RigSpray - Developing a tool for prediction of loads from marine icing on offshore structures

Background and objective In offshore oil and gas operations in arctic areas like the Barents Sea, several additional environmental load conditions need to be considered compared to similar operation further south on the Norwegian Continental Shelf (NCS). These additional conditions relate e.g. to sea ice, icebergs, snow and icing. This project focused on icing. Icing occurs when there are strong winds in combination with waves and low temperatures. In such conditions sea spray is transported up on the installation and freezes and forming a layer of ice. Considerable amounts of icing may accumulate during winter storms. Observations have shown accumulated icing up to 500 tonnes on one offshore installation. Icing accumulates mostly close to the sea surface above the wave washing zone. The parts where waves hit the installation will normally be "wave washed" and free of icing. The main objective in this research project was to develop a software which could be used to estimate the amount of icing on an installation at sea. The software uses a weather condition, specified by air temperature, wind and wave conditions, etc. as input and calculates the amount of icing that forms on the different parts of the installation. To support the main objective the project also performed measurements of sea spray on offshore installations. The prediction of sea spray is the greatest knowledge gap associated with models for icing. To perform such sea spray measurements the project developed instrumentation that could operate in harsh conditions and under the strict regime that exists on oil and gas offshore installations. The project was executed by DNV GL in collaboration with SINTEF, University of Oslo and Equinor. Instrument development and offshore measurements The project successfully developed instrumentation for use offshore. The main challenge was to create a device which can measure sea spray. The developed sea spray measurement system was inspired by the conventional "tipping bucket" which is used to measure rain. The "modified tipping bucket" was tested theoretically with real ship motions (movements) and practically in a wind tunnel. The wind tunnel tests were performed to check the performance in wind and ?spray? conditions. The wind tunnel tests were performed with heavy rain and wind speeds up to 100 km/h. The theoretical and practical tests showed that the instrument performed well. An offshore measurement campaign was performed on the Norne FPSO, a ship-shaped installation, in 2017 and 2018 i.e. two winters. The instrumentation consisted of a combination of onboard measurements (motions, waves, wind etc.) and project specific instrumentation. The project specific instrumentation consisted of a video camera system and the ?modified tipping bucket? located on six different locations. After the measurements offshore, a spray model was developed based on the measurements. It was challenging to interpret the spray measurements as the measured water was a combination of rain and sea spray. The final model for sea spray was incorporated in the developed software. Prototype development The project developed a software for estimation of marine icing. The software is based on solution of a complex mathematical formulation for ice thickness, fluid film thickness, and salinity on a surface geometry. The results give how much of the water that is freezing and how much that runs off each object on the structures. Input to the model are environmental conditions such as wave conditions, temperature, humidity and wind speed and direction. The model can use input from Computational Fluid Dynamics (CFD) analysis for heat transfer and estimate long time series of icing. The latter is useful in establishing relevant loads for design. The prototype tool has been compared with documented lab-scale and full-scale icing events. The comparison showed that the prototype estimates accumulated icing loads with acceptable accuracy. Basic research A small part of this project was to study the physics of wave-structure generated sea spray by computational means. This work was performed by the University of Oslo. The first step was to study a plunging jet and generation of bubbles in water where the numerical simulations were compared to experiments. This benchmark was performed to test the numerical model on a simple case. The next step was to generate breaking waves and spray in laboratory conditions. Two articles were published on bubble entrainment and jet impact on a free surface. Another topic was to study the feasibility of using Reinforcement learning and optimal control in fluid mechanics. One of the published articles (Rebault et al. (2019)) was one of the first applications of deep learning in fluid mechanics where deep reinforcement learning (DRL) is applied to the model problem of drag reduction of a flow past a cylinder. This work has attracted a lot of attention and has many citations.

Icing may have significant effect on the structural and operational integrity and may pose a risk to the stability of floating structures. The project have addressed the main gap in marine icing estimates for offshore structures by performing full scale measurements of sea spray. The development of the prototype tool makes DNV able to provide advisory services to various problems related to marine icing for offshore structures in a cold environment. This is primarily a challenge for oil and gas production but other coastal structures in a cold climate are also relevant to assess. DNV will consider to provide guidance on icing for offshore structures in relevant service documents such as Recommended Practices or Standards. The project has supported UiO in their long-term research on sea spray generation. The research collaboration between UiO and Princeton University and CNRS, Sorbonne Université will continue on the subject.

A key element of the project will be the development of a marine icing model that is able to estimate ice accumulation on offshore structures using historical metocean data. This will be obtained by refining an existing numerical model to represent heat transfer and spray flux more accurately whilst maintaining an efficient numerical code. This tool shall be able to estimate marine icing loads that enables the user to calculate relevant annual probability of exceedance for loads from marine icing for a given installation and location. The rate of ice accretion is determined by the amount of water hitting a surface per time and the heat transfer from the surface, and both must be accurately predicted for marine icing models to give reliable results. Due to a limited number of available sea spray measurements, all existing models are associated with a large uncertainty in the amount of sea spray. The project aims to reduce the uncertainty related to prediction of sea spray production to an acceptable level by performing measurements on a number of installations offshore. The project includes the development of suitable equipment for reliable long-term measurements of sea spray flux, frequency and duration. The results from these measurements will be used as a basis for a sea spray model included in the tool for calculation of marine icing.