Thursday, February 3, 2011

More on Offshore Drilling

A jack-up rig consists of a movable platform which can be jacked up and down the (usually) three supporting legs. The video clip below shows one of the typical designs. These provide a common means of drilling in water, where the water depth is relatively shallow - say, 50 to 400 feet.

Jack-ups will be floated out to location and the legs then lowered independently until they are bedded securely and the platform is level and above wave height. It is clear that their use would be restricted when
there are strong currents or an unstable seabed.

A semi-submersible is a floater drilling rig. In this case, a deck is supported by a tubular structure, and by two hulls to provide buoyancy. Again, the deck carries equipment, accommodation modules, a helicopter pad and typical layout is shown in the below video clip.
Semi-submersibles can move easily from one location to another either by being towed or under their own thrusters power. They are mainly used, therefore, for exploration and appraisal drilling where this ease of movement is essential.

When on location, the semi-sub (as it is often helipad called) takes on water ballast (into the two hulls, etc). This will lower the structure in the water and lower the centre of gravity. In this position it is shielded from the effects of rough water at the surface and achieves a high degree of stability.

A semi-sub can operate in deeper water than a jack-up. Its maximum operating water depth depends on the type of mooring system employed. Some semi-subs use anchors with wire and chain to hold them on station. Others use dynamic positioning which is a system of computer controlled thrusters, to maintain their position. Modern semi-subs using anchors may, in exceptional circumstances, drill in water up to 3,000 feet deep.

Semi-subs using dynamic positioning systems are capable of drilling in even deeper waters, up to 6,000 to 10,000feet deep.

The drilling equipment in the semi-submersible is more sophisticated compare to a jack-up and more costly as the semi-sub has various sea motions while afloat and the drilling system on board has to be able to function at the same time deal with the heave motions while the rig is afloat. Some drilling systems may be able to discoupled from the vessel in case of severe and uncontrollable situation such as unexpected storm or harsh environment and for the safety of the crew, the drilling riser may be released after the sub-sea LMRP/BOP is being shut from the well.

In comparison to semi-submersibles, a jack-up has some advantages:

a) Lower construction costs. Semi-sub usually cost almost double of that of jack-up or more
b) Less personnel required to run the rig. Jack-up has about 120men compared to Semi-sub with 200men onboard
c) Because of (a) and (b) lower day rates.
d) The possibility to work over a fixed platform.
e) It is cheaper for the operator to use a jack-up:
 -Less powerful tug boats to move the rig while it is afloat
 -No mooring system required, no lost time to run anchors. But some rigs now come standard with anchors
 -Less maintenance costs.
 -Surface BOP without sub sea system.
 -Simple well head assembly.
f) Less down time:
 -No wait on weather due to motions.
 -Drilling equipment can be handled faster and easier.

However, the jack-up have some disadvantages:

a) Limited water depth. The maximum water depth for the largest JU is 450ft. Semi-sub could work up to 10,000feet on the latest design.
b) Depends on bottom condition. The bottom soil conditions may cause a punch through or deep leg penetration. Semi-sub does not have such issue as it is afloat while drilling operation is carrying out.
c) In case of a blow-out the rig can not move off location whereas a semi could.
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.

The table below show some of the different jackup designs ( excluding KeppelFELS design of A-class, Super A, B-class, Super-B,etc ) and its capability in terms of size, VDL capacity,etc :

Types of Drilling Rig Capability

Some key personnel on board the drilling rig and their roles and responsibility :-

Tool pusher - In overall charge of rig operations, implementing the drilling plan and compliance with all safety requirements. Reports to the company representative.

Driller - In charge of the drilling process and operations. Responsible for compliance with the drilling plan and for the drilling crew. Reports to the tool/tourpusher.

Assistant driller - Assists the driller. Usually responsible to the driller for the operation of bulk storage equipment (for handling mud chemicals, etc.) and for the mud flowline system. Reports to the driller.

Derrickman - Responsible for the storage and movement of tubulars in the derrick and monitoring the mud systems. Reports to the driller.

Roughneck - Works on the rig floor. Responsible for general rig floor activities under the direction of the driller/assistant driller. Reports to the driller.

Rustabout - A member of the general workforce, assisting with the movement of materials,cleaning, painting, etc.

Formulas for Drilling and Prod

Rig Functions

Drilling Mud Technology

Drilling Mud Technology

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