Litgrid along with PSE Polish Power System Operator prepares the largest synchronisation project of the Baltic States with continental Europe – the construction of the Harmony Link interconnection. During the implementation of the project one of the most significant preparatory works of the project was completed – the seabed survey. At the site of the future interconnection, in the territorial waters and exclusive economic zones of Lithuania, Poland and in the exclusive economic zone of Sweden, the seabed was surveyed for about half a year.

We are talking to Evaldas Žibaitis, the project manager of the Harmony Link group of Litgrid’s Strategic Infrastructure Department, about his view how the survey was conducted and what its most important conclusions were.

– Why was this survey needed?

When planning the Harmony Link, which will reach approximately 300 km at sea, it is important to assess the natural and artificial objects that may affect the cabling process. A seabed survey is very important for potential contractors when planning what technical parameters of cables (the connection consists of more than one which will be placed as bundled or next to each other) will need to be manufactured, how to lay them, and so on. Not only naturally formed rocks or boulders lie on the seabed, wreck, anchors, fishing nets, ropes and other objects were found in the survey area, which will need to be assessed by contractors when planning further actions. In addition, part of the Harmony Link route passes through a chemical munitions dumping ground located in the Gotland Deep in the Swedish Exclusive Economic Zone and in the Lithuanian Exclusive Economic Zone. According to the Polish company MEWO S.A. which led a consortium of contractors, the Baltic Seabed in the area is covered with a large amount of unquantified waste, wrecks and warfare remnants. Luckily, no unexploded ordnance was found on the planned cable route. These objects lie on the seabed for a variety of reasons. Some of them have been specially dumped, others have been there due to accidents at sea, and still others are remnants of military operations. Therefore, it was necessary to assess all potential underwater hazards that could interfere with the smooth construction process of the Harmony Link cable. Submission of the survey report together with the procurement documents to potential cable-laying contractors will help them to manage the risk of laying and dredging, to plan the works realistically.

– To what extent was the work carried out?

Four types of surveys were carried out: offshore geophysical survey, offshore geotechnical survey, nearshore geophysical and geotechnical survey, and target/crossing offshore geophysical survey. The final report of the main part of the survey consists of an interpretation of the results of all four seabed surveys. In addition to geophysical and geotechnical survey an environmental survey was also performed which included inter alia taking water and sediments samples and sea mammals observations. The main survey works at sea took place in January-July 2021, the total length of the route was almost 300 kilometres, and the total investigated area is almost 100 square kilometres, which is equivalent to the territory of Klaipeda city. In the surveyed places, the sea depth reached 115 meters.During the survey, more than 160 specialists in various fields worked – surveyors, laboratory specialists,  vessel crew members, geology, mechatronics engineers, etc. The survey was carried out using as many as 8 vessels flying the flags of Lithuania, Poland, Panama, Sweden and the United Kingdom.

– What challenges have you encountered in conducting such a large-scale investigation?

Along with our colleagues from PSE we had to deal with many surprising issues as offshore projects are much more different from the onshore ones. However, the survey went smoothly, but it is natural that there were various challenges in a project of this size. Initially, it took an unexpectedly long time to obtain a permit to carry out seabed survey works in Swedish Exclusive Economic Zone. However, after a long process of harmonization, in which top-level politicians were involved, the permission was finally obtained on a symbolic day for Lithuania – March 11, 2021.The main difficulties were related to unpredictable weather conditions, which were challenging during the survey – due to large waves, floating ice, strong winds, a significant part of the survey time had to be spent on waiting for favourable weather conditions for the planned works. Curious situations have also occurred, such as parts of equipment falling and sinking at sea. The survey was continued using spare equipment, and eventually the sunken machinery was found and extracted.

– What are the main findings of the Harmony Link seabed survey? What jobs await in the near future?

The final report of the seabed survey provides information on the depth of the sea and the nature of its seabed surface, the type of seabed sediment, and describes the structure of the sediment. Along the planned route, objects of human activity were found and the geological structure was described. Many marks of bottom trawling have been observed. Trawling damages the seabed and the flora and fauna that live there, as well as may cause damage to laid cables.However, the most important for cable construction will be the Cable Burial Risk Assessment (CBRA) following the seabed survey, which will provide more detailed information relevant to the cable contractor. The CBRA is needed to assess the specifics of cable burial at different locations on the seabed, assessing the risks of damage to laid cables, while ensuring a reliable cable position in the long term.It should be noted that an environmental survey report is currently being prepared to help assess the potential impact of the connection on the Baltic Sea environment and to ensure that the best results are achieved in terms of sustainability. The survey report is scheduled for later this year.