What is tender assisted drilling rig?
TADs were the rig of choice in the 1950s and early 1960s in the
GOM for development drilling off fixed platforms. However, the
monohull tenders tended to lose location with mooring failures
during storms. This occurrence, along with severe motions of the
tender, resulted in their losing favor, except for use in very mild
or benign environments, such as in the Far East and West Africa.
There are about 25 TADs in existence today, with most being
monohull tenders. Four are semi tenders and offer the motion
characteristics to drill in mild to somewhat severe environments.
The TAD advantage is that its DES is relatively lightweight,
one-quarter to one-fifth the weight and one-third the space of a
standard platform rig. Most TADs carry the DES on the tender hull
and are self-erecting, so no workboat or derrick barge is required.
They are particularly attractive for situations in which there is
an old platform with reduced load-carry ability and/or space, such
as when a platform was drilled with a standard platform rig and
then production equipment was loaded onto the platform, thus
eliminating space and load-carry capacity. It is not unusual for a
platform to deteriorate with age and then be unable to hold up a
standard platform rig when additional wells need to be drilled. The
TAD is an option for this situation. Of course, if the platform is
in jackup water depth range, the jackup may also do the drilling if
its cantilever can reach the well centers with adequate load
capacity and if there are no incompatible spud can holes and/or a
severe punch-through condition.
For spars and TLPs in deep water where weight and space are at
an absolute premium, TADs, particularly semi TADs with their
lightweight DES, have significant advantages in some cases over a
modular platform rig. This is usually true for spars and TLPs with
> 9 or 10 wells up to a maximum of ≈ 24 wells. For spars and
TLPs with < 9 or 10 wells, their load and space availability are
too small for any type of platform rig or DES, and those with >
24 wells are large enough to support a modular platform rig without
a large weight and space penalty assuming all other factors are
equal.
Semi TADs also have the advantage of acting as construction
barges for platforms that are commissioning production equipment.
Their large rig-up crane, open decks where the DES is stored and
transported, accommodations, and general facilities offer a
relatively inexpensive construction platform compared with a
construction derrick barge.
Why would anyone want to use a TAD? They may be particularly
attractive for standard platforms in water depths over jackup-rig
rating and where space and/or load limits are a major factor, for
deepwater spars and TLPs with the right number of wells, and for
any platform where weight and space for long ERW are limited.
Generally, a TAD costs more than a platform rig, especially the
modular type, but they are a very attractive option for certain
situations.
http://petrowiki.org/PEH%3AOffshore_Drilling_Units#Tender_Assist_Drilling