A New Combination of Conditional Environmental Distributions Jan-Tore Horn a,b,* , Elzbieta Bitner-Gregersen c , Jørgen R. Krokstad b , Bernt J. Leira b , Jørgen Amdahl a,b a Centre for Autonomous Marine Operations and systems (NTNU AMOS), NTNU, Trondheim, Norway b Department of Marine Technology, NTNU, Trondheim, Norway c DNV GL, Veritasveien 1, Høvik, Norway Abstract In this paper, a joint distribution of all relevant environmental parameters used in design of offshore structures including directional components is presented, along with a novel procedure for dependency modelling between wind and wind sea. Probabilistic directional models are rarely used for response calculation and reliability assessments of stationary offshore structures. However, very few locations have the same environment from all compass directions in combination with a rotationally symmetric structure. The scope of this work is to present a general environmental joint distribution with directional descriptions for long term design of stationary offshore structures such as offshore wind turbines. Wind, wind sea and swell parameters will be investigated for a chosen location in the central North Sea. Keywords: joint distribution; environmental parameters; directional effects; design; offshore wind turbines 1. Introduction The present work presents a general multi-dimensional joint distribution which is fitted to data from the site of a future offshore wind farm in the central North Sea. The aim is to obtain a statistical representation of combinations of all relevant environmental variables for design of offshore wind turbines where 5 absolute and relative load directions are important for response analyses. The proposed model is useful for full long term analyses to calibrate simplified de- sign methods, and finding probable combinations of environmental parameters for extreme sea states and simplified ULS design [1]. Environmental variables include wind, wind sea, swell and tide, as well as their respective directions. 10 * Corresponding author Email address: jan-tore.horn@ntnu.no (Jan-Tore Horn) Preprint submitted to Applied Ocean Research January 10, 2018