Please explain how the cantilever skid works for a derrick
Hi, I only have knowledge of onshore drilling and know that offshore workers get paid better. Also, I like the idea of world travel and working the offshore rigs seem exciting to me. I've been doing a bit of research on Jack-ups and am intrigued about how they move around and get in position for pumping oil. So, I have a question:
I understand the cantilever structure that the derrick is fixed to. I'm fuzzy on the meaning of skid, is that allowing for the time it takes to get traction on the ocean floor? for example with an uneven bottom, the drill bit might slide/skid and needs a movement capability at the surface?
Comments
Hi Bryce, sorry for the delay. This question was out of my area of expertise so I asked Steve Hauxwell to explain in detail.
Here's a cut and paste of my Skype conversation with Steve:
Steve: Skidding is the processes of moving the cantilever. I am sure you have picked up that the drilling package (the derrick and area within, commonly known as the 'rig floor'), can be moved about on a set of cantilever beams. This process of movement is known as skidding. I am sure you've also figured that this happens after a Jack-up is in place with the legs supported at the seabed.
Jason: So when is skidding needed? Why would you need to move the Derrick around? Surely any approximate sea bed entry point is the same?
Steve: Skidding has many purposes. the early jack-ups skidded the derrick over Slots (holes) in the vessel hull. This meant that all the wells were drilled immediately under the rig itself, (i.e. in-between the legs). This was perfectly fine in the early days and served the industry well.
With the advent of offshore platforms to house wells (wellhead platforms), there was a problem with efficiency. It's highly efficient to start a project by first installing a wellhead platform which is piled into the seabed. It will then have a series of guides for each of the wells. Anything from 3 to 40-guides.
Imagine a wellhead platform sticking up 40 feet above the sea level. You now see the problem. You cannot get a Jack-up, (or semi-sub) above the platform.
So you come alongside the platform, as close as you can, position the Jack-up legs on the seafloor, jack the hull up to a height above the platform, and then skid the derrick out over the side above the platform. It is then moveable to the centre of the derrick can be placed above each well slot.
Now you can see the benefit of modern Jack-ups with huge derrick cantilever skid capacity. I am dealing with this day in day out. In fact tomorrow I have a meeting with a platform manufacturing company to ensure a new wellhead platform can be matched to the available North sea Jack-up fleet. The older jack-ups had limited cantilever skid (25ft), now we're seeing a 110ft capability.
You might now start to see the benefit of our constructor database. A quick and easy guide to matching jack-ups with platforms. Also, bear in mind that rigs do come back to existing platforms to re-work old wells.
Jason: "series of guides" what do they look like?
Steve: And in the North Sea, it's not just a case of moving the rig to the other side of the platform. The wind and waves topple Jack-ups if they are not correctly aligned to the prevailing weather conditions.
They are massive I-beams, the same as the ones used in modern building structures, but significantly larger. One on either side of the derrick. The derrick is skidded out or sideways by a series of hydraulic pistons. As with any cantilever, it's all about anchoring the supporting end, (i.e opposite end to the derrick).
Jason: Finally, so there might be a series of wellhead platforms, that break the surface, but don't have a gym or cafeteria. Then the Jack up has the rig crew and goes from one wellhead to the next wellhead locking on?
Steve: The derrick is physically attached to the end of the Cantilever Beams. it is the beams that move with the derrick, not the derrick skidding along the beams.
Usually, there is a walkway added from the rig. Often these wellhead platforms are referred to as 'Normally Unmanned Platforms' -NUI's). Often, after the wells are drilled a 'topside deck is added, which might include basic accommodation and a helideck.) If this is the case then it often prevents a traditional jack-up from returning to the platform.
Sometimes, as in the case of huge developments, the field will include several fixed platforms with walkways in-between. You will have seen these pictures. There may be several separate wellhead platforms and a central production platform.
If the production platform also has wells underneath its structure, then a jack-up can't drill these wells. In these instances, the platform will have a built-in drilling module (it's own drilling derrick). Think about the huge Brent, Forties etc.) and you have the idea.
In the deeper North Sea, the sea water depth is too great for jack-ups. semi-submersibles can also (obviously) not be floated above wellhead platforms, so typically there are two solutions. Install platforms with their own drilling modules, or develop the field with a semi by means of seabed structures. This is very common these days. so common that even jack-up are developing seabed fields (i.e. no visible platforms rising out of the sea).