The crew of the Far Senior dealing with some very large mooring components. Photo Far Senior Crew.

This is the fourth of a set of articles written at the turn of the century for an offshore magazine. The only changes to have taken place since then have been the introduction of even larger ships than those described and the development of torpedo anchors in Brazilian waters. The description of the Bourbon Dolphin accident in "A Catalogue of Disasters" provides information about the practical aspects of this sort of work.

If one reads any set of papers on the subject of anchoring and mooring one becomes aware that it is an extremely technical business, and some of the papers are so technical that it is unlikely that they will be understood by anybody except the presenters and others directly involved in whatever their particular field of expertise may be. The papers sometimes use words that are not in the vocabulary of even those well versed in the use of the English language. Take for instance the word "dilatancy". The New Oxford Dictionary of English describes this as "The phenomenon exhibited by some fluids, sols and gels in which they become more viscous or solid under pressure". The word appeared in a paper presented at a recent conference in Aberdeen (in 1999) in conjunction with "undrained shear strength" and other catchy phrases.

Anchoring has come on a long way since the first semi's were positioned in the North Sea. BP's triangular rig the Sea Quest was positioned at the Forties Field by tiny ships which  just dumped something heavy on the seabed as far away from the rig as they could get. The anchor positions were marked by buoys laid by survey vessels which used Decca Hifix and if the buoys were washed away before the rig arrived it was the job of the survey ship to maintain station at the anchor position while the anchor-handler steamed out at full power towards it. Fortunately for the survey ship the position was always optimistic and the little ships never had sufficient power to get out that far, which resulted in them having to scatter much extra metal in the form of clumpweights and piggy backs on the seabed beyond the anchor itself. It is extremely unlikely that the word "dilatancy" was in anyone's vocabulary at the time.

In addition to the technospeak to be found in these papers it is significant that the presenters are spending more time telling the potential audience how to use the systems which they are championing. This may be due to the fact that there seem to be two areas of enthusiasm developing, one for the use of prelaid moorings using suction anchors or one of their derivatives, and one for conventional anchors which are laid at the time of the rig being positioned on the location, just as has always been done in the past. This is because in the past suction anchors required larger and more elaborate craft to undertake their deployment than conventional anchors. Now anchor-handlers are large enough and generally provided with sufficient hardware and software to use DP to maintain station and launch the ROV which is now becoming essential for all underwater tasks and so are able to deal with either suction anchors or conventional anchors, curiously the increase in the level of instruction for the deployment relates more to keeping the anchors in one piece than to the technicalities of laying them. In all case the vessels now doing the work are light years away from the little ships which went out there more than 20 years ago to position the first semi-submersibles.

Having said this it now would seem to be an appropriate time to discuss what the industry sees and being the vessel requirement for the job, indeed it was intended to get to this in the last issue, but it seemed necessary to first go into the equipment which ships are now being expected to be capable of deploying before moving on. In this article reference will be made to the conference on "Continuous Advances in Moorings and Anchorings" held in Aberdeen in May of 1999 but the papers will be discussed in more detail in a later issue (one assumes that they were).  This is the third conference on the topic held in Aberdeen, and significantly this is the first which actually discussed the manner in which supply vessels might deploy the moorings.

This is the Alex Chouest, a sister ship of the Gary Chouest. Note the large reels above the winch space. Photo Oddgeir Refvik

The advocates of the suction anchor and the prelaid moorings describe the manner in which a single anchor-handler can be used to deploy suction moorings. One of the papers presented at the conference was concerned with the prelaying of suction pile moorings for the Transocean Marianas in the Gulf of Mexico using the new AHTS Gary Chouest. The piles are 60 ft long 12 ft in diameter and connected to 12000 ft of 96mm wire. Although details of the ship were not available at the conference, and there seems to be a general reluctance on the part of some American shipowners to disclose the capabilities of their craft, the paper says that the ship was able to carry and deploy three moorings before going back to port to load a further set.

This means that the vessel has a drum capacity of 5500 meters of 96 m wire. Since an ROV is used extensively during the mooring operation it is also provided with some means of deploying it. Additionally there is a good chance that it is fitted with some form of basic DP system to maintain station while the engaging in landing the suction pile and sucking it into the seabed. Of course little bollard pull is required because all that is required once the rig is on station is to connect up the rig mooring to the pile mooring, although almost certainly twin sharks jaws, or Karmforks will also be fitted to make this particular part of the job easy.

In order to be able to carry out just this sort of task Halter Marine have produced a number of designs, and the one which precisely fits the specification is the HLX255 a 14,000 bhp 255 ft anchor handler. One such vessel is the Seacor Vanguard delivered by the yard in October 1998. This vessel is can store 41,200 ft of 3.5" wire has two triplex sharks jaws and is provided with a Kongsberg-Simrad DP system. By European standards the ship would seem to be a little underpowered, but it would certainly be adequate for the suction anchor task. This class of vessel is provided with a feature that is at present unique on anchor-handlers, a traction winch. Traction winches have so far only been used in the offshore industry as a means of tensioning moorings on semi-submersibles while actually storing the wire in the pontoons. They consist of two closely positioned drums with the mooring wire running from one to the other. Hence when operational the wire is always at the first wrap and so the as the mooring is recovered the maximum pull remains available.

The Seacor Vanguard possibly still unique in being provided with a tension winch. Photo: Oddgeir Refvik.

Of course, suction anchors may be less suitable for use in the North Sea where the seabed is a little harder, and hence the conventional high holding power anchors marketed by Bruce and Vryhof are  likely to come into their own. These manufacturers are currently facing some credibility problems due to the extremely large and powerful vessels now being used to deploy and recover their anchors. Back in the days of the Sea Quest the ships found it extremely difficult to deck anchors in 300 ft of water, and ten years ago they found it difficult to deck anchors in 2000 ft of water, today the craft are so powerful that they can deck anchors in virtually any depth of water. The Vryhof paper presented at the Aberdeen conference contained, in addition to the phrase "undrained shear strength", instructions as to how Stevpris might be recovered without them being damaged. This is because the problem for the manufacturers is that it is now possible for the ships to destroy the anchor during recovery. It is also possible for these very powerful ships to pull bits out of the older semi-submersibles and it is becoming common practice for towmasters to select the least powerful of the vessels available to connect to the tow to prevent such misfortunes.

One of the extremely powerful vessels capable of reeking this sort of havoc in the North Sea is the Farstad UT722 Far Senior. This vessel is one of several UT722s being offered for charter from both Scotland and Norway with 180 tonnes bollard pull and a winch capable of pulling 400 tonnes at the first wrap. As yet it has hardly been tested since North Sea and Atlantic margin deep water work has as yet been somewhat limited, but its designers appear to have thought of many ways in which the vessel may be operated efficiently in addition to the massive tow and workdrums and the enormous chain lockers with which it is fitted.

Probably its most remarkable and remarked on feature is the small but powerful crane which at the touch of a button can be made to rise from the deck next to the towing pins and be used for manoeuvring the heavy pins and shackles required to connect the sort of moorings used in deep water. Its robust construction means that in addition to lifting it can be used for tugging turning or pushing. In addition to this crane there is a further lightweight crane, sometimes known as a cherry picker which can be pulled down the side of the deck.

The winch itself is provided with a work drum and a tow drum which have a combined capacity of nearly 6000 meters of 96mm wire. The 1300 meters of 83 mm tow wire takes up little space on its drum. Each of the drums is divided into a large section and a small section so that connections will be so positioned that they will not damage the main part of the wire. The wires can be guided into position with spooling gear which is controlled from the bridge and each set of spooling gear is provided with a CCTV pointing towards the drum so that the possibility of loose turns can be minimised. In addition to being able to view the drums from the CCTV it is also possible to physically see the drums from the aft control position - this is as opposed to viewing the drums entirely on CCTV screens as is the case with many modern craft. Despite the wonders of technology there is no substitute for a proper view of the equipment.

The Far Senior also features the latest developments in aft controls. Once more one cannot help making the comparison with the early supply vessels which, despite the fact that the bridge personnel might spend days driving from the aft controls, were either not fitted with any form of seating or else were fitted with adapted typists chairs. Over the years seating has gradually improved until, on the Far Senior the ultimate may have been achieved. The seats at the control positions feature a full set of winch controls built into arms of the Chief Engineer's seat and a full set of ship controls built into the arms of the captains seat.  Both positions are equipped with a joystick. Whether the positioning of the controls in this way is actually an advantage is probably a matter of personal taste, but it looks great.

The Far Senior still doing its stuff and now festooned with additional equipment out in Brazil. Photo: Jan Plug.

The UT722 is not the largest type of vessel available to the industry for deep water work, however there is a tendency for the operators to opt for the largest vessels available in the hope that all eventualities will be covered. As a result the costs of mooring a rig in deep water may be extremely high. The Atlantic margin has already proved itself capable of absorbing large sums of money even in what might be now considered to be moderate water depths. Those familiar with Atlantic weather will be aware that it would probably not be possible to build a ship large enough to be able to work in anything but what passes in that area for good weather. Hence it would seem logical to ensure that bollard pull is adequate and that the ships are provided with systems which will ensure that delays in the operation are minimised. If this is indeed the appropriate approach then the Far Senior is well positioned to take advantage of the move into deep water in the Atlantic - if it happens. 

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