The Sovereign Explorer and the Maersk Seeker. Photo Victor Gibson

This description of an actual rig move in what on the UK Continental Shelf is still deep water, was written in 2000. The Sovereign Explorer, one of the few semi-submersibles built in UK, was always a bit of a problem, and has now gone to the breakers, as has the Maersk Master. The Jack Bates is now in Australia.

At the beginning of April 2000 the Sovereign Explorer featured prominently the British national press when it was been boarded by members of Greenpeace, who scaled the legs and bivouacked an external ledge at main deck level, in the style of mountaineers overnighting on the North face of the Eiger. At the time it was "stacked", the oil industry term for being laid up, in the Cromarty Firth and was being prepared for a Marathon Oil deep water charter. 

 Marathon Oil UK, the charterers and Transocean Sedco Forex, the owners, obtained an injunction to have the protestors removed and as silently as they had arrived they disappeared and are still being sought by the police. The visit followed a similar attack on the R&B Falcon semi-submersible Jack Bates, where the protestors chained themselves to parts of the moorings. Enterprise, the charterers of the Jack Bates are suing Greenpeace for a million dollars for interrupting their programme. Marathon are not suing anyone, taking the view that mobilising a semi-submersible from lay-up is a major task, subject to delay and difficulty, and occupation of  an exterior surface of the rig by a couple of protestors was one of the more easily solved problems of those they were facing.

Greenpeace are of course targeting rigs bound for the Atlantic, claiming that their presence will have an adverse effect on marine life of all sorts. Such claims appear to be entirely subjective, but it is possible to say that an oil rig takes up very little space on the surface of the planet, and marine life appears to thrive in the immediate vicinity of oil rigs, platforms and pipelines.

Both the Jack Bates and the Sovereign Explorer were heading for water depths of about 1320 metres (4500 ft)  60 miles west of Harris, one of the better known of the Scottish Western Isles. On the UK continental shelf this is deep water in a hostile environment.

Marathon and Enterprise decided to co-operate on the venture, since the schedules for the two rigs were compatible. The Jack Bates was to be taken out and moored first, and the Sovereign Explorer second, the same ships being used for both operations.

The ships chosen were the venerable Maersk Master and the newly delivered Maersk Seeker and Maersk Searcher. The latter two vessels were built to Maersk designs derived from their B class vessels, in Singapore. For the Sovereign Explorer operation Marathon also hired the Swire Pacific UT720, Pacific Blade.  The Maersk S class vessels can achieve 211 tonnes bollard pull with 18,250 BHP and have a single work drum with capacity for the required 2000 meters of 83 mm wire and the Maersk Master is provided with two workdrums each with a more limited capacity which allows for 1000 meters of 87 mm wire. The Pacific Blade has 12,200 BHP and a resulting 152 tonnes bollard pull although this capability was not relevant to the task for which it was engaged. It is interesting to note that this vessel has the longest tow wire at 1436 metres.  

The Maersk Master out in Brazil. Photo Tony Poll

The operators found that their co-operation had been fortuitous since there was in the event a finite amount of 83mm wire in 2000 meters lengths available in Aberdeen. The Maersk Searcher and Maersk Seeker had the bare end clamped into the securing point on the work drum, and the Maersk Master, designated as the towing vessel was fitted with 2x 1000 meter wires, one on each its twin workdrums. Each of these wires was fitted with a swivel at the rig end.

The Sovereign Explorer was originally designed to work in a maximum depth of 3500 feet, and so had to have an additional 200 metres of chain fitted. This gave the rig 1400 metres of 76mm chain and 1800 metres of 90 mm wire. The bitter end of the chain is permanently connected to the outer end of the wire, which is stored on reels in the pontoons and paid out, tensioned and recovered by means of tension winches mounted above the windlasses on the corner columns. The anchors are 12 tonne Stevpris. Despite the extreme environment in which the rig is intended to work it is not dimensionally large, extending to 90 meters in each direction.

Having ensured the removal of the pressure group members and had the extra 200 metres of chain added to the mooring system, the rig departed the Cromarty Firth being towed by the Maersk Master at 1600 on 4th April. Apart from 24 hours hove to in adverse weather the tow northward through the Fair Isle passage was uneventful and the rig with its attendant vessels arrived on the location at 0130 on 16th April.

Shortly after 0200 the job of locating the rig was started in earnest with the chasing pennants for Numbers 1 and 5 anchors, the forward starboard and port aft moorings being passed down to the Maersk Seeker and Maersk Searcher. The two ships, therefore at opposite corners of the rig started out, the operation following the designed procedures which were prepared for Marathon by the Aberdeen Marine Consultants, Trident Offshore.

Each mooring was run out using the load-sharing technique with which readers of this column will by now be familiar, and which was pioneered in the Gulf of Mexico. Briefly, for those who have not followed the development of deep water mooring, the technique involves the rig and the ship alternately deploying the mooring and work wire, thereby keeping the tensions in the system at an acceptable level. A computer programme is used to identify the stages of the process, and one of the principle requirements is to provide a figure for minimum bollard pull to keep the mooring off the rig bolsters which are extremely vulnerable to wear.

Deep water rigs with combined wire and chain systems must be provided with a means of changing over from the windlass to the winch, which is always difficult in one way or another because the chain is piled up in a locker and the wire is stored on a drum. Some systems are automatic, complex and expensive and some are conducted entirely by the rig crew, who split the chain and connect up the end of the wire, often working on a small platform below the windlass and sometimes with no mechanical aids.

The Sovereign Explorer falls somewhere between the two. The end of the wire is permanently attached to the end of the chain, but the transfer of load from the windlass to the tension winch is undertaken with the assistance of a guiding system which is hydraulically driven and manually controlled. The system was designed to accept the vertical loading from the anchor chain of 90 tonnes, which had now been exceeded by the addition of 200 metres of chain. Hence the presence of the Pacific Blade.

When the moorings reached what is known as the transition point, the Pacific Blade was required to J-hook the chain close to the rig and reduce the weight on the guiding system. This initially proved to be difficult because the rig was not fixed in any way, apart from by the two anchor-handlers pulling in opposite directions. It is easy to J-hook a horizontal mooring and impossible to J-hook a vertical mooring and so achieving any horizontal component required some co-ordination. However the initial difficulties were overcome and by 1500 on the first day both the initial anchors had been laid.

The Pacific Blade. Photo Peter Taylor.

To illustrate the process the running of No 8 anchor by the Maersk Seeker is described in greater detail.      

The chasing pennant for No 8 anchor was passed to the Maersk Seeker at 1522 on 16th April, about 13 hours after the start of the mooring operation. The ship's crew connected the end of the pennant to the link at the end of the 2000 metre work wire with a 120 ton shackle and the anchor was lowered off the bolster. The ship heaved up on the work drum until the anchor could be sighted at the roller.

100 metres of chain was then paid out to allow the ship to run out and respool its work wire and this task was completed by 1938 when the rig crew started to pay out No 8 chain and the Maersk Seeker started to run the anchor. At around 2030 the 1400 meters of anchor chain had been paid out and the Pacific Blade was J-hooking for the bight so that the transition from chain to wire could be accomplished. Once the chain was hooked the transition took place and was completed at 2106; the Pacific Blade then released itself from the chain.

The rig and the ship then resumed alternately paying out wire. When the rig stopped paying out wire at 2122 the Maersk Seeker paid out 650 metres of wire, maintaining the bollard pull required by the programme. When  this was completed the ship was 1775 metres from the rig. The rig then paid out a total length of 900 metres of wire and the ship paid out a further 650 metres wire while still maintaining the required bollard pull and there-after increased power to achieve a distance from the rig of 2900 metres. The rig then paid out a further 950 metres of wire which was completed at 2346. At this stage of the operation the ship was 3500 metres from the rig and was being required to apply 105 tonnes of  pull, the highest level necessary during the whole operation. The Maersk Seeker then paid out a further 480 metres of wire and moved away to a maximum distance of 4000 metres. This is two and a half miles away in old money!    At twelve minutes past midnight number eight anchor was on the bottom and for that mooring it was all over bar the shouting, except for the return of the chasing pennant to the rig. The chasing back and the return took a further one hour and forty minutes.

On the morning of 17th with only two anchors to run the weather blew up. In the western Atlantic and the northern North Sea depressions can change direction unpredictably and the since each anchor was taking something approaching 12 hours to run it was possible for a ship to be caught part way through a run by a change in the weather. So it was for the Maersk Searcher which was forced to abandon the operation for weather six hours into running Number 2 anchor.  However, modern navigation and positioning systems are sophisticated and the ship was able to lie to the rig anchor, bow to the weather until the morning of 19th when it completed the laying of Number 2 and then went on to lay the last anchor which was Number 6.

So, the job was done. The insurance tensioning of the moorings was carried completed at 1900 on 19th April and the Maersk Searcher, now the only anchor-handler left on the location, was allowed to depart for Aberdeen. Both vessels were required to de-tension their work wires before returning to port. De-tensioning is now routinely carried out after deep water operations because otherwise the motorised reels used to recover the wires from the work drums are just not up to pulling out the turns. The usual technique is to drop a five tonne anchor over the stern on the end of the wire and then pay it out, in shallow water - or deep water, depending on how the mood takes you. Whatever technique is used, the operator is particularly keen to get the wires back in good condition. They are, after all, longer and stronger than the average tow wire, and while shipmasters treat tow wires as if they were delicate parts of their anatomy, they have not yet learnt to adopt the same approach with very long work wires.

The whole operation was carried out with very few problems apart from two winch failures, due to the electronics refusing to believe that the chain length had been increased from 1200 to 1400 metres. Well of course - the experts will be saying - but wait till they come to pick it all up again!

 
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