Recommended practice for shallow water DP operations

Summary

Analysis of IMCA Station Keeping Event reports for vessels operating in shallower water environments suggests that the relevant guidelines have not always been followed.

Most operators and suppliers regard a water depth of less than 30 m as shallow. However, there are no clear overall limits on water depth for dynamic positioning (DP) operations, and each task should be considered individually on its merits. Thrusters and power generation systems may have to work harder, and the vessel’s capability may be degraded in shallow water. It is recommended that the vessel owner take all necessary steps to anticipate these changes, including calculations of the environmental loading limits and DP capability in proposed shallow water conditions. The vessel and DP system are normally designed for use in a particular operating environment, and proper consideration should be given to whether this is primarily deep or shallow water operations. When changing operations between deep and shallow water, a proper evaluation of changes in vessel DP performance is required.

Shallow water DP operations can be affected by:

• degradation of thruster performance
• reliability of position reference systems (PRSs)
• mud or debris stirred up by thrusters
• increased kelp
• increased amount of floating debris
• increased wave frequency motions
• strong tidal current or changes in current force coefficients
• turbulence, and
• reflections from the surface or structures on the seabed.

All may lead to degraded performance and reduced operating limits but, provided the vessel operator has considered them for the location and operations in hand, they should not lead to an abrupt position loss.

This document provides a specific reference when considering shallow water DP operations in compliance with existing IMCA guidelines. Operations within energy fields located in shallow water are likely to involve complex simultaneous operations and challenging geographical constraints in the form of numerous fixed assets. Therefore, the document concentrates on the following five elements of DP operations:

• key DP personnel
• PRSs
• environmental challenges
• thrusters and power generation
• operational control.

Contents

• Introduction
• Guidelines and Regulatory Requirements
• Key DP Personnel
• Position Reference Systems
• Environmental Challenges
• Thruster and Power Generation
• Operational Considerations for Shallow Water DP Operations
• Conclusion
• References

Version history

October 2023 – initial publication