An Approach for Using Integrated Geoscience Data to Avoid Deepwater Anchoring Problems
Authors: Young Alan G. , Phu Dianna R. , Spikula Daniel Robert , Rivette Jill , Lanier Daniel L. , Murff James 
Published: July 2009

MODU's have been traditionally anchored using conventional drag anchors and more recently with vertically loaded anchors, suction piles, and gravity anchors. The scope of a geotechnical site investigation associated with the design of an anchor mooring spread is usually kept to a minimum. This approach is successful when the soil properties are uniform and easily predictable. However, when risks of anchor problems are high, it may be prudent to conduct more detailed site investigations.

This paper describes a proposed site assessment risk matrix that can be used with existing geophysical and geotechnical data to help evaluate the risk that anchoring problems may occur within the proposed area of a mooring spread. The site assessment risk matrix includes the following factors: 1Difficult Soil Conditions, 2Soil Data within Area, 3Spatial Soil Variability, 4Type of Seafloor Strength Profile, 5Anchor Experience in Area, 6Confidence in Computed Anchor Capacity, 7Type of Mooring, and 8Seafloor Topography. Finally post-installation anchor performance is compared with predictions to provide confidence in anchor performance or to determine the need for additional risk assessment data.

Each factor is given an importance weighting value of two or three. Further, each site risk factor is given a risk rating from one to five where one indicates the lowest level of concern and five the highest. The score for each site risk factor is the weighting value times the risk rating. The sum of the site risk scores is the overall site assessment risk score and is an indicator of the risk of anchor problems. Available geophysical and geotechnical data are important inputs to evaluate each of the factors and to indicate adjustments needed to afford the anchors the best chances of achieving their desired performance. Case study examples will be presented to explain the rationale in selecting the weighting values and to show how each of the site risk factors may be evaluated.

The paper argues that potential anchor problems may be minimized when experienced geologists and geotechnical engineers evaluate the available geophysical and geotechnical data for potential anchoring risks. When site conditions indicate high risks such as soil variability or strong soils that are difficult to penetrate, then additional geotechnical data may be needed. One option in this case is the use of Jumbo Piston Core (JPC).

The approach provides a framework for highlighting the potential risks associated with installing anchors in complex geology exhibiting highly variable soil conditions. This approach is a particularly useful tool in the early stages of a project when high-resolution seismic data and detailed geotechnical site characterization are yet to be acquired, and continues to be useful throughout the anchor design process. The mooring spread can be planned to avoid anchor locations where subsurface conditions indicate the greatest risk for unsatisfactory performance. The risk matrix can be updated when new site information is obtained. The benefit is potential avoidance of anchor problems that may cause damage to nearby offshore infrastructure or induce costly delays to the anchoring operations.