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Conventional ship and barge-shaped rigs
In the early days, ships were very attractive and the most common floating mobile offshore drilling units (MODUs).
Complications and disadvantages
Ships mobilized quickly and could carry a large amount of operator consumables, such as casing and bulk mud. However, their motions in weather proved to be a significant disadvantage in even mild environments. If a ship-shaped unit was hit on its beam with even moderate swells, the roll could raise havoc with efficient productivity. (Fig. 1) shows a typical spread-moored drillship from 1970.
The Offshore Co. (now Transocean) developed and patented the turret mooring system (Fig. 2). This system solved some of the motion problems, but other problems remained:
- Decks were sometimes awash with green water
- The turret could store only a limited amount of mooring wirerope because the winches were all located on the turret “plug,”
- The subsea blowout preventer (BOP) usually had to be stored on the drill floor
Over time, long mobilizations decreased, while the number of MODUs and semis increased. The semi, with its vastly superior motion characteristics, became the MODU of choice for floating work. Another factor is that, even though ships could carry large amounts of consumables, their space utility and connivance were limited by their cigar shape. The heyday of these units was the late 1950s to late 1960s, with a few being built in the early 1970s. Not until the late 1990s were more drillships built in the form of Dynamic Positioning (DP) ultradeepwater units.
This page refers only to the spread-moored units, which were usually rated at less than 1,000-ft water depth unless their mooring system was supplemented with mooring line inserts (i.e., mooring lines were inserted into the MODU’s own lines by use of anchor-handling boats). For instance, 1,500 ft or more of mooring wirerope may be inserted into the mooring line of the drillship’s own lines, thus increasing its line length and scope. With the inserts, some units have rated themselves at greater than 2,000 ft, but mooring MODUs in this manner is time-consuming and expensive.
Dynamic positioning (DP) units: alternatives to moored units
The alternative to moored MODUs is DP units, with their self-positioning thrusters and propulsion. Barges, or non-self-propelled units, are also not discussed here because these units, which are few in number, are used in lakes, bays, and buoys, not in offshore areas. Today, there are very few moored drillships left, and they operate only in the mild, benign environments of the Far East and West Africa. Most are more than 25 years old and, generally, have not been upgraded technologically, which is another of their disadvantages.
Drillships may still be a valid choice in certain situations:
- If a location with a very benign environment is under consideration,
- If a conventional well is to be drilled,
- If the well is in a remote location where logistics is a primary consideration
- If mobilization of another type of unit is costly.
In all the above situations, price is the driving factor in utilizing a drillship.