e. Thames Estuary, Banks off Great Yarmouth, the Wash, Bristol Channel, Goodwin Sands, The Varne, The Shingles, The Shambles etc…), over the 12 months or so whilst the sinker sits on the seabed, the effect of the tide / current moving the sand & sediment along the seabed, it has a tendency to effectively ‘bury’ the sinker underneath a layer of sand (or mud/silt). This only helps to keep the sinker in position for the time it is on station, however when it comes to recovery, it means not only are we trying to lift a 3-5 tonne sinker off the seabed, we are also trying to pull it out from where it is buried underneath sometimes 2 or more metres of sand. This puts additional strain on the mooring chain, sinker shackle and sinker lewis. The chain is prone to parting at this point if the sinker is very heavily sanded and the chain is already worn, which presents an additional safety hazard to those working on deck, as well as the financial loss of losing a certain length of mooring chain along with the sinker. You would be surprised how much strain this can place on a chain and how stubborn they can be to come up. In addition, the base of the sinkers are usually slightly concave, so that they stick to the seabed much like a bath plug. Add to that suction the weight of a few cubic metres of sand, and suddenly a 5 tonne sinker can cause our 16ton capstan real problems as it’s too heavy to lift.

BM_TH_GAL_2015_HI_166_.jpg

Sometimes we use the natural buoyancy & stability of the ship to try and assist – when inclined to an angle, the ship will naturally want to come back upright. This righting lever can help to slowly pull the sinker out rather than mechanically.

Sometimes the rising tide is also used to free them. If the capstan cannot pull the sinker out from all the sand it is buried under, then the mooring chain can be stoppered off and we wait whilst the tide rises. This rising tide lifts the ship in relation to the seabed and thus helps pull the sinker out from its buried spot.

How do we prevent it – visit the station more often so that the sinker doesn’t get buried under so much muck! Some TH stations are only lifted once every two years, however if we find these sanded or difficult to recover, we recommend that they return to an Annual Inspection to prevent heavily sanded sinkers.

BM_TH_GAL_2015_HI_167_.jpg