Assessment of navigational risks imposed by gas pipelines

Natural gas has been transported through Nord Stream, the offshore gas transportation system from Russia to Germany, since 2011. The pipelines are routed along the seabed of the Baltic Sea within the Swedish Exclusive Economic Zone (EEZ). Nord Stream 2 AG is currently investigating the possibilities for extending gas transportation by adding new pipelines to the existing system. By using AIS data and anchoring simulations, SSPA is assessing the maritime risks brought about by the presence of the two new pipelines along the buffer zones close to Hoburgs Bank and Norra Midsjöbanken to the south-east of Gotland.

Results from a similar study performed by SSPA for the first Nord Stream project indicated that the presence of the pipelines may influence the decision-making process by the master of a drifting vessel regarding where and when to find an appropriate place for emergency anchoring.

At present, there are two different routing options in the Swedish EEZ: one is to the east side of the existing pipelines; the other, to the west side. The current study aims to identify the best routing option with regard to maritime safety and pays particular attention to the risk of grounding on the protected bank areas as a consequence of delayed emergency anchoring.

Analysis of AIS data

The risk assessment is carried out in accordance with established risk analysis standards and the IMO´s Formal Safety Assessment (FSA) methodology, where applicable. The study is backed by a thorough traffic analysis in which AIS data of vessel passages in the area in 2014 is combined with trend analyses to establish projections of credible traffic scenarios for 2025. SSPA collects and stores AIS data and has developed tools for processing and analysing big data on sea-traffic patterns as well as any vessel-specific data contained in the AIS data. In addition to these projections, the AIS data also serves as a basis for identifying which vessels are to be used in simulations of critical scenarios with loss of propulsion due to machinery failure or blackout.

Representative vessels are selected both with regard to their frequency in the area and their potential impact in the event of a grounding accident. In addition, various drifting characteristics for different ship types are taken into account. For the current analysis, a tanker of Aframax size was chosen due to its high frequency in the deep water (DW) route and the risk of a severe oil spill in the event of grounding. The tanker is complemented by a container vessel with a large wind area, which offers different drifting characteristics compared to the tanker.

Emergency anchoring

Simulations of vessels drifting from critical positions in adverse wind conditions were conducted in order to investigate whether additional pipelines along the existing Nord Stream system would potentially reduce the possibilities for successful anchoring in the event of drifting.

The simulations were performed using the SSPA SEAMAN simulator so as to describe the ship dynamics in all six degrees of freedom, including both first and second-order wave forces. The SEAMAN anchoring model describes the walk out of the anchor cable and its catenary behaviour. The horizontal decelerating anchor force exerted on the drifting vessels is dependent on the scope of the cable, the type and weight of the anchor, and the seabed holding ground.

When machinery failure with loss of propulsion is simulated, the vessels normally turn windward, gradually losing speed before finally drifting downwind sideways. The depth of the DW route is too large for anchoring but the areas between the banks and the DW route are designated as buffer zones for emergency anchoring. The areas leeward of the pipeline route generally offer favourable depth and plenty of room for safe anchoring. None of the anchoring simulations showed dragging distances in excess of 100 metres. The option whereby the planned new pipelines are routed east of the existing system will not cause any potential delay of emergency anchoring, nor will it reduce the available area leeward of the pipeline route. It is therefore considered favourable from a risk perspective.

Quantitative analysis of grounding probabilities

A grounding incident would result from a chain of events where each of the constitutive elements turned out in an undesirable way. The probability of a grounding incident was estimated based on frequencies of black-out and rudder failures, together with wind statistics, simulation results of drifting time and with regard to the number of vessels having a draught large enough to potentially ground on the banks.

The probability of an unsuccessful emergency anchoring and the subsequent drifting grounding was found to be low: approximately 5x10-5 events/year, i.e. once in 20,000 years. Compared to the probabilities of powered grounding and collision accidents, 5x10-5 events/year are considered low. The presence of additional pipelines in the area is assumed not to have any impact on the probability of grounding compared to the present situation and the existing Nord Stream pipelines.

Experience over the five years in which the first Nord Stream pipelines have been in operation without any incidents supports the conclusions of the previous risk analysis. Both projects are examples of how SSPA’s toolbox of simulations, risk analysis and AIS data are used, in consultation with the relevant authorities and governmental bodies, to assist stakeholders in the permit application process.

Photos and illustrations:

Route of existing Nord Stream pipelines and options for the routing of Nord Stream 2. Illustration: Nord Stream 2 AG.

Construction of the Nord Stream Pipeline in early April 2010 with the pipelay vessel Castoro Sei (C6). Photo: Nord Stream AG.

Chain of events leading to a grounding incident. Illustration: SSPA.

General structure of risk analysis and its application for the present study. Illustration: SSPA.