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RigSpray – predicting sea spray on Arctic offshore structures

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Ekeberg, Ole-Christian, DNV GL Ole-Christian Ekeberg
Senior Engineer

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RigSpray - Sea spray icing poses a threat on multiple levels
Sea spray icing poses a threat on multiple levels from personal safety and hindering operation of essential components to jeopardizing the stability and integrity of offshore structures and vessels.

Objectives

The primary objective of RigSpray is the development of knowledge, models and a tool to estimate marine icing loads required for design. Design requirements are given by the regulatory bodies, such as the Petroleum Safety Authority (PSA), and specified in for example NORSOK N-003 and ISO 19906.

Secondary objectives that will support the main objective are:

  • Development of instrumentation for long-term measurement of sea spray (flux, frequency and spray-duration)
  • Measurements of sea spray on offshore structures (semi-submersible, ship shaped and cylindrical shaped)
  • Development of a sea spray model suitable for implementation in the icing model (mathematical and/or physical)
  • To study and quantify uncertainties in the sea spray icing model (water transport, heat transport, melting)


Project Summary

In offshore oil and gas operation in artic 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 relates e.g. to sea ice, icebergs, snow and icing. Icing and snow is expected in the entire Barents Sea while sea ice and icebergs may also be expected in the northerly parts of the Barents Sea. This project focus 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, forming a layer of ice. During winter storms, it is expected that considerable amounts of icing may accumulate and observations on the NCS have shown icing accumulating to more than 300 tons in one offshore installation. Icing is mostly accumulated close to the sea surface above the wave washing zone. The parts where waves hit the structure will normally be ‘wave washed’ and free of icing.

The main objective in this research project, is to develop a tool which may be used to estimate the amount of icing on an installation. The tool should use a weather condition, specified by air temperature, wind and wave conditions, etc. as input and estimate the amount of icing that forms on the different parts of the installation.

To be able to estimate the amount of icing the project shall also perform measurements of sea spray on offshore installations. Today the prediction of sea spray is the greatest knowledge gap associated with models for icing. To perform such sea spray measurements the project need to develop instrumentation that can operate in harsh conditions and under the strict regime that exists on offshore installations. The project is executed by DNV GL in collaboration with SINTEF and University of Oslo.


Update November 2017:

The project has now successfully completed the instrument development phase. The main challenge in this part of the project was to create a device which can measure sea spray offshore on several oil and gas producing installations. The sea spray measurement system inspired by the conventional ‘tipping bucket’ which is used to measure rain. The ‘modified tipping bucket’ is expected to work offshore and has been thoroughly tested both theoretically and practically.

The instruments were also tested theoretically using real ship motions (movements) which may be expected during the real measurement’s. Further, a wind tunnel test was performed to check the performance in wind and rain conditions. This wind tunnel (the Velux wind tunnel in Øst-Birk Denmark) can perform tests with heavy rain and wind speeds up to violent storm (100 km/h). From the tests, it was concluded that the instrument performs as expected in in all assessed conditions.

The instruments were installed on a ship-shaped installation (FPSO) for measurements of sea spray in September 2017. The instrumentation consists of a combination of onboard measurements (motions, waves, wind etc.) and the spray instruments. The spray instruments consists of a video camera system and the ‘modified tipping bucket’ located on six different locations. The measurements offshore are now live and shall continue for about 6 months.

In the end of 2017 an early prototype tool for estimating marine icing, will be made available to the project participants. This will ensure user input and suggestions for the future development. The icing model is based on solution of a complex mathematical formulation (solves coupled partial differential equations) for ice thickness, fluid film thickness, and salinity on a surface geometry. This tells us how much of the water that is freezing and how much that runs off each object. Input to the model are environmental conditions such as wave conditions, air temperature, relative humidity and wind speed and direction. The model can also use input from Computational Fluid Dynamics (CFD) analysis for heat transfer and estimate long time series of icing.


Update May 2018:

Data from one winter with measurements on a ship-shaped installation (FPSO) has now been collected. The data is being analysed and a new sea spray model is under development. Currently the data from the first season of measurements looks promising. The project is now working towards pro-longing the campaign on the ship-shaped installation with one more winter and is also about to instrument a second offshore installation. If all goes according to plan two installations will be instrumented for sea spray measurements the winter 2018/2019.

The prototype tool for estimating marine icing has been compared with documented full scale icing events. The comparison shows that the prototype estimates accumulated icing loads with acceptable accuracy.


Budget and schedule

RigSpray JIP has a budget of 21 million NOK. The project started in 2016 and should last for three years. The project receives financial support from the Research Council of Norway (PETROMAKS 2), Equinor, ENI, OMV, Aker BP and Lundin. The project is managed by DNV GL and executed in collaboration with Sintef Digital and University of Oslo.


Last modified on 22 August 2018