Operational Scour Management for Safe Jack-Up Operations

Abstract

Scour is an imminent threat for jack-up operations in shallow waters with sandy seabed conditions, strong tidal currents and/or a harsh wave climate, such as the North Sea. Jack-up operations in scour-sensitive areas require jack-up operators to evaluate the risk of scour and to adapt a scour management strategy in order to safeguard against foundation failure. Jack-up deployments are characteristically short term operations. This can be in the range of one day (construction project jack-ups) to several months (drilling units). Often, the strategy is to monitor scour and take remedial measures if and when required. An operational scour forecast and hindcast system was developed in order to assist jack-up operators with their management of scour. The system is based on automated importing and processing of metocean data (water levels, waves and currents) from operational metocean models or measurement stations and calculating forecasts for the temporal scour development with semi-empirical relations, derived from physical model tests for various types of foundations. During an operation, the results provide the basis for a periodic assessment whether, how much and where scour protection is required and when surveys should be performed. After an operation, the hindcast results serve as calibration for the scour prediction model and as evaluation of the scour management strategy. This paper presents two case studies for which the system was applied in the southern North Sea. In the first case study, the system was used to forecast scour around unprotected spudcans. A few weeks into the drilling operation, the operator decided to apply scour protection based on the warning that the predicted scour depth was reaching a critical value. A field survey performed after the scour protection installation confirmed that the predicted scour depth had been in line with the measured scour depth. The second case study comprised a drilling operation before which scour protection had been installed. The main objective of the system was to predict the stability of the scour protection and provide recommendations for scour protection maintenance and scheduling of scour inspection surveys. The field survey performed after the main storm event revealed that the predicted scour depth had been overestimated. A hindcast was performed to calibrate the scour model for future operations. It was concluded that the improved scour forecast system is an important milestone for scour management and contributes to improved safety in future jack-up operations.