The historical development of mooring has been concerned with the shipping business, and therefore reflects the operational practices of that industry. Mooring of offshore structures has no real historical precedent, and must meet different operational needs. So a new technical must grow out of the old. The commonlinking factor is the marine environment. Even the largest ships would normally anchor only in relatively calm conditions in a sheltered haven lying to one or two bower anchors and being free to swing about these anchors to lie in the position of least resistance of the combined effects of both wind and current.
In the situation where the weather deteriorates then the ship can reduce the forces on her moorings by using her main engine or by lifting her anchors and proceeding to sea until conditions improve. Moored offshore structures on the other hand using basically the same equipment have to maintain a constant position within relatively close tolerances irrespective of weather.
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They cannot be allowed to align themselves in the position of least resistance although due to position of least resistance although due to their symmetry this factor is not likely to be greatly significant. The rig must be restrained almost equally to all points of the compass, and therefore for any given prevailing direction the mooring system is largely redundant to the order of 75 per cent.
This in practical terms will mean anchor's cables being usefully employed at any one time each showing the forces to some varying degree dependent on their direction. The magnitude of these forces remains for the moment one of the great mysteries, since not a great deal is known of the ultimate or worst weather conditions that can be expected for any given location. The pioneers. The study of the problems posed by the oscilla- tory motion of already moored ships in Table Bay, South Africa, was entrusted to Dr.
Basil W. In his epoch-making pa- pers, published just a few years before the Suez crisis, he showed, just by resort to an analytical treatment, how long waves of height so small as to pass unnoticed can bring the ships to move in resonance with them. The rapid increase in ship dimensions since the Suez crisis and the consequent pro- blems this created, gave rise to further studies of the same nature by Robert C.
Russel and F. Kilner, regarding the be- haviourofmoored ships, and by Andre Pages, F. Vasco Costa and B. Saurin, in the study of berthing operations. Ayers and R. Baker, P. Callet, I;Descang and L. Eggink, Foster and R. Lutz, F. Vasco Costa and F. Visiol et al. At the XX Congress Baltimore, papers were presented by. Abbet and A. Levinton, M. Bars and M. Khales, M. Deschenes and M. Dubois, N. Diunkorvski and A. Kasparson, J. Gillespie et aI, A. Gonzalez and F. Agos, C. McGowan, L. Greco et aI, E. Lackner and W. Hensen, and F. Vasco Costa and J.
Agema et al, P. Fagerholm, L. Van Houten, S. Kastner and K. Wendel, A. Pages and A. Girardin, C. Steward and F. Wilson, John T. Russel, Antonio G. Portela and Jean Sommet; and on the berthing of ships by F. Vasco Costa, Jean Somrr. Saurin, John T. O'Brien, Andre P. Pages, Robert C. Russell and Albert Steenmeyer. Among the partici- pants who more actively participated in the discussions the names of S. Kastner, A. Torum, J. Lebreton, T. Lee, J. Rendle J. Gervasio , A. Leite, P.
Tryde, V. Bratianu and P. Giraudet are to be mentioned. At this meeting the use of physical models, of mathematic models, and the stochastic nature of the phenomena under study were discussed at length. Mentioned among the pioneers should also be O. Grim and Van Loewven for stUdies of the added mass concept. Institutions that promoted studies. Among the institutions that promoted the study of the problems posed by the bert- hing and mooring of ships the most active was certainly PIANC- Permanent International Association of Navigation Congresses.
Such subjects were not only discussed at the above-referred Congresses but as well at the XXII Congress held in Paris in and also at the Congresses in Ottawa and Leningrad.
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At those Congresses and at Committees especially created for the purpose, important roles were played by A. Dickson, J. Gervasio, A. Leite Chairman of the II Int. Oil Tanker Commission , J. Brolsma, S. Paape, E. Harlow and A. Furbother, among several others. Army Corps of Engineers at Vicksburg, the U. Keck Laboratory, Pasadena Ca. Among such papers special reference is due to those presented by Brendan F. Unoki and A. Oortmerssen thesis, presented in , entitled "The motions of a moored ship in waves". In Dr.
Wilson, G. Goulston, Ib. Svendsen, Walter W. Massie, James M. Keith, Ludwig H. Seidl, George H. Lean, J. Van den Bunt, James F. Wilson, J.
Randle, and F. Langeveld, A. Rendle and Roger Maari were parti- cularly active in dealing with the single point mooring pro- blem. Besides those already mentioned from oil companies staffs, J. Feben and D. Martin of British Petroleum are deserving mentioning. Recent Lmportant contributions. Among the authors that publi- shed recent contributions the following are noteworthy of rea- ding and of study by all those interested in the improvement of berthing and mooring operations; - regarding the statistical approach - I.
Svendsen, P. Tryde, G. Viggosson, D. Cooper, P. Tomlinsen and T. Fontijn, A. Vrigec, Ib. Svendsen, Shigeru Ueda, G. Birt, J. Moes and S. Holroyd, J. Wilson, L. Seidl and J. Fontijn, Paul Bastard, Eugene H. Harlow, A.
Advances in Berthing and Mooring of Ships and Offshore Structures
Brolsma, C. Lawrence, S. Nagai, Nikerov et al and T. Tryde, H. Vrijer and Shigeru Ueda. Fylling and Roger Maari. We are lucky enough to have as lectures at this NATO-ASI some of the authors mentioned, and you will hear from them how new means available, namely monitoring equipment, transducers and the computer, can be used to improve berthing and mooring op- eration and to render them safer, even under adverse local conditions. Forces applied by Displ.
Further, some methods and application of risk analysis to shipping safety issues will be presented. At last an introduction to some risk problems associated with offshore mooring will be given. The accident rate has gone down significantly over the years, but still there are high number of losses and casualties to ships each year. Information and statistics on marine casualty types, causes and location for the worldwide fleet is mainly collected by the Underwriters. In addition more specific data may exist in national statistics for some of the fleets of the main countries of registry. Ships are engaged in a multitude of trades with large variations with regard to size and age of ship, technical standard, sailing conditions, frequency of port calls, as well as crew qualifications.
One should therefore be careful about making to wide conclusions from general loss and casualty statistics. The average number of ships lost per year was Total losses Total tonnage losses Percentage Percentage No of of total of total Type of casu1aty vessels by type g. Average per year World tonnage, average The annual average loss ratio in tonnage terms are 0. The data in Table 2 includes Norwegian vessels and drilling units of 25 gross tons and above. Table 3 gives information on registered Norwegian vessels during the same period.
The loss rates as a percentage of the total Norwegian fleet is as follows:. Annual loss percentages No of vessels Tonnages. Total losses: 0. The lives lost due to marine casualties represent an annual rate of 0. Loss rate: Total losses: 0. In Tables 5 and 6 are presented data on various circumstances related to place and conditions associated with grounding and collision casualties for Norwegian registered vessels above 25 g. From the data in Table 5 and 6 it is interesting to see that most grounding and collision casualties seems to take place in clear weather with good visibility and calm and moderate wind conditions.
Table 5: Norwegian vessels partially lost by grounding or collision, by circumstance of casuaJty. Totally collision and grounding casulaties were reported with the following subdivision on types of casualties:. The average annual number of port calls during the years was If we assume the same traffic intensities during the other years for which the casualty data have been collected, the casualty rate per port call will be 7 x , i.
A subdivision of the casualties according to causes has been carried out and the resul ts are presented in Table 7. Table 7: Causes of Collision and Grounding casualties in the Elbe in the years Collisions Contact between damages Groundings ships Failure of bridge instruments 2 Failure of steering systems 3 4 3 Failure of propulsions 5 2 Misjudgement of situation 15 2 Erroneous navigation 10 3 3 Wrong speed range 7 2 Violation of "Rules of the Sea" 6 Human loss 3 3 Wrong communication 5 2 Low visibility - Fog 17 Windage 1 3 Current drift 5 2 I "Squat" effects 7 I o Failure of other ship 3 A voidance manoeuver 4 Acts of God 4.
Table 6: Norwegian vessels partially lost by grounding or collision. Circumstance of casualty by percentage. Groundings Grounding Collision Other collision! Circumstance of casualty and No. Total J00 66 79 25 27 9 Place of Casualty: Open sea 3. Weather conditions: Fog July and August have the lowest casualty rates. These methods include:. The two first methods are the most useful for estimating the probability of marine accidents. Fault tree methods are, however, useful in spill probability assessments for terminal operations such as loading and unloading of hazardous materials.
Each of these methods generally require some subjective treatment, based on experience and engineering judgement, for applications to specific sites or routes or for particular types of cargo or vessel design. The approach using historical casualty data requires two major assumptions:. This premise implies that human error has been and will continue to be the major contributing factor in the occurrencce of marine casualties, and that the basic nature of human error has not and will not change over the period of interest.
This assumption relates to the "base level" as defined for sailing procedures and currently established traffic measures. By introducing new sailing procedures and traffic restrictions the accident rate may be reduced. The underlying causes which generate accidents in one harbour are the same as those which generate accidents in similar marine operations in other harbours and coastal areas anywhere in the world. This assumption is necessary since marine accidents are rare events, and the data base for any particular seaway or site is generally small.
Other similar techniques include the kinematics method developed by R. The probability of collisions and groundings are influenced by factors such as:. Not all of the above factors are explicitly taken into accQunt in this rather coarse risk model, but they should be considered in a more detailed shipping study. The results of some of them are briefly summarized in this chapter. Experience has shown that it is mainly impact damages due to collisions and groundings that are causing massive leaks and spills.
In the results presented here we have therefore only included accidents with impact damages i. When comparing the results from the various studies and ports, one should bear in mind the following:. In a recent study carried out by A. Porsgrunn 11 British harbours average for 10 harbours 18 Rotterdam 70 Tokyo Hamburg 17 As can be seen from the figures there seems to be very little correlation between the number of movements per year and the probability of collision per movement.
In another study carried out by Arthur D. Little, Inc. The results are presented in Table 9.
North Sea lj. A brief presentation of the data basis and the results for some of the studies referenced in Table 9 is given here. A total of arrivals per year of various types of ships are expected in the harbour area per year. Nearly arrivals per year will be of ships carrying hazardous cargoes, of which the more important are ammonia, chlorine, LPG, fuel oil, sulphuric acid and vinyl chloride. The calculations are based on accident statistics available for the Porsgrunn harbour area , for other ports in Europe, and for the world-wide operation of liquified gas carriers.
Total ship traffic, distributed by size, was obtained for the year period During this period, a total of marine casualties were recorded, distr ibuted into 21 groundings, 27 collisions with other ships, 52 ship-to-jetty collisions, 3 ship-to-ship at jetty collisions, 6 fires, 7 collisions with objects, and 5 adrift.
Over this year period, the total casualty rate was determined to be 4. By type of casualty, the rates would be as follows:. Groundings: 3. Of the total casualties, the London Port Autority classed only one as a "major" casualty, 29 as "moderate" casualties, and the remaining as "minor" casualties, with little or no damage. A significant factor in this relatively low level of damage incurred in these marine casualties is likely the transiting velocity in the Thames which has been controlled to eight knots within some of the regions of the fairway.
A study developing risk levels for LNG importation to the Netherlands considered marine casualties over a year period and concluded that the casualty proability for all tonnage classes and vessel types were 5 x collisions per transit and 1. Another study concludes that the collision probability for port approaches in the southern North Sea is 1 in A study of ship collisions in the English Channel, Dover Strait, and southern North Sea analyzed a worldwide data base of 20 years through and resulted in a total of collisions, with an average number of ships in service equal to This result suggests a yearly collisions probability of 0.
Henry Co. Considered in this study were all tankers over gross tons approximately DWT ; were in service during this period. Coast Guard Casualty Reports. For both casualties and spills, the data were broken down further into distributions for the various location categories by type of casualty. Since exposure of these tankers, in terms of transits along particular waterways or time spent in the location categories, was not studied in this survey, casualty rates cannot be determined in any direct manner.
However, an estimate has been developed, using reasonable assumptions about tanker operations. A strong correlation was found between tanker casualties and port calls. The data included vessels with gross weights in excess of tons approximately DWT or lengths greater than feet. For distribution of the casualties by type, this report refers to the data given in the J. On the basis that the J. Henry data are applicable in this regard, the corresponding collision, grounding, and collisions-at-pier probabilities would be:.
These statistics, based on information in Lloyd's List, includes serious casualties to selfpropelled, ocean-going, oil and chemical tankers and combination carriers in oil trading, above Also included were liquefied gas carriers of greater than Since the report, which included statistics for the period, IMCO has updated this survey yearly. The most recently published analysis, covering the year period , indicates a total of casualties over 39, tanker-years, giving a casulty rate of 0.
Of these casualties, caused pollution spills. Thus, the spill rate was 0. A distribution of the casualties into type was made for the data. The exposure of those tanker vessels, in terms of port calls or time in restricted waters, was not indicated in the IMCO survey. However, an estimate has been developed, using reasonable assumptions about the operations of this tanker fleet.
The study was restricted to tanker sizes above 10, DWT. The overall rate for collisions and groundings combined in port areas, based on such incidents and an average of 3. Assuming, as before, that tankers above If the incident rates per year were transformed into incident rates per port call the difference between small and large tankers would be even more pronounced, since the smaller vessels would on average make more port calls per year than the larger vessels. A number of such models exists of which 3 are briefly described here: These models are: R. Solem In this model the probability of a ship accident grounding, collision is interpreted as a product of two probabilities.
One is the probability of encountering an obstruction in the course, which in the absence of a manoeuvre would lead to an accident. The second is the probability of a mismanoeuvre in the face of an impending accident. The encounter probability is calculated for various collision scenarios. For grounding scenarios, necessary course alternations are interpreted as corresponding to encounter events.
Available accident statistics makes it possible, in conjunction with calculated encounter rates, to obtain an estimate of the mismanoeuvre probability. A model for this probability as a function of parameters associated with the manoeuvre is suggested. The probability models described, may be of use in predicting ship accident rates in waters for which no accident records are available.
The resulting models for calculating the probability of collision and grounding are as follows:. Based on worldwide accident data, accident data specific to several American and European ports, and upon an analysis of the MacDuff collisions, the value is estimated to be 1. In this method, based on the determination of an optimum path of maneuvrability for a particular ship transit, the probability of collision with another ship is deduced from the statistical patterns of the traffic environment. To follow this development, for estimation of collision involving the target ship as the struck ship, let.
It is assumed that this function is independent in V and and hence can be expressed simply as the product of these two uniform density functions. It is shown by Sharma that the collision probability is approximated in a conservative fashion by the relation:. The effect of visibility on the ship traffic collision risk has been discussed by R. The visibility was divided in the following ranges:. Groundings: VAF 0. Most of the above parameters are well controllable during the mooring system design or the chain production process.
The corrosion and wear processes are at present not fully controlled. However, the degradation process by these factors are slow and the structural soundness of the chain is controllable by inspection. Chain handling must be considered a fairly uncontrolled operation and should be restricted as much as possible. The experienced chain anchor line fracture frequency is as indicated in fig.
The probability of line failure is dependent on anchor handling. In ref. Investigation of chain fractures has shown that most chain failures are brittle fractures. This test should not be interpreted as typical behaviour of K3 chain. In service experience has shown that some rigs never get line failures while for other rigs the line failure rate is very high. This indicates a significant spread of chain quality, the test referenced above clearly shows that the breaking strength of a low quality K3 chain is significantly lower than MBL. Similar test for K4- chain has not been performed.
However, inservice experience again indicates a significant scatter of chain quality. The conclusion derived from the inservice failure investigation, the Veritec test, and discussion with manufacturers, are: - The main reason for the high mooring line failure rate is the scatter in quality a few failures may be due to fatigue or overloading.
Advances in Berthing and Mooring of Ships and Offshore Structures by E. Bratteland
Most probably due to increased quality in the manufacturing processes. Substandard quality is here defined as chain having low toughness i. CTOD values less than 0. A satisfactory manufacturing process is absolutely essential in order to obtain a reliable chain. No amount of NDE, inspection and testing can substitute for an inferior manufacturing process. The stress distribution in the links will result in the largest stresses at the outer surface of the link, i. The conclusion of this is that a QA system which is capable of detecting outer surface cracks of 3 mm depths and more, and ensuring a CTOD value of more than 0.
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Many of the reported fractures are caused by cracks developed when the chain passes the fa-irlead. Optimal fairlead design will reduce the additional stress level in the chain and thus the probability of developing such cracks. Optimizing of fair lead design may among others be to use more pockets in the fairlead and to avoid the use of kentner shackles. A fairlead designed for chain only may have smaller tolerances and smaller fairlead induced stresses than a fairlead designed both for chain and shackles. The catenary mooring system for a mobile offshore structure can be modelled as a redundant load-sharing system.
Element i. Hence, the event of failure Ei is defined as:. The failure probability of a line system consisting of n-l remaining lines is then given as In addition, we included the system failure probability in case of thruster blackout, pT. This probability can also be obtained from equation 1 by substituting. Here FT is the force provided by the thrusters which in case of thruster blackout represent an equivalent load to be carried evenly by the remaining n-1 lines.
Two different mooring line strength probability distributions have been applied both of the Weibull type to see how this affects the results:. Fitted Weibull distribution according to test data for anchor chain test specimens. The distribution is as follows:. The reason for distribution II is: The mooring wire test data showed small variations in breaking strength. This gave a fitted Wei bull distribution with a cut-off at approximately tons, i. To approach our calculations in a conservative manner, we maintain that in the safety factor range of 1 to 2 below tons there is a fixed probability of failure of 0.
The mooring failure probability curve in Fig. Some conclusions with regard to the results of the calculations: - The mooring system failure probability is considerably reduced with increasing safety factor in particular for systems with several parallel load-sharing elements. Jahrgang - Nr. Journal of Navigation, p. Particular attention is paid to the use of winches installed on piers or quays for berthing and mooring. It is a common misunderstanding that de- veloping countries are mainly the exporters of bulks and im- porters of manufactured products.
The USA, Canada, Australia and Sweden all export large bulk quantities ore and grain while typical developing countries like India, Pakistan and China have very little bulk export. The tremendous increase in the carrying capacity of bulk vess- els has brought large areas of the world, previously inaccess- ible due to their distance from developed area, into the prod- uction system. Due to the trend of the large-scale development of ports, one may expect that future superports will be arranged within a large frame relying on deep natural or artificial channels con- necting the open ocean with the port.
The port area may be very large in water as well as land area and it may consist of a number of single facilities including bulk terminals of various kinds of oil, gas, ore, grain, container, general cargo and special terminals for fruits, frozen products, etc. An exception may be fishing ports and small craft harbors, which for reasons of economics, are usually pub- licly owned. In this way, modern ports are becoming represent- atives for the abilities and accomplishment of free enterprise rather than examples of the general tendencies towards socia- lization.
Advantages of bulk handling include: minimum labor requirements, no packing or packaging, one bill of lading, usually one port of call, and the ability to haul large volumes over long dis- tances. Disadvantages of bulk handling include: not all ports Bulk cargo operations depend upon the existence of adjacent industrial plants such as, flour and lumber mills, refineries, iron and steel mills which require water front access to ex- port their products.
The distance from the water front to such plants depends upon the handling techniques involved. Liquid pipe commodities may be located several miles from the water front. Bulk cargo handled by conveyors, may be loc- ated several hundred y'ards from the water front. Bulk terminals generally require deeper channels and larger turning basins than other types of terminals due to the draft of the vessels.
They are often loc- ated on the deeper side of other commercial docking areas. The total height of such breakwater may therefore be about 40 m, and with a crown width of 10 m and slopes of It is obvious that it may sometimes be difficult to justify thecost of such breakwaters and that it therefore becomes nec- essary to find less expensive alternative s or to reduce their length to a minimum.
A breakwater is placed to protect the ships against wave act- ion above an acceptable limit. Converted into practical port engineering, this means that the movements of a vessel at berth must not exceed certain functional limits. With respect to handling, for example, under ideal conditions it should be possible to unload up to about 30 containers per hour. This limit is seldom reached. Most container operations handle some 20 containers per hour, while others may sometimes come down to just a few per hour.
This happens in particular where waves are very long and tend to set up seiche motions in har- bor basins or roadsteads. Ports on the Pacific coast are par- t. Seen from an operational point of view, protection of a vessel at berth should be interpreted as a hindrance to or decreasing of movements of the vessel.
Such movements, however, can be stop- ped or minimized directly by means of moorings without the assistance of protective works. While such a procedure has been followed in numerous cases in industry, it has not yet been used in port technology, undoubtedly because it introduces some additional risks. Tying the vessel down to "zero" or to "elastic movements" in fenders and moorings only, seems to be a more acceptable idea. The question, however, is of how much movement can be allowed without introducing adverse efects on the efficiency of oper- ation and on safety at berth.
The movements which we are concerned with are: surge parall- el to quay , sway perpendicular to quay , yaw turning around the vertical center line of vessel and pitch turning around the horizontal center line perpendicular to the vessel. The various movements are not equally important for different kinds of vessels.
Surge is relatively less important for tan- kers. Bruun 4,5,9 discusses the socalled "allowable move- ments" for various kinds of vessels but at the same time points out that any movement of a vessel at berth is undesirable, be- cause it is always adverse to operations as well as to safety at berth. Innumerable practical cases have proven how easily a mooring system can fail, most often due to resonance effects of the outside forces and the forces in the mooring ropes.
It is logical to assume that a proper mooring and fendering system will be able to replace a breakwater - or at least part of it. This open pier intalla- tion for coal bulkers up to , DWT has now functioned sa- tisfactorily since After long discussions and testing it was decided to build an unprotected facility with a m pier at m depth and a 1, m trestle carrying con- veyor belts. Design details are described in a later paragraph entitled "Replacing Breakwater by Adequate Mooring and Fendring" Only little tug assistance is needed.
Similar piers or termin- als with no breakwater protection are planned i. At HADERA, tension-mooring was provided by ships winches, but tests were run for higher tensioned pier-based winches which can be put in, when it is considered desirable or necessary. At the same time it has sev- eral operational advantages, particularly for terminals in exposed locations. It may be located inside or outside a nor- mal port area, depending upon space availability and depth re- quirements.
The oldest terminals were the ferry terminals which were usually rail-connected and placed inside a port area. The development of transportation systems for oil pro- ducts made it - for reasons of safety - necessary to establi- sh tanker terminals either in remote areas of existing ports or outSide existing ports. This immediately brought up the question of site selection for facilities handling inflammable and explosive cargoes. The situation further aggrevated with the introduction of LNG and LPG-vessels, which increased the demands to operational safety.
Safety requirements in the principal areas are recognized universally, but so far no in- ternational standards have been adopted. Attempts are now be- ing made to do so through committees working on national and international levels. With respect to dry bulks it has been common pract. But gradually - as experi- ence became available - concerns regarding pollution grew, and at present many rules regulating industrial development, smo- king or smelling commodities, have been established on local and national levels.
At the same time the absolute size of the loads increased, particularly influencing depth require- ments. This established another very important boundary con- dition for new, more effective and environmentally acceptable facilities. The ultimate result was the introduction of the socalled "deep water terminals", which are referred to as bulk, mainly oil and gas products,but during recent years also to coal and ore handling facilities.
Proceedings of the NATO. Keywords: operational conditions, moored ship response, physical The work carried out over the last decades led to the development of advanced tools, able to Advances in berthing and mooring of ships and offshore structures. Proceedings of the 7th international conference on coastal and port. Offshore Structures, Trondheim, Norway, , E.
Bratteland, ed. Design of Marine. Holden, C. Plenary talk at the 10th European Workshop on Advanced Control and. Examples of computed moored ship motions and experimental verification.. A vessel will moor with its lines or anchor chain connected to shackles,.