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DESIGN
AND OPERATING CONSIDERATIONS FOR AHTS SUPPORTING DEEPWATER DRILLING AND PRODUCTION
This is a paper which was
presented to an Offshore Support Conference in 1999. It provides a journalistic view of
the effect on the supply vessel business of the move into deep water.
Synopsis
The oil industry has been drilling holes
in the seabed for about 40 years. In its first 20 years offshore it moved from about 60 ft
of water to 600 feet. In the next 10 years it moved from 600 ft to 2000 ft and in the last
10 years the move has been from 2000 ft to 6000 ft.
In each case no-one could imagine how
progress into deeper water could be made, except by means of dynamic positioning. Today
dynamic positioning is more straightforward than it has ever been but moored rigs are
still preferred.
This paper outlines the development of the
anchor-handler in line with the move into deeper water and considers the possible changes
in the future.
CONTENTS
Introduction
Historical Perspective
Depth Versus Distance
High Holding Power Anchors
Suction Anchors
Vertical Lift Anchors
Use of Fibre Moorings
Mooring Connections
How Today's Ships Cope with the Tasks
So What of the Future
Changes in Rig Design
Conclusion
Introduction
I have often thought how wonderful it must
be to be the editor of a newspaper, or a leader writer or columnist, able to write in the
first person, and therefore able to express an opinion. Most of what I write is in one way
or another intended to express some-one else's opinion or even no opinion at all. I am
often forced to use phrases like "in some quarters it is considered that" or
"conversely there is a body of opinion which suggests," when really I would like
to write - "I think this" or "I think that." So bearing this in mind,
and since what follows can be little more than speculation I have decided to indulge
myself with a paper written in the first person.
Firstly I make the assumption that the
words 'deep water' in everybody's minds refer to water depths in excess of 5000 feet, and
the requirements of the title refer to the ability of the AHTS to moor the vessel being
used for deepwater drilling and production. This is probably a little at variance with the
actual experience out there. Anyone who has been involved in lifting or laying anchors in
2000 ft will testify that it seems pretty deep. I was once involved in the recovery of a
3" mooring from 2000 ft. Two ships were used, and the plan was to tow the whole
mooring into shallow water, where it would be possible to recover it to the deck of the
largest of the two. In only 1000 ft we lifted the bight of chain onto the deck of a UT704
and managed to secure it in the shark's jaw. However before we had lowered the bight back
against the jaw one of the legs of the 100 tonne grapnel broke off. The chain rushed back
down the deck, turned the jaw through 90 degrees and disappeared back over the stern. It
took 3 days just to press the remnants of the jaw back into the deck.
Historical Perspective
Years ago 600 ft was deep water. That's as
long ago as I can remember, but of course before I started in the industry it may have
been that 300 feet was deep water. In fact, laying and recovering moorings was a task only
undertaken by the Admiralty to allow them to secure warships in places like Scapa Flow. I
remember the mooring patterns described by the Admiralty Manual of Seamanship. They were
different for different sizes of ship and for various water depths. The deepest moorings
were laid in less than 100 ft of water, and of course the technique used was to lay them
over the bow of a mooring vessel equipped with staghorns.
However I think we will forget about the
Admiralty, whose mooring techniques were for ever assigned to the history books when, in
1982 in the Falklands, the Wimpey Seahorse laid the same number of moorings in three days
that the Admiralty would have laid in three weeks. It would seem first of all therefore to
be necessary to compare the vessels which were capable of laying moorings in 600 feet with
those capable of laying moorings in 2000 feet in the Atlantic and as it turns out 5000
feet in the Gulf of Mexico.
The vessels which laid the moorings in 350
feet in the Forties field developed just over 4000 bhp, they could pull 150 tonnes on the
first wrap of their anchor-handling drums and had a bollard pull of 48 tonnes. They were
extremely small. Today there are ships with well over 200 tonnes bollard pull and winches
capable of lifting 500 tonnes. Large enough you would think for any task.
It seems to me that the first thing we
should be doing when we are thinking about what is to come in the future, is how we got
from the little ships which started laying moorings to the vast craft today doing the same
job but in deeper water. It may be worth noting that may of the rigs going to work in this
deep water are either the same one which went to work in the shallow water, or modified
versions of them, non of the mods involving a great increase in size.
Shipbuilders in the 1960s and 70s were
able to build large ships so why did they not design and build supply vessels bigger? The
answer is of course that they did not want them running into oil rigs. The ability of
ships to manoeuvre was virtually unheard of apart from harbour and salvage tugs which
seemed to the uninitiated to operate as a result of the willpower of the master rather
than as a result of any installed systems. So, they felt that by giving a small ship two
engines two propellers, two rudders and sometimes a single bowthruster they were doing the
best they could. The bowthrusters on some of the ships were so puny that you had to be
something of an expert to know whether they were working. Indeed the major problem these
early ships had was not so much water depth as friction on the seabed. However, because
the ships were so small there was little windage and so they were fairly manoeuvrable.
The engines which powered these craft were
either generator engines or railway engine engines, neither of which were suited to the
task, particularly since there were asked to vary their revolutions all the time in
response to the throttles - until the arrival of the CP propeller. They had to be small to
fit under the deck and usually they took up virtually all the main part of the hull.
The very earliest ships were provided with
A-frames on the stern in order to handle anchors and a small roller in the centre allowed
the wire to be paid out. Some on the other hand had no A-frame and were unable to pull the
anchors onto the deck at all. I believe the first time a ship pulled an anchor onto the
deck over the roller it was an accident.
Today ships can be made as powerful as
anyone thinks necessary. Typically the recently delivered UT741 Far Sovereign develops
27,400 bhp giving a bollard pull of 280 tonnes, and what might be termed run of the mill
vessels in the North Sea, the KMAR404 and the UT722 both develop around 180 tonnes
bollard. They are all very large ships and essentially owe their being to the improvement
in the equipment which turns the power into thrust, and the means of controlling speed and
direction.
This is not positioning, although DGPS and
fanbeam has helped with the task there is little doubt that the multiple thrusters engines
and rudders can be, and at least for some of the time probably are, controlled by a driver
with suitable skills. It is only the numbers of thrusters combined with either diesel
electric set-ups which allow infinitely variable revs, or CP systems which produce the same
result, which allow the manoeuvring to take place.
There may be those who remember the early
GEC systems which filled half the bridge and were connected to two engines, two rudders
and a thruster, and offered joystick control with the necessity for the driver to select
the engines to be put into the ahead and astern modes. Well they worked after a fashion
but they were hardly worth the space they took up.
Hence it seems obvious that the first
requirement for a modern vessel tasked with the laying of moorings is the ability to
position itself exactly where required, and if the means of doing this is automatic so
much the better.
Depth Versus Distance
That being said some problems became
evident once the depth became a significant component in relation to the total length of
the mooring. It would for instance be perfectly acceptable today for the anchors to be
placed 1500 meters from the rig in most water depths. In water depths of 100 meters the
depth can be shown as 6% of the distance; in water depths of 800 meters the depth is 53%
of the distance, and so on.
There has been a tendency for those in
charge of mooring activities to require the anchor-handling vessels to continue to steam
out to the required 1500 meters, or what-ever, with the anchor at the stern roller, but
what was possible when the water depth represented 6% of the distance may not be possible
if the depth represents 50% of the distance.
BMT's Deploy programme illustrating the
fact that there is 179tonnes of tension with 6000 ft of chain at 5000 ft distance.
In this case the combined forces act on
the anchor handling drum while it is completely full of wire and as a result there has been
a tendency for brakes to slip in the middle of the operation, causing general distress.
Subsequent to the distress they try something else. Often, out in the Atlantic they have
tried putting a second ship in the middle with a J-hook. This seems to work, but of course
it then means that two ships are occupied with one mooring extending the rig shift time.
This can be crucial if an Atlantic low is rushing towards the location.
A pair of anchor-handlers configured to
J-hook a mooring out to the required distance
In deeper water in the Gulf of Mexico the
technique of "load sharing" has become more common, where the anchor is
gradually lowered towards the seabed and the ships gets further away. However, in order to
load share and still get the anchor out to the notional 1500 meter distance the ship needs
to be able to store great lengths of large diameter wire.
Another problem which has become evident
only as semi-submersibles have moved into deeper water is the vulnerability of the rig
bolsters, the means of stowing the anchors when the rigs are under way.
Of necessity the bolsters stick out from
the columns of the rigs well beyond the sides of the pontoons, and as a result it is
almost inevitable that the mooring will possibly damage the bolster if insufficient pull
is used. Experience has shown that the chain may wear through the horizontals of the
bolsters and become trapped or possibly cause damage to the actual structure of the rig.
High Holding Power Anchors
Once rigs start to work in deep water it
ceases to be a good plan to lay piggy backs behind main anchors or to have to go through
the process of lifting and rerunning anchors. It all takes too long. Hence in virtually
all deep water situations, and here I realise that I am thinking deep water as being
anything over 2000 ft or 700 meters, it is a good plan to lay an anchor that is going to
hold the rig in position if it is laid right.
Both the Bruce and Vryhof high holding
power anchors are complex fabrications, such that the chasers lodge at the end of the
shank rather than travelling down the shank towards the crown. Hence in order to place
them correctly on the seabed the ship must lower away gently and apply just the right
amount of pull. Bruce anchors claim that theirs will turn over, regardless of how they are
laid which is probably why the ends of the flukes look more like some sort of origami.
However Vryhof's anchors rely on the skill of the shipmaster doing the laying.
Laying high holding power anchors requires care and skill
In addition to the requirements relating
to the skills of the personnel carrying out the laying operation there is the need to have
substantial equipment on the deck of the ship to ensure that the laying is done properly
and that the necessary equipment can be stored.
Additionally, in order to use these drag
embedment anchors in deep water it is an absolute requirement to use considerable lengths
of chain to prevent uplift. Usually either 1000 or 1500 meters are used before changing
over to wire, since wire is always included in the system in one way or another once we
get much deeper that 2000 ft.
Further problems have become evident as
the bollard pull and lifting capability of the new tonnage has been applied to the
recovery of high holding power anchors. The anchors are extremely efficient and given the
right fluke angle they will bury themselves deeply into the seabed during the insurance
tensioning process. When the moorings are chased out the chaser will not travel up the
shank of the anchor as has already been mentioned. Instead it will lodge on the end of the
shank. Hence it is possible for the force from the AHTS to be inappropriately applied. The
anchors need to be gently eased out of the mud, and probably in deep water not decked
until the rig has recovered the mooring. Any other plan may well result in considerable
damage to the anchor - or even complete destruction.
Suction Anchors
One identified answer to these and many
other problems is to pre-lay the moorings, and so far suction anchors have been favoured,
particularly in the Gulf of Mexico where the seabed is particularly soft. There are
proposals for systems which use suction anchors to embed plate anchors at specific depths
and precise positions and proposals for what might be called self drilling piles, although
the latter would seem to require a rig which seems to defeat the purpose.
Two ships engaged in the deployment of a
plate anchor by means of a suction pile.
Suction anchors used to be deployed by
barges or large construction or crane vessels, but the technique has been refined so that
now a sufficiently large anchor-handler to house a number of piles on deck and enough wire
on its storage reels can do the job. In addition it requires a means of launching and
recovering and operating an ROV.
One assumes that in order to place the
pile in the appropriate position it also requires DGPS interfaced with its propulsion
steering and thrusters. Well, no problem.
The motivation for this approach is of
course that the moorings can be laid independent of the rig, so that if day rates for
semi-submersibles are particularly high some money can be saved. Whether such an approach
would be valid in the Atlantic margin is debatable since a couple of top of the range
anchor-handlers might equal the day rate of a semi-submersible, and the weather window is
all.
Vertical Lift Anchors
In addition to pre-laying suction anchors
it is possible to pre-lay conventional anchors and high holding power anchors and to
pretension them using one of the patented tensioners produced either by Vryhof or Bruce.
Of course this operation has been limited to shallower waters in the past and, all in all
the technique is not really practical since the lengths of chain required are
unacceptable.
However, as if by magic these two
difficulties are overcome by the Vertical Lift anchor, which ceases to be dependent on the
horizontal component of the mooring to hold it in position. Vertical Lift anchors may be
placed on the seabed hanging from the mooring wire, the ship can then apply the required
forces by bollard pull to break a tension link. The anchor then takes up a new attitude.
This allows a single but powerful vessel to lay and tension anchors, which in themselves
are dimensionally easy to deal with. An alternative might be for the ship to embed the
anchor in the seabed but leave the rig to carry out the tensioning operation.
Use of Fibre Moorings
Despite the lack of long term experience
with fibre moorings, they are already being used extensively in the Campos basin. Their
advantage is that they do not in themselves have weight under water. Chain and to a lesser
extent, wire must use a certain amount of their strength purely to support themselves, so
buoyant moorings would appear to be the way forward.
The downside is that they are
dimensionally much larger than wire for the same strength and have a greater bending
radius. As a result very large winches are required to store them, and very large reels
required to move them. The general feeling in the industry is that unless a new and cost
effective material is discovered it may eventually be necessary to site all plant for
making up such ropes on the dockside so that they can be immediately reeled into the
ships.
This approach would be similar to that
used by the telephone cable manufacturers who reel trans-ocean cables directly into the
ships.
Mooring Connections
Something which tends to be forgotten in
all this bandying about of winch capacities bollard pulls and deck areas is the fact that
the bigger the wire or chain the bigger the connections, and if one is going to apply
large loads to these system by means of the ship it is almost certain that the crew will
have to connect up some large shackles and links.
It is probably that the links and possibly
even the shackle pins are too heavy to lift, and so the crew are forced to use
considerable ingenuity or some pretty ingenious equipment top carry out the task.
How Today's Ships Cope with the Tasks
Although we Europeans look out in
amazement at the extraordinary feats being carried out in Brazil and in the Gulf of
Mexico, both these areas of operation have one thing in their favour. It is proximity to
the shore. Water of considerable depth may be only a few hours from a suitable base port,
or at least shallow water, where moorings and anchors may be conveniently stowed on the
seabed for later collection.
Typically the Gary Chouest one of the
latest Edison Chouest anchor handlers was able to moor up the Transocean Marianas in 6000
ft of water all on its own. The rig was moored with suction anchors and the ship could
carry three at a time, together, one assumes with the required lengths of wire. Hence it
had to return to port twice for further equipment during the mooring process, all of which
took place prior to the arrival of the rig.
No such serious testing of existing
vessels has taken place in the Atlantic where there still appears to be considerable
difficulty in 2000 ft (750 meters). In the Northern North Sea and out in the Atlantic all
activities are controlled by the weather and so one would expect that enough ships would
be hired to carry all the equipment at once, what-ever sort it was.
Some work has been done mooring FPSOs and
shifting some mobiles, and operators have often been caught out by the weather. Since they
are under the impression that they need very large vessels indeed, these have turned out
to be extremely costly operations in all cases. It should also be emphasised that the
activities took place in the shallows - compared with the Gulf of Mexico.
Meanwhile companies such as Farstad are
putting time and effort into developing ships which will be able to cope with anything the
industry can conceive as being possible. In addition to the Far Sovereign mentioned
earlier. The Far Senior, a UT722 has many features which make it suitable for deep water
work. 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. 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.
Some ships have been specifically modified
to be capable of deploying the large diameter fibre ropes. One of them is an early Vik and
Sandvik design VS476, now one of the Maersk C class, Maersk Chieftain, and is incidentally
a very useful modification of what was essentially an ill conceived design, offering a
very large platform, a high bollard pull, but a very poor winch. It is worth noting that
the lack of the winch has meant that the whole class has spent virtually all their working
lives on straight supply work. - Its easy to get it wrong.
On the other hand, the Maersk Mariner and
Master, another odd design with their hatch forward of the bridge and a resulting
shortened after deck, have carried out or been involved in most of the major marine
operations offshore during the last ten years. Even though the hatch turned out to be
pretty useless, their enormous work and tow drums - two of each, and fifteen thousand
horsepower the client can count on have ensured their continued popularity.
So What of the Future
Is the Atlantic going to be too much of a
challenge even for the oil companies? Particularly now that concept of throwing money at
problems seems to be disappearing, to be replaced by the idea that to avoid making
mistakes its better not to start out on the job. Of course carried to its logical
conclusion we will eventually have to look for a new energy source.
If the Atlantic is off the map as far as
oil exploration is concerned, then it might well be appropriate to keep on doing just what
we are doing. Build or modify a few more ships to be able to deal with fibre rope and lift
suction anchors over the stern and that's it. Surely not!
Despite the fact that the oil industry as
a whole fails to appreciate the contribution made to its offshore activities by
traditional marine expertise - and this is, as I said at the beginning, a personal view -
the industry accountants are able to equate bollard pull with dollar signs, and it is
therefore logical that the industry will look for an economic way of doing the deep water
work.
This applies more to exploration and
development drilling than the siting of the final field production equipment, what-ever
it may be. During the drilling phase the rigs need to be moved about at frequent
intervals. It is currently possible to drill about five wells per year to between 15,000
and 20,000 ft in the North Sea. However the FPSOs or storage of what-ever sort only need
to be positioned once.
Hence, I think it likely that the industry
will turn its attention to means of reducing the bollard pull required to do the job. In
fact they have found one way already, by using suction anchors. The 15,000 or so
horsepower being used in the Gulf of Mexico is now more or less standard in the North Sea.
A second way may be by prelaying VLAs
(Vertical Lift Anchors) and then connecting the rig up to them. In this case the ship only
needs to be capable of embedding the anchors sufficiently for the final tensioning to be
carried out by the rig. However in both these cases the ship requires an accurate position
fixing system, and a means of transferring the information to the ship's manoeuvring
systems.
Changes in Rig Design
Who is to say that the design of
semi-submersibles will remain the same in the future - just because they have changed only
in details in the last 30 years?
It may be that it is just ceasing to be
practical for a rig to carry enough mooring of one sort or another to anchor in 2000
meters. If this turns out to be the case then it would be likely that the ships would
carry the mooring system. It would after all be much more practical for the moorings to be
carried external to the rig compartments, because if there is a failure it is a massive
task to make the changes. Additionally rigs either have to carry the wire and its reels in
the pontoons or the whole lot on the deck - either system has massive drawbacks for the
rig operators. So if the wire is to be carried externally it might as well be altogether
external.
A combination of vertical lift anchors and
plenty of wire would provide a transportable mooring system. Ships such as the Maersk
Mariner and Master could carry a single mooring on each drum and four anchors, and by
using dedicated anchor-handling vessels and dedicated towing vessels it would be easy to
develop a process which involved virtually no delay in rig moving.
A less drastic change would be to assign
the carriage of the anchors to support vessels. This would allow the bolsters to be
removed and as a result make the whole process of load sharing more realistic. One of the
major problems is the prevention of damage to the bolsters in deep water, resulting in the
ships having to pull hard to keep the mooring clear of the steelwork.
Finally, anyone who visited the Aberdeen
Oil Show might have seen models and Videos of a delightful U-shaped semi-submersible
designed to enfold platforms and dismantle them. A key system of this proposal is the
provision of a suction anchor on each corner. The rig manoeuvres itself into position and
then lowers the anchors. Once they have been sucked into place the rig deballasts to
become a sort of tension leg platform. What the advantage of this system could be over a
conventional mooring system in a few hundred feet of water was not explained, except
possibly the ability to dispense with attendant vessels. However, the idea of a rig
carrying its own suction anchors could catch on.
It might have to be assisted round its
moorings by a couple of ships, but the final footprint could be extremely small, making
the total lengths of wire used much less that that required for conventional mooring where
the rig must carry the complete length of the mooring and the ship must carry the wire for
at least one positioning system.
Conclusion
It seems to me that one can conclude a
couple of things from the forgoing.
The acronym AHTS stands for
anchor-handling tug supply (vessel). It is possible that the industry might move in the
direction it did back in the late 1970s and build some ships which are intended to do less
than the whole job. In those days the specialist ships were AHTs, and they were found to
be more efficient than ships with longer decks which were trying to do the supply job as
well.
Perhaps we are entering the era of the AH
(Anchor-handler), the PH (Pile-handler) and the TS (Tug Supply).
It would be nice to be able to say that
ship-owners and designers might get some guidance as to what the oil industry will require
of them during the 21st Century. However the pattern is unlikely to change. The owners are
going to have to look into their crystal balls, some ship-masters are going to have to do
some magic with ships currently afloat, some project managers are going to see the light
and then some-one is going to build a groundbreaking ship. Everyone is then going to say
"they must have been mad to build that."
However, regardless of the designs of the
ships and the requirements of the industry and the winches and the positioning systems,
the job of mooring floating objects is always going to be a difficult task requiring great
skill, so when it comes to operating considerations it seems to me that it is necessary,
even now, for the owners and the oil companies to co-operate in ensuring that the
necessary skills will remain in the industry.
Wonderful software such as the BMT Deploy
programme help with the planning of deepwater tasks, but operations which look OK on paper
or on the computer screen are likely to be quite different on the deck of an
anchor-handler in an Atlantic swell. It is at such times that the skill of the shipmaster
and his deck crew is all important.
I can only say that I have yet to see, in
any of the many learned and technical papers I have read in the subject of deep water
mooring, an acknowledgement of the requirement for this skill.
Vic Gibson. September 1999
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