Optimized Operability and Fuel Consumption Through Forecast-DrivenMooring and Riser Response Prediction

Abstract

When operating a mobile drilling unit on subsea wells, operational decisions are typically based on a Well Specific Operating Guideline (WSOG) where all key operational limitations are summarized. The WSOG is typically derived from marine riser and mooring analyses performed separately. The operating envelopes are based on worst-case scenarios, adding conservatism, which ensure safety but can result in unnecessary downtime and increased fuel consumption during offshore drilling campaigns. In recent years, electronic versions of WSOG have become more common. This paper aims to demonstrate how combining forecast-driven dynamic operating limits with mooring line failure analysis can expand the allowable operating window without compromising on safety. The focus is on quantifying potential gains in operability and fuel efficiency for drilling operations conducted in the harsh North Sea environment, where weather-related downtime remains a major cost driver.

This paper compares a more traditional electronic WSOG approach with a new approach where the mooring and riser system analyses are integrated. In the traditional approach, actual and forecasted weather data are combined with a riser model to predict the limiting vessel excursion. The integrated method employs a digital tool for calculation of real-time operating envelopes, in combination with mooring line failure simulations to determine maximum rig excursions.

A typical North Sea drilling campaign was selected as the case study. Actual rig operations carried out under standard well-specific operating guidelines (WSOG) were compared with the method proposed herein. The analysis included comparisons of allowable vessel offset thresholds, and estimates of potential operating hours gained. Thruster usage was analyzed to assess how less conservative operating limits could reduce the number of active thrusters required, thereby lowering fuel consumption and associated emissions.

The combined riser and mooring line analysis method consistently increased the allowable operating envelope compared with conventional electronic WSOG. The analysis showed that significant wave height (Hs) and rig offset limits could be increased without exceeding riser or mooring integrity criteria. In addition, the expanded limits enabled periods of operation with fewer thrusters online, lowering overall fuel consumption and associated emissions.

This paper presents a new approach integrating the riser and mooring line failure analyses into a live offshore decision support tool. The study shows that the proposed method could increase operational uptime and environmental performance without compromising safety margins. The results highlight the potential for forecast-driven dynamic approaches to enhance both economic and environmental outcomes in modern offshore drilling operations.

Authors

Jan-Erik Hygen
Engineer at 4Subsea

Harald Holden
Lead Engineer Drilling at 4Subsea

Kai Roger Nilsen
Director Of Engineering at Delmar

Jan Erik Bergsvik
Principal Engineer at Delmar