Jack up drilling rigs represent about 60% of the worldwide Mobile Offshore Drilling Units (MODU's) fleet. Compared to other type of drilling rigs, the jackup is rather special since it involves specific problems such as leg penetration, punch through and moving with the legs fully raised during the ocean or field tow, dry or wet.
The intent of this information is to introduce the basic concepts, which have involved the operational aspects of drilling jack ups. There are many recommendations in the specific Rig Operations Manual and they are a result of the analysis and compilation of the rig specific set out by each individual rig operator.
Historically the JU was built to operate in mild environments up to 250ft of water depth. The modern largest JU's are built to operate world wide with at present up to maximum water depth up to 450ft.
ADVANTAGES AND DISADVANTAGES OF A JACK-UP :
In comparison to semi-submersibles, a jack-up has some definite Advantages:
a) Lower construction costs. A semi drilling rig is probably about 2 to 2.5 times of a jackup.
b) Less personnel required to run the rig. A semi has about 2 times the numbers of jackup crew personnel.
c) Because of (a) and (b) lower day rates. A jackup day rates is about half the semi.
d) The possibility to work over a fixed platform.
e) It is cheaper for the operator to use a jack-up:
Less power full boats to move the rig. No mooring system required -no lost time to run anchors.
Less maintenance costs. Surface BOP without sub sea system. Simple well head assembly. However, some semis' have thrusters installed for DP mooring.
f) Less down time:
No wait on weather due to motions.
Drilling equipment can be handled faster and easier.
However, the jack-up’s have some Disadvantages:
a) Limited water depth. The maximum water depth for the largest JU is 450ft. Semis' can work up to 10,000 feet water depth while afloat.
b) Depends on bottom soil condition. The bottom soil conditions may cause a punch through or deep leg penetration.
c) In case of a blow-out the rig can not move off location. For semi, the riser could be disconnected and move away from the location, if emergency case.
d) More fragile. Many incidents and damages during moving and because of a punch through. Statistics have shown that over 75% of the incidents occur under tow or during jack-up/jack-down operations.
e) Safe operations require strict procedures and many different ways of preloading the rigs to ensure that the rig is spudded firmly on the seabed before it is jackup to the operational airgap, usually ranging 30-60 feet airgap.
Introduction to rig equipment at drillfloor,etc :-
Intro Drill Rig Equipment 1