Offshore,; Engineering by J.B. Cook, O.B.E., C.Eng.
The lecture dealt with the extraction of oil and gas from underwater sources, with particular reference to the seas around the U.K. It outlined some of the problems, some solution and some possible areas for development.
First the seismic survey aimed at evaluating the key characteristics and exploring the areas of uncertainity. Rapid improvement in computer power over the last few years has led to increasing accuracy and the development of quite reliable 3-dimensional representations. Results of the survey, and knowledge of others in the area, enable a decision on drilling to be made.
Thus the engineer starts with a knowledge of the uncertainties; the cost of a wrong decision is high. After the survey, we can assess:-
1. The uncertainties
(a) Size of field.
(b) Production rates, from each well and from the field as a whole. Also for each fraction, from methane to the heaviest crude.
(c) Drive mechanism. In the North Sea the most common is water injection to encourage natural lift .
2. Safety. Information required will include
(a) Wind and wave formation. North Sea structures .are much heavier than those in the American Gulf, where occasional storms are very severe, but there are long periods of calm.
(b) Seabed conditions. These include position of wrecks (7000 in North Sea) and results of “soil” testing.
(c) Water depth, currents at each depth, tidal conditions (the Hydrographic Survey)-.
and from these estimate
1. Production profile. Income depends on this..
2. Number of wells required.
3. Manning level.
4. Cost. Including that of full removal.
5. Development schedule.
These items are inter-dependent.
If the project goes ahead, each phase must be clearly set out to ensure proper control, avoiding detail before concept is accurately defined. The conceptual design, on which everything depends, can occupy 100,000 man hours for a North Sea Platform. Many of the parameters will be known in probabilistic terms only and Monte Carlo analysis is used to indicate optimum) coverage of a wide range of situations.
Rig types include:-
1. Concrete platform (max. depth 1,000 ft).
2. Guyed tower,.
3. Floating platform with single anchor (suitable to 600 ft.)
4. Catenary, anchored.
5. Tension leg. The hull, which is buoyant, does not,, go up and
down with the waves, but the leg tension varies (up to 1800 ft).
6. Sub-sea for greatest depths. : Likely to be used in future. Expensive. Best suited to the smaller field’.
The crane barge has revolutionised the off-shore industry. The one described has two cranes, each with a lifting capacity of 6,000 tons, giving a twin capability of about 10,000 tons. Draught 28 m. Living accommodation for 750 is provided. Accurate positioning, relative to fixed transponders on the bottom is obtained by computer control of propellers. Rapid arid accurate movement of ballast is essential
to maintain stability during lifts.
Use of barges has changed bias from concrete to steel for off-shore structures, saved up to 30% steel and weighted decisions towards fixed rather than floating platforms.
The modern computer has enabled overall solution of the structural design of what is, in effect, a Portal Frame in three dimensions. There are complications such as fatigue loading and temperature stresses caused by hot oil stored in the legs. One of the most complicated designs had 900,000 degrees of freedom.
Difficult fabrication at nodes can now be avoided by use of steel castings for sizes up to 30 tons.
Under-water pile drivers are commonly used, care being needed to avoid stressing the structure.
“Top side1’ design must provide for
1. Drilling
2. Fluid separation, remembering that composition of the extract can vary over the life of the rig.
3. Pumps.
4. Compressors. Gas 2,000 p.s.i,, rig to shore Reinjection 7,000 p.s.i.
5. Water supply. Untreated for drilling. Treated for reinjection (filtered to 2 microns and deareated; to 5 p.p.m.) and domestic (reverse osmosis of evaporation) use. ‘
6. Power generation. Normally at 60 Hz, the highest industrial frequency available from manufacturers. –
7. Supply facilities, e.g. helicopter pad and cranes for access : to boats, which are not normally moored alongside.
8. Material storage-..
9. Safety facilities
within an overall requirement for
1. Safety, bearing adverse weather in mind,
2. Continuous operation.
3. Cost of space,
4. Cost of manpower (say £70 per man per hour)
A flow diagram for a Brent platform showed how the product constituents are separated, first to oil and gas, then to the different fractions.
Early experience led to a Project Management organisation emphasising
1. Single point responsibility,
2. A structured system based on Critical Path Analysis.
3. A review, by people not directly involved, at the end of each phase of the project.
In conclusion, it can be said that, though we now have the techniques to exploit most fields on the continental shelf, there is still a long way to go. Standardisation, computerisation, and equipment development can facilitate exploitation of smaller fields economically. Satisfactory operation has been achieved at depths of up to 7,000 ft. and extraction from sources under very deep water is a firm prospect.
Industries can learn from each other. Off-shore companies, for example can offer advanced Quality Control systems and can learn Value Engineering, particularly in the field of safety.
Off shore, branches of engineering combine, learning from each other’s experience. An off-shore engineer must have an appreciation of disciplines other than his own.
The lecture was followed by a film depicting the life of the crew on a North Sea oil rig in production.
Points which came out in discussion included
1. 40% – 50% of the oil reservoir under the North Sea is extractable
2. Companies normally reach mutual agreement on the division of output from a field, but there is facility for arbitration,
3. Rig design is usually for a safe fatigue life of 30 years. Re-use is unlikely, but the new crane barges permit removal of casings on the smaller rigs.
4. The project programme must cover “conception to removal”, consent procedures differ from those for nuclear plant, but ministerial approval is required.
5. Gas extracted has to be processed before suitable for use in the aircraft type turbines generally used on rigs. Dual fuel systems are rare.
6. Maximum off-shore distance is limited by helicopter range.
