# Economic Superiority of PIP Slip Joint Compared to Conventional Tubular Joints
## 1 Introduction  
Offshore wind energy is a key player in the global transition to renewable power, offering a sustainable solution for energy generation. As wind farms increase in complexity, optimizing structural components is essential for enhancing both efficiency and reliability. Various foundation systems, including gravity bases, tripod structures, suction caissons, jackets, and monopiles, support these turbines. Among these, monopile foundations have emerged as the preferred option for moderate water depths of up to 35 m, thanks to their strength and adaptability [1, 2, 3]. One of the key structural elements in offshore wind turbine foundations is the tubular joint system, which connects monopiles or jacket structures to turbine towers. Traditionally, grouted joints and bolted flange joints have been commonly used for connections in offshore wind structures. Grouted joints are primarily utilized to connect the monopile and transition piece (referred to as the MP-TP joint). In this arrangement, the transition piece (outer sleeve) is placed over the monopile (inner pile), and the annular gap between them is filled with high-strength cementitious grout, as illustrated in Figure 1a. Load transfer is facilitated through shear keys or surface roughening, with the grout playing a vital role in transferring axial and bending loads [4, 5, 6]. Grouted joints have been the dominant choice in offshore wind applications due to their ease of installation and tolerance for fabrication and installation deviations [7, 8]. Conversely, bolted flange joints are used at the top of the transition piece to connect the wind turbine tower and are sometimes employed in preassembled MP-TP systems. This type of joint consists of large-diameter ring flanges welded to each tubular section, which are then connected by high-strength preloaded bolts, as per Figure 1b. Load transfer in bolted flange joints occurs through bolt shear and flange compression/friction, allowing the joint to effectively handle axial, bending, and torsional loads [4, 5, 6]. Bolted flange joints are increasingly preferred in the industry because of their superior fatigue performance, reusability, and the ease with which they can be inspected and maintained [9]. Both joint systems, grouted joints and bolted flange joints, have significant drawbacks that must be weighed against their advantages. In the case of grouted joints, long-term issues such as grout cracking, fatigue, and slippage have prompted the development of improved detailing techniques, including high-friction interfaces and shear keys. Additionally, their reliance on precise installation, extended curing periods, and the challenges associated with inspection and maintenance make them less desirable compared to more modern, cost-effective, and installation-friendly alternatives [4, 10]. Figure 1: Different types of joints used in tubular section: (**a**) grout joint, (**b**) bolted flange joint, (**c**) PIP slip joint. On the other hand, bolted flange joints face several challenges, including the need for high precision in flange manufacturing and assembly, as well as strict control over bolt preload to prevent fatigue failures. There are numerous potential failure mechanisms related to bolts, flanges, and shells. Significant parametric studies have been conducted to identify key factors that influence ultimate strength, assess failure mechanisms, and evaluate the effects of different aspect ratios and flange segment widths. The design criteria are also evaluated considering these failures [11; 12]. The forging process of flanges is also noteworthy in this context. Innovative forging technologies have been developed to eliminate welding operations in conveyor driver forgings, utilizing numerical modeling and automation to optimize the process. Furthermore, cold forging techniques for producing hollow flanged parts from tubular blanks have been analyzed using finite element methods to assess product geometry quality and process parameters [13; 14].