Detailed Sea Bed Mapping For A Pipeline Across The Norwegian Trench

Transcript

In tern a tion a l H ydrographic Review, M onaco, L V II (2), J u l y 1980 DETAILED SEA BED MAPPING FOR A PIPELINE ACROSS THE NORWEGIAN TRENCH by M artin HOVLAND a n d A rne IN D R EEID E E n gineering D ep artm en t, S tatoil, Stavanger, N orw ay P a p e r p re s e n te d a t th e 1st I n te r n a tio n a l H y d ro g ra p h ic T e c h n ic a l C o n feren ce, O tta w a , C an ad a, M ay 1979, an d re p ro d u c e d b y k in d p e rm is s io n o f th e O rg an iz e rs. ABSTRACT In 1976, two y ears a fte r the discovery of the S ta tfjo rd oil field in the N orth Sea, the S tatoil/M obil G roup decided to go ah e ad w ith an evalu ation of th e feasibility of laying a su b m a rin e pipeline fro m S ta tfjo rd to the w estern coast of N orw ay. Due to the rough n a tu re of th e N orw egian w est coast a n d unexpected featu res (pockm arks) in the N orw egian T ren ch , the S tatfjord-N orw ay pipeline ro u te called for a detailed sea floor m apping. F o r the general m apping, boom er, sp a rk e r, side scan so n ar an d pitch, roll a n d heave-com pensated echo so u n d ers w ere used. F o r th e detailed su rveying of the ro c k y shore ap p ro ach areas, a dense profile grid w ith the above-m entioned system s w as used in ad d itio n to m an n ed subm ersibles ca rry in g video e q u ip m e n t a n d precision profiling equipm ent. In all, nine shore a p p ro a c h ro u tes w ere studied, of w hich th ree w ere subject to m a n n e d subm ersible surveying before one shore a p p ro ach route w as selected. V aluable experience w as gained in th e course of th e su rv ey program m e, in cluding experience on th e com plexity of su b m arin e navigation in rugged te rra in , au tom atic m ap p in g system s a n d the difficulty of p ro d u cin g side scan so n ar m osaics over larg e areas. 1. — INTRODUCTION The Statfjord field, 160 km w est of the Norwegian coast, in the northern North Sea was discovered in February 1974, and was declared com m ercial in August of the same year. The Norwegian part of the field is owned by the Statoil/M obil Group of w hich Statoil (the Norwegian State Oil Company) ow ns 50 % and the field operator Mobil Exploration Norway Inc., owns 15 %. In 1976 the Norwegian authorities requested an evaluation of the feasibility and the cost of two alternative crude oil transportation system s from the Statfjord field, nam ely, offshore loading by oil tankers, and pipe­ line transportation across the Norwegian Trench to a shore based terminal on the w est coast of Norway. Statoil undertook the pipeline project on behalf of the Statoil/M obil Group. The main objectives of the project, w hich w as termed the Statfjord Transportation System Project, were to study the feasibility and cost of installing, constructing and operating a large diameter (36 inch) crude oil pipeline from Statfjord to a term inal processing plant on shore. The project has posed a range of engineering questions, m ost of which were related to pipelaying in water depths of 300 to 350 m in the Norwegian Trench. 2. — REQUIRED ROUTE INFORMATION One m ain objective of the project w as to locate and describe an acceptable pipeline route. This necessitated a com prehensive program for acquisition of all necessary hydrographic and geophysical data. It was evident from the start of the project that the greatest routeing problems would be presented by the rugged sea floor in the coastal region, where crystalline bedrock constitutes the major part of the seabed. A large dia- F i g . 1. — A pe rsp ec tiv e view , lo oking fro m the n o rth at the N o rw eg ian T rench ( v e r t i c a l sc a le e n h a n c e m e n t ) . F ig . 2. — Map sh o w in g a r e a s surveyed a n d th e lo ca tio n of S ta tf jo r d and S o t ra . meter submarine pipeline requires a route which is devoid of horizontal sharp bends, (curvature radii should not be less than about 2 km ). Since the vertical bending tolerance of the pipe is also relatively small, the route should preferably not present any abrupt changes in slope. Further­ more, the pipe should be able to rest upon a seabed, ideally consisting of gravel, sand or clay. The aim of the first reconnaissance survey, along the west coast of Norway in 1974, was to locate any possible seabed features in the cry­ stalline nearshore areas providing an obvious approach route to shore. The survey was run with boomer and echo sounder, but did not result in the identification of any clear-cut approach route. After surveys both nearshore and offshore in 1975 and 1976, the island of Sotra near Bergen (figure 2) was chosen as the potential pipeline landfall area, due m ainly to the deeper waters further north in the Norwegian Trench, the width of the coastal zone, and the possibilities of finding a suitable terminal area on Sotra Island. All the survey work performed between 1974 and 1976 was done by Geoteam A /S, Norway. 3. — DETAILED NEARSHORE SURVEYS — 1977 A narrowing-down method was applied for the detailed hydrographic and geophysical mapping of the areas identified in the coastal zone as po ten tial shore a p p ro ach areas. T he procedure consisted of a step-by-step m apping, w hereby th e areas w ere m apped to an increasingly detailed level, for each step reje ctin g some areas, while the best ones w ere selected for fu rth e r surveys. T he n earsh o re detailed surveys off S otra w ere p lanned a n d executed in the follow ing sequence : Step 1 : P ro d u ctio n of b ath y m etric an d sedim ent d istrib u tio n m aps of th e a p p ro ach areas previously not surveyed, to a scale of 1:10 000 . Step 2 : P ro d u ctio n of detailed b ath y m etric and sedim ent d istrib u tio n m aps to a scale of 1 :5 000 of prom ising shore app ro ach routes identified in Step 1 and in previous surveys. Step 3 : P ro d u ctio n of precise seabed info rm atio n along the m ost prom ising ro u te s identified in Step 2. 3.1. Nearshore survey methods, equipment and time Step 1 w as c a rrie d o u t in the spring and early sum m er of 1977 em ploy­ ing a 64 ft fishing boat. P ositioning w as done by “line of sig h t” rad io navigation, th e m ain h y d ro g ra p h ic and geophysical eq u ip m en t being a pitch, roll an d heave com pensated echo sounder, a su rface tow ed sub­ bottom profiler (boom er), and a side scan sonar (figure 3). The sonar fish w as tow ed a t a m ean a ltitu d e of 40 m above th e rugged sea floor. T he line spacing in Step 1 was set to 150 m. The m ain profiling direction w as N-S in o rd er to detect p rom ising geological fo rm a tio n s ru n n in g E-W . The d a ta processing, in terp retatio n , plotting an d contouring w as c arried out on land. F u r th e r details on the equipm ent a re given in table 1. In Step 2, five p o ten tial shore approach routes becam e subject to m ore detailed m apping in the sum m er of 1977. The sam e ship and equip­ m en t w ere used as in Step 1, except for one area w here th e vessel and eq u ip m en t from the offshore survey w ere used. A 32 ft cabin cruiser equipped w ith a roll com pensated echo sounder and a boom er w as used to m ap areas close to shoals, islands a n d shore. The positioning of the sm all vessel w as done w ith two theodolites. T he line spacing in Step 2 w as 75 m generally, a n d about 25 m along identified p o tential route corridors. Step 3 w as begun in the a u tu m n of 1977 w ith an evaluation of the detailed 1 :5 000 m ap s produced in Step 2. None of th e routes w hich w ere identified could provide a n y solutions avoiding som e sea floor p re p a ra tio n s p rio r to pipe laying. The final step in th e shore approach m apping sequence, th erefo re, called for a thoro u g h ex am ination an d p re ­ cise docum entation of the critical sections of the po ten tial routes. Follow ing th is evaluation th re e areas were chosen for survey and m apping by m anned subm ersibles, two outside L0no Island, a n d one fu rth e r n o rth in the H ja rt0 y area. T he in fo rm atio n gained by th e subm ersible surveys w as then su b ject to th o ro u g h engineering analyses for selection of the best pipeline route. F ig . 3a. —- S id e s c a n s o n o g r a m o f a n e a r s h o r e a r e a . F ig . 3b. — T h e c o r r e s p o n d i n g s u b - b o t t o m p r o f i le r r e c o r d (see fig. 3a). The m anned submersible surveys were carried out in two stages with two different contractors. One survey was done late in 1977, the other one early in 1978. The main instrum ents carried by the submersibles were precision bottom-profiling system s and video tape system s. Further equipm ent details are given in Table 1. A profile grid with a line spacing of 25 m was run in critical route sections, w hile only one profile was run along the other parts of the prom ising approach routes. Some submersible tracks for the L0no area are shown in figure 7b. Mapping sequence, time, vessel, contractor, survey system s and equipm ent specifications for nearshore surveys conducted in 1977 and 1978 Mapping sequence, tim e, vessel and contractor Survey system Type and specification Positioning Motorola Mini-Ranger III (MRS), short range radio positioning. Echoso under Simrad EK-S scientific, 38 kHz, 7° beam angle transducer, papertape and analogue datalogging. Pitch, roll and heave compensation by use o f a gyroplatform made by the Continen­ tal Shelf Institute, Trondheim. Sub bottom profiler EG & G Uniboom system (500 Hz at 300 J). Analogue and magnetic tape recording. Side scan sonar Klein Hydroscan model 4 0 0 (1 0 0 kHz). Analogue paper recording. Step 2, Summer 1977, M/K "Lotroll", Same as above Same as above "Pegasus” 32 ’ Noteby-Blom Joint Venture, Norway Positioning Echosounder Intersection by two Wild theodolites. Atlas 4 7 0 ,3 3 kHz, 6° beam angle, roll stabilized transducer. Sonarmarine Ltd. U.K. See Table 2 Step 3a, Autumn 1977, M/S “ Vickers Van­ guard”, 269’ support ship with two minisubmarines : “Pisces II” and “Pisces VIII” . Vickers Oceanics Ltd., U.K. Surface positioning Motorola Mini-Ranger III (MRS), short range radio positioning. Submarine positioning and position log ATNAV, long base acoustic trans­ ponder navigation system. A dual axis Doppler sonar position log, mounted on “Pisces II” . Precision depth profiling Combination o f precision Digiquartz pressure sensor and high frequency short range echo sounding system. Video recording Sub-Sea Systems under-water camera, Sony recorder. Step 1, Spring 1977, M/K "L o tro ll" 64’ form er fishing boat, Noteby-Blom Joint Venture, Norway Stereo photography Two 35 mm UMEL Deep Sea cameras. Sub-bottom profiler Parametric sub-bottom profiling sys­ tem , 7° beam angle. Side scan sonar Klein Electronics (100 kHz), magnetic tape recording, mounted on “Pisces II” . Step 3b, Spring 1978, Surface positioning M/S, “InterSub Three”, 249’ support ship with Submarine positioning one minisubmarine : "PC 1201” . Kvaemer Precision depth Intersub, Norway. profiling Video recording Motorola Mini-Ranger III (MRS), short range radio positioning. PA21, long base acoustic transponder under-water navigation system. Combination o f precision CZ9029 pressure sensor and ELA precision depth sounder Type D S11 (170 kHz). Hand-held Sony camera, Sony recor­ der. Video shot through the dome­ shaped plexiglass nose o f “PC 1201 ” . 4. — DETAILED OFFSHORE SURVEYS — 1977 It has been generally know n th a t the sea floor in th e n o rth e rn N orth Sea is generally even an d consists of soft clay in th e deeper p a rts (N orw egian T rench). It w as also know n th a t th e sea floor in som e areas h as a varying degree of ro u g h n ess in th e form of gullies a n d local depres­ sions. T he surveys in 1974 an d 1976 show ed th a t these seabed featu res w ere of a sim ilar type to features, term ed p o ck m ark s, observed in the B ritish N orth Sea. P o c k m ark s a re local depressions of a circu lar, oval or irre g u la r shape, varying in diam eter from less th a n 10 m to 300 m, an d ran g in g in depth below th e m ain seabed level from 1 m to 10 m. T he purpose of a detailed offshore survey in 1977 w as th erefo re tw ofold : a) To produce detailed b ath y m etric profiles an d in fo rm a tio n on soil types and thicknesses along th e chosen offshore pipeline ro u te corrid o r. b) To m ap the position, size, form an d d istrib u tio n of p o ck m ark s in an are a of the p o ck m ark s region. T his w as done p rim a rily fo r ro u te alignm ent, b u t also in o rd er to a tte m p t to u n rav el th e m ode of p o ck m ark form ation. T he are a chosen as a p o c k m a rk reference a re a w as m apped such th a t a la te r resu rv ey of the are a w ould enable th e detection of possible changes in the p o ck m ark p a tte rn . T he are a s m ap p ed offshore are show n in figure 2. 4.1. Methods and equipm ent used offshore T he vessel em ployed for th e detailed offshore survey (see tab le 2) w as a 185 ft fo rm er ste rn traw ler. T he survey was r u n on a 24-hour basis. Table 2 Survey system s, type and specifications on the offshore survey conducted in the sum m er o f 1977 w ith M /T Criscilla, a 185' form er stern trawler. The contractor was Sonarm arine Ltd., UJC. Survey system Type and specifications Positioning Hi-Fix/6 with Decca Pulse 8 as back-up system. (Decca Trisponder nearshore). Echosounder O.R.E. Model 323 A with shallow-towed, heave compen­ sated, 9* beam angle transducer. Sub-bottom profiler Modified Huntec deep towed boomer (540 J) with analogue paper and magnetic tape recording. Side scan sonar Sonarmarine deep towed sonar fish with modified Kelvin Hughes 48 kHz transducers. Range : 375 m and 750 m. The main hydrographic and geophysical equipment was : a precision echo sounder with a shallow towed, heave compensated transducer, a deep towed, depth compensated, high resolution boomer and a deep towed, single channel, high resolution side scan sonar (figure 4). In the pock­ mark area the line spacing was 135 m in order to provide sufficient overlap and coverage from both sides. In the remaining offshore pipeline route corridor the line spacing was 600 m. . POCKMARKS th SEA aaTT0hv MULTI LAYERED — r - CUTS I'ig . 4a. — S a m p l e o f d e e p t o w e d sid e sc a n s o n a r r e c o r d f r o m t h e N o r w e g i a n T r e n c h , b. — S am p le of deep to w ed b o o m er reco rd in g fro m the sam e area. 5. — SURVEY RESULTS AND EXPERIENCE 5.1. Nearshore The field work in Step 1 of the nearshore surveys took l \ months, during which 900 km of bathymetric and geophysical profiling was carried Flo. 5a. — R e c o n n a i s s a n c e p r o f i le s r u n i n 1974 in o n e o f t h e n e a r s h o r e a r e a s close to th e Isla n d of L 0no w e st of S o tra m a in Island, h. — B a t h y m e t r i c m a p o f t h e L 0 n o a r e a . T h e m a p is p r o d u c e d f r o m s o u n d i n g s d o n e b y t h e N o r w e g i a n H y d r o g r a p h i c Office a r o u n d 1930. F ig . 1). — T h e 6a. —- A d d i t i o n a l s u r v e y l i n e s r u n i n 1975 a n d 1976 i n t h e L 0 n o a r e a . re su ltin g m o d ific atio n of th e c o n to u rs ach iev ed by th ese su rv e y lin es c o m p a r i s o n t o t h e m a p i n f ig u r e 5b. in out over a 120 km 2 area. Due to numerous islands, shoals and the proxi­ m ity to shore, only daylight working hours were used in order to assure safe navigation. The shore-based data processing, plotting, geophysical interpretation, and contouring were commenced two weeks after the field work started. A set of three map sheets to a scale of 1:10 000 were thus completed. The map sheets contained information on bathymetry (with a contour interval of 10 m ), the soft sedim ent thickness and its distribu­ tion, th e d istrib u tio n of rock and coarse sedim ents a n d th e sea floor m orphology. An ad d itio n al 1900 km of profiling in Step 2 yielded detailed m aps for five areas to a scale of 1 :5 000 w ith a contour in te rv a l of 5 m , an d seabed in fo rm a tio n as m en tio n ed for Step 1. T he step-w ise su rveying a n d th e re s u ltin g im p ro v em en t of the b a th y ­ m etric in fo rm a tio n is illu stra te d in figures 5 to 7 fro m th e L0no area, one of the p o ten tial sh o re ap proach areas. In Step 3, 63 km of m an n e d subm ersible surveying yielded detailed depth profiles, c o n tin u o u s video recordings, stereo colour p h o to g rap h s, a n d som e side scan so n ar in fo rm a tio n along th re e p o ten tia l sh o re ap p ro ach ro u tes in two lan d in g areas. T his in fo rm a tio n provided a ro u te d escrip­ tion enabling the c o n stru c tio n of detailed d e p th profiles to a scale of 1 :2 000 along critical sections of th e ro u tes. T he experience gained d u rin g the tw o-year m ap p in g sequence describ­ ed for the coastal zone m ay be su m m arized as follows. a) Time and costs T he m ain lesson le a rn t in the course of th e p ro je c t w as n o t to u n d e r­ estim ate th e m apping req u irem e n ts. E ven th o u g h it w as k n o w n th a t the coastal zone w ould p re se n t th e greatest ro u te in g problem s, m o re tim e th a n a t first a n ticip ated w as sp e n t in search of a shore a p p ro a c h ro u te. T he search for an easy shore ap p ro ach ro u te becam e a se arc h fo r a technically feasible ro u te w hich re q u ire d th e least a m o u n t of sea floor p re p a ra tio n s p rio r to pipe laying. b) Side scan sonar handling P rio r to th e field w o rk in Steps 1 and 2 it w as k n o w n th a t th e side scan so n ar h a n d lin g w ould req u ire a lo t of effort in o rd e r to a cq u ire good re su lts in th e n e a rsh o re area. In o rd er to tow a s ta n d a rd so n a r fish a t an o p tim u m m ean h e ig h t over the rugged te rra in , each line w as carefu lly p lan n ed on h y d ro g ra p h ic ch arts. A w in c h o p e ra to r w ith d ire c t com m uni­ cation to th e bridge w as em ployed. F a irly good reco rd s w ere a cq u ired from a to ta l of 2100 km o f n e a rsh o re side scan tow age. T h e so n ar fish only grazed a su b m a rin e m o u n tain to p once. T his h a p p e n ed on a line w here no problem s w ere expected a n d th e w in ch m an h a d gone for lunch. In stead of em ploying an e x tra m an fo r th e w inch, it w ould be m uch b e tte r w ith eith er a steerab le fish or a fa s t w in ch o p erated d irectly by th e side scan so n a r operator. c) The echo sounder system and vessel It was difficult to find an ideal com bination o f vessel a n d echo so u n d er for th e n e a rsh o re area. A narro w -b eam tra n sd u c e r is re q u ire d fo r ra p id dep th v ariatio n s. T his again calls fo r a stable p latfo rm . H ow ever, a large Fig. 7a. — Detailed nearshore route survey lines run during mapping Steps 1 and 2 in 1977 and previously, in the L 0 no area. (stable) vessel cannot operate as close to shore as required. A compromise solution was therefore chosen. A narrow-beam transducer was installed in a fairly small vessel, together with a pitch, roll and heave sensor. The transducer’s movem ents were compensated for in the data processing done on-shore. This solution led to a reduced sea state tolerance for the survey, which had to be interrupted when the vessel’s roll (in particular) became too large. d) Submersible operation In the rugged nearshore terrain the long base acoustic bottom navi­ gation system turned out to be difficult to operate due to shadowing of paths and m ultiple reflections of transponder pulses. Even though maps to a scale of 1 :5 000 were provided, and great care was taken in planning, both contractors had problems at first finding locations for the bottom transponders which gave good positioning. Quite some time was spent at the beginning of each survey in a trial and error process of trans­ ponder positioning. The end result was a fairly narrow transponder pattern where the transponders were placed 10-20 m above the sea bottom. Field time could have been saved if a numerical sim ulation model had been at hand, whereby the optimal long base transponder positions could be computed in relation to the topography and the water temperature lapse rate. I*'hi . 71). — The final b a th y m e tr i c map of the L0no area resulting mainly from the 1977 survey lines. Some manned su b m e rsib le tracks are also shown for the two routes in t h i s shore approach a r e a . 5.2. Offshore T he offshore field w ork done in A ugust an d Septem ber 1977. D urin g th is period a to tal of 2900 km of profiling w as com pleted, covering a n are a of 350 k m 2. T he in te rp re ta tio n and processing of the d a ta w as done onshore and resu lted in, am ongst others, a set of m aps at a scale of 1:10 000, w ith 2 m d ep th c o n to u r line interval, show ing sedim ent th ic k ­ ness an d stru ctu re, all p o ck m ark s w ith a diam eter larg er th a n 10 m, w recks, and the in te n sity of tra w l scars (figure 8). Some of th e pock­ m ark s were found to be som ew hat m ore irre g u la r in shape th a n previously expected. Strings of sm all p o c k m a rk s and shallow fu rro w s (lineations) a p p a re n tly connected to larger pock m ark s were also discovered and m apped. F ig . 8. — S am p le of a m a p p ro d u c e d fo r th e p o c k m a rk a re a a n d a lo n g th e offshore ro u te in th e N o rw eg ian T rench. T he experience gained from th e offshore survey m ay be sum m arized as follows. a) Positioning Due to its 24-hour capability, Pulse-8 was chosen as the p rim a ry positioning system offshore. H i-F ix /6 was included p a rtly as a back-up system , and p a rtly to increase th e accuracy in th e p o ck m ark s region. However, it tu rn e d o u t th a t H i-F ix /6 w as stable th ro u g h o u t 24 h o u rs and w as th erefo re used as the p rim a ry positioning system , a n d w as operated in range-range mode. P ulse-8 w as used fo r lane-setting (of H i-F ix /6 ) and as a back-up system . D u rin g the survey H i-F ix /6 seem ed to provide a rep eatab ility of 3-4 m, and an absolute accu racy of 10-15 m . b) Side scan sonar T he increased know ledge about p o c k m a rk featu res re su ltin g from th is survey, w as m ainly achieved by th e side scan so n ar w h ich w as towed a t an o p tim u m altitu d e (15-20 m) above th e bottom , reg a rd le ss of the (actual) w ater depth. T his w as done w ith no rm al profiling speed (3-4 k nots), w ith a fairly reaso n ab le lay back of th e fish (about 350 m in 300 m w ater d epth). An a tte m p t w as m ade to co n stru ct a side scan so n ar m osaic p ictu re of th e p o c k m a rk area. How ever, th e re su lts w ere n ot as good as expected, an d did not add significantly to the p rese n tatio n of p o c k m a rk features. c) Deep towed sub-bottom profiler In relatio n to experience gained fro m previous surveys w ith sta n d a rd surface tow ed equipm ent, it w as found th a t the deep tow ed boom er (tow ing depth 100-250 m) gave tw o m ain advantages. Due to the proxim ity to the sea floor, the sou n d pulses travel th ro u g h less w a te r, reducing the am o u n t of noise and a tte n u atio n , a n d th u s providing an increased d a ta quality. T he deep tow ed system s are less influenced by th e sea su rface condi­ tions an d a re therefore less w e a th er dependent. A disad v an tag e is, however, less position control. 6. — THE PIPELINE ROUTE A ro u te fo r a p o ten tia l cru d e oil pipeline from the S ta tfjo rd field to the w est coast of N orw ay h a s in the course o f th e p ro je c t been identified and m apped in such detail th a t engineering on th e pipeline could be p e r­ form ed. T he ro u te is show n in figure 9. Along the offshore section, the pipeline ro u te is aligned so th a t larger pockm arks will be avoided. M any possible p o c k m a rk form ation m odes have been review ed. T he theories studied ranged fro m p o ck m ark s being caused by biological activity to w a rtim e depth charge explosions. It is p rese n tly believed th a t pock­ m ark s are form ed in so ft clays by a particle-b y -p article rem oval of m aterial, caused by slow gas m igration. In the shore ap p ro ach section th e ro u te is p a rtly covered by sand and gravel. B ut rock b lastin g can how ever not be avoided a n d w ill be necessary in w ater d e p th s dow n to ab o u t 40 m . Rock filling will also have to be done along som e shallow w a te r sections. In deeper w a te r dow n to 240 m, long pipe sp a n s a n d overstresses in th e 36 in ch pipe w ill be F ig. 9. — T he o v e ra ll p ip e lin e ro u te fro m S ta tfjo rd to S o tra Islan d . prevented by the in sta lla tio n of special supports. Several boulder areas in th e shore a p p ro ach section w ill fu rth e rm o re have to be cleared p rio r to pipe construction. The to tal su b m arin e length of the route is 183 km , w ith 99 km in w a te r d ep th s g reater th a n 300 m. The m axim um d ep th is 354 m only 8 k m from the N orw egian coast. The pipeline ro u te continues onshore in a len g th of 24 km fro m the lan d fall site to the term in a l site. T his p a rt of th e ro u te includes 11 island-to-island w ater crossings an d 3 tunnels. T he p e rm a n en t tra n sp o rta tio n system for crude oil fro m the S ta tfjo rd field, w h eth er offshore loading or pipeline, will be chosen later. 7. — CONCLUSIONS L ooking back on th e survey period, from our (S tatoil’s) point of view, som e im provem ents of the survey equipm ent an d m ethods w ould have been desirable. T he follow ing item s indicate som e are a s w here w e th in k th a t increased rese a rc h an d developm ent could pay dividends. F o r sub-area con stru ctio n purp o ses there is a stro n g dem and for quantified areal in fo rm atio n , w hereby in terp o latio n of profile d a ta is avoided. Such a possibility probably lies in the developm ent of a “ste re o ” side scan sonar system or an acoustic “c a m e ra ”. Moving into deeper w a te rs there is a dem and fo r a com m ercial deeptowed m u ltisen so r c a rrie r in co rp o ratin g good han d lin g facilities, steering possibilities, a n d a reliable positioning system . A n obvious d em an d fo r th e client is to increase the su rv ey speed w ith o u t reducing d a ta q u a lity . U sing a com bination of h y d ro g ra p h ic and geophysical system s, a typical survey speed today is 3-4 k n o ts. A 10 knot capability in deeper w a te rs w ould obviously m ean a g rea t ad vantage. T he m u ltisen so r c a rrie r m en tio n ed above could possibly provide a n answ er. T here is also a g reat need for im proved su b m ersib le-m o u n ted su b ­ bottom profiling a n d so n ar reco rd in g system s, in co rp o ratin g on-line p ro ­ cessing of the d a ta , w hereby v a ria tio n s in the subm ersible’s a ttitu d e are corrected for. A great deal of tim e in the field could be saved w ith an im proved subm ersible positioning system , giving im m ediate on-line positions. Sub­ m ersible surveying is today a fa irly slow an d laborious process, due m ain ly to the fact th a t th e subm ersible h as to stop to get a reliab le position fixing. T here are, how ever, g rea t expectations to the developm ent of a com m ercial in e rtia l n av igation system , or a reliable doppler system .