The present oil and gas production of the free world is 40 million barrels per day. The offshore production accounts for 65 million barrels per day, or 16 per cent of the total pro-duction and it has been estimated that by 1980 more than 20 million barrels per day will be produced offshore,
amount-ing to 33 per cent of the total production.
Essentially, at present all the offshore production is controlled and subjected to primary processing in the same way as on land, by marinized conventional equipment mounted above water on platforms. From these it is piped ashore for
storage and transfer or stored in
floating storage before loading into tankers for bulk transportation. Except for the platforms to support the production equipment and the pipe-lines themselves, no equipment is placed on the seabed.With increasing depths of water andincreasingly severe weather in areas in which oil and gas finds are made, the first tentative steps are being taken to place equipment on the seabed in order to reduce the number, size and com-plexity of equipment support platforms. This is discussed in greater detail later in the paper.
Commercial production of
oil and gas
is currently limited to water depths not exceeding 350 feet. This is well within practical commercial diving depths for temperate and tropical waters and has been so for some considerable time. The present depth record for diving is 1500 feet. This was a simulated dive carried out at the Royal Navy Physiological Laboratories in a chamber. The open sea depth working record stands at 780 feet for extended periods under water, when some typical workwas carried out on a dummy
wellhead off the coast of Corsica.Exploration drilling operations can and do take place
in much greater depths. Without making an exhaustive search, the deepest commercial drilling known to the author at the time of writing was 1200 feet. On a recent scientific mission
a core was recovered from the mid-Atlantic by drilling,
after re-entering the hole in a water depth of 16 000 feet. It should not be inferred that oil and gas wells can be drilled commercially at these depths, but it is a beginning. This does, however, highlight the fact that production techniques rather than drilling at present limit, and for the foreseeable future will limit, the water depths in which the production of hydrocarbons can be commercially undertaken.
In the following paragraphs, the author has assumed the widest possible definition of the word 'submersible'any unit controlled directly or indirectly by
man to carry out
specific tasks or observation under water. On the basis of this definition, submersibles can be split into the following main types:stationary work chambers, towed submersibles, tethered submersibles, free swimming submersibles; * Technical Manager, Seal Petroleum Ltd.
8
Deift University of Technology
Ship Hydromechanics Laboratory
Library
Mekelweg 2, 2628 CD Deift
The Netherlands
Phone: +31 15 2786873 - Fax: +31 15 2781836
THE POTENTIAL USE OF
UNDERWATER
VEHICLES IN THE OIL AND
GAS INDUSTRIES
R. I. Walker, T.D., M.A., M.I.C.E.*
and where manned, the above categories are sub-divided into wet (diver transport) and dry (with a life supporting atmosphere) versions.
No matter what the form of the
submersible, all are dependent to a greater or lesser extent on a surface support vesselgenerally to a greater extent. In the past, and to somedegree now, owners and operators of
submersibles haveturned a blind eye to the need for adequate surface support with their submersibles, or have at the eleventh hour impro-vised one. One can well appreciate that reasons for doing so are financial. An effective surface support unit is likely to ex-ceed in value the submersible which it services. The handling system for recovering and launching a submersible ïs at the heart of the surface support vessel andone does not have to be a mariner to appreciate and envisage the problem of
recover-ing a submersible, weighrecover-ing in all probability five tons or more, in any sort of sea, and of landing it securely on the deck of a vessel.
The extent to which a submarine vehicle can be used, and hence its cost, is dictated by the number of effective working hours that such a vehicle can be used. Looking at sea states in the North Sea, admittedly a badarea, weather data collected over the past decade shows the following significant wave heights to be expected throughout the year:
Less than three feet 16 per cent Less than five feet
Less than eight feet 64 per cent86 per cent Less than twelve feet 95 per cent In the writer's experience, there are few vehicles that can be operated in seas of wave height more than four feet, and none that can be operated inseas of six feet wave height. Operators, not unnaturally, claim appreciably higher figures and may be on the way to increasing these limits with novel single purpose vessels designed purely as a mother vessel.
It is pertinent to review the range of jobs to be performed under water by the industry. First comes seabed survey for exploration drilling
operations. This, by the very nature
of the work, can take place in water depths beyond existing commercial production depths. At this stage information is only required on the anchor holding ability of the seabed in the case of floating drilling units and on seabed bearing capacity in the case of bottom supported barges. Additionally, both types require a general topographical survey of the seabed prior to positioning for drilling, for the presence or otherwise of obstructions, including wrecks. This data can be obtained more economically by indirect methods of echo sounding, side scan sonar, and shallow penetration high resolution continuous seismic profiling. Shallow-scrape samples of seabed material can also be more easily obtained from other than submersibles with their present range of sampling devices. The extent of seabed and soilsmechanics information required for permanent structures on the seabed is so great in detail that the current range ofsubmersibles and accessories has little part to play.The Potential Use of Underwater Vehicles in the Oil and Gas Industries
In the Gulf of Mexico and the North Sea and other
areas heavy in traffic, an attempt has been made to bury a pipeline, or rather to dig a trench around the line. The trench subsequently fills and buries the line by the normal processes of deposition. The submersible has no part to play in these areas, other than to check that the line is covered and remains covered. In some areas pipelines are laid directly on the hard seabed without any attempt at covering. It is probably only in these areas, on the rare occasions that a pipeline inspection
is judged to be necessary, that a
sub-mersible truly becomes competitive with diving and indirect methods of survey.It is in the area of pipeline repair, maintenance and installation that the first category of submersiblethe dry ambient pressure work chamberhas already found a use in the oil industry. A number of pipelines have been repaired
by welding T-junctions cut in and made to pipelines previously laid. Similar static chambers have been used for the repair and inspection of platforms already installed. Access to these work chambers is made by working divers, who enter the chamber through a water lock. The chambers are initially installed from a surface support vessel, and throughout their utilization on the bottom, they are powered and life-support maintained from the surface.
One or two partially successful attempts were made in the early sixties to visually survey unburied pipelines by
divers towed along from a bottom supported or swimming towed vehicle. Due to the problems of communication between divers and the surface these were only partially successful, and the author knows of no recent attempts to make use of towed submersibles for pipeline inspection. This paper does not discuss towed mid-water or bottom supported instrumented "fish" used in indirect survey methods, such as
side scan sonar, magnetometer. etc. These methods are exten-sively used by the industry.
With the exception of tethered vehicles, which are discussed in the paragraph dealing with the future trends in the industry, free swimming vehicles have found limited use in the industry and in the author's view will continue to do so although in number they form the majority of vehicles. They have been used, however, in emergencies, where
con-ventional means have failed. Their prime virtues are in the search and observation role where, hopefully, the industry has few needs for such operations which are associated with
emergencies and malfunctions. One or two cases of the
successful recovery by submersibles of high cost equipment lost or dropped from a drilling rig have been recorded.Turning next to the future of the industry, the author believes purpose designed, special purpose tethered sub-mersibles will have a part to play. With the move of the industry into deeper water, there will come a time when it
is more economic to carry out the simplest control
production functions directly on the seabed. It is for servicing these units and carrying out low frequency control functions that tethered submersibles will be required. These submersibles are unlikely to look like the conventional fish-shaped vehicle. They will be handled from the surface support unit, guidedto a specific location on the seabed where they will be
required to perform some pre-planned specific task such as operating valves designed for this purpose as opposed to those designed to be operated by man, replacing whole sub-systems and assemblies for maintenance on the surface, thus transferring men to a dry controlled environment where tasks can be performed below the water. The precise water depth in which it becomes economic to transfer these func-tions below water is at present the big question mark in the industry. It is, however, significant to record that there are five commercial organizations working to this end. It is theauthor's view that for some functions this
critical waterdepth has been reached.
From the foregoing paragraphs, it should be clear why subsea vehicles have been little used in the past by the oil industry. Firstly, the industry generally requires to "do rather than see". Submersibles to date have been built to observe rather than to perform a specific task.
Except for exploration drilling, as opposed to exploitation drilling, all work for the oil and gas industry has taken place in water depths in which commercial diving services are available.
Even so, all commercial production at present relies on platform mounted surface production units. There are a few quasi-commercial subsea production exercises being
con-ducted, such as the ADMA Subsea Production Scheme. These, however, are information gathering exercises directed
at future production systems.
The chief virtue of conventional submersibles are their mobility. With the exception of pipelines, the oil and gas industry requires to work at specific locations rather than
over a wide area.
ACKNOWLEDGEMENTS
The author would like to acknowledge the assistance in writing this paper from. his present colleagues and his former ones, with the British Petroleum Company Limited.
