RRS Ernest Shackleton Diary — 2 July 2000
Continuing the Pipeline Survey. Fish and anemone spotting. Part five of the Shackleton history
Date: Sunday July 2, 2000
Position at 0800 UTC +1
Latitude 57°43′ North
Longitude 00°45′ East
Current Weather: Fine, clear and sunny
Wind: 5 knots
Sea state: Rippled
Air temperature 13°C
This whole week has seen us following a single pipe, the Forties 36″, out from Cruden Bay to the Forties Oil Field. This means that there is not a great deal to report, with the ROV having been in the water for nearly the whole period.
On Thursday morning a helicopter came out from Aberdeen and a change of charterer personnel took place, with fifteen joining and fifteen departing the ship. Charterer personnel normally work for twenty-eight days and then go on leave, and their personnel are split so that there will be a change every fourteen days. The helicopter also carried some cargo and up-to-date newspapers, something that is missed whilst at sea.
Thursday was also Captain John Marshall’s birthday, so by way of a present we let him be the Helicopter Landing Officer and stand out on the Helideck and wave to the aircraft as it landed and then run the operation of getting people and baggage on and off the aircraft. Standing under a helicopter for twenty minutes, with the engines and rotors running is a very exciting thing to do.
By Saturday evening the end of the pipeline was approaching. This means that the ship will once again be operating within the 500m zone of the Forties Platforms as we continue the pipeline survey for BPAmoco.
The Forties Field consists of five production platforms and one riser platform and is about 110 miles to the east-north-east of Aberdeen.
There are four fixed installations, Alpha, Bravo, Charlie and Delta, which are self-contained production and processing units. Platforms Alpha to Delta were installed between 1974 and 1975 with Echo being installed in 1986. There is also an unmanned riser platform called Unity. The platforms sit in about 130 m of water.
Forties Charlie is a fixed installation with accommodation for some 149 personnel and is the production and drilling facility within the Forties Field. It also acts as a collection installation for the Forties Field importing oil and natural gas liquids from Alpha, Bravo and Delta.
Some of you may well have seen the BBC 2 programme the other week on the epic voyage of Shackleton in the James Caird from Elephant Island to South Georgia. I understand that this was a very good documentary on the journey and the hardships endured.
The following is taken from the BAS in-house newspaper, June edition, and refers to the naming ceremony.
“For me, the BAS highlight of last month was the renaming ceremony of the RRS Ernest Shackleton on 13 May in Hull. After her highly successful first season for BAS there was a real sense of enthusiasm and pride from ship’s officers and crew. It was apparent that the 200 guests, including the local mayors, the Chairman and Chief Executive of NERC, representatives from Polar Ship Management, other senior dignitaries, and present and past members of BAS and their families, were all duly impressed. Alexandra Shackleton’s rousing speech evoked both the spirit and the achievements of her grandfather, whilst the music of the Kirkbymoorside Town Brass Band provided a musical backdrop which was as informal as it was flawless.
Our Norwegian visitors were especially impressed by the rendition of their national anthem. The band, along with other guests, were eager to take part in the guided tours around the Shackleton and James Clark Ross. I would like to thank all of you who made the ceremony a day for BAS to be proud of, and I look forward to the service and opportunities which the new vessel will provide".
Director, British Antarctic Survey
Continuing the series by Simon Allen.
Types Of Survey
General Imaging (Acoustic) Survey
General Imaging techniques are based around traditional acoustic sensors such as echosounder and sidescan sonar. The intention of the General Imaging survey is to cover as much of the pipeline as possible in a fast and relatively cheap manner. The sidescan sonar provides a good general indication of the situation of the pipe and surrounding seabed. The sidescan, which can be a towfish, or ROV/ROTV mounted, is moved parallel to the pipeline at a fixed offset. The height of the sidescan is varied dependent on the situation of the pipe.
The sidescan sonar is now generally deployed as the primary sensor on a Remotely Operated Towed Vehicle (ROTV). This box-kite like device has control vanes that allow some control over the ROTV's height and cross course. With greater control over the sidescan position, the range of the sidescan can be reduced allowing higher frequencies and better target resolution.
Sidescan sonar is the primary sensor used for General Imaging surveys. Despite its many shortfalls, there is no other tool that can provide such a good overview of the pipe and its surrounding seabed.
The principle of a sidescan sonar is simple, a tightly formed acoustic pulse with approximate dimensions of 1.5° horizontally and 60° vertically is transmitted to strike the seabed at an angle. The strength of the returning echoes are plotted against time with the scale resetting to zero with each new transmission.
Old style analogue systems amplified the returning signal and literally burnt a moving roll of paper to a varying degree dependent on the strength of the “return” and the gain settings. Crude, but remarkably effective. Modern sidescan sonars work on the same principle except the returning signal is digitised in the fish and the results passed back through the tow cable to display and recording equipment in a digital format. This has the advantage of being less susceptible to noise when transmitted over long cables and the records can be copied with no loss of quality. Digital signal processing may also be performed on the collected data to enhance the record.
The frequency of a sidescan sonar can vary from 50 to 650 kHz, the frequency effects the resolution and range obtained. The lower the frequency the higher the range, but the poorer the resolution. Pipeline inspection from an ROTV, where the position of the sensor to the pipeline can be controlled normally uses sidescans which operate in the 300–500 kHz range. The horizontal range is normally set to 50m at these frequencies.
The identification of features is largely down to the experience of the geophysicists, the measurement of features is based on assumption and trigonometry. As explained above the raw data from sidescan is intensity of return against time, sidescan sonar does not measure angle, but if we assume a uniformly flat seabed then it is possible to derive “measurements” from the sidescan records. Figure 3 shows a short section of a sidescan records with a diagram showing how this relates to distance on the ground.
The first thing you will notice is that the linear record, is not linear on the ground. With the “fish” flying at 10m as it is in this example, the first 15m of the record are all a bit cramped on the record. Most modern sidescan systems offer the option to display the records “range corrected”, that is to say with the full width of the record representing the linear horizontal range, but little is gained in expanding the first 5–10m of the record, and most geophysicists prefer to interpret features from the uncorrected records. Range corrected sidescan records have a place in mosaicing multiple records or developing geo-referenced co-ordinates from a record.
With our assumption of a uniform flat seabed solving the range to a target and the height of a target is a case for Pythagoras and similar triangles. Taking Hf to represent the sidescan height, Rs the slant range to target, then the horizontal range to target is Hr = (Rs²+ Hf²)
To determine the height of the target we know:
by similar triangles, therefore
To determine the along track (length) and across track (width) dimensions we have to make further assumptions about the nature of the target. For measurement of the length, we assume the sidescan maintained a steady heading and was towed at a constant velocity for the whole time that the target was visible to the sonar. In the example below we see a trawl board resting in close proximity to a pipeline.
If for example the fish sidescan was being towed at two metres a second (approximately four knots) and the time difference between T1 and T2 was one second we would calculate its length to be two metres. In some cases however these assumptions can be flawed, particularly if the sidescan is being tugged by vessel movement in the tow cable. Quality control of the collected data is therefore of the highest importance.
The width of a target can be measured in a number of ways, all of which require some knowledge of the possible shape of the object being observed. In the above example we are fairly lucky because we know the shape of a trawl board and know that we are most likely seeing the flat face, its width is therefore the difference in the horizontal range from the start of the target to the end of the target. For a target such as that seen in figure 4, the only assumption we can make is that it is probably broadly symmetrical and therefore its width is double the horizontal length of the insonified face.
Fortunately we know the size and shape of the subsea structures that have been placed on the seabed otherwise we would have trouble providing measurements for the likes of this Subsea Intervention Valve cover.
Features of note on this reverse coloured record have been annotated:
A Section of pipeline in span between the gravel dump and SSIV. A highlights the shadow, the pipeline itself can just be seen as a white vertical line going into the top right hand corner of the SSIV.
B A pipeline leading into the SSIV clearly showing two flanges. On the original record some of the flange bolts could be distinguished.
C Signs that the sidescan has been tugged. This is characterised by the vertical lines across the width of the record just below the C. This evidence calls into question any calculated length for span A as this is also effected by the yaw of the fish.
D Another span, again from the gravel dump to the SSIV, but showing no signs of distortion.
The angle at which the acoustic signal strikes an object can also determine what is seen. To determine whether a pipeline is spanning it is ideal that the strike angle is in the order of 10–20° from the horizontal. If however the pipeline is trenched, then the sidescan must be positioned to see over the top of the trench walls, leading to a strike angle of 20–40°.
Figure 7 is a negative image of a pair of umbilicals in a trench, using the negative image it is easier to see the shadows. The record was “illuminated” from the left and as you can see there are small shadows on the left hand trench wall. If the sidescan was flown any lower then the umbilicals would be in shadow, any higher and the umbilicals would present less of a target and be more difficult to detect.
This is shown diagrammatically in figure 8, where the fish was towed in position B, if the sidescan had been towed in position A the umbilicals would have been shadowed, and in position C any spans indistinguishable.
As with all features detected by sidescan, the detection and measurement of freespans is made easier if the seabed is flat. This is rarely the case, but by careful monitoring and comparison with span lengths and heights from visual survey sidescan is a useful tool for monitoring trends. Figure 7. Shows a number of idealised situations. Situation B shows a span over a flat seabed, if the hard return from the pipe has a width greater than the pipe radius before displaying the pipe shadow then the pipe is spanning. Simple trigonometry allows the calculation of span height. In Situation C the pipe may or may not be spanning, with sidescan it is difficult to tell, and in situation D the pipe is spanning, but any attempt to determine the span height from the sidescan records will result in a wrong answer.
With all the vagaries of sidescan interpretation and measurement, why do we still use it? The answer is simple. There is no better tool for getting a fast overview of the pipeline and the surrounding seabed. No other tool can provide such resolution over such a wide area, and most importantly the records are directly interpretable the moment they are recorded. There is a very low processing overhead.
Swath systems have recently been put forward in a variety of forms as a possible replacement for sidescan sonar, but no existing swath systems have the resolution to replace sidescan. Whilst swath data can be presented in “sidescan like” ways, (Figure 10) it is important to examine the mechanics of swath in order to assess its limitations and strengths.
The resolution of swath, like sidescan is dependent on the frequency of the transducer and the distance of the transducer from the target, but unlike sidescan the “beam” is not continuous, but split into discrete rays. Because the angle at which these rays are transmitted is known the range measured by individual rays can, when integrated with vessel attitude and position information, be translated into depths and absolute positions.
High frequency swath transducers can be vessel, ROV, or ROTV mounted, but the meaningful processing of the collected data is dependent on highly accurate position and attitude information.
Consider the footprint of a one degree beam looking directly down and at 45° to the vertical at two ranges, 30m, and 100m. In 30m of water the corresponding footprint widths are approximately 0.5m and 0.8m respectively. In one hundred metres of water the footprint widths are 1.7m and 2.7m. In both these water depths a towed sensor or ROV can be positioned at an optimum position.
In Figure 11 we can see a typical trench profile overlaid with swath footprints of 0.5 m and 1.5 m and we can see what is lost. Based on the intensity of the return it is possible to ensure that the pipe is detected (where visible) but still the resolution is a problem.
Vessel-based swath is an excellent tool for monitoring the erosion/deposition of seabed material in areas subject to large changing forces such as landfall areas, or for assessing the volume of seabed deposits such as drill mud mounds. ROV-based swath is an excellent tool for pre-lay survey. The processing of swath data will be covered in a later section.
Ernest Shackleton Part 4
The epic voyage of the James Caird to South Georgia
Meanwhile, the James Caird was making 3 mph between the icebergs. Worsley imagined structures and creatures etched into the mighty bergs as he described, “Swans of weird shape pecked at our planks, a gondola steered by a giraffe ran foul of us, which much amused a duck sitting on a crocodile’s head. Just then a bear, leaning over the top of a mosque, nearly clawed our sail… All the strange, fantastic shapes rose and fell in stately cadence with a rustling, whispering sound and hollow echoes to the thudding seas…”. They were making a fairly good distance each day… some 60 to 70 miles. But the going was very rough. The sleeping bags became soaked making it increasingly difficult to find warmth. The boulders taken aboard for ballast had to be shifted continually in order to trim the boat and give access to the pump, which became clogged with hairs from the moulting sleeping bags and finneskoe. The four reindeer sleeping bags shed their hair freely from the constant dampness and soon became quite bald. Their legs were chafed by the wet clothing, which had not been changed for seven months. The insides of their thighs had been rubbed raw with seawater increasing the pain. Meals were regular in spite of the stormy weather. Breakfast, at 8 a.m., consisted of a pannikin of hot hoosh made from Bovril sledging rations, two biscuits and some lumps of sugar. Lunch, at 1 p.m., was more Bovril sledging rations, eaten raw, and a pannikin of hot milk. Tea, at 5 p.m., had the same menu. They had 6½ gallons of fuel for the oil lamp which complemented their supply of candles. On the fourth day out, a severe storm hit them. During the afternoon they spotted small bits of wreckage, the remains probably from some unfortunate vessel that had failed to weather the storm. The next day the storm was so fierce that they had to put out the sea anchor in order to keep her heading into the sea, take in the double-reefed mainsail and hoist the small jib instead. A thousand different times it appeared the small boat would capsize but she lived on. The south-westerly gale was born above the Antarctic continent and with it came temperatures near zero. The sea spray froze on the boat, coating everything with a heavy layer of ice. The boat became so heavy that the men were forced to use what tools they had to continually chip away the ice as it froze. By the next day the weight of the ice became a serious problem as she became more like a log than a boat. The situation called for immediate action. They first broke away the spare oars, which were encased in ice and frozen to the sides of the boat, and threw them overboard. Two of the fur sleeping bags went overboard… they weighed a good 40 pounds each since they were so wet and besides, they were frozen stiff as a board. About 11 a.m. the boat fell into a trough, losing the sea anchor in the process. They had no choice but to set sail and trust that it would hold. They beat the canvas until the bulk of the ice had cracked off and, fortunately, it worked as the little boat came up to the wind again. Frostbite became a serious problem as large blisters developed on exposed fingers and hands. By the dawn of the seventh day, the wind had subsided. Once again the course was laid for South Georgia… it had been six days since an observation had been made. The sun came out and the men hung their sleeping bags to the mast and spread their socks and other gear all over the deck. The ice began to melt away as porpoises came blowing alongside the boat. Cape Pigeons and an occasional Stormy Petrel swooped within a few feet of the tiny craft. Wild “snapped” the sun and determined they had gone over 380 miles and were nearly half-way to South Georgia. The eighth, ninth and tenth days of the voyage had little to report. On the eleventh day (May 5), a tremendous cross-sea developed and at midnight, while Shackleton was at the tiller, a line of clear sky was spotted between the south and south-west. Shackleton wrote, “I called to the other men that the sky was clearing, and then a moment later I realized that what I had seen was not a rift in the clouds but the white crest of an enormous wave. During twenty-six years’ experience of the ocean in all its moods I had not encountered a wave so gigantic. It was a mighty upheaval of the ocean, a thing quite apart from the big white-capped seas that had been our tireless enemies for many days. I shouted ‘For God’s sake, hold on! It’s got us.’ Then came a moment of suspense that seemed drawn out into hours. White surged the foam of the breaking sea around us. We felt our boat lifted and flung forward like a cork in breaking surf. We were in a seething chaos of tortured water; but somehow the boat lived through it, half full of water, sagging to the dead weight and shuddering under the blow. We baled with the energy of men fighting for life, flinging the water over the sides with every receptacle that came to our hands, and after ten minutes of uncertainty we felt the boat renew her life beneath us”. The cooking stove was floating around in the bottom of the boat and portions of their last hoosh seemed to soak everything. It was 3 a.m. before the stove was finally functional again. The next day, May 6, Worsley determined that they were not more than a hundred miles from the northwest corner of South Georgia… two more days of favorable wind would put the island within sight. Thirst took possession of them. Their mouths were dry and tongues were swollen. On the morning of May 8, about 10 o’clock, a little bit of kelp was passed. An hour later two birds were seen sitting on a big mass of kelp and at 12:30 p.m., McCarthy caught a glimpse of the black cliffs of South Georgia, just fourteen days after departing Elephant Island.
They looked for a landing place but the presence of blind rollers proved the existence of uncharted reefs along the coast. Here and there were rocks close to the surface and over them great waves broke spouting thirty to forty feet in the air. The rocky coast seemed to descend sheer to the sea. Night was drawing near and despite their craving thirst for water, there was no choice but to wait until the following morning to make shore. At 5 a.m. the wind shifted to the northwest and increased to one of the worst hurricanes ever experienced by Shackleton. The little boat was tossed around in the raging sea and when dawn appeared, no land was in sight. At 1 p.m. land was once again sighted but sheer cliffs with roaring breakers was all that awaited them. Evening approached and suddenly, when disaster seemed imminent, the wind shifted and the small boat was once again free to locate a safer landing place. The night wore on and as dawn arrived on the morning of May 10, there was practically no wind. They sighted an indentation which they thought was King Haakon Bay. Shackleton decided this would be their landing place as the bow was set towards the bay. Soon angry reefs were on both sides with great glaciers reaching the sea. About noon they sighted a smooth stretch of water that reached the head of the bay. A gap in the reef appeared and they made for the opening but suddenly the wind shifted and blew straight against them right out of the bay. That afternoon, after tacking five times into the strong wind, they made it through the small entrance into the wide mouth of the bay. A small cove, guarded by a reef, made a break in the cliffs on the south side of the bay and they turned in that direction. The entrance was so small that they had to take in the oars but in the gathering darkness, the James Caird ran on a swell and touched the beach. At 2 a.m. on the first night ashore, Shackleton woke everyone, shouting, “Look out boys, look out! Hold on! It’s going to break on us!” It was a nightmare…Shackleton thought the black snow-crested cliff opposite them was a giant wave.
Unfortunately, the men were 17 miles from the Stromness whaling station: a journey over South Georgia’s mountains and glaciers awaited them, an effort no one had ever accomplished. McNeish and Vincent were too weak to attempt the trek so Shackleton left them in the care of Macarthy. On May 15, Shackleton, Crean and Worsley set out on their adventure. They climbed over icy slopes, snowfields and glaciers until reaching an altitude of 4500 feet. Looking back they could see a fog rolling up behind them. The ridge was studded with peaks and since they had no sleeping bags or tent with them, it was imperative they find a lower elevation before night set in. They managed to descend 900 feet in two or three minutes by sliding, like children, down a snowy slope. The country to the east was an ascending snow upland dividing the glaciers of the north coast from those of the south. Another meal was had at 6 p.m.; Crean was the cook as Shackleton and Worsley broke the wind from the cooker. Night was upon them and for an hour they plodded along in nearly complete darkness. About 8 p.m. a full moon appeared from behind jagged peaks, lighting their pathway. By midnight they were once again at an elevation of about 4000 feet. After 1 a.m., the Primus was started again and the men ate hot food which renewed their energy. By 1:30 a.m. they were on their feet again, still heading towards Stromness Bay. A dark object in the distance looked like Mutton Island, which lies off Husvik. Their high hopes were soon shattered as crevasses warned them that they were on another glacier… Shackleton knew there was no glacier in Stromness and realized it must be Fortuna Glacier. Back they turned and tramped up the glacier again. At 5 a.m. they were at the foot of the rocky spurs of the range. The men were exhausted as they sat down, under the lee of a rock, and wrapped their arms around each other to keep themselves warm. Within a minute, Worsley and Crean were asleep but Shackleton realized that it would be “disastrous if we all slumbered together, for sleep under such conditions merges into death”. After five minutes rest, Shackleton woke them up, told them they had slept half an hour, and gave the command to begin again. They were so stiff that for the first 300 yards they couldn’t bend their knees. A jagged line of peaks loomed before them. This was the ridge that separated them from Stromness Bay. They found a gap in the ridge and went through it at 6 a.m. with anxious hearts and weary bodies. The twisted rock formations of Husvik Harbour appeared right ahead in the early light of dawn. While Worsley and Crean started the cooker, Shackleton climbed a ridge above them in order to get a better look at the land below them. At 6:30 a.m. Shackleton thought he heard the sound of a steam whistle calling the men from their beds at the whaling station. Shackleton descended to the others and told them to watch the chronometer for seven o'clock as this would be the time the whalers would be called to work; right to the minute the steam whistle sounded. Never had they heard such a sweeter sound.
The men cautiously started down the slope of the ice-clad mountainside. The only possible pathway seemed to be a stream flowing to the sea below. Down they went through the icy water, wet to their waist, shivering cold and tired. Then their ears heard the unwelcome sound of a waterfall. The stream ended in a waterfall that dropped 30 feet, with impassable ice-cliffs on both sides. They were too tired to look for another way down so they agreed the only way down was through the waterfall itself. They fastened their rope around a rock and slowly lowered Crean, who was the heaviest, into the waterfall. He completely disappeared and came out the bottom gasping for air. Shackleton went next and Worsley, the most nimble member of the party, went last. They had dropped the logbook, adze and cooker before going over the edge and once on solid ground, the items were retrieved, the only items brought out of the Antarctic, “which we had entered a year and a half before with well-found ship, full equipment, and high hopes. We had ‘suffered, starved and triumphed, grovelled down yet grasped at glory, grown bigger in the bigness of the whole.’ We had seen God in His splendours, heard the text that Nature renders. We had reached the naked soul of man”. Shivering with cold, they set off for the whaling station, now just a mile and a half away. They tried to straighten themselves up a little bit before entering the station, but they truly were a sight to behold. Their beards were long, their hair was matted, their clothes, tattered and stained as they were, hadn’t been washed in nearly a year. Down they hurried and as they approached the station, two small boys met them. Shackleton asked them where the manager’s house was and they didn’t answer… instead they turned and ran from them as fast as their legs would carry them. They came to the wharf where the man in charge was asked if Mr. Sorlle (the manager) was in the house.
‘”Yes,” he said as he stared at us.
“We would like to see him,” said I.
“Who are you?” he asked.
“We have lost our ship and come over the island,” I replied.
“You have come over the island?” he said in a tone of entire disbelief.
The man went towards the manager's house and we followed him. I learned afterwards that he said to Mr. Sorlle: “There are three funny-looking men outside, who say they have come over the island and they know you. I have left them outside.” A very necessary precaution from his point of view.
Mr. Sorlle came out to the door and said, “Well?”
“Don’t you know me?” I said.
“I know your voice,” he replied doubtfully. “You’re the mate of the Daisy.”
“My name is Shackleton,” I said.
Immediately he put out his hand and said, “Come in. Come in.”’
They washed, shaved and dined on ‘coffee and cakes in the Norwegian fashion’. Worsley boarded a whaler headed for Haakon bay while Shackleton prepared plans for the rescue of the men on Elephant Island. The next day Worsley arrived to find the three men waiting under the upturned James Caird. They all returned to Stromness Bay and the next morning Shackleton, Worsley and Crean left on the Norwegian whaler Southern Sky for Elephant Island. Sixty miles from the island the pack ice forced them to retreat to the Falkland Islands whereupon the Uruguayan Government loaned Shackleton the trawler Instituto de Pesca but once again the ice turned them away. They went to Punta Arenas where British and Chilean residents donated £1500 to Shackleton in order to charter the schooner Emma. One hundred miles north of Elephant Island the auxiliary engine broke down and thus a fourth attempt would be necessary. The Chilean Government now loaned the steamer Yelcho, under the command of Captain Luis Pardo, to Shackleton.
As the steamer approached Elephant Island, the men on the island were approaching lunchtime. It was August 30 when Marston spotted the Yelcho in an opening in the mist. He yelled, “Ship O!” but the men thought he was announcing lunch. A few moments later the men inside the “hut” heard him running forward, shouting, “Wild, there’s a ship! Hadn’t we better light a flare?” As they scrambled for the door, those bringing up the rear tore down the canvas walls. Wild put a hole in their last tin of fuel, soaked clothes in it, walked to the end of the spit and set them afire.
The boat soon approached close enough for Shackleton, who was standing on the bow, to shout to Wild, “Are you all well?”. Wild replied, “All safe, all well!” and the Boss replied, “Thank God!” Blackborrow, since he couldn’t walk, was carried to a high rock and propped up in his sleeping bag so he could view the scene. Within an hour they were headed north to the world from which no news had been heard since October, 1914; they had survived on Elephant Island for 105 lonely days.
With special thanks to Gary Pierson, Webmaster of South-Pole.com for allowing me to use this information from his site. More to follow in the coming weeks.
Forthcoming events: More pipeline surveying!
July 2, 2000