January 16, 2017, 3 weeks and 4 days at sea.
It was already yesterday that we had to interrupt our work on deck.
During the morning’s mooring recovery a shimmering, oily patch was noticed on the sea surface, the origin of which, of course, had to be investigated right away. Polarstern uses a very pure and light fuel, causing even smallest amounts of it to stretch out widely into a thin oil film (which also evaporate quickly due to its thinness), yet it is our declared goal to operate absolutely devoid of leakages, no matter how small.
Based on the location of the sporadically appearing films, a first suspicion emerges: the port side’s propeller shaft seal might have become untight. The shaft is relieved from any stress and the crew, while constantly scrutinizing the sea surface for slicks, applies various loads to the hull by successively operating the manifold machinery aboard. If the shaft seal is the source, no more patches should occur. However, at sea a damaged shaft seal would be irreparable and we wound need to head for the next suitable shipyard on a single propeller. Certainly not the most urgent matter, but nevertheless a thought present to all scientists aboard: This would imply the end of this, and maybe even the next expedition’s field work. Should we really meet that same mishap as in 2014/15?
January 2017, 3 weeks and 5 days at sea
Our first suspicion was not confirmed and the crew relentlessly persists in their effort to find the source of the leakage. Scientific deck work is postponed until further notice, as is the day to day planning of the expedition.
A new suspicion develops. A tank could have sprung a capillary crack, leaking fuel. Polarstern, built in 1982, features “outside” tanks, which use the hull planking as part of the tank design. In theory, the outside water pressure should press water into such a tank, rather than to allow fuel seeping out. However, depending on the ship’s vibrations, a capillary crack might allow one or the other drop to creep out. Right away the suspected tank’s content is inter-tanked into an empty one, which is vented and inspected while following strict safety measures, a most stressful task: Teams of two need to climb through narrow man-holes into and within the tank to check all welding seams for intruding water. Being aware that respectable but nevertheless only 36 mm of steel keep the water masses out of this void, this job also bears its psychological challenges.
Yet this tank is tight. Systematically, the crew inter-tanks one tank after the other, keeping a close watch of the ship’s stability. An arduous, time-consuming job: Polarstern has many tanks, which are successively vented and checked for leakages. In the evening, finally, a glimpse of hope: An overflow pipe of the ballast water system, which is piped through a fuel tank to the outside, sprung a leak inside that tank. Fuel might have hence first leaked from the tank into the overflow pipe and from there to the outside.
18. January 2017, 3 weeks and 6 days at sea
Throughout the night the damaged pipe was sealed with a sleeve, the tank will remain unused until the ship’s return to Bremerhaven and the pipe’s replacement. The ship is testing various load states, yet no more oil films are discovered. Job done, everybody aboard is most relieved. The scientific party regains hope: Yes, we lost three days of station time, but owing to the relentless and decided effort of the crew another 2 weeks of research are ahead of us. A big “THANK YOU” to the crew for that. At 1 PM on the hour the scientific work is resumed, starting with the redeployment of the mooring that was recovered three days ago. It is only now, being back on track, we sense how big the tension had been during the last days. Once again it became clear: Our old lady Polarstern requires special care, which she certainly receives year-round. Some things however, like this presumed fatigue crack in a ballast water pipe, is beyond predictability or maintenance plans.
19. January 2017, 4 weeks at sea
It is not before evening that we reach the next mooring position. All moorings to be serviced after our call at Neumayer Station have been in the water since 4, if not 6 years. Do you, esteemed reader, expect your car to start up after 4 years on the parking lot? Well, the releases are also powered on batteries, and this time they simply refuse to open the hook to the anchor – at least not at once. For more than half an hour we try to release from different positions, before finally the mooring starts rising, surfacing in a lonely field of ice, of all things (Fig. 1).
20. January 2017, 4 weeks and 1 day at sea
This morning, just after 2’o clock, a further Argo floats was deployed, another will follow in the course of the day. For today this will be it with regard to station bound activities, because we are in transit to a mooring position in the far South. Argo floats freely float in the ocean, surfacing every 10 days to determine their position and to upload their data via satellite link. However, can this possibly work in the Weddell Sea, which is ice covered for more than half of the year? To facilitate the use of floats in this region, we taught them to sense whether it is likely for them to surface in open or ice-covered waters on the basis of the temperature profile they are measuring while ascending. In the latter case, they abort their surfacing attempt to try again 10 days later. This can result in us having to wait for 1 to 2 years before the float, usually in austral summer, suddenly emerges under the ice to retrospectively transmit all past profile data. Yet how can we know where the float has been throughout its long drift under the ice? That is what the aforementioned moored RAFOS sound sources are used for, which, once daily, transmit a coded tone. The floats, synchronized with the sources, listen for these tones to determine their time of arrival, from which we can calculate the floats’ positions. To maximize the range within which the floats may detect the sources, we tune their resonance pipes, much like those of an organ, to the local sound velocity, such that they attain sound levels similar to those of the calls of the great baleen whales.
21. January 2017, 4 weeks and 2 days at sea
Today we possibly face the most difficult mooring recovery. The sea ice map depicts the mooring’s position just outside the area of high ice densities (Fig. 2). Wind and tide may hence push the ice field over the mooring at any time. At about 8 AM, with another 6 miles to go towards the mooring, the sky ahead darkens, indicating open water in this region. Having reached the location, thick fog hampers the view, yet the ice radar reveals that the mooring is located under a large ice floe. We position Polarstern alongside, going “adrift” to determine the drift of the ice (Fig. 2). We seem to be lucky, the drift sets 0.1 knots to the North and, if continuing, in 2 or 3 hours the floe will have drifted past the mooring. We patiently wait, not taking the eyes off the radar and navigational screens when suddenly the northward drift dwindles to a halt. Is this it? Half an hour later the floe slowly resumes its northbound drift, slowly but steadily pushing past the mooring. Finally – it is about 1 PM – it is in open waters and we trigger the release. Yet nothing happens – defying all our attempts– the mooring does not respond.
This is Fiona's (our new little ROV, Fig. 3) finest hour. Its first deployment for real. We will try to attach a recovery line to the mooring line close to the top floatation at about 220m depth. If successful, we will then try to pull up the entire mooring by this line, including its anchor. Sending Fiona to 220 m depth, we drive her towards the mooring, supposedly some 70 m starboard abeam, while hoping for an echo of the mooring’s top floatation on the ROV’s sonar image. And indeed, after a few – seemingly endless – minutes, we suddenly spot a little red-orange dot on the sonar screen. The distance reads some 50 m according to the sonar, yet the visual video only shows close-ups of zooplankton. We aim for the little red dot, give full throttle, but Fiona advances only very slowly, why, we do not know right now, which only increases the tension. The dot in the sonar image slowly but steadily grows, we switch to the more sensitive black-and-white camera and finally, at about 10 m distance, we spot the apparitional shape of the top floatation (Fig 4 and 5).
Minutes later – the final approach is hampered by currents – we are face to face with the mooring rope, trying to snap the carabiner (to which the recovery line is attached) into the mooring rope.
Two, three attempts – it is hard to guess the rope’s distance without a steric view – and the mooring is hooked (Fig. 6). The ROV lets go of the carabiner and is cautiously pulled back aboard. A great success, allowing hopes that this approach will come in handy to recover further intractable mooring, even though this time all our efforts ended in vain, when at last the recovery line broke during heaving.
22. January 2017, 4 weeks and 3 days at sea
A well-deserved transit day bound northwestward without any deck work after yesterday’s toil. With the recovery of the old mooring postponed to 2018/19 (quitting is not an option), a new mooring was deployed and a CTD was cast at safe distances to ensure continuity of our time series and calibration of the instruments which for now remain at sea.
Everybody on board is in good health and looking forward to the last leg of this expedition, an intense sampling of the transition from the deep Weddell Sea to the Antarctic shelf.
Olaf Boebel
