The Maersk Provider out in Brazil, with its retrofitted deep water winch. Photo Tony Poll  

This is the third of four articles written about deep water mooring for a marine magazine back at the turn of the century. Looking at stuff on the internet it appears to be applicable today, and worth a read.

In 1801 Horatio Nelson at the Battle of Copenhagen famously put his telescope to his blind eye when it was pointed out to him that the part of the fleet commanded by Admiral Hyde Parker was flying a signal requiring the whole fleet to withdraw.

"I see no ships", he said. It would have been inconvenient for him to have seen any ships. He was doing just fine without anyone putting their oar in and telling him that the odds were against success.

The message and the sentiment must have been uppermost in the minds of the oil industry development managers when they decided that it would be a good idea to look for oil in water a mile deep. There are a number of things against such an enterprise and one of them is the requirement to lay moorings, and therefore the need for ships to do it.

These gentlemen could hardly have been thinking of ships when the 1996 licensing rounds took place in the US Gulf, resulting in the prospect of over 100 wells being drilled in water depths of more than 1800 meters. The operators in the Gulf also expect to be able to drill in water depths of more than 2000 metres in the near future, and for readers who still think in imperial, these numbers are about a mile - deep.

In the previous article in this series the technique of load sharing was described which is one way of getting the anchor out to the required distance without losing it or damaging the ship doing the job. However such a technique requires that the rig have on board all the mooring.

A second solution which has been proposed by a group headed by BP Exploration Inc and includes Aker Maritime, Diamond Offshore and Camerons. It is called "Installation in Advance", and incidentally these three words are not actually three words, they are now a trademark and the intent of "Installation in Advance"™, or IIA, is to put sufficient systems in place for a semi-submersible to be floated in and start drilling without having to run moorings or set the riser. Hence the systems include not only the mooring system, but also the initial casing and the 1500 to 2000 metres of riser.

Since our interest is in the mooring system no further mention will be made of the casing or even the "drop casing hammer" which is apparently an essential component or the means of putting the incredible length of riser in position to await the arrival of the rig. Such a length of riser is, by the way, so specialised that the loss of 1500 metres of the stuff from the drill ship Peregrine 1 resulted in it being out of service for 12 months .

The main tool to be used to position the mooring system for this operation is the SEPLA, or Suction Embedded Plate Anchor. The anchor is a development of the suction anchor which has been about for some years but has the disadvantage of not being easily recoverable and hence of limited use for semi-submersible mooring. However Aker Marine Contractors carried out some tests with recoverable suction anchors in 1996 and also did some work with the VLAs (Vertical Lift Anchors) which were discussed in the last issue. In their opinion VLAs are not totally suitable due to the indeterminate depths at which they may trip. It is probably worth saying at this point that the development is taking place in the Gulf of Mexico where the bottom is extremely soft and so the horizontal movement of the VLA prior to tripping may be unacceptably great.

Obviously an advantage of a suction anchor is that it moves vertically downwards a required distance and once in position the suction plate on the top can be recovered leaving what is now a pile with a mooring attached under the seabed.

In promoting their latest development Aker Marine Contractors say that the major disadvantage of the suction anchors is that each one would weight more than 70 tonnes and would be dimensionally difficult to handle, and it is true that suction anchors are often 60ft long and 12 ft in diameter. Indeed earlier uses of this form of anchor have usually required the employment of diving or construction vessels of considerable dimensions, or sometimes crane barges, and obviously if one is promoting a cost effective way of dispensing with the on-rig systems, the employment of vessels which might be even more expensive than the rig should probably be forgotten.

Since the dimensions of suction anchors are considerable, Aker Maritime has reasoned that they can be used to embed a plate anchor in the seabed and then be removed for re-use. The technique is to take an extremely long suction pile, about 80 ft and slot the anchor plate into the bottom of it, securing it in position. One of two ships will then lower the pile with its anchor to the seabed and the second ship will lower away the mooring.  When the pile has first of all been allowed to self-penetrate and then been sucked down to the required depth an ROV is used to release the SEPLA from the pile. The pump is then put into reverse and the pile is pumped back out of the seabed and recovered to the ship. The other ship completes deploying the mooring and buoys it off.

If the mooring was chain or wire or a wire/chain combination a second slightly smaller wire would be required, and the larger wire would have to be laid out on the seabed and the smaller wire buoyed off. Apart from the expense and the amount of room the second wire would take up on the ship there would then be the risk of a complete loss of one or more of the moorings before the arrival of the rig which would more or less negate any possible savings.

Hence, instead of using wire the system will use polyester ropes for the majority of the length although wire is to be used for the last section between the rope and the anchor where the mooring passes through the soil since apparently seabed soils degrade polyester rope.

The use of polyester rope is not unique. The Brazilians are using this form of mooring for tying down floating production systems, however the jury is still out in the UK where it is generally considered that this form of mooring has been insufficiently tested to be trusted.

The Brazilians are known for fearlessly going where no project manager has gone before and as long ago as 1995 hired the ME606 Maersk Provider which was already a big ship but which was enhanced for its Petrobras contract with a winch of extremely large dimensions. She has been engaged in laying deep water moorings ever since and in 1996 they hired the Maersk Chieftain a large but now ageing VS476, the Far Sea, a late model ME303 and the Normand Neptun. Some of these vessels have been modified to deal with the polyester rope which has the disadvantage of having a very large working radius and so requiring a very large or modified roller and a large winch drum.

The rope makers have been pretty active in the field since it became apparent to them that their services might be required, and the Installation in Advance mob are allied with Marlow polyester ropes. Another player in the market are Bridon who like the other rope makers have been supplying coils of stuff of one sort or another to the marine industry for years. Bridon like Marlowe have chosen the polyester rope which has been used for years and which is in ready supply and which in water is only about one third of its weight in air.

Undeterred by the problems associated with it one company has chosen to produce rope from an alternative product. "Deeprope" is a product of the Belgian company HPFC Le Lis-Vermeire. Deeprope and is made from HMPE or as it might be better known High Modulus Polyethylene. Few of us have much understanding of the differences between different types of polymer, however what they have in common is the fact that they are composed of monofilaments and so do not have to be woven or laid as do natural fibres, although the lengths of parallel fibres are commonly encased in a woven cover for convenience. The greatest advantage of the polyethylene used by "Deeprope" is that it is only three quarters of the diameter of the equivalent breaking strain polyester. And is buoyant in water.

Meanwhile back in the North Sea operating experience with on-rig systems is increasing, not without some pain, and the requirement for means of testing possible rig move procedures to ensure that the vessels hired are capable of doing the job has been identified. Suitable formulae for calculating the tensions set up when running anchors and the resulting catenaries have been about for years but it is a long winded process to vary the possible parameters until the required result is achieved.

To assist with this process BMT Fluid Mechanics , who used to be British Maritime Technology, but are now a part of ASCO Marine and Underwater Services of Aberdeen, offer a programme called "DEPLOY". Writers may be relieved to know that the word "deploy" does not appear to have become a trademark and so they will still be able to use it.

The programme allows the planners to feed in the mooring system configuration the water depth the slope if any, and the intended bollard pull of the anchor-handlers, and then to consider the anchor deployment in stages. The programme will indicate any violations of the initial parameters. It will tell you if the chain or wire is burning its way through the bolsters, if the tension in any of the connections exceeds safe limits or if the required bollard pull to achieve the result exceeds that of the vessel being considered.

One of the possible advantages of this programme is that it may allow those planning the rig moves to chose vessels which are the optimum size for the job, rather than some which may be too large, and which would therefore possibly violate the safety margins for the connections rather than the ones for the tension at the roller. Another is that it will provide precise information as to whether it is practical to allow the mooring to ground on the seabed, or indeed, how much of it should be allowed to ground on the seabed, always considering the angle of departure and therefore whether the bolster is going to be worn away.

It’s a far cry from the old seat of the pants stuff which used to go on out there.

The programme was initially developed at the request of Transocean who had anticipated have some difficulty deploying the moorings of the Transocean Leader. This Aker H4.2 is now fitted with a chain wire combination mooring system of 3 1/2" chain and 96mm wire, and while almost any decent sized anchor handler could pull out the wire it requires a substantial craft to pull out the chain. To start with the boffins in BMT produced figures at the request of the rig owner, faxing the results to the rig, but now it is possible for a rig owner or an operator to test the mooring plan on their PC and if it works go out into the field with considerable confidence.

The programme might also be used to test the design of rig mooring systems before installation, since, as has been pointed out before, the siting and size of the various components can make all the difference.

And as for "seeing no ships", well Lord Nelson did not have a computer programme to help him out of a jam.

 
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