Experiments by Invited Researchers


Investigation of local ice loads and their dependence on mooring stiffness

Project acronym: HyIII-HSVA-8_02
Name of Group Leader: Vegard Aksnes
User-Project Title: Investigation of local ice loads and their dependence on mooring stiffness
Facility: Large Ice Model Basin (LIMB)
Proceedings TA Project: Investigation of response of moored ships in level ice
Data Management Report: There is no Data Management Report available for this project

User-Project Objectives

Moored ships are believed to be feasible for oil and gas operations in ice-infested waters. Previous model tests have often been performed with a special concept in mind and the main objective has been to test the concept in extreme ice conditions, for instance in large pressure ridges. Dynamic response, which is important for fatigue assessments among others, has not been well addressed. For numerical modelling of dynamic response, it is important to know how to relate ice actions to vessel response and mooring forces. Transfer functions between ice forces and mooring forces are in general unknown.

To gain information about relations between ice and mooring forces, simple model tests were executed. The tests involved a simple hull shape with one degree of freedom; surge, and a simplified mooring system, consisting of linear springs. The bow was segmented in to six panels, each of them equipped with a triaxial load cell, such that ice forces could be measured. In addition, mooring (global) forces were measured. Spring (mooring) stiffness and ice drift speed was varied, while all other vessel and ice properties were constant.

The main objectives of the tests were the following:

  • Study and compare dynamic properties of ice and mooring forces at different drift speeds
    and spring stiffness’s.
  • Study correlation between ice forces on the different panels in the bow waterline.
  • Study breaking and friction components of ice forces in the bow.

Short description of the work carried out

Model tests were performed during in April 2009. Three tests in level ice were executed. Two different spring stiffness’s were used, in addition to one test run with the vessel fixed and no possibility to move in any mode of motion. Two different ice drift speeds were used in each test run. Open water tests were performed to identify open water resistance of the model. The natural period and open water damping of the model was found through free decay tests.

Global forces, panel forces in the bow, surge displacement as well as surge and heave accelerations were measured. Above and under water videos were recorded.

Some minor difficulties were experienced with the setup:

  • Water inside the bow due to the segmented bow exerted forces on some of the waterline panels, however this caused only minor problems for the analysis.
  • The displacement sensor for surge motion in some cases gave error signals. In practice, this caused virtually no problems as the displacement is proportional to the global force in surge direction and these measurements can be used instead of displacement measurements.

Highlights of important research results

Note that the results presented below are only preliminary and deeper analysis is required to fully understand the test results.

The mean mooring force was largest for the softest spring configuration for both ice drift speeds, but largest for the lowest speed. The stiff spring configuration and the fixed configuration gave similar mean mooring forces, about half of the soft configuration for low speed and slightly increasing for the highest speed. Standard deviation can be understood as a measured of how dynamic the mooring force was. For the stiff and fixed configurations, the standard deviation was constant with respect to ice drift speed, with the standard deviation for the fixed twice as large as the stiff one. The standard deviation for the soft configuration was equal to the stiff configuration at the highest speed, but three times the stiff configuration for the lowest speed.

The mean ice force on the centered waterline panels was the same for the stiff and the fixed configuration. It was slightly increasing with drift speed. The mean ice force for the soft configuration followed the same trend as the stiff and the fixed, increasing with drift speed. However, the mean force was 50-70% larger for the soft configuration for both drift speeds. The standard deviation for the ice force behaved similarly to the mean, increasing with drift speed and being 30-50% larger for the soft configuration than the stiff and fixed configurations.

Model before application of bow segments

Bow segments applied

Mooring system simulated by springs

Fully equipped model