## 1 Introduction  
In recent years, with the rapid development of offshore engineering, it is significant to understand the mechanical engineering properties of seabed soil. Submarine sediments are characterized by loose structure, high water content, and high sensitivity, which make drilling and sampling difficult, and their engineering properties mainly rely on in situ field tests [1].  
Standard penetration test (SPT) is a common in situ testing technique with the advantages of simple equipment and wide application. The engineering and mechanical properties of cohesive and sandy soils can be determined by analyzing the blow counts of SPT (N) [2; 3; 4; 5]. However, the SPT equipment is not a precision instrument, and the measured N is affected by various factors such as human operation errors and equipment differences. Therefore, it is usually solved by stereotyping equipment specifications, standardizing operation methods, and limiting the scope of application.  
For SPT in shallow marine soft soils, since the survey hole is deep (generally tens of meters), the blow counts are affected by the rod length; therefore, the length correction must be carried out. Some suggestions have been given in domestic and foreign related norms and studies on the rod length correction of the measured penetration blow counts in the analysis of the results of the SPT [6; 7; 8; 9; 10; 11]. In addition, many scholars have also studied SPT from the hammer impacting energy for an accurate evaluation of the N value [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. According to the theory of one-dimensional rod stress wave propagation, [PERSON] et al. [25] obtained the correction coefficient for the rod length within the range of 36 m through the indoor model test. In addition, [PERSON] et al. [26] obtained the rod length correction coefficient in the range of 83 m by indoor model tests and found that the rod length correction coefficient was independent of the foundation soil material properties by comparison tests of soils with different properties. [PERSON] et al. [27] studied the axial stress and hammer impacting energy distribution along the probe rod based on the field dynamic penetration test and established the relationship equation between hammer impacting energy and rod length. Using experimental and numerical simulation methods, [PERSON] et al. [28] analyzed the hammer impacting energy distribution in the probe rod and gave the rod length correction coefficient in the range of 72 m rod length. However, the existing studies mainly focus on onshore dynamic penetration test, while the influence of the deflection and deformation of the probe rod and the shaking of the ship-mounted drilling platform on the rod length correction coefficient in the offshore environment is not considered [29].  
In this study, a series of on-site in situ tests was carried out to analyze the axial stress distribution and acceleration changes during the hammering process of the probe rod in the offshore environment. The difference in the transmission efficiency of hammer impacting energy of probe rods with different lengths is further revealed, and a rod length correction method suitable for the offshore SPT is proposed, which provides a reference for the offshore SPT.