3 ms·

Can you say a bit more about the anchors? I have in mind just large pieces of metal dragging on the ocean floor but I'm sure that's not representative.

dogma1138 6y ago
Offshore anchors aren’t that different than what you imagine https://www.franklinoffshore.com.au/news/offshore-anchors-purpose-types/ https://www.franklinoffshore.com.au/news/offshore-anchors-pu... They are designed to lodge and get partially buried in the sea floor soil. Some oil rigs might be anchored more permanently to submerged concrete blocks or very heavy metal anchors, you basically drop them off a ship then when the rig moves into place you have divers connect the anchor cables to the previously submerged anchors. Some also use steering engines for station keeping but that is usually for smaller exploration and mobile rigs.
- ghaff 6y ago
  >Some also use steering engines for station keeping but that is usually for smaller exploration and mobile rigs. Yeah, I'm just familiar with exploration rigs. I was actually trying to remember if the Ocean Odyssey, which ended up being used by Sea Launch, for a launch platform, had dynamic positioning or not. I think it may have but I left before it launched and wasn't involved in that aspect of its design.
  - dogma1138 6y ago
    I think for sea launching dynamic positioning can be used the engines should be powerful enough and you have more control on under which sea conditions you’ll be operating under. At that point the exhaust might be the biggest problem but if you design your exhaust deflectors correctly they themselves can be used for station keeping as long as you don’t vent everything from one side of the rig I think you’ll be all right. Deep enough ballast tanks and dynamic positioning should be more than stable enough for launch. I think the rigs in the med use dynamic positioning even on a semi permanent basis because the Mediterranean is much calmer compared to say the North Sea or even the Atlantic.
    - ghaff 6y ago
      Yeah, I would think you'd want DP for that. With drilling, your big constraint was stresses on the marine riser (the big pipe that connects the drill floor to the subsea blowout preventer) and the parts connected to it. As I recall you wanted to keep your horizontal deviation at less the 2-3% of water depth.
ghaff 6y ago
Sure. I actually did a fair bit of work related to mooring system analysis. Basically, the rig would be towed on station. Each of the four columns in the corners of the rig largely consisted of chain lockers and there were two big anchor windlasses at each corner. Each anchor would be lowered down in sequence and a work boat would carry the anchor out some distance with the chain playing out and drop it. It would then be tensioned using the windlass so that the anchor digs into the sea floor. This operation was then repeated. So you basically end up with a pair of anchors coming from each corner in some sort of a spread configuration. I'm sure there were later designs that used dynamic positioning rather than anchors but those were just on drill ships when I was working.

dogma1138 6y ago

Offshore anchors aren’t that different than what you imagine https://www.franklinoffshore.com.au/news/offshore-anchors-purpose-types/ https://www.franklinoffshore.com.au/news/offshore-anchors-pu... They are designed to lodge and get partially buried in the sea floor soil. Some oil rigs might be anchored more permanently to submerged concrete blocks or very heavy metal anchors, you basically drop them off a ship then when the rig moves into place you have divers connect the anchor cables to the previously submerged anchors. Some also use steering engines for station keeping but that is usually for smaller exploration and mobile rigs.

ghaff 6y ago
>Some also use steering engines for station keeping but that is usually for smaller exploration and mobile rigs. Yeah, I'm just familiar with exploration rigs. I was actually trying to remember if the Ocean Odyssey, which ended up being used by Sea Launch, for a launch platform, had dynamic positioning or not. I think it may have but I left before it launched and wasn't involved in that aspect of its design.
- dogma1138 6y ago
  I think for sea launching dynamic positioning can be used the engines should be powerful enough and you have more control on under which sea conditions you’ll be operating under. At that point the exhaust might be the biggest problem but if you design your exhaust deflectors correctly they themselves can be used for station keeping as long as you don’t vent everything from one side of the rig I think you’ll be all right. Deep enough ballast tanks and dynamic positioning should be more than stable enough for launch. I think the rigs in the med use dynamic positioning even on a semi permanent basis because the Mediterranean is much calmer compared to say the North Sea or even the Atlantic.
  - ghaff 6y ago
    Yeah, I would think you'd want DP for that. With drilling, your big constraint was stresses on the marine riser (the big pipe that connects the drill floor to the subsea blowout preventer) and the parts connected to it. As I recall you wanted to keep your horizontal deviation at less the 2-3% of water depth.

ghaff 6y ago

Sure. I actually did a fair bit of work related to mooring system analysis. Basically, the rig would be towed on station. Each of the four columns in the corners of the rig largely consisted of chain lockers and there were two big anchor windlasses at each corner. Each anchor would be lowered down in sequence and a work boat would carry the anchor out some distance with the chain playing out and drop it. It would then be tensioned using the windlass so that the anchor digs into the sea floor. This operation was then repeated. So you basically end up with a pair of anchors coming from each corner in some sort of a spread configuration. I'm sure there were later designs that used dynamic positioning rather than anchors but those were just on drill ships when I was working.