A dynamically positioned (DP) vessel relies on a computer system to maintain position and heading or stay on a predetermined track. DP is used for a variety of operations. Within the offshore industry it is, for example, used for offloading, drilling, diving, subsea intervention, seismic and construction operations. All DP operations are inherently threatened by loss of position, and operators often only have a short response time to correct or mitigate such losses (Chen & Moan 2005). 

IMO (1994) has defined three DP classes. The basis of the classification is the worst single failure modes.

• Equipment class 1: loss of position may occur after a single failure,
• Equipment class 2: loss of position should not occur after a single failure of an active component,
• Equipment class 3: Loss of position should not occur from any single failure (including flooded watertight component and fire in subdivision.

A manifold of human and organizational factors, technical (design) failures, environmental conditions, or a combination of these, can initiate a loss of position. Loss of position can result in major accidents, such as collision, loss of well integrity, damage to subsea structures, rupture of loading hoses, etc. Operations where loss of position may have severe consequences typically require DP equipment class 2 or 3.

Despite these redundancy requirements, statistics from recent years (Vinnem 2013, Lundborg 2014) reveal that the frequency of accidents during the last ten years for FPSO (Floating Production Storage and Offloading) shuttle tanker offloading DP operations is significantly above the risk acceptance criteria. The frequency is roughly one order of magnitude higher than what is deemed acceptable. It is  therefore imperative that effective preventive and mitigating actions are implemented to reduce the operational risk and the potential for major accidents.

During the operational phase, real-time data is available that can serve as input for risk assessment. The Norwegian petroleum safety authority (PSA) requires that risk analyses are updated from design and construction to operations (PSA 2015). An online risk model for operational risk to support DP related decision-making is therefore a logical next step in improving the safety of DP operations (Vinnem et al. 2015).

 Vinnem et al. (2015) have developed a conceptual online risk management framework (see Figure 1). The framework is meant to present the operators with a real-time risk picture and give early warnings of deviations in the system. The online risk management framework Is based on online risk models that build on date from various sources, for example: historical data, sensors, measurements, experience data.

An online risk management framework solution can be developed as an independent software solution integrated into the bridge or central control room or the solution can be integrated with existing software.

The online risk model will warn the operator of deviations in the system, but the model could also be further developed to instigate action if the appropriate operator response does not occur (Vinnem et al. 2015). An online risk model and decision support tool may give the operators early warning signals of events that may lead to a loss of position, giving the operator more time to respond and handle the situation.

Sources:

Chen, H. & Moan, T., 2005. FPSO - shuttle tanker collision risk reduction. Journal of Offshore Mechanics and Arctic Engineering 127: 345 – 352.

IMO, 1994. Guidelines for vessels for dynamic positioning system. London: International Maritime Organization.

Lundborg, K.E.M. 2014. Human Technical Factors in FPSO-Shuttle Tanker interactions and their influence on the Collision Risk during Operations in the North Sea. M.Sc. Thesis: NTNU.

Petroleum Safety Authority (PSA) 2015. Management Regulations §16: General requirements for analysis, retrieved from ptil.no on 09.12.2016.

Vinnem, J.E. 2013. Offshore Risk Assessment: Principles, Modelling and Applications of QRA Studies (3rd ed., Springer Series in Reliability Engineering). London: Springer.

Vinnem, J.E., Utne, I.B. & Schjølberg, I. 2015. On the need for online decision support in FPSO–shuttle tanker collision risk reduction. Ocean Engineering 101: 109-117.