What is tender assisted drilling rig?

Answer 1

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

Answer 2

An offshore rig, assisted by a 'tender,' or probably several, which are just the specialized boats that supply the rig, etc.