fenders design

Berthing structures for large oil tankers

synopsis The ease with which oil can be distributed allows considerable latitude in the choice of sites for oil terminals. This has led, in the post-war period, to the development of natural deep water harbours and to the use of crude oil tankers with displacements well in excess of 100,000 tons. The berthing of such vessels has called for reconsideration and amplification of design data which were previously largely empirical. Recordings of velocities and impacts of vessels berthing at British Petroleum’s crude oil terminal a t Finnart in Scotland are assisting in the rationalization of fender design, and have also provided a measure of the hydrodynamic mass associated with a berthing vessel. The amount of energy which has to be absorbed when large vessels berth even at modest velocity has led to the introduction of long stroke fenders and flexible berthing dolphins of large deflexion in order to protect both the vessel and the berth. Careful attention is also given to the disposition and strength of mooring facilities due to the magnitude of the wind forces on unladen tankers. These factors, together with other considerations peculiar to the design of berthing structures for large tankers, are described.

The Report of the Committee of Inquiry into the Major Ports of Great Britain, published in 1962, des- cribes the oil industry as a prime example of the vertically integrated industry, in that one and the same company will prospect for oil, extract it from the earth, transport it in its own or chartered tankers, process it at its own refinery and sell it through its own marketing organiza- tion.

With regard to the import of petroleum into this country, the Report confines its remarks to considerations as to whether existing and potential terminals are likely to cope with the requirements of the country in the next 10 to 20 years.

The broad conclusion which it reaches is that the industry, in co-operation with port authorities, will be capable of expanding existing terminals and developing new ones in accordance with the needs of country and industry.

This conclusion no doubt results from considerations of the past record of the industry in developing import facilities in step with the rapidly expanding consumption of oil in the country. This has been especially noteworthy during the past 10 to 15 years with the development of major oil ports at Milford Haven, Finnart, Fawley, Tranmere, the Isle of Grain, Thames Haven, etc. Many of these, it will be noted, are apart and in some cases remote from industrial areas.

Siting of Terminals It will be inferred from the foregoing that terminals for the discharge of tankers can be relatively independent of established port facilities. This is due in the main to the

Table 1--Tanker Dimensions

*G. E. DENT BSc(Eng) AMlStructE AMICE

ease of distribution of oil, either by pipeline or by reshipment by coastal tankers. The industry is, therefore, in the somewhat fortunate position of being able to take the maximum advantage of natural deep water harbours in which it can construct its own self-contained terminals. The immediate proximity of industry and marketing centres is not essential, although of course this would be ideal.

Similar considerations apply generally to loading terminals in oil-producing countries. Sites must naturally be within reasonable distance of producing fields, but the most advantageous natural harbours can be selected over a fairly wide area.

Sizes of Crude Oil Tankers The average size of all foreign-going vessels has seen a steady increase in the past decade but, due to the relative independence of oil tankers of depths of water at established ports, the increase in the case of crude oil tankers has been by far the most rapid. Not many years ago a vessel of 30,000 tons dead weight was commonly referred to as a ‘ super tanker ’. Now there are vessels of over 100,000 tons dead weight in service and many more in the range of 50,000 to 100,000 tons are under construction. An indication of the size of such vessels can be obtained from Fig 1, which shows a tanker of 85,000 tons dead weight, with attendant tugs, approaching British Petroleum’s Finnart Terminal in Loch Long, on the West Coast of Scotland.

The economics of bulk transportation are beyond the scope of this paper, but it may be of interest to record that the unit cost of transportation in a vessel of 40,000 to 50,000 tons dead weight is approximately half of that in vessels of 15,000 to 16,000 tons. There is, therefore, plenty of incentive to use the largest possible vessels on all routes.

Tanker Dimensions Table 1 sets out the approximate leading dimensions of vessels ranging from 12,000 to 100,000 tons dead weight. The figures most relevant to the design of berths are draught, displacement tonnage and windage areas.

Approaches to Terminals The minimum depth of water required in approaches to terminals naturally depends on the exposure and incidence of swell in the area. The minimum operational requirement is a bottom clearance equivalent to 10 per cent of the draught of the largest vessel using the termi-

I Tanker Approx. Overall Beam Draught Freeboard Maximum dead weight displacement length laden laden windage area

8,000 12,000 420 56 25 -0 5 . 0 14,250 16,000 2 1,900 547 69 30 -3 7 -6 19,000 32,000 42,500 665 86 35 -0 11 -6 34,000 50,000 65,000 760 97 41 -0 13-0 41,000 70,000 90,000 S15 113 44 -0 14.0 44,390 100,000 126,500 918 128 49.0 17 * O 48,400

* Senior Civil Engineer, British Petroleum Co. Ltd.

Paper to be read before The Institution of Structural Engineers at 11 Upper Belgrave Street, London SW1 on Thursday 13 February 1964 at 6 pm.

THE STRUCTURAL ENGINEER FEBRUARY 1964 No 2 VOLUME 42 39

Fig l-85,000 tons dead weight tanker approaching Finnart

nal. An additional allowance is necessary in exposed areas and where channel dimensions are such that a vessel will squat unduly when traversing the channel. Where much dredging would be involved in providing access a t all times, an acceptable compromise is to restrict the largest vessels to entering on any high tide.

Types of Crude Oil Loading Berths The ease of distribution of oil, to which reference has already been made, is not restricted to the land, and the simplest loading terminal consists of a buoy berth served by a submarine pipeline with flexible pipes a t the sea- ward end for connecting to a tanker’s manifold. Buoy berths are not always adequate, however, and if they were little structural design would be required at the terminal. The inherent disadvantages of this type of berth are the limitations in respect of the number and size of hoses, loss of time in awaiting a suitable state of the tide to berth, excessive maintenance of flexible hoses and general difficulties of communication.

Some of these difficulties are overcome by the provision of an island berth, again served by submarine pipeline, and such berths are now becoming quite common where the required depths of water exist only at a considerable distance from the shore.

For maximum efficiency, however, a piled approach serving one or more berthing heads is desirable. Experience has shown that the efficiency of this type of terminal, measured in terms of annual throughput, is much greater than that of a buoy berth.

Types of Crude Discharge Berths Full jetties with road approaches are almost invariably provided at these terminals. Crude oil is not infrequently discharged direct to a refinery, in which case the same jetty may be equipped for receiving crude oil and for loading products tankers. The large number of pipelines involved in the latter operation makes submarine lines impracticable and the requirement, in cooler climates, for heating and insulating some of the pipelines is also an important consideration.

Layout of Berthing Facilities The layout and design of buoy berths is outside the scope of this paper and structural design as such arises only in the case of island or jetty structures.

All cargo connexions on an oil tanker are grouped conveniently in a position approximately amidships. The basic requirements of an oil berth, therefore, consist solely of a small central platform on which manifolds and gantry equipment can be mounted, together with protective structures alongside which a vessel may berth, and adequate mooring dolphins to hold the vessel safely alongside. It is relevant to note that the integral nature of the industry, to which reference has already been made, involves the designer in giving consideration to the forces imposed on both vessels and structures. No difference of opinion therefore arises as to whether fendering is intended to protect the jetty from the vessel or vice versa. This explains why fendering tends to receive greater attention at oil berths than is generally the case at most conventional berths.

The correct orientation of a berth relative to the tidal currents in the area is of very great importance to the design engineer, and detailed site observations of direction and velocity of currents are essential before a berthing line can be established and design criteria decided upon.

Design Data for Fendering The design of fendering is the most important, and unfortunately the most complex, problem in the design of a berthing structure. It involves consideration and assessment of berthing velocities, the mass of a ship and the water moving with it, the likely and allowable deflexion and the stress in the ship’s hull, the desirable deflexion and thrust of the fender and even the absorption of part of a ship’s energy by heat and by movement of jetty piles in the sea-bed.

These factors in the past have been assessed chiefly on a combination of faith, hope and experience. The strength of the hull of the large modem tanker is not

40 VOLUME 42 No 2 FEBRUARY 1964 THE STRUCTURAL ENGINEER

greatly in excess of its much smaller predecessors, however, and it is obvious that the safety of both vessel and berth depend upon the restriction of the berthing thrust to reasonable proportions.

In view of this British Petroleum has installed equipment at its Finnart Terminal to measure both the energy and thrust from vessels berthing at the larger of the two jetties. Velocities of impact are also recorded in order to assist in theoretical analysis, and to enable the results to be adapted with reasonable confidence for design purposes at other sites.

These recordings are now providing useful data for future designs and each berthing is proving to be amenable to analysis on the following basis:-

where E is the measured energy absorbed by the fender, v is the measured velocity, W is the displacement tonnage of the ship plus

the hydrodynamic mass, CS is a ' softness ' factor dependent on the

relative elasticity of the fender and the ship's side,

CE is an eccentricity factor which takes into account the fact that part of the kinetic energy is retained by the berthing vessel (the vessel swinging after impact) when fender contact is not opposite the centre of mass of the vessel.

Observations in respect of the point of contact at any given berthing provide the necessary data for CE to be evaluated. In addition the reasonable assumption may be made that the distribution of energy between the ship and fender at contact is directly proportional to deflexions. A direct indication of hydrodynamic mass can therefore be assessed from each berthing record. Observations to date suggest that the combined mass of a vessel and its 'envelope ' of water, at sites comparable to Finnart, is 1 -3 times the displacement tonnage of the ship, i.e. the hydrodynamic mass is 30 per cent of a vessel's displacement when berthing approximately broadside.

This, however, is purely of academic value. For design purposes the overall energy requirement for the fendering systems is of far more importance than the analysis of the individual contributory factors, and this can be readily assessed from the data. Over some 75 berthings recorded at Finnart to date the energy absorbed by the fenders has varied between zero and 25 in. tons per 1000 tons displacement, although there has been one exceptional recording of 112 in. tons per 1000 tons. It is admitted that the data are not yet as extensive as could be desired in view of the fact that the jetty will, it is hoped, survive some 3000 berthings during its lifetime and the upper value of readings so far recorded will certainly be exceeded. Pending the accumulation of additional data it is therefore necessary to apply a method of estimating extreme values over a given range from statistical data over a smaller range. Such a study indicates that for 3000 berthings at Finnart

l=-- - - l \

SIDE TRANSVERSE

L- - - 1\ BOTTOM T R A N S V E R S E

BOTTOM SHELL I . IO'

Fig !&-Section th~ough hull of a tanker (725 f t 0 in . , 104 f t 0 in., 52 f t 6 in . , draught 39 f t 0 in., aP$rox. 50,000 tons dead weight, typical wing tank structure)


the maximum normal fender impact is likely to be 37 to 38 in. tons per 1000 tons displacement.

These recordings and assessments refer of course to controlled berthings only, during which vessels are brought to a virtual standstill off the jetty and warp themselves in on their moorings with added assistance from attendant tugs.

Unfortunately full details are not known of the single exceptional impact to which reference has been made. It is known, however, that the vessel was by no means out of control.

It would, therefore, seem necessary in design to make some provision for small errors of judgment, or to cover for instance a case where a vessel is affected by a sudden squall at the time of berthing.

Pending the accumulation of additional data the following appears to be a realistic provision, based on the dead weight tonnage of the largest loaded ship which is to use the jetty:-

(1) an energy capacity of 40 in. tons per 1000 tons dead weight in main fenders without exceeding normal working stresses;

(2) a reserve capacity of a further 40 in. tons per 1000 tons dead weight before fenders are fully compressed or yield point stresses are reached in the case of independent flexible dolphins;

(3) where fenders form part of the main jetty structure a further resistance to thrust after the fenders have been fully compressed.

Dead weight tonnage is used in the above in preference to displacement tonnage purely for convenience. The ratio of dead weight to maximum displacement for large crude oil vessels is approximately 1 :l .3.

It will be noted that on this design basis working stresses are likely to be exceeded if the anticipated maximum normal impact of 37 to 38 in. tons per 1000 tons displacement is associated with the largest vessel that will use the berth. Such a combination is extremely unlikely, however, and should it eventuate the energy transmitted would still be comfortably within the elastic limit of the fendering.

This basis of design can be justified only at a site of equivalent protection to Finnart and also where fenders are comparably spaced. Until more data are available a t less protected sites the basis of design remains a matter of engineering judgment, although it is considered that Finnart may reasonably be used as a ‘ control ’. Strength of Ship’s Hull to Withstand Berthing Loads A typical section through the hull of a tanker is shown in Fig 2. The spacing of transverse frames varies between 10 ft 0 in. and 12 ft 6 in. and, as might be expected, the dimensions of the horizontal bulb angles spanning between these frames vary from a maximum towards the bilge to a minimum at deck level. Unfor- tunately the maximum fender thrust is’ likely to occur in the region of, or a little above, the laden water line where the strength of the hull is near its minimum. For this reason it is necessary for the plan length of the contact face of a main fender to be at least 12 ft 6 in. in order to prevent possible overstressing of the hull longitudinals in bending.

The transverse frames are considered to be capable of resisting a force of approximately 175 tons at normal working stresses, and ideally this figure should be used as the maximum thrust from a jetty fender under normal working conditions. Thrusts in excess of 300 tons could under certain circumstances cause permanent deformation of the hull.

Detailed Fender Design Several basic types of fender have been developed to meet the rather stringent requirements of high energy capacity a t low thrust. Large deflexions are obviously

necessary. Assuming a constant rise of thrust with deflexion, a working fender capacity of 4000 in. tons may be obtained with a deflexion of 40 in. and a thrust rising to 200 tons. The additional 4000 in. tons capacity would be obtained at a total deflexion of 569 in. and a thrust of 282 tons, which is just within the ultimate allowable thnist calculated for the ship’s hull.

Flexible dolphins formed of steel piles cantilevered from the sea-bed provide this load-deflexion characteris- tic. Such a dolphin, consisting of five cylindrical high tensile steel tubes and having a capacity of approximately 4100 in. tons at yield, is shown in the background in Fig 3. The capacity of such a dolphin can, of course, be increased in direct ratio to the number of tubes, although the deflexion will be constant, and the thrust will increase pro rata to the number of tubes.

Fig %Flexible berthing dol9hin

Cylindrical rubber blocks arranged in series behind a fender head can also be designed to give considerable deflexion. Fenders a t the Finnart Terminal are of this type (Fig 4). The load-deflexion graph is somewhat less favourable (Fig 5) and a greater deflexion for equivalent capacity is required compared with a flexible piled dolphin. It will be noted that the thrust associated with a capacity of 4000 in. tons is 270 tons. This particular jetty will resist such a thrust comfortably, but it is harder on the vessel than the berth equipped with flexible dolphins shown in Fig 3.

Gravity fenders provide a load-deflexion graph somewhat similar to the fenders incorporating rubber blocks in compression. Such fenders at Kuwait each consist of four 75-ton units suspended in such a manner as to lift 28 in. whilst retracting 48 in. Heavy gravity fenders (Fig 6) are also provided at Angle Bay in Milford Haven and both these and the Kuwait fenders are giving satisfactory service. This type of fender has the disadvantage, however, of very rapidly rising thrust at the later stages of deflexion.

The choice of the type of fender for any particular locality depends to a large extent on local conditions. For instance, in poor ground conditions the use of flexible steel dolphins may be quite inappropriate. One considerable advantage of the flexible dolphin, however, is the reserve of energy in the plastic range. Under a heavy impact, in the accident class, it is therefore likely that a vessel will be halted without damage to its hull, as the thrust would remain constant during plastic


L. W. 0. S. T ._ -

l.---

: . -

I r

SECTION ON LINE A-A. (OUT FOSITION)

I

I l \ ,-+x

j \DRY RAMMED CONCRETE IO'-b" __ct

SECTION ON LINE B-B

Fig 4-Main fender at Finnart

THE STRUCTURAL ENGINEER FEBRUARY 1964 N o 2 VOLUME 42 43

7,500 i 7,O 00 .-

0 e

l - 7 -----+ 400

0 IO 20 30 40 STROKE (INCHES)

50

DESIGNED FOR VESSEL OF 65,000 TONS DEADWEIGHT

WITH OCCASIONAL REQUIREMENT FOR 100.000 TONS DEADWEIGHT

Fig 5-Load-deflexion graph-Finnart fender

deformation of the dolphin. Subsequent replacement of the dolphin would not present a serious problem either practically or financially.

In all types of fendering it is necessary that the fender face should be capable of conforming to the angle of the ship’s side when berthing and also of resisting forces parallel to the berthing line. Finnart observations have shown that the so-called broadside berthing has an average inclination to the jetty face of 1 in 16.

Such angled berthings are easily accommodated at Finnart as the fender heads are designed to swivel. Gravity fenders also meet the requirement reasonably well in view of the fact that the several units forming the fender head can deflect independently. Flexible dolphins also take up the alignment of the ship’s side without difficulty, although the design must take into account the torsion on the dolphin caused by eccentric loading.

One important aspect of fendering design has not yet been mentioned, i.e. spacing of fenders. Theoretically the required energy capacity decreases as the distance between fenders increases. The lines of a tanker are

such, however, that the parallel side extends for less than half of the ship’s length, and the vessel must lie against the fenders when in berth. Main fenders are therefore sited at a distance apart equivalent to approximately 0.3 of the length of the largest vessel likely to use the berth, with secondary fenders of reduced capacity sited intermediately to support smaller vessels within the parallel middle body.

Design Data for Mooring Dolphins Those who have seen a modern oil tanker in unladen condition will readily appreciate the very considerable wind forces involved when a vessel in this condition is exposed to broadside winds. Such an occurrence is not uncommon as terminals frequently afford little shelter from wind, especially as there are no transit sheds to provide protection as there are on dry cargo wharves.

Most vessels in excess of 50,000 tons dead weight have some permanent ballast tanks which assist to some degree in minimizing the windage area, but the forces are still considerable, as can be seen from Table 2. Pressures

f - M

Fig 6-Gavity fenders at Angle Bay

are taken from the British Standard Code of Practice for Buildings CP3 and the velocities are those for a one-minute wind at an elevation of 40 ft.

Efficient resistance to offshore wind on a vessel alongside can be provided only by breast mooring ropes and the appropriate mooring dolphins should be so disposed that these ropes are as near as possible to 90" to the berthing line (Fig 7-the layout adopted at Finnart). The head and stern ropes, being angled to the berthing line, also assist in resisting the wind force, and, in the case of the largest vessels, secondary breast ropes can be taken to dolphins which are provided for the head and stern ropes of smaller vessels. As a general rule all dolphins are designed to resist one-third of the total wind forcegiveninTable2withafactorofsafetyof 1 -7inrespect of overall dolphin stability. As a precaution the ultimate load thus obtained is checked against the breaking load of the mooring ropes provided for the ' design ship.

Dolphins are located at a distance of 150 to 200 ft behind the berthing line in order that all moorings shall have an adequate length. This guards against snatching in gusty conditions and also limits the amount of mooring attention required during the loading or discharging of a vessel. It will be appreciated in this respect that the deck level of a vessel may well vary 50 f t or more during the period alongside due to a combination of tidal variation and change of draught.

The head and stern moorings perform their chief duties during the actual berthing operation in that they restrain longitudinal movements whilst the vessel is being manoeuvred into such a position that her manifolds are correctly located relative to the jetty connexion. Once the vessel is fully moored the head and stern ropes take little load, as her shorter spring ropes, attached to bollards on the face of the jetty head, prevent movement in a longitudinal direction.

SECONDARY FENDERS

Fig 7-Berthing layout at Finnart

rJatt. CONTOURS BELOW CHART DATUM.


Table 2-Wind Forces on Tankers

Tanker (max. one minute (max. one minute dead weight

Exposure D Exposure C

mean wind speed at

Total wind Pressure Total wind Pressure

40 f t height = 72 mph) 40 f t height = 63 mph) mean wind speed at

force (tons) (lb/ft2) (tons) (lb/ft2) (tons) force

8,000 11 - 3 72

360 18-2 280 14.1 70,000 277 18-2 214 14-1 32,000 129 15-2 99 11 - 6 16,000 94 14.8

100,000 14 -3 310 18-6 405

-

Design of Mooring Dolphins The sole requirement of a mooring dolphin is to resist horizontal or near horizontal pull. Unlike berthing dolphins, no deflexion under load is required. In fact deflexion is a disadvantage if the dolphins are provided with a piled access walkway.

I t is usual, therefore, for these dolphins to incorporate raking piles to take tension and compression. Vertical piles, in association with raking piles, are theoretically uneconomic but are sometimes necessary to provide an initial platform through which the rakers can be driven. Pile rake is limited by practical considerations and assuming a rake of l :Z& the compression and tension forces are approximately 22 times the horizontal pull, Provision of the necessary tension resistance often presents difficulty, especially where pile penetrations are small. The frequency with which this problem arises is no doubt due in part to the fact that where deep water is encountered at a reasonable distance from shore it is quite likely that the hard sea-bed will be found to have little overburden.

In such circumstances the required tension resistance can be partly offset by the provision of a heavy capping deck to the piles. Alternatively tension piles can be anchored. At Finnart a novel but economic solution was reached by tipping bunds of surplus rock on the shore- ward side of the dolphins and providing horizontal ties and struts from the dolphin deck slab to anchorages in the bunds.

All mooring dolphins should be accessible by launch and simple fenders protecting access ladders are essential.

Design of Jetty Head and Approachway It has already been stated that the loading or discharge of oil cargo presents a far simpler problem than that of handling dry cargoes. In its simplest form the only requirement is the provision of means of connexion between ship and shore manifolds. The deck area required is not great and, with the exception of the hose gantry, superimposed loadings are small.

Where fenders are housed on the main jetty structure the required lateral strength is determined by fender thrust. Otherwise horizontal loading is restricted to the pull from spring bollards and expansion forces from pipelines, which are normally anchored on the jetty head. As a general rule all these horizontal forces are resisted by raking piles, as indicated in Figs 8 and 9. In the latter case the jetty is of the finger pier type and berthing thrusts may therefore be experienced from either side.

In layouts incorporating berthing dolphins advantage can be taken of their lateral strength, and spring bollards may be mounted on the dolphin decks. Pipe thrusts then constitute the total horizontal loading on the jetty heads and where these are not great raking piles may be provided on a purely nominal basis.

At Finnart (Fig 7) deep water lies so close to the edge of the loch that it was possible to transmit horizontal

loads direct to shore through reinforced concrete struts. The jetty, therefore, consists of a horizontal frame propped up by piles driven to, but not anchored in, the steeply shelving rock bed of the loch.

Approachways consist of a roadway and pipetrack spanning between pile bents. Where the approach is long only a single traffic lane is provided together with traffic passing bays at approximately 500 ft centres. The pipelines span unsupported between the bents, which may be as much as 50 ft apart, small pipes being given support from the larger ones as necessary. A typical approach structure is shown in Fig 10.

Materials of Construction It will probably have been noted by those engineers who have visited oil terminals that hollow steel piles are frequently used. This is due not to lack of faith in reinforced concrete but to the fact that rapidity of construction favours steel. Underwater corrosion is no longer a serious matter due to advances made in cathodic protection techniques and durability of pile coatings. Where construction time is not critical, or where availability or price of steel piles is unfavourable, reinforced concrete piles are used for verticals and compression rakers, and steel piles only as tension rakers.

Reinforced concrete comes into its own in the deckwork. Terminals frequently include long jetty approaches which are admirably suited to precast and prestressed construction. High quality work and rapid construction are both possible.

The author is not in favour, however, of prestressed work in the jetty heads due to the complications that would arise in repair in the event of damage.

Cargo Handling In design of jetty and shore pipelines an economic balance has to be reached between the capital cost of the pipelines and the time of turnaround of the vessel. The cost of maintaining a typical vessel in service may be well in excess of L1500 a day, with the result that pipelines are frequently large in order that loading and discharge rates may be high and the vessel time alongside short. The largest crude oil vessels are able to load and discharge at approximately 10,000 tons an hour and turnaround time, allowing for deballasting at loading terminals, seldom exceeds 24 hours.

If all crude tankers were of 100,000 tons dead weight and all turnarounds were achieved in 24 hours, the annual capacity of one crude berth at 60 per cent

, J

,, / I I

Fig LKwanana-section through jetty at fenders


occupancy would therefore reach the phenomenal figure of 22 million tons per annum. At present it is seldom that the average tanker size at any berth exceeds 30,000 to 40,000 tons, with the result that the throughputs are unlikely to exceed 6 to 8 million tons, but this figure may well be doubled in the next decade.

In order to achieve high loading and discharge rates unrestricted flow of oil must be possible. This accounts

for the large flow boom structures now being used on jetties (Figs 1 1 and 12). The heights of the structures are determined by tidal rise and change in draught and must allow coupling up of hoses at the most adverse combination of draught and tide. The installation shown in Fig 12 is that of an island berth in the Arabian Gulf. The operating platform in this instance is elevated some 30 f t above sea level in order to be clear of the

Fig lO-A$@oach structure at Angle Bay


maximum storm wave in the area. The platform manifolds being installed at this height enables the length of loading boom to be kept to a minimum.

The booms serve the double purpose of conveying the oil and adjusting the height of the flexible connexion to the vessel. Four hoses and booms are normally provided at a crude oil berth. These are usually of 10 or 12 in. diameter but 16 in. is likely to be a common size in the near future. The normal flow rate through one, 10 in. hose is approximately 1250 tons an hour and through a 12 in. hose 1800 tons an hour, at an oil velocity of 30 f t a second. An increase in hose size is therefore called for in order to take full advantage of the potential rates of the largest tankers and also to reduce the pressure drop through the hose structure.

allowed within the same security zone whilst dangerous cargoes are being handled. Special tools of non-sparking metals are provided for all work within the restricted area.

It is also necessary to take rigid precautions to ensure that all electrical equipment and all pipelines and structures are adequately earthed. Where jetty structures incorporate concrete piles, special earthing conductors are required to provide a direct path for any lightning discharge.

Static charges generated by the flow of oil through the loading system leak away to earth through the jetty earthing system or through the vessel and water.

Insulating flanges are provided at the jetty end of all flexible hoses in order to guard against the possibility

Fig 11-Flow booms at Finnart

The replacement of hoses by articulated swivel jointed pipes is now an established but far from universal practice as there are many, the author included, who are reluctant to depart from the principle of having one naturally flexible unit between a rigid structure and a buoyant and ever-moving vessel.

In order to retain complete control of loading at a high rate it is essential that remote control of loading pumps is possible a t a moment’s notice from the jetty; also that valves are so located on the jetty head and approach that the flow of oil to the jetty in an emergency can be stopped without delay.

Safety of Operations Stringent safety precautions are obviously necessary in the handling of petroleum, and all electrical equipment within a radius of 300 ft from the ship-shore connexion is normally flameproof. The use of naked lights is obviously not allowable and only authorized persons are permitted in the jetty areas. No craft or vehicles are

of incendive sparking at the manifold when connecting or disconnecting. Until recently it was universal practice to earth vessels in berth through the jetty structure but this has been discontinued due to the frequent use of cathodic protection for the jetty structure.

Fire-Fighting Facilities In addition to the elaborate precautions taken to minimize fire risk it is necessary to provide adequate foam equipment for emergency use. In the author’s opinion this is best located on standby craft, either on tugs or specially designed fire floats, full manoeuvrability of equipment thus being obtained. Even with such provision, however, jetty equipment in the form of fixed or portable monitors is desirable.

Emergency Removal of Ships from Jetties The provision of quick release hooks, which can be tripped even when moorings are under load, is desirable on dolphins, especially for wire moorings. It should not


Fig 12-Flow booms at Das Island, Arabian Gz~lf

be assumed, however, that it is advisable for the vessel involved in an emergency to be removed immediately from the jetty. In fact it is considered to be sound policy to keep the vessel alongside, where full use can be made of all emergency appliances. At large installations, however, where several vessels are in port simultaneously, the quick removal of neighbouring vessels may well be necessary.

Prevention of Oil Pollution Discharge of ship’s ballast in port prior to loading can be a potential source of oil pollution, and this applies to products carriers at refineries as well as to crude vessels at loading terminals. Special deballast lines are therefore frequently provided and the vessel’s ballast is discharged by pipelines ashore to separating tanks. This procedure occupies a substantial proportion of a vessel’s time alongside at loading terminals.

Any oil which may leak from jetty valves and manifolds, together with surface water falling on the operational area of the jetties, is drained to sumps slung below the jetty deck. These sumps are discharged by automati- cally controlled pumps into the deballast lines (or separate slops lines) and the efluent is likewise taken through shore separating tanks.

Several types of boom have been developed to prevent the spread of an accidental leakage. One interesting type consists of an air bubble curtain, on the lines of the hydraulic breakwater, which has the unique advantage of containing oil within the enclosed area whilst still allowing the vessel to pass through the boom.

Summary The foregoing gives an indication only of how the oil industry in general is developing its berthing facilities to meet the ever-growing demand for petroleum and the rapidly increasing size of tanker. Port facilities constitute only a small link in the chain between the oilfield and the consumer, but the use of the very large vessels, together with adequate means of loading and discharge, plays an important part in the overall economic picture. Despite the advance in development of new forms of offshore facilities incorporating submarine pipelines, the services of the structural engineer will still be necessary to design jetty facilities, as for reasons of communication and overall control such facilities cannot be rivalled for efficient cargo handling and rapid turnaround of vessels.

Bibliography 1. McGowan, C. W. N., Harvey, R. C. and Lowdon, J. W.,

Persian Gulf ’, Proc. Inst. Civ. ETgrs., June 1952. ‘ Oil loading and cargo handling facilities at Mina al-Ahmadi,

2. Palmer, J. E. G. and Scrutton, H., The design and construction of Aden Oil Harbour ’, Proc. Inst. Civ. Engrs.; July 1956.

3. Murray, P. and Collett, D. N., ‘ Kwinana jetty , Proc. Inst. Civ. Engrs., December 1956.

4. Lackner, E. and Hensen, W., ‘ Principles for the formation of

example of the Wilhelmshaven Oil Jetty ’, XXth International discharging terminals for large tankers, illustrated by the

Navigation Congress, Baltimore, 1961. 5. Little, D. H., ‘ Some dolphin designs ’, Proc. Inst. Civ. Engrs.,

6. Little, D. H., ‘ Some designs of flexible fenders ’, Proc. Inst. November 1946.

7. Baker, A. L. L., Paper on fenders contributed to Section 11, Civ. Engrs., February 1953.

8. Wright, H. J., Cathodic protection , Proc. Inst. Civ. Engrs., Question 2, Pryc. 18th Int. Cong. on,Navigation, Rome, 1953.

April 1962. 9. Scrutton, H. and Donelan, B. J. O’C., ‘ Some features of the

Inst. Civ. Engrs., October 1963. civil engineering work in Cammell Laird’s shipyard ’, Proc.

10. Stephenson, R. A. and Barfod, 0. T., ‘ Tanker accommodation in the Mersey a t Tranmere ’, Proc. Inst. Civ. Engrs., October 1963.

A Fortran programme for the solution of skewed plates * J. B. KENNEDY BSc PhD

The following is a precis of a paper which is filed in the Institu- tion’s Library (reference X(6)). Copies are available for borrowing or can be consulted at the Institution.

Synopsis Acomputer programme, written in Fortran, to obtain an analytic Fourier series solution for skewed stiffened plates, loaded uniformly, is presented. Various combina- tions of parameters were tested. Convergency was ex- amined by comparing solutions with different numbers of harmonics. The paper may remind the structural engineer about the ease of communicating with an electronic computer through compiler languages.

Analytic Solution Fig 1 shows a skewed plate stiffened by edge beams, with 8 = skew angle, and 2a and 2b the skewed dimensions. For a plate with a uniform lateral load q, it can be shown that the solution for the deflexion function W in the fourth order partial differential plate equation in skewed dimensionless co-ordinates q and can be represented by:

W = D { A10 + A20q2 + A30t2 + A40 [q4’

* Assistant Professor of Civil Engineering, University of Saskatchewan, Saskatoon, Canada, and formerly Research Assistant in the De9artment of Civil Engineering, University of Toronto.

THE STRUCTURAL ENGINEER FEBRUARY 1964 N o 2 VOLUME 42 49

fenders design

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