**Abstract**

Securing the return on investment for commercial floating wind farms by a proper estimate of the operation and maintenance (O and M) downtime is a key issue to triggering final investment decisions. That is why crew transfer vessel (CTV) weather stand-by issues should be assessed together with new floating wind floater concepts, to boost their cost attractivity. However, such issues as the numerical investigation of the landing manoeuvre of a service ship against a floater reveal complex to calculate. Based on similarities with seakeeping, we investigate various floater geometries. To estimate the weather limitations associated with each configuration. Most recent works find that calculation compares with 5% accuracy to an experiment from a test tank at a model scale. Method description: (A) Vessel seakeeping: (1) assess vessel responses (amplitude and phase angles) and (2) compare them with vessel responses of available publications, as a benchmark. (B) Vessel berthing: (1) model both vessel and floater, (2) account for the wave masking effect of existing floater designs, and (3) compare the ratio of wave vertical force over wave horizontal force and the grip coefficient at the interface between the vessel fender and the floater boat landing. Findings: The wave masking effect calculation for a square floater is cross-checked favorably with an existing demonstrator.

**Keywords:** operation and maintenance, crew transfer vessel, floating wind farm, significant wave height, wave period

### **1. Introduction**

The development of floating wind farms implies the issue of offshore O and M workers safety. It is therefore of upmost importance to know the constraints and acceptable conditions for berthing a CTV.

For berthing with the "bump and jump" method, a CTV comes and pushes its fender against the boat landing ladder. The fender studied here is the stiff fender [1].

The present work relies on the results of a study performed by HSVA [2] and endeavors to meet the results obtained in its CTV model tank test. However, our approach here is different from HSVA numerical berthing calculations, which are more sophisticated.
