GeoRW
03.05.2021 4:10:38
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Global Marine's Charles Keaton said "it depends on how long you are going to be on a spot for a period of time. Most people would prefer to have moored, because the operating costs, once it is moored, are much cheaper, and there are fewer uncertainties. The economics are really driven by the water depth." [links]
On the other hand, with the time involved for mooring, a moored vessel is geared towards development drilling. This is not to say that either type expressly limits the rig to that type of drilling. The economics tend to lean more toward that direction. more
There are many reasons why the debate has gone on so long. The main one is superiority of one technology versus the other. R.E. Steddum, Manager of Transocean's Structural and Naval Architecture group, explains: "I don't think that we have ever been able to establish that (superiority) conclusively. We have not seen a compeling analysis that one is superior to any other one."
At the same time, DP does not require the use of heavy-duty anchor handling boats and downtime to set anchors. The use of these boats can also eat into the operating budget and they must be used each time the rig moves location. This also brings up the availability of the vessels to handle anchors and the increased risk of something going wrong during the mooring setup.
Transocean's Steddum says that the arguments are strong on both sides, with no clear cut winner. The only way to figure the optimal choice is on a case-by-case analysis. "It is fair to say that most operators come to that conclusion. I believe that if you can moor passively, that is what you would prefer to do, although it is not based entirely only economics. [links]
Diamond Offshore has used the pre-set moorings and agrees with the economical advantages. "We cut our teeth on upgrades and deepwater with EEX and the Ocean Voyager, using pre-sets. We really liked it a lot and it worked well," stated Shawn Vigeant, Manager of Contract Services for Diamond. more
Older British boats usually have a solid pulpit and climbing over the bows may require gymnastics.
With long lines at the stern, it is easy to untie the lines on board and re-tie them on the opposite quarter, so it is a relatively simple process to moor in the box and then cross the stern lines once secure. here
The important point is that you are stopped by a post, not by someone’s prized boat. [links]
It is always worth choosing a vacant box rigged like this if you have the option.
A slip line is run across the box between the side ropes, with a clip at its centre that can be fastened to the bows. [links]
Putting a line around the post allows us to slip it as we leave, and is a lot simpler. more
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Avoiding chafe at the pennant is only one part of the equation. You also need to protect your lines from chafe at the boat. The best chafe gear is fire hose, which you can get for free at most fire stations (they may give it to local marinas or the Coast Guard auxiliary, too). Rather than tying it to the line — which can stretch and the chafe gear end up in the wrong place — we tie it to the boat or cleat and then it stays where it is needed, regardless of how much the line stretches.
It helps if the mooring ball has a long pennant, but basically the way we do it is for Dave (stronger, longer arms) to be on the bow with the boat hook. I’m at the helm (it’s what I’m best at). I sloooowly motor up to the mooring ball, with Dave using our headset to talk me right up to it. He has one end of the dock line already cleated on the boat, with the free end passed over the gunwale and looped back onto the deck with the end easily “grabbable.” He uses the boat hook to pull the pennant up (I am at the wheel keeping the boat in position — using a little reverse first to stop the boat’s motion, then perhaps just a touch of forward if it’s windy), grabs it in one hand, quickly sets the boat hook down and grabs the end of the line, passes it through the eye and cleats it. It doesn’t matter how long the line is at this point. Then he will pull on the first line to get the eye in reach again and put the second line through. If it’s windy, I may have to use a little forward so he can get to the eye in the pennant again. Once both lines are through and cleated, I leave the engine running but in neutral and go forward to help adjust them, put on chafe guard, etc. When it’s particularly windy, we only adjust one side at a time so both are never uncleated at the same time.
My version, for a catamaran. My best try at preventing chafe and twisting.