This is the first of two articles about the difficulties faced by the oil industry when they aim to recover hydrocarbons in deep water. It was written just as the steps were being taken to build vessels capable of working in 10,000 feet and as people were becoming resigned to the prospect of operating DP vessels. It appears that today (in 2016) a lot of the old stuff is being sent to the breakers, and modern deep water craft are being hired out at moderate day rates, all due to the reduced oil price. So the question today might be will lower spec craft be built to cater for the lower oil price?

 

The Atlantic Kingfisher Running an Anchor

Back in 1977 the operators in the North Sea were having seismic work carried out beyond the edge of the Continental Shelf out in the Atlantic. Just getting the seismic lines run was a major problem due to the frequency of bad weather and the length and height of the swell even In good weather. The main problem was that when the stern of the ship rose and fell there was a sort of whip effect along the streamer causing it to ripple at the same frequency, and if the vertical movement of the streamer exceeds the required parametres then shooting the line must be aborted. The seismic ships doing the work spent weeks at a time sheltering behind convenient bits of the Shetlands waiting for the conditions to improve. Despite the drawbacks the work was done in the anticipation that in the future the technology would be available to allow drilling to take place.

When the rigs first followed the seismic vessels out into 700 metres of water to the West of the Shetlands in the early eighties, it seemed, to the pessimistic that there would be no possibility of recovering hydrocarbons, because in addition to the water depth there was the weather, and the swell, and to make matters worse extremely strong currents which changed direction at different depths.

Probably the key component in deep water drilling is the riser, which for those unfamiliar with drilling equipment, is the tube which extends from the drill floor to the seabed. Drilling fluid is pumped down the centre of the drill string and returns to the surface bearing the drill cuttings in the tube itself. Down the outside of the main body of the tube are two small pipes which are the choke and kill lines. It is sufficient to say that these lines are essential in well control situations. The whole lot is supplied in convenient portable lengths and is bolted together and lowered to the seabed to be connected by hydraulically operated latches to the well head. Obviously the deeper the water, the more lengths of riser there are to be stored, sometimes on the rig and sometimes ashore with supply vessels ferrying the bits to the location. Additionally it becomes necessary to provide the thing with buoyancy so that its weight at the rig floor can be carried by the riser tensioners. All  drilling units have riser. Drill ships have a riser, DP semis have one and lastly moored semi-submersibles have one.

In all cases the ship or rig must have the means of maintaining position, such that the angle the riser takes from the vertical must not exceed a predetermined angle. This is usually something under five degrees. The horizontal distance at the surface between the actual position of the unit, and the position it would be in if the riser was vertical is known as the excursion. The reason for mentioning the riser is that the two things which are challenging to the drilling industry with regard to deep water are, firstly the riser and secondly the means of maintaining position. Riserless drilling is on the way. It has been thought of and that seems to be enough for the oil industry. Drilling without moorings has been with us for the last twenty years but moorings are still being used, probably because they have always been cheaper than dynamic positioning.

It is difficult today to see which way it is going. DP drill ships of enormous dimensions are being constructed, with a view to providing a stable platform which is not complex and which can weathervane. DP rigs are being resurrected as a means of maintaining position but drive offs remain a nightmare for the drilling fraternity and the constant of operation of 20,000 bhp can consume a great deal of fuel. And worse, it is not only the cost of the fuel which is a bind, it is the constant supply by large ships, which must be hired, today at substantial rates.

Why bother then when there are still plenty of shallow seas in the world? Perhaps there is no straight answer to this question but it is probable that the solution lies in the decisions made when the results of the seismic surveys hit the desks of the top cats. This is where it is possible to see the point of sending lone seismic craft out into the more inhospitable areas of the world searching for hydrocarbons. The surveys provide the oil companies with objectives, and comparisons may be made in terms of difficulty, because the job is never easy. They will consider the difficulty of exploration, and if the exploration proves fruitful the difficulty of extraction, and should the extraction prove possible, the difficulty of transportation, and finally whether the size of the field is likely to merit such attention.

Oil exploration at sea is carried out by mobile units, but they do require support and the more distant their operation from the recognised areas of activity the more expensive the support may be, and, if the exploration is fruitful the more expensive the transportation. So the decision makers probably have a choice of moving into new areas or moving into slightly deeper water on the edges of the current areas of exploration, and here we really are talking about the edges. In the North Sea, which is a continental shelf on the edge of the Atlantic, even the more distant oil fields are little more than 24 hours steaming from the support bases and the deep water is probably not more than 36 hours away. Most of the oilfields are in water depths of about 150 metres. In comparison the average depth of the Atlantic Ocean is 3,926 metres (12,800 ft) and at its deepest point the ocean floor is 8700 metres below the surface (28,000 ft).

In the Gulf of Mexico many jack-ups are still employed virtually within sight of land and semi-submersibles operate only a few hours steaming time from their base ports. The deep water, and we are talking here about water depths in excess of 700 metres is only a couple more hours away and another hour away the water depths may exceed 1000 metres, 1500 metres or 2000 metres. Hence the requirement here is to overcome the vertical distance rather than the horizontal distance, and operators are encouraged by the fact that the existing infrastructure in terms of platforms and pipelines may only be a few miles away from the new but deeper fields. In this, and some other areas of the world weather conditions are usually benign, fostering the use of marine equipment which might not survive in the Atlantic and therefore making the task of overcoming the vertical distance more attractive. In some places, such as the Mediterranean coast of Israel, this preference is even more obvious since the deep water is only 25 miles from the shore.        

The providers of marine services have a specific influence on the course of events. Right from the first time the need for anchor handling was perceived the shipowners have designed ships which have more power, have higher bollard pull, have winches which can pull more and have larger drums for bigger wire and more storage - the list goes on. And it is subsequent to the arrival of such craft on the market that the steps further out have been taken.

Rig operators have also taken specific steps to move into deeper water, although they have often been helped by operators who have paid in part for some of the upgrade work to turn shallow water semis into deep water semis. 

So, getting back to the means of operating in deep water and specifically the moored semi-submersible, it may be worth looking at what makes a semi-submersible capable of operating in deep water. It is obviously the length of the mooring. If the rig is capable of carrying all the mooring required for the greater water depth, and if it is also capable of carrying all the riser which is required for the deep water, this indicates very large overall dimensions which in turn may require moorings of large dimensions.

Experience has shown that in water depths of up to 700 metres it may be possible to lengthen the chain moorings, and despite the fact that the chains seem to descend straight down off the bolsters, there is still sufficient horizontal component close to the surface to prevent too much excursion, which those who have paid close attention to these words will know is related to the angle of the riser from the vertical. Once the water gets deeper it is necessary to use wire to provide the horizontal component. Most rig owners have opted for a combination of chain and wire since the chain can be used for the initial length to ensure that the anchor remains buried no matter what the weather.

Typically the Transocean Leader, owned by Transocean Offshore Inc was docked in the early part of 1997 and upgraded to work in 1350 metres (4500 ft) of water, in the Atlantic to the West of the Shetlands. The Transocean Leader is an Aker H4, one of two in existence, and is now over 40,000 tonnes all up. It is a substantial structure. It is fitted with traction winches and storage drums for 92mm wire and  operates a wire and chain combination which, as has already be discussed, has become as close as one can get to a standard technique.

It occurred to some intrepid souls in the Gulf of Mexico and probably in Brazil as well that even if the rig was not capable of carrying all the necessary wire and chain and the necessary riser, some suitable marine support could achieve the same result. A rig with wire mooring could have five hundred or a thousand metres of chain attached to each anchor by a suitable anchor handler - or indeed in the right environment by a well equipped barge. The key element here is time and one should either have plenty of time or plenty of ships; after all, a single ship is capable of mooring a rig in a few days given continuous good weather and support close to hand.

A more usual technique is for the ship to carry say a thousand metres of wire per anchor and for the chain to be broken at some point and the wire inserted. This sounds fairly simple if you say it quickly but it at least requires tow means of securing wire and chain on the deck and the requirement has on its own virtually ensured that all modern anchor-handlers are fitted with two securing devices side by side.

If all this seems like too much trouble and the operators opt to have their holes drilled by drill ships or by dynamically positioned semi-submersibles, they still have to extract the oil, and the techniques once more depend as much on horizontal distance from support as vertical distance to the surface. In the Gulf of Mexico where the platforms are often in shallow water close to the edge of the deep, subsea completions may be appropriate or even such oddities as guyed towers which, one suspects, also require a modicum of good weather for installation.  But in Brazil, where the fields are a bit further from the edge and where consequently they must be more self supporting, moored production facilities are favoured.

If one starts talking about mooring a floating production facility in more than 1000 metres of water then the life of the mooring, and the buoyancy required by the unit just to support the moorings begin to feature. When it comes to the life of the moorings, the use of wires deployed from the unit may be inappropriate since chasing is the single most destructive activity to take place during the life of a mooring wire.

It is a fact that if you lower a very long string down a very deep well, eventually the weight of the string itself will cause it to break, and of course the same thing applies to wire in water. To overcome this particular problem the rope makers have been at work and have produced buoyant moorings of equivalent strength to the wire or chain heretofore used for tying oil rigs to the seabed. Of course there are penalties. The first is the very large dimensions of the material and a working radius which is greater that that provided by the stern rollers of even the largest anchor-handlers. The second is the fact that since the material is buoyant there is uplift at the anchor, and therefore conventional anchors cannot be used. Even this problem has been solved, and practical aspects of the solution will be discussed in the next issue.

 

 
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