... system riser draught mooring system modification upstream oil & gas fpu hull conversion pontoon upgrade Deegan , F.J , Loffman , M. & Odufuwa , D. 2014 . The Conversion of Mobile Offshore Drilling Units to Floating Production Systems – Issues, Opportunities...

... 20366 wave excitation sustainability relative wave motion excitation subsea system generator body 1 social responsibility draught upstream oil & gas sustainable development pitch motion energy conversion ocean energy wave energy conversion renewable energy reservoir characterization...

... subsea system fpso substructure crack length draught hull vessel calculation mass distribution hull module floating production system tandem hull consideration structural analysis inter-hull column tandem hull vessel offshore technology conference structural integrity platform...

... draught upstream oil & gas offshore technology conference configuration parametric analysis requirement transit ballast root vicker ltd convergence module subsea system consideration hydrodynamic coefficient pontoon constraint payload excursion coefficient allowable...

... platform base caisson otc 6347 offshore technology conference floatout emplacement concrete gravity substructure installation pipework construction draught OTC 6347 Innovation in Concrete Gravity Substructures: The Ravenspurn North Platform and Beyond J. Roberts, Ove Arup & Partners Copyright...

... agreement upstream oil & gas uncle john draught jackson subsea system wave frequency frequency response operational draught rao wave direction 135 surge full-scale measurement wave height british maritime technology OTe 5824 Full-Scale Measurements on the Semisubmersible "Uncle John...

... offshore technology conference draught buoyancy unit otc 5549 riser operation storage tank subsea system anchor line application non-weathervaning production storage OTe 5549 Semi Spar: Integrated Non-Weathervaning Production Storage and Offloading Unit by P. Balleraud, Single Buoy Moorings...

... minimum value for the ratio: limited numerical accuracy in calculating hydrostatic characteristics in damaged con- dition (can lead to errors in draught and heeling angle) effect of mooring system upon stability is not taken into account limited accuracy in determination of wind and constant heeling...

.... At this site, a barge draught survey was performed. The deck weight for mating was predicted to be 18,013 tonnes. Also, final dimensional control surveys were made at the mating interface points. Table 1 gives the load condition for the deck and barge. With the deck positioned midway along the barge...

... positioning system caisson mobile arctic caisson molikpaq installation arctic kiggiak upstream oil & gas operation berm preparation otc 4942 subsea system draught set-down operation herschel bay mac approx molikpaq terry fox positioning OTC 4942 Installation...

... very well with the strip theory calculations. Especially for the shallow draught conditions the dis- agreement is quite pronounced. This may be due to the fact that strip theory is not very apt for estimating the forces on the columns. In general, it is fair to say that the measured heave force...

... calculation replacement upstream oil & gas displacement otc 4213 objective reference foint adjustment necessity subsea system semi submersible drilling rig stress level semisubmersible drilling rig rearrangement ballast new section draught diagonal drilling equipment keel...

... moments to be used in stability calculations. For that purpose, the tests are per- formed at various draughts, normally three, e.g. survival, operational and transit conditions. Usually, several tests are performed at each draught, one at even keel and a number of tests at different angles of heel/trim...

... emergency rig platform upstream oil & gas operation draught buchan development project modification production platform subsea system generator weld control room penetration shutdown riser OTC 3958 BUCHAN DEVELOPMENT PROJECT - CONVERSION OF ADRILLING RIG INTO AFLOATING PRODUCTION...

... fender stiffness coefficient initial motion clearance sway initial motion compression phase spring rate expression equation tanker size collision loading condition means ship impact model test model test draught fender OTC 3664 ON HYDRODYNAMIC ASPECTS OF SHIP COLLISION...

...<lge has been developed which provi- des a solution to the problems attendant to statio- nary dredging operations in open sea and which has a semi-submersible hulL This paper deals with a de- sign developed fora Semi-Submersible Dredge (SSD! destined for operations in so~thern areas of the North Sea, and in which areas it can operate for 95% of the year. The SSD offers the oppor'tunity to supply sand to coastal areas from the open sea under circum- stances in which conventional dredging equipment is unable to operate; INTRODUCTION In recent years it appears that the areas of operation for large-scale dredgi~g operations have shifted from inland waters to the open sea. SUffi- cient dredging equipment has been developed for in- land water operation such that this work can be car- ried out without great difficulty. However- the situ- ation is otherwise for operations carried out in the open sea due to the conditions and circumstances existing there. Currently employed dredging equip- ment is unsuitable for the execution of dredging work in its entirety in open sea including the pro- cesses of suction, transport and discharge of dredged material. The stationary suction dredges undergo very considerable hindrance through motions by waves due to the sensitivity of the dredging process to vertical motions. Of all conventional dredging equip- ment the trailing suction hopper dredge is the most suitable for operation in open sea, and it is only the discharge of the dredged material via a pipe- line that gives the problems. Special equipment re~ui~es to be developed for dredging work that is carried Qut"maiply in open References and illustrations at end of paper. sea, and for the building up of artificial islands and the re-establishment of beach and coastal areas. As a result of extensive erosion of beach and coas- tal areas, particularly in the United States, such re-establishment is vitally necessary. From prac- tice, it has been shown that many attempts to re- establish beach and coastal areas have been diffi- cult, or have led to damage to the dredging instal- lations due to the barren circumstances under which the work had to be performed. At the OTC of 1974, a paper (1) was presented dealing with offshore dredging systems for what is termed "Beach nourishment" projects. In this paper comparisons were made between dredging systems that were possibly applicable to this type of work. Amongst other installations, the semi-submersible dredge (SSD) was quoted as possible being the most principally favoured installation for beach nourish- ment operations. This occasioned the making of a study having the object of producting a completed design for a SSD. In the study carried out prior to the presentation of this paper, all the operational facets of the dredging art which could be of impor- tance in the design of a SSD were placed on the agenda for consideration. The philosophy behind the design was that the dredging installation must be capable and suitable for carrying out the dredging process in it's enti- rity without having to "run for port" every time heavy weather set in and, to this end, making use of the semi-submersible principle of operation as now generally known in the practice of the offshore technique. By application of semi-submersible prin- ciple a situation is achieved where in the dredging installation and the process thereof is less sensi- tive to waves, and such that the number of workable days is greater in relation to those obtained with conventional dredging installations. This paper deals with the crux of the problems involved in the design in general, and to one specific design in particular. It is self-evident that a universal design is just not possible. The design must be executed on the basis of well established data or a given ope- rational situation. In the design to be presented, 851 it w.as taken that this concept of a SSD w.ould ope- rate in the southern reaches of the North Sea where, in offshore terms, particularly unfavourable weather conditions prevai~. The design has been realised jointly through, and on the basis of, other realis- tically considered starting points later to be quo- ted herein. The SSD design based on starting points which are valid for one particular case, or set of circumstances, need not however be unsuitable for another operating situation. Adaption of the design to other situations and circumstances is also natu- rally possible. Moreover the design resulting from this study is not the only one possible for a SSD. The presentation of this design points to the direc- tion in which thought can be given to the realisa- tion of a SSD which is suitable for, and capable of, carrying out necessary dredging in open sea. 1. DEFINITION OF THE DESIGN REQUIREMENTS The southern reaches of the North Sea, where depths of approximately 30 meters are encountered, have been selected as a representative operating area for the installation. The prevailing circum- stances, in the area which the installation is des- tined to work, as regards to wind, waves and current are coped with by the design of the SSD. The dred- ging installation must be capable to work for 90 to 95% of the time. Moreover, the SSD must by able to survive in open sea under extreme circumstances without running for shelter. In order to obviate interruption of the suction ,dredging process as much as possible due to reposi- tioning of the dredge, ~t is necessary that the areas to be dredged have a considerable over depth in rela- tionto the seabed. On the other hand, an increasing suction depth necessitatesouse of costlier and larger dredging installations. This in turn implies an in- creasing cost per cubic meter of transported sand. On the grounds of these foregoing considerations, it is thus possible to determine a favourable wor- king depth. For the_design of the subject SSD a suction depth of approxima~ely 50 meters was taken as a starting point. _ .. The draught with empty ballast tanks of the dredging installation must not exceed 5 meters ap- proximately. This requirement is necessitated rela- tive to transport of the installation from harbour to the area of dredging operations during which pas- sage shallow waters etc. may be encountered. The choice of pump capacity for the dredging installation is determined-by the distance over which the sand/water mixture is to be transported. For this particular project, a pumping distance of approximately 2000 to 5000 meters has been chosen. A production capacity (expressed in cubic meters of sand per hour) was chosen which took into account the quantity of sand/water-mixture which could be handled at the point of discharge thereof. In beach nourishment projects, too large a capacity results in an increase in the percentage-loss of sand through flow-back into the sea. As a design requi- rement, 1650 and 500 cubic meters of sand per hour pumped over distances of 2000 and 5000 meters res- pectively were chosen. The -sort of material is of great importance in the calculation of the produc- tion potential of the dredging installation. In the chosen working area, the material is mainly and substantially "moderately fine" sand with an average grain size of 0.235 millimeters. The speci- fic weight (with the pores filled with water 1 is approximately 1.85 ton' per cubic me,ter. The concept provides for fully continuous dredging with a fuel storage capacity aboard such that two weeks of continuous operation would be achieved. 2. SSD DESIGN 2. 1. Hull design In the general design of the dredging instal- lation, the design of the hull itself stands central in as much that it's characteristics determine the success of the design as a whole and to a major de- gree. The hull must be considered as a "carrier" for the dredging installation proper with the demands thereon that it is seaworthy in the southern areas of the North Sea in which it is primarly designed to operate. The term "seaworthY" in this context implies a~solu~ely that the hull will only exhibit slight mo- t~ons ~n waves and that it's safety and that of the manning is ensured at all times whatever the circum- stances. Of the operational and technical requirements laid down in general for the dredging installation as a whole, a number of specific requirements for the hull can be briefly stated herewith. In order to carry out dredging operations over the largest num- ber of days possible, the hull must be less sensitive to,wav:s. This chara?teristic can be obtained by ap- pl~cat~on of the sem~-submersibleprincipe. further- more, the hull must be able to survive under extreme circumstances at sea. Relative to the suction depth, the pumps require to be positioned within the hull below the waterline in order to reduce the suction head as much as possible. Parts of the installation prone to wear must be easily accessibe for repair. The manoeuvering and general handling of the suction pipe must be accomplished in a reasonably simple man- ner',and in addition to which inspection and possible repa:r must b7 enabled. The safety of both manning and ~nstallat~on as a whole must be ensured in the event of collision or in the event of the occurence of sea-~o~ng perils, and this particularly in respect of stab~l~ty and a reserve of buoyancy over which strong demands are made. Representative conditions, under which the in- stallation will operate in the southern areas of the North Sea, have been compiled from statistical data thereon. These conditions are set out in Table 1 and wherein a division has been made between "surviv~l conditions" and "working conditions". For design pur- poses, working condition is to be construed as the condition in which the SSD is still capable to carry out dredging work. A water depth of 30 meters has been taken into account in the design, and with res- p:ct to the ,hull ~esign effort has been made to pro- v~de a conf~gurat~on which gives favourable seaway Characteristics coupled with a practical form and d~en~ions. A bas~c requirement is that heaving, p~tch~ng and roll~ng motions must be slight. Heaving motions are generally small when the natural heaving period of the dredge is large in relation to the wave period. In the working area wave periods arise up to about maximum of 12 se- conds. In view of the fact that resonance must be 852 avoided, the hull's natural heaving period of some 20 seconds would be SUfficiently great. This means relationship between the waterline surface and the displacement is substant:l.ally determined. The ' pitching and rOJ..ling motions are generally smaYI when'the metacentre height is small. From the as- pect of pitch and roll excitation through waves, it is generally more favourable'when'the centre of buoyancy lies under the centre' of gravity. This means that the initial stability is formed by the moment of inertia of the waterline. In respect of these considerations, the dimen- sions of the hull have not been set down. These will mainly determined by ,the function of the hull as a "carrier" for the dredging installation. The motion of the hull will become smaller as the size of the hull becomes larger. However it is of great importance that the hull is not necessarily large and costly, and thus a number of points' will be considered which are determinant for the dimen- sions of the hull. Relative to the suction depth, the pumps must be located as low as possible in the hull. This requirement in combination with an acceptable suction pipe length and a permissible attitude in relation to the seabed will result~in a necessary draught to reach the suction depth of 50 meters. Due to the fact that the pumps are loca- ted at the bottom of the construction as a whole, it.follows that they occupy rather a voluminous part of the hull which must serve as a combined pump and engine room. Since it will be necessary to lift out parts, such as pump housings and impellers, for re- placement it will be necessary to provide a shaft for this purpose leading up, out 'of the engine room. In addition to the use of th1.s shaft as an exit, another means of exit must be available for reasons of safety for personnel. A number of variations to the configuration of the hull have emerged from the investigation therin- to. In the assessment of the most suitable of these variations, the considerations of the aspects of trim, stability, safety and suction pipe handling have played a role in. relation to the hydrodynamic aspects. These considerations have led to a multiple symmetrical underwater portion of the hull with three vertical columns equispaced radially at j20 degrees apart and with a centrally located fourth column (see Fig. 3.1. The multiple sy.ffimetrical design of the hull means that there is no preferential heading to waves or current. The engine room and fuel :tanks are located be- neath the central column so that the trim remains un- affected by fuel consumption. The three peripherally arranged columns have an underwater communication passage with the engine room and through which quick emergency exit therefrom can be made. The four co- lumns provide a sufficientdegree of safety by exit therefrom in the event of collision or leakage . The hull will be ofa fully double-walled construction so that by (hypothetical} damage thereto no access space will be damaged. This means that the cross sec- tions of the four columns are determined in such a manner whereby, at the same time, displacement to achieve the natural heaving periOd is near enough fixed. Handling of the suction pipe is achieved by the provision of a guidance system arranged between two columns on the outside of the hull. These particular columns are surmounted by a platform onto which the suction pipe can be laid. In relation to the dimen- sions of the hull this means that the distance between the columns on the outside is determined by the length of the suction pipe. Operation of the dredging installation of the SSD makes no special demands in respect of stability. Thus it is possible to accept a small initial stabi- lity which is favourable to the seaway characteristics The required initial stability must be obtained throug! the moment of inertia Qf the waterline, because the centre of gravity lies above the centre of buoyancy in connection with the associated favou- rable influence on the seaway characteristics. The moment of inertia of the waterline is of Suffi- cient magnitUde when the distance from the columns coincides with the distance that is required for the guide tracks for the suction pipe, and with this the main dimensions of the underwater portion of the hull are fixed in fact. The stylisation of the hull has been realised on the basis of hydro- dynamic and constructional starting points. More- over, in this stylisation attention has yet been given to the fact that the empty draught amounts to no more than 5 meters approximately and that the stability is sufficient during ballasting operations for all draughts. In order to ensure safety in the event of ha- zardous situations, the SSD possesses sufficient reserve buoyancy above the waterline. At the same time there is herewith achieved a stability range of such magnitUde that there is no tendency Of cap- sizing under any circumstances. The reserve buoyancy is obtained by broadening the columns above the waterline where they are connected to each other by tubular beams and Wherein, amongst other equipment, the accomodation is located. In order to prevent slamming the deck construction is located sufficient- ly high enough above the waterline. Maximum draught can be maintained uptill conditions as quoted in Table 1 under "maximum draught condition". In wor- sening conditions the draught can be reduced to "survival draught" at the utmost. 2.2. Hydrodynamics Calculations have been made to determine the motions of deep draught SSD and mainly in accordance with the theories propounded in (2) and (3). The in- fluences of water depth have been discounted in the calculations. The amplitude of the motions over the 'Whole reach of the wave heights and periods have been determined for such as those thereof occuring in the working area. From these calculations it appears that the SSD exhibits no pitChing or rolling motions, and the yawing motions are also ignorably small. The motions of the SSD thus comprise exclusively those of heave, surge and sway. These last two mentioned motion components are relatively large as a result of wave motion in shallow waters. Due to the multiple symme- tric form of the hull, the SSD has SUbstantially no preferential heading in waves. In fact one can only speak of horizontal and vertical motions. Table 2 shows the calculated motion amplitudes under diverse conditions with significant wave heights and periods. Fig. 2. shows the heave response curves for maximum draught and survival draught. From the figure it will be seen that the draught has a great influence on the magnitude of the heave amplitudes. 853 This, amongst other matters, is a result of the shal- tely ~300 kW (1750 hp) per PUMP till be necePsarY and low water effect where against the draught has hardly *hey will run at a speed of approximately 350 revolu- any influence on the horizontal mot?on wnplitudes. tions per minute. The pumps will be designed as dou- ble walled types, this implies that the space between In order to assess the correctness of the calcu- the inner and outer housings will be adapted to the lations, observation tests have been carried out in operating pressure of the dredging system. Through the Netherlands Ship Model @asin at Wagen%ngen. The the adoption of these features, the demands made on observations were directed to a model subjected to ir- the mechanical strength of the inner housing of a dou- regular waves the spectrum of which corresponds with ble walled pump will be somewhat less stringent. It that of the working area. The oEservati onsexhiEi- vill be possible therefor to employ harder, and con- ted a good likeness to the calculations. The ver- sequently vear-reqistant, materials for their manu- tical motions were slight, whilst the horizontal facture. In the absence of a pressure difference over motions were relatively large. Indee-d,no pitching the wall of the inner housing, it will be possible to or rolling motions were ascertained. allow this part of the puzapto completely wesx away. In order to determine anchoring requirements, The suction line has an internal diameter of drift forces under diverse condi t?onswith i.nflu- 650 millimeters. The suction pipe is provided with a ences of waves and currents were calculated and for hinge joint an& am universal joint by which means the which, among other matters, use was made of (4 suction pipe is positioned according to variations in the working depth, and to provide sufficient flexibi- 2.3. Anchoring lity thereof tq cope with potential interruptions to the process through external influences such as tidal The SSD is moored in it~s working position by currents and motions of the hull. The lower end.of four anchors, each with its cable of approximately the suction pipe is provided with a mantle to prevent 900 meters. Two attachment points and two winches blockage of the latter in the event of collapse of a are located on the fore column, whilst each of the steeply sloping sandwall thereon during the dredging two rear columns is provided with one attachment operations. point and one winch. In the interests of inspection and maintenance, The SSD is self-emplac?ng in the area to be it is necessary to haul the suction pipe inboard worked by haul in or pay out of the anchor cables out of the water. To this end the suction pipe is by means of the winches. With the SSD moored by provided with three suspension points, one at the it s anchors as shown in Fig. 3 and, fn combination suction mouth, one at the universal joint and one with the discharge line, a working area of some where the suction pipe connects to the piping sys- 230.000 square meters can be covered. With an ave- tem within the hull of the dredge. Due to the fact rage sub-seaEed depth of 7 meters for example, an that the working deck lies high above the waterline, hourly production rate of 500 cubic meters of sand there is a great risk that the suction pipe will slam (pumping distance of 5000 meters~ and with a total against the hull during hoisting in. In order to ob- production time of 20 hours Wer day, 160 working viate this risk, three guiding tracks are provided days are available before it is necessary to remoor to extend vertically upward from the underside of the SSD in an adjacent working area. In the case the hull to the working deck. The method of hoisting of an hourly production rate of 1650 cubic meters =d bringing the suction pipe inboard with the aid of sand (pumping distance of 2000 meters~ under the of the guiding tracks and other equipment therefor same circumstances, 49 working days axe available is as follows. before relocation of the installation is necessary. The suction pipe is first hoisted from its wor- In order to withstand the survival condition, king position to engage carriers therefor which are the SSD must make use of more of the four available arranged to slide in the guiding tracks (the suction anchors. In respect of this _requirement, the SSD pipe is in permanent engagement with one of the car- must be maneuvered timely to a more favomable po- riers at the position where it connects to the shell), sition in relation to the wave direction. Fig. 4 and on further hoisting the carriers slide vertical- illustrates a favorable position as an example. Du- ly upward in the guiding tracks carrying the suction ring the monthly storm (working condit?onl the sur- pipe to the working deck. Upper parts of the guiding vival position must be adopted when the wind reaches tracks located above-decks terminate on a mobile force 9 Beaufort. When the wind force exceeds 9 to gantry which is movable from the deckside inboard 10 Beaufort, the discharge p?peline must be uncou- and with the aid of this facility it is possible to pled. In practice, it ~rks out that about 3% or 4% haul the suction pipe inboard where it can be secu- of the time is occupied in taking precautions to red to the deck. Inspection and maintenance of the bring the SSD to safety in extreme conditions. sucti onpipe is now possible without the necessity for anyone going outboard to accomplish these tasks. In view of the fact that the SSD has no self- propelling facilities, it must be towed to the wor- Although motions of the SSD are relatively king location, and whereat %he anchors are set out sltght, swell compensators are provided for compen- by a supply tender. sating the motions with respect to the seabed. Here- by the hoisting cables are maintained taut at all 2.4. The dredging installation times between the suspension points on the suction pipe and the winch. In view of the pumping distance of between 2000 2.5. Dischaxge line and 5000 meters, it is necessary to adapt two dred- ging pumps to work in seriesflIn order to achieve the This discharge line is 600 millimeters in dia- desired production, two pumps will be installed each meter and consists partially outboaxd from the hull of 1750 millimeters diameter and each provided with of a horizontally arranged pipe srm of some 25 meters a 5-bladed impeller. A driving capacity of approxima- in length and pivotable about a vertical axis through 854 an angle of about 180 degreesThispivotting is ef- fected from the working deck by means of winches and steel cables. In this manner it is possible to hold- off the discharge line from the vertical columns of the SSD. The end of the arm is provided with a disconnectable tall joint which is remotely control- led, and thus it is possible to disconnect the dis- charge line from the SSD at any time. In the high- riding condition of the SSD (sea tramsport condi- tion).this ball jo;nt coupling is located above the water. A pipe length of some 40 meters is couple& to the ball joint and further provided with flota- tion units by which means the forces acting on the ball joint coupling and exercised by the p$pe length are minimised. A rigid pipe has been chosen for this position to prevent the disch~rge like from coming into contact with the anchor cables of the SSD. A flexible floating line is connected to the rigid pipe, this line has a length of approximately 48o meters. This part of the discharge line is con- stituted by steel pipe lengths of 18 meters separa- ted by flexible rubber hoses of 6meters in length. Both parts of this section of t~e discharge line are provided with annular circumscribing flotation units the volumes of which are such that if the dis- charge line is filled with water it still floats (see Fig. 5In the event of pumping a sand/water mixture having a particular specif?c gravity, the discharge line sinks to the seabed (see Fig. 61. Lengths of chain are fixed to the dikcharge line at discrete intervals therealong, and By varyihg the weight of each extending lengtliof chain it is pos- sible to fix the course of the discharge line in a floating condition. The Eiggest advantage of a discharge line with flotation units and chains is that the influences of waves and cuxrent on the flexitiledeli veryline are el?m?nated during dred- ging operations. By pumping water ttiroughthe flexi- ble discharge line so that it surfaces, unobstruct- ed repositioning the SSD ?s then made possible. After approximately 48o meters of flexible discharge line, there follows another fixed prtion of variable length lying on the seabed. This por- tion comprises lengths of steel pipe of 18 meters long, these lengths are coupled by ball joints which permit the fixed portion.of the discharge line to follow uneven contours of the seabed. The coupling between the fixed portion and the flexi- ble portion of the discharge line is formed by a swivel bend arrangement on the seabed. The beginning of the dredging process must take place at a correctly chosen time to lay the discharge line in the correct position or maintain it there, The previously quoted discharge pipe arm and the rigid extension piec=ethereon should not lie in axial prolongation with.respect to one ano- ther, this requirement is necessary in order to re- duce mechanical stresses being applied thereto through hull motions of the SSD. In Fig. 7 the SSD with the complete delivery line is shown. 2.6. Stability The metacentric height has been determined for a number of important working conditions. Table 3 details a number of values for draughts of 22.00 meters, 16.50 meters and 11.50 meters. It appears that the metacentric height is smallest with a draught of 11.50 meters. With smaller draughts the metacentric is greater as a result of the greater waterline surfaces of the outer three Columus. Although the initial.stability is small, the SSD has a sufficient stability range. The righting levers have been calculated for various angles of list and various directions in which listing takes place. As exanples, these values have been set out for a draught of 22.00 meters in Fig. 8. The direc- tion O degrees indicates that the construction lists forward. From Fig. 8, it is concluded that the SSD exhibits no inclination to capsize. Stability under damaged conditions for a num- ber of cases thereof have been calculated. In all cases the stability appears to be sufficient and the final attitude of the SSD acceptable. 2.7. Workability The maximum draft can be maintained over 93% of the time (see Table 1). The motions of the SSD are then so slight that operations can carried out on without interruption. The draught must be redu- ced in conditions worse than the maximum draught con- dition. The drift forces on the SSD and the discharge p?peline are so large moreover, that the SSD must adopt a favouxable position with respect to the an- chors. If necessaxy, the SSD itself must manoeuver to such a position by use of its winches and anchor cables. Under the working condition set out in Table 1, the SSD can still be operative over a quar- ter of the reachable area. The motions of the SSD axe stil acceptable to allow dredging work to be car- ried on. In more worse conditions than the working condition, the survival condition must be adopted. This situation arises for about 1% of the time. The motions of the SSD can then be of such magnitude that dredging work has to be stopped. The whole working area within reach can be wor- ked for 93$ of the time. This is valid for a quarter of this area over 99% of the time. On the grounds hereof, an average workability of 95% can be achieved. 3. GENERAL ARRANGEMENT PLAN The general arrangement plan is illustrated in Fig. 1, and the main details are outlined in Table 4. The internal division of the hull is as follows. Two diesel engines are located in the engine room umder the central column for driving the dredge pumps, each engine has a power of approximately 1400 KW (1900 hp). TO provide easy transition from the low pressure pump to the high pressure pump, an sxrange- ment has been chosen in which the suction side of the high pressure pump is directly connected to the delivery side of the low pressure pump (see Fig. 9). The suction line leads from the shell connection to the engine room via one of the rear columns and a communicating passage therebetween. The delivery line from the high pressure pump is directly connected to the earlier described pivotable arm. Furthermore, two diesel alternators of 350 KW (450 hp) each are located in the engine room. In normal operation the power delivered by one alter- nator is sufficient, however in particular opera- tion the second alternator is needed. A lift is installed to provide communication 855 between the control room ~~ the engine room, a hours, and for which fuel consumption will amount staircase is also prov~ded in addition thereto. An to 37Q0 ton of gasoline. The fuel consumption of escape passage is provided between the engine room the supply tender has been estimated at 300 ton per and the tween deck via each of khe outer columns. year approximately. The annual fuel cost, with gasoline at $ 200 per ton, will amount to $ 800.000. The fuel tanks are located above the engine room, whilst the remaining space within the double- 4.2.2. Lubricating oil cost walled coristructionis availalle for water bal- last. The necessary fixed ballast requ?red to main- Consumption of lubricating oil is estimated to tain a stability in semi-su13nergedcond?t?ons is be3.3 grem/KW/hour (1.0 grsn/hp/hour] or, annuallyY applied proportionally to the dou?51e-?30ttomand 24 ton. On the basis of a lubricating oil price of double-walls of the eng?ne room. appraimately $ 1400 per ton, an annual cost of $34.000 results. Anchor winches, vari ousworkshops, stores, ser- vice spaces, and five.two-persw caBins are located 4.2.3. Manning costs on the tween deck. An emergency alternator ?s also located on this deck and provides an output of 50 KW (TO hp). The central control room is locatedon During dredging operations the crew will com- prise three men per shift aboard the SSD, and wor- the upper deck and from which the operations of king a three-shift system 5 days per week. The sup- the engine room installations, dredgi nginstallations ply tender requires a two-men crew whilst the pro- and anchor w?nches are controlled. ject, in its entirety, will be rum by a supervisor. Reserve personnel must also be taken into conside- A hytiaulical telescopic crsne is accommoda- ration together with watchmen and such like. On an ted on the working deck for the purpose of l?f ting annual basis, it is estimated that cost of 13 men out parts of the pump and engi neroom equ?pment. will come for the account of the dredging project. This crane straddles a greater part of the working Average personnel costs have been estimated as deck and can also be used -forcarrying out work on $35.000 per man, resulting in a total of $455.000. the suction pipe and for the transfer of goods smd crew on board and off. 4.2.4. Insurance costs 4. ECONOMIC ASPECTS OF THl SSD It has been taken that these costs will be in the order of 2$ of the investment value. With pum- The prices and costs quoted under this heading ping distances of 2000 and 5000 meters respective- are based on prevailing circumstances in the Nether- ly, the insurance premiums will amount to $ 222.000 lands, however the amounts have been converted into and $ 240,000. US dollars. 4.2.5. Maintenance and repair costs 4.1. Investment These items are difficult to determine since The construction costs of a SSD, as described the SSD cannot be compared with existing types of in this publication, amount to approximately dredgihg i nstal.lations. $ 9.000.000. The purchase price for the discharge However these costs have been calculated to be$ 300 per production hour. This pipeline can be split into the following amounts. resulting in a maintenance and repair item of - for the floating portion (48o meters]: $ 150.000. $ ~.5oo.000 annually. for the sunken portion (with a pumping distance 4.2.6. In%erest and depreciation costs of 5000 meters]: $ 1.3T0.CJOO. for the sunken portion (with a pumping distance of 2000 meters): $ 460.000. The constant annuity method was used to deter- mine depreciation. With a 10% interest rate and.com- A supply tender is needed for the transport of plete depreciation over 20 years, the annual costs- amount to 11.7% of the invested capital. For pumping personnel, stores, and parts, for anchor emplacement and laying of the suction pipeline etc. The new distances of 2000 and 5000 meters respectively this from the yard cost of this vessel mounts to approxi- smounts to $ 1.300.000 and $ 1,406.000. Table 5 gi- mately $ 1.500.000. ves a summation of the annual costs for each of the specified pumping distances. For pumping distances of 2000 and 5000 meters 4.3. production figures respectively, the total investment for each amounts to $ 11.110.000 ant 12.020 .000. The production figures, per hour, are 1650 and 4.2. Operating cost 500 cubi cmeters of sand respecti~ely for pumpingdistances of 2000 and 5000 meters. On a basis of operating costs have been calculated on the ba- 5000 production hours per year the annual produc- sis of realistic prices ana existing cost norms (5). tions for the foregoing pumping distances are 8.25 A starting point of 5000 net3 production ho~s Per million and 2.5 million cubic meters respectively. year has been taken in these calculations. The follo- 4.4. Costs per cubic meter wingly quoted costs are of importance: 4.2.1. Fuel costs The cost price per cubic meter tith a pumpingdistance of 2000 meters is: A basic requirement is a continuous supply of 4.311.000 = $ 0 52 3250 KW (4400 hp] throughout the 5000 production .8.250.000 856 and, with a pumping distance of 5000 meters: goingly descr?bed herein is capable to pump sucked 4.435.000 material directly ashore from the open sea with a = $1. 77 large workability which, for southern areas of the 2.500.000 North Sea, is 95$ of the time. 4.5. Contract price per cubic meter The calculated cost price per cubic meter of sand is at a reasonable level in view of unfavoura- If a figure of 10% is taJsenas a basis for a ble conditions under which the SSD must operate. return on investment with a pumping distance of 2000 meters then the income therefrom must amount to: The SSD is a specially designed installation $4.311.000+$ ~.111.000=$5.422.000. and for this reason is not universally applicable,however in certain cases, application of a system The attendant price per cubic meter will thus be: with the SSD has good prospects. 5.422.000 = $0.(6 Acknowledgement 8.250.000 The design of the SSD has been executed in the In the case of a pumping distance of 5000 meters, cadre of a graduation assignment to Mr. L. Goossens the income on the same basis must amount to: by the University of Technology at Delft, Netherlands. $4.435.000 + $1.202.000 = $5.637.000 The definition of this assignment came into being in co-operation with IHC Holland..In relation hereto, and the price per cubic meter: the authors tender their grateful thanks to Prof.Dr. 5.637.000 ~ ~ z *5 Ing. C. Gallin and Ing. J. Punt of the Naval Archi- . tecture and Marine Engineering Department of the 2.500.000 University. 5. CONCLUSIONS REZV3RENCES The application of semi-submersible hulls to 1. Nonner R., Dooremalen J.J.C.M. van, and Ziegler R.B., Offshore Dredging Systems for Beach Nou- suction dredging installations.makes the latter sui- table for operation in heavy weather conditions. rishment Projects, Offshore Technolo~ Conference 1974. The embodiment of the SSD presented here is of 2. Hooft J.P., Hydrodynamic Aspects of Semi-submer- a tiesignthat satisfies the design requirements laid sible Platforms, Thesis, Delft 1972. down therefor. A design on the basis of other requi- ~ 00 K.M., rements could result in a different hull configura- . and Miller N.S., Semi-submersible de- tion. sign: the effect of differing geometries on hea- ving response and stability, The Royal Institu- tor beach nourishment projects where sand is *ion of Naval Architects, spring Meetings 1976. unavailable in a reachable vicinity, the SSD can be 4. Pijfers J.G.L. and Brink A.W., Calculated drift used in combination with trailing suction hopper forces of two semi-submersible platform types dredges which suck sand elsewhere and dump it in the in regular and irregular waves, Offshore Tech- area of the SSD. nolo~ Conference 1977. As opposed to a combined system, the SSD fore- 5. Operating Cost Standards for Construction Equip- ment. Nivag 1977. Table 1 Operational conditions Survival condition 50 year storm Significant wave height (m) 8.00 Maximum wave height (m) 16.00 Mean wave period (see) 11 Mean wind speed (m/see) 30 Current (m/see) 1 Frequency of exceeding 2.9 x 10-3 Working condition monthly storm 3.50 7.00 8 18 1 1.2 Maximum draught condition 2.50 5.00 7 14 1 7 857 Survival condition Working condition Naximum draught condition TABLE 2 HORIZONTAL AND VERTICAL MOTIONS Horizontal motion amplitude (m) 3.81 0.73 0.26 Dimensions: circumferentialradius depth centre CO1umn diameter outer CO1umn diameter maximum draught survival draught empty draught length over al1 breadth over al1 maximum suction depth suction pipe diameter delivery pipe diameter TABLE 3 DATA ON INITIAL STABILITY Vertical rotion Draught (m) 22.00 16.50 11.50 amplitude (m) Displacement (ton) 4448 4051 3691 1.27 Centre of gravity 0.33 above keel (m) 7.88 7.01 6.73 0.14 Wetacentre height (m) 0.79 0.50 0.12 TABLE 4 MAIN PARTICULARS OF SSD 21.03 m 29.50 m 6.@l m 4.00 m 22.@3 m 16.50 m 4.W m 38.@3 m 45.@l m 50.03 m 0.65 m 0.60 m Installedpower: dieselengines 2 X 1400 kW (1900 hp) diesel alternators 2 X 330 kkl (450hp) emergencyalternator 50 kW (70 hp) Productioncapacitiesat suction depth 50 m: pumping distance 2000 m 1650 m3/h pumping distance 5000 m 500 m3/h TABLE 5 REVIEW OF ANNUAL COSTS Annual costs 2000 metres delivery distance Fuel 800.000 Lubricating oi 1 34.000 Crew 455.000 Insurance 222,C+30 Maintenanceand repair 1.500.000 Interestand depreciation 1.300.000 Displacements: at draught 22.00 m 4448 ton at draught 16.50 m 4051 ton at draught 4.00 m 1781 ton Weights: steel 1152 ton dredging installation 146 ton machinery installation 95 ton anchor system 49 ton acccmnodation 15 ton fixed ballast 324 ton Capacities: fuel 227 ton waterbal1ast 2786 ton lubricatingoil, provi- sions, fresh water etc. 16 ton Acconsnodation: 10 persons Spare parts dredging installation: 2 pumphousings 11 ton impellers and suction hoses 6 ton Annual costs 5000 metres delivery distance 800.000 34.000 455.000 240.000 1.500.000 1.406.000 Total 4.3KOO0 4.435.000 43 .wLYLEL I / / \ . :+E FIG, lB / // 1/ \ \ . & lEWA.A \ FIG, lC ~ F /i wI /1I ./ ./i. > / W / / FIG, lD.. . .. 1 =- ~ FIG, 1 - GENERAL ARRANGEMENT PLAN, JMEAVE RESPONS SS13 1.J II DRAUGHT 22.00 MDRAUGHT 16.50 M I // o X/ v 5 10 15 20 WAVE PERIOD (SEC). FIG, 2 - HEAVE RESPONSE CURVES, I 500 M 1 500 M 1 I /A OPERATIONAL ARFA 1 500 M 500 M FI IYERY_. LmE_ FIG, 3 - POSITION OF ANCHORS...

... deals with sea keeping and mooring tests on one hand and towing tests for two headings and three draughts (transit, survival and operation) on the other hand. The experiments were performed with two models at a scale of 1:50 and 1: 100. The results of the sea keeping tests, i.e. the transfer functions...

... Units in offshore industry, not necessarily oil offshore industry the general design philosophy with the influence of major parameters like draught, column and stability is shortly treated. Considerations regarding the strength during fabrication upending and use, as well as the fabrication...

... are slip formed afloat in an adjacent basin. The structure is then towed to the Inner Sound of Raasay on the West Coast of Scotland for deck installation. floatlng gravity structure aspect amplitude upstream oil &amp; gas assembly site draught engineer transverse motion exploration...

... system is large because of changes in draught, the minimum distance bet~ ween the hose connection to buoy and manifold is not reduced. This makes the system suit- able for modest waterdepths. The weathervane effect is optimum, as the centre of rotation lies in front of the vessel. Bow or stern- thruster...