Sunday, May 15, 2011

Jackup Terminologies and Types

In early 1955 ( before I was born in '59 and I started to work only in 1980, see my blog article on pressure vessel design ), the first 3-legged jack-up appeared on the offshore scene. The rig was the R.G. LeTourneau jack-up, the Scorpion, for Zapata Offshore Company. The Scorpion, an independent leg jack-up, used a rack and pinion elevating system on a truss framed leg. The rig worked very successfully for several years but was lost during a move in the Gulf of Mexico. The Scorpion was closely followed by The Offshore Company Rig No. 54. For Rig No. 54, however, a hydraulic jacking system on a trussed leg was used. These jack-ups were followed by Gus II, a mat supported unit using a hydraulic jacking system, which was built by Bethlehem Steel Corporation.

Those early breed of jack-ups were primarily designed to operate in the U.S. Gulf of Mexico area in water depths up to 200 feet. Wave heights in the range of 20 to 30 feet with winds up to 75 mph were considered as design criteria for these units. In most cases, in the event of a pending hurricane, the rigs were withdrawn to sheltered areas. Jack-ups can be either self-propelled, propulsion assisted, or nonpropelled. The majority of jack-up rigs are non-propelled. The self-propelled unit, although very flexible, requires a specially trained crew of operators as well as a better trained rig drilling team.

Jack-ups have been built with as many as 14 legs and as few as 3 legs. As the water depth increases and the environmental criteria become more severe, we find that to use more than 4 legs is not only expensive but impractical. The prime forces on a jack-up are generated from the waves and currents, hence, the less exposure to the waves and currents the fewer the forces being developed on the unit. From this standpoint the optimum jack-up is the monopod or single leg unit.
Problems other than wave forces, however, must be overcome with the monopod type unit. But in areas such as the North Sea with very rough' seas there is a need for the monopod jack-up.

When evaluating which type of jack-up to use, it is usually some of the criterias to consider :

1. Water depth and environmental criteria.
2. Type and density of sea bed.
3. Drilling depth requirement, environmental conditions.
4. Necessity to move or stop during hurricane or storm season.
5. Capability to operate with minimum support.
6. How often it is necessary to move.
7. Time lost preparing to move.
8. Operational and towing limitations of the unit.

The independent leg unit depends on a platform (spud can) at the base of each leg for support. These spud cans are either circular, square, or polygonal, and are usually small. Nowadays, spudcan bottom comes with tips for better holding on ground. The larger spud can being used to date is about 56 feet wide. Spud cans are subjected to bearing pressures of around 5,000 to 6,000 pounds per square foot, although in the North Sea this can be as much as 10,000 psf. Allowable bearing pressures must be known before a jack-up can be put on location.

Jackup Slides Ckw

Le Thourneau rigs have been the majorities in the Gulf of Mexico and most of them operating in the region are coming to thirty years or more in operating life. Some have gone through many upgrades, eg, increasing the cantilever outreach and hook load increase.

Le thourneau jackup


Prabhakar Tiwari said...

luckily visited this blog and found a perfect place for me. I am a naval architect working on the jack up design development. currently studying the Super Gorilla 450ft design. kindly clarify the following-

1) max storm elevated weight means pontoon lightship + variable load. Am I correct in my understanding?

2) since this is 450ft water depth design, how much max variable load during drilling condition should be assumed keeping in mind that more variable load is good but it will increase the structural natural period, almost 10-11 sec. These periods are close to the wave periods. please suggest on this.

If you have some other technical guidance or design database for a good study and learning process, please share with me on my personal email :-

Thanks and Regards,

Prabhakar Tiwari
Naval Architect

Choong Kim Whye said...

Nice to hear from you and how long have you been in Keppel? Jackup does not has pontoon, maybe you are referring to the traiangular shape hull of the rig. Elevated includes the variable ( VDL ) and you are right.

To briefly answer to your point 2, VDL depends on few factors,i.e. wind, wave, drilling load,etc and all these will affect the global structure of the leg, hull, spudcan,etc The water depth affects the leg length but other factors like drilling airgap, wave current, and other environmental factors as well as the leg penetration into the seabed affects the global strength of the rig. Normally rigs work on 40feet to 60feet airgap and the rigidity of the legs is the criteria of the structural leg design and this in turns affect the jacking house that holds the 3 leg structures.

The rig designer has a lot of computer analysis on the global design, which includes the main structure of the cantilever, drillfloor and holding claws which is main hold down of the cantilever to the main hull.

THese are just briefs and the design is not as simple as we speak.

Hope it gives you some idea.. read on...

Prabhakar Tiwari said...

Thanks for you comments.

I was in keppel mumbai office for more than a year and then i joined another design company in singapore. Till then I have designed almost 3 jackups for 300ft, 400ft and 450ft water depths. I do global analysis and stability analysis.

actually I was looking for little more in this post.

My prime concern is that when i understand the various harsh environment JU rigs for more than 400ft water depth, their variable load seems to be very high, sometimes upto 10000 t. for a deep water unit when unit itself has long natural period, if the unit is designed for such a higher variable load, operator is to maintain almost same variable load everytime as lesser variable load will cause more dynamic amplification in those cases.

i am looking for guidance on this side. appreciate if you can help me. also it would be nicer of you if you could suggest me some guidance for jack up designs.