The modern deepwater drill ship Stena Forth at work in the GOM. Photo by Paul Morris in 2015.

When this was written at the turn of the century the oil price was a bit lower than it is today (April 2016), and obviously since then there have been developments both with the construction of mobile units and anchor-handlers. But it all still seems pretty relevant today.

Well - how long is a piece of string? The oil industry, in its quest to overcome deep water problems, has at least began to define the parameters. Part of the answer to the question is almost certainly " not as long as the chasing pennant required to position a rig anchor in 5000 feet of water".

 

As the millenium approaches and, despite the current lack of enthusiasm for drilling holes in the seabed (back at the turn of the century), the pundits in the industry consider that deep water exploration will continue because that is where the oil is going to come from in the future. Hence, regardless of the current low oil price, the techniques for recovering reserves from great depths must continue to be developed so that the operators will be ready with the product next time there is a demand.

The magic figures for the year 2000 are still being measured in feet, and they are 5000 feet for moored rigs and 10,000 feet for dynamically positioned ones and oil rigs are now frequenty moored in water depths in excess of 5000 feet in the Gulf of Mexico and floating production systems are being located in water depths in excess of 3000 feet in the Campos Basin. Several of the very large drill ships which are under construction or being fitted out are supposed to be capable of drilling in 10,000 feet.

The tools being used for these tasks are barges, big cranes, suction anchors, fabricated high holding power anchors, drag embedment vertical lift anchors, large anchor-handlers with winches of breathtaking dimensions, tension winches both on the rigs and on the ships, large diameter wires, chain-wire-chain systems, special chasers and a great deal of sweat.

A Vertical Lift Anchor on the deck of a Maersk anchor-handler out in Brazil somewhere.

Having presented a list of some of the possible solutions it is necessary to describe some of the problems. The first is the difficulty which semi-submersibles have in relation to deck load. Broadly speaking the stabilising weight of a semi is contained in the pontoons, the buoyancy in the columns and the combination of the two allows a certain weight on deck. Therefore in addition to requiring more of everything which is needed to drill the hole, riser, casing and drilling fluid, in all probablity the rig also needs to be able to accommodate the means of mooring it. Some rigs have chosen to maintain the existing chain and added a wire reel on the deck above or adjacent to the original windlass. Eight such reels greatly add to the deck weight. An alternative is to fit tension winches on the deck and to put the storage reels down the legs or in to pontoons, but while this solves the prime problem it is possible that many minor ones will be created due difficulties resulting from threading a mile of large diameter wire through complex leads and sheaves into the pontoon areas.

Another way of solving this problem is to prelay the moorings using barges, or anchor- handlers suitably equipped, but whichever technique is used there is no doubt that only anchors which are going to stay put can be used. Both Bruce and Vryhof have high holding power fabricated anchors on the market which do seem to work and suction anchors only seem to have the disadvantage of not being re-usable, and requiring a somewhat sophisticated craft to install them.

It is of course possible to insert a length of wire into a conventional chain system, but this is likely to be less successful than prelaying the moorings and then connecting the rig up on its arrival, however, neither technique has found much favour in the North Sea which illustrates the greater difficulties likely to be experienced in Atlantic weather. Indeed the current somewhat depressed market may give the rig designers the impetus which is needed for them to think up some really efficient mooring systems. As things stand, at present no matter how ineffective the mooring system may be, it is the operator who pays for the move, and so the effectiveness or not of the mooring system has no financial implications for the rig owner. Hence one assumes that at a time of depressed oil prices, rig owners may be spurred on into improving the mooring systems so as to make their rigs more attractive to their clients. It is worth noting that it is also the operators who hire the support vessels and their awareness that the more difficult the job the more effective the ships must be, has resulted in the amazingly rapid development of the anchor-handler as a vessel type.

Operators in the North Sea are beginning to learn that if rig mooring systems require particularly benign conditions for operation, this in combination with the very large ships which are inevitably being used, can result in a rig move bill which may be at the very least a significant part of the cost of the well.

One would not want to give the impression that there has been no research into deep water mooring by the oil companies; there has, and one of the results has been the acceptance of a technique known as "loadsharing". This has been developed using fairly simple computer programmes which minimise the load at the rig fairlead and the ship's roller and also minimise the amount of chain dragging along on the seabed at any time.

Obviously when one is working in deep water, it is just not possible for any ship to get to the required distance from the rig with the anchor at the stern. To do so the cable would have to be virtually in a straight line between the fairlead and the roller. There is bound to be catenary. Apparently in recent years some venturing to the west of the Shetlands have resorted to J-hooking chains in the middle to get the lead tug out to required distance. This is one way of doing it, the catenary effect is reduced if the cable is supported in the middle. However it is not quite such a good idea as letting out a bit of cable and pulling the thing out with the anchor just above the seabed.

A typical loadsharing programme would be as follows:

At a distance from the rig of 400 ft the rig lowers away 2500 ft of chain; the anchor-handler has the anchor at the stern, requires a bollard pull of two and a half tonnes and experiences a tension on the winch of 75 tonnes. The anchor handler moves out to 1200 ft and lowers away 4000 ft of work wire, the rig also lowers away 4000 ft of wire. The bollard pull required is 5 tonnes and the tension on the winch is 100 tonnes. In this programme it is now time to change to the second winch drum, and so the work drum must be capable of accepting 100 tonnes of tension on the first wrap.

The ship changes drums and then moves away to 8000 ft from the rig and lowers away a further 2000 ft of wire. The rig also lowers away another 2000 ft of wire. By this time each unit has 6000 ft of wire out and the chain is on the bottom in a water depth of 5300 ft. The tension on the winch is 130 tonnes and the required bollard pull is 65 tonnes.

When the point is approached when the anchor is to be lowered to the bottom, the ship is 14,500 ft from the rig with 7000 ft of wire out, the rig has 9000 ft of wire out and all the chain is either on the bottom or close to it. At this time the tension on the winch is 170 tonnes and the required bollard pull is 110 tonnes.

All these weights and tensions are well within the capabilities of a middle aged anchor handler. The computer programme tells those doing the job something which is fairly obvious to those who have an affinity with this type of task, but may not be quite so obvious to those who do the job by rote. This is the moment to note that, in terms of lengths of pieces of string, the ship is two and a half miles away from the rig and they are connected by over three miles of wire and chain.

One of the tools now used in deep water mooring. Fibre rope being loaded onto a Solstad A101.

For semi-submersibles then, it may be said that for deep water the systems are the same except that they are larger and more difficult to handle. More connections are required and the anchors have to be laid the right way up. Indeed the only disadvantage of the fabricated anchor is that it should be laid with its flukes down, and they are generally of such large dimensions that the chasing collar rests at the joining shackle. However it is claimed by Bruce that their heavy duty anchors turn themselves the right way up even if laid incorrectly  and by Vryhof that it is only a matter of following the instructions. One thing is certain, one does not want to be considering laying back-up anchors, or in fact having to relay anchors, so it is best to get it right the first time.

Bruce have supplied anchors to a number of semi-submersibles which have been upgraded for deep water work including the Transocean Leader, upgraded specifically to work in depths of 4500 feet west of the Shetlands and the Transocean Rather upgraded to work in deep water in the Gulf of Mexico as long ago as 1992. They have also suppied anchors to the Diamond M-Odeco upgrades which include the Ocean America, Ocean Voyager and Ocean Valiant.

The means of mooring floating production systems seem to be a little further advanced. Both the major anchor producers have developed unconventional anchors which might be described as drag embedment, vertical lift anchors. These anchors can be deployed in a more or less conventional manner, and when put under tension will disappear into the seabed until a sufficient tension is reached to break a link which allows the anchor to change alignment. Instead of now being an anchor which naturally takes up an angle to the mooring, it takes up an angle at 90 degreees to the mooring , allowing an exceptionally high weight to be applied. The existence of such anchors has allowed the designers of the systems to consider eliminating the chain which is normally necessary to prevent uplift at the anchor, and so giving them the oppportunity of using wire only or else the one of the several polyester rope types on the market.

It is just possible that ingenious use of the vertical lift anchor might reduce the possibility of contact between the mooring and the bolsters of semi-submersibles and therefore reduce the wear on both. This problem was already in evidence in the last decade when rigs will all chain moorings were working in 700 metres of water, the greatest difficulty being experienced during recovery when the cable was capable of sawing through the bolster and rendering the whole system inoperative.

In deeper water with chain/wire combinations there is a tendency for the wire to clear normally constructed bolsters by a minimal distance which obviously depends on the water depth the height of the fairlead above the bolster, the thickness of the wire and the tension which the winch is able to exert. The situation can be made worse by weather. With conventional chain moorings in moderate water depths it is standard operational practice to slacken off the lee moorings when unacceptable levels of tension are observed in the weather moorings. This reduces the tension in the whole system and allows the rig to maintain station, even though it may be unable to drill.

In a similar situation in very deep water the wind and wave action will push the rig towards the lee moorings so that they begin to saw on the bolsters. Rather than being able to reduce the tension in the whole system it is necessary to increase it, risking the breaking of the weather moorings or if it is left there is a risk of either parting the lee moorings or sawing through the bolsters. On the plus side for the supply vessel owner these possibilities increase the necessity of keeping large anchor-handlers on station just in case they need to be used to take the partially drifting rig in tow.

Although it is apparent that some of the problems relating to deep water mooring are being solved all the solutions involve a great deal of hard work and, more importantly, time. There is a general reluctance on the part of those engaged in deep water activites to release information because in most cases the job is taking so long and is obviously still fraught with difficulty. The anchor suppliers have numerous single anchors on their client lists providing an indication of mooring failures, but it is the need to find the answers to these sort of problems which will ensure the continuance of deep water work, even during the current downturn.

 
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