17 Jun - All about ROVs
RRS Ernest Shackleton Diary
Position @ 1200: 53°43'North 001°07'East. Inside 500 m zone of West Sole Bravo Platform.
Conditions: Wind N x W x 5 kts; Barometer 1013.2 mb; Air Temperature 12.0° C; Sea Temperature 10.9°C; Cloudy, fine and clear, rough sea.
Map of survey sites (See below).
This week has seen RRS Ernest Shackleton continue on the rounds of BP Amoco platforms in the southern North Sea with the ROV's spending more and more time in the water now that the strength of the tides is dropping off slowly. Last Sunday saw us working within the Leman Field until Tuesday, when we headed for Davy Platform and the Indefatigable Field. The Bessemer Platform was surveyed on Wednesday and on Friday we moved to the Camelot Field. In the early hours of Saturday the ship was working at the Hyde Platform (the picture of this platform is used in the index page to the North Sea Updates) and later in the day we arrived at the Cleeton Field. By Saturday evening we had moved to West Sole Bravo Platform, where the vessel is currently waiting on the tide and weather before completing the platform survey.
Thursday afternoon saw visits from two helicopters to effect a crew change, with sixteen persons departing the vessel and thirteen joining. The majority of these were charterer personnel. Also leaving the vessel was Captain Marshall, who handed over command to Captain John Harper, who will be in charge of the vessel for the next four weeks.
In a bid to help you follow our travels, I have located the following map which should give a much better idea of where we are. This map will be available on all future North Sea Updates from RRS Ernest Shackleton.
On Saturday afternoon it was thought a whale was in our vicinity as something could be seen surfacing ahead of the vessel, but as it was not visible for very long it was difficult to identify it correctly. About half and hour later another siting was made and this time being much closer it was positively identified as a seal!
West Sole: The West Sole reservoir is located in the southern North Sea some 70km off the coast of Yorkshire. The field was discovered in 1965 and by 1975 was supplying almost all of Britain's gas. Further information on the West Sole field is available from the BPAmoco web site.
Remotely Operated Vehicles by Sean Mahoney, for Agnes.
We are now into the second week of our North Sea charter, and currently in the Leman Field off the coast of Great Yarmouth. This year we have seen some new additions to our vessel, these being two Lynx's and a Tiger, and before you think we have been turned into Noah's Ark, let me point out that these feline names are actually the make of ROV (Remotely Operated Vehicle), we are using for this year's inspection programme.
The basic concept of an ROV, or Remote Submersible, is that because it is unmanned it can be used in any underwater environment, including depths of up to 3000 m, thus reducing the risks to divers. The work schedule for "our" ROV's will include GVI (General Visual Inspection) of sub-sea structures and pipelines, incorporating FMD (Flooded Member Detection), and CP (Cathodic Potential) surveys. The vehicles are fitted with "Tooling Packages" to enable them to perform these tasks. The GVI will be looking for any obvious damage or debris, which could have an adverse effect on the integrity of the structure. FMD is used to ascertain whether any parts of the structure members have had water ingress. CP is used for measuring the "potential" of the structure, whereby a voltage measurement is taken at various locations on the item being inspected, and this has to be within a pre-determined threshold. This may all sound very technical, but we assure you that the personnel operating all of this equipment are highly trained and very competent, or so they tell us !
Just for the sake of name dropping, here are the motley crew that are operating these systems: On the Lynx's - Sean Mahoney, Tony Felgate, Peter Bruce, Mike Tisdall, Chris Bryant, Malcolm Downie, and Dave Leslie. And on the Tiger - John "Benny" Benson & Steve Crewdson.
The inspection programme being carried out will take in the majority of BP's platforms. This will be from the Southern sector (off the coast of Great Yarmouth), right up to the waters off the Shetland Isles. In the southern sector the Vessel and ROV's are governed by a fairly strong tidal current, this could be anything up to 4.0 knots. The depth around this area ranges from 18 to 60 m.
Technical bit - During the phase of a full Moon, the tides become much stronger, these tides are called "Springs". As the Moon wanes, although the tide is still quite strong, it is much less than when we have a full Moon, this phase is called a "Neap Tide". This is all due to the Moon's gravitational pull.
During "Spring" tides, we have to wait for "Slack" water (current below 1.5 knots), before the vessel can move onto location, and the ROV's can start diving. This slack water can be anywhere between four to six hours apart. A typical dive could last up to 30 minutes before the current starts to increase again. On a Neap tide, a typical dive could last anything from 45 minutes to 2 hours. In the northern sector of the North Sea i.e. off of Aberdeen, tidal movements due to the depth of the water, which can range from 90 m to 200 m, very rarely restrict the ROV operations. However, it is very common to have violent storms in this area, and due to the diving limits imposed for safety reasons, the ROV's cannot operate above 35 knot winds, and a wave height of 3.5 m.
Forthcoming events. Continue survey of platforms and manifolds.