One of the things that you can find out is what types of platforms operate world wide, but it can be time consuming. Also when I started in the industry the descriptions of each one quickly went into the paddling pool of tech speak. I thought I would put together a newbie guide to offshore platforms. Wikipedia does a good job, but I decided I would take their list and put some video of each type to go along with a description.
Which is the Safest Design of Platform?
There is no single design because there are no single set of challenges for any given type. It entirely depends on where the platform is to be placed, what it is to be used for and what facilities are within striking distance of the oil and gas field.
Until relatively recently one of the many ways of testing a design has been to place a model in a large tank of water then simulate the way the sea pushes it around.
More recently companies like Technip have developed advanced computerised versions of these. One advantage is that it cuts the experimental time down to a matter of weeks. Data is fed into a computer program that runs the concept through all the known variations of the area such as wind force, currents and other geographical considerations.
Common Designs Used in the Oil and Gas Industry
These were the pioneer designs and the basic principle is still used today. In short, and very basically, you do the research and then build a framework with fixed legs, tow it out and tip it up so that it rights itself in the water. Then you secure the legs down into the sea bed so it doesn’t move. After that you bring in the decking, the machinery the pipes and the accommodation for the personnel. All that, like the frame, can be built onshore then brought out and bolted on.
In amongst the variations of these is the Complaint Tower. These are, as the name suggests, less table shaped, narrower and can be sunk in deeper water. The slender profile and flexible design of the structure mean they can take more force from the locality while remaining stable.
These are ships in that they float rather than being giant tables upended into the sea. The ridged platforms are clever, very clever but here the cleverness scale starts to climb even higher.
Semi Submersibles can go anywhere and even use their own systems to go it alone. Ballast is used to either raise them or lower them at sea as they anchor over the intended site. They can also use automatic systems and their engines to float in exactly the same position no matter the current and wind.
MODUs (Mobile Offshore Drilling Units)
Jack up rigs, in other words. They do what it says in the description. Early ones were converted barges, some ex-WW2 military ones were used in early Gulf exploration. MODUs are taken onsite, the legs lowered and the platform is jacked up to the required height.
One of the refreshing things about this topic is that most of the titles tell you about half of what each platform does. Here we have surface vessels with the capacity to drill. Simple sounding,however, a ship traditionally moves a lot more than a platform. Here is a nice video ( music is a bit suspect) that explains how it it can be done today.
TLP (Tension Leg Platform)
I know that engineers will probably find fault with my simplistic description but TLP are to my mind balloons. You get a balloon at the fair and it bobs around as you walk about. The shorter the string the less it can move. Here the correct lengths of ‘strings’ called tendons are attached to the sea bed. A platform is towed to the location. They match the buoyancy of the platform with the correct length of the tendons so that it cannot bob about like your balloon.
Have look at this brief video. It goes into detail about how TLPs are placed. For a quick view of how they stay where they are meant to, with the minimum of bobbing, check out the last minute or so.
Gravity Based Structure (GBS)
It takes a fair amount of fascination to ignore the sunshine that is rarely seen in the UK and instead spend an hour watching a video about the massive Troll platform. Sad or just obsessed, you decide which I am. A GBS (like the Troll) is a structure built, towed and sunk into the required location and anchored partly due to it’s weight but mostly by increasingly ingenious methods. The Troll for example is held in place by vacuum to the sea bed. Principally a GBS is not tipped into place, it is lowered. The Troll is supported by reinforced concrete legs that are water tight and so huge they have lifts going down them. Think of a building in the sea and you will get the drift.
Spar platforms are like pencil fishing floats. The platform remains stable because of the weight that is suspended under them and the anchor system that keeps them precisely in place. These are mobile in that they can be redeployed relatively easily and are good in deep water. The spar that is under the platform can be 100s of feet in length and their enduring use in the Gulf of Mexico, for example, is tried and true.
Here is a video by Technip. Now the lady presenting might give you the impression she is trying to sell you one… if you have $1.5 billion give her a call.
There are other platforms, specialist support designs and all manner of other vessels but the above covers the major types in use today. I hope that helps sort out which is which. As always I welcome any suggestions or comments.
Take care, Christine