# Economic Superiority of PIP Slip Joint Compared to Conventional Tubular Joints
## 1 Introduction
Despite these advancements, bolted flange joints are not easily detachable. Moreover, the forging process for flanges incurs substantial costs, mainly due to the significant heat required [15; 16; 17]. This process involves heating steel billets to temperatures between 1150 \({}^{\circ}\)C and 1250 \({}^{\circ}\)C (2100 \({}^{\circ}\)F to 2300 \({}^{\circ}\)F) to achieve the necessary malleability for shaping. The high-temperature requirement increases energy consumption and operational costs, highlighting the need for substantial resources and craftsmanship to produce high-quality products. Therefore, it is essential to explore new hybrid connections that facilitate easier installation and mobility when needed. Given these considerations, PIP slip joints emerge as a highly effective alternative to traditional tubular joint systems, as they proficiently resolve the aforementioned challenges. Pile-in-Pile (PIP) slip joints are innovative structural connections that enable the surfaces of circular hollow sections (CHSs) to slide apart, as depicted in Figure 1c. By employing friction-based connections, PIP slip joints significantly enhance reliability and minimize the risk of failure, proving to be a superior choice over other options available [18; 19]. Furthermore, due to their slender and lightweight design, wind turbines are particularly vulnerable to lateral loads such as wind and wave action [20]. In this context, utilizing PIP slip joints provides a structural advantage by introducing a degree of semi-flexibility at the connections. This semi-flexible behavior is mechanically governed by frictional resistance and geometric tolerances at the slip interface, and experimental studies on similar friction-based joints suggest that the allowable rotational flexibility typically ranges between 0.01 and 0.03 radians--sufficient to absorb service-level lateral displacements without compromising structural integrity [18; 19]. Additionally, incorporating a ring stopper to resist axial loads facilitates a self-locking mechanism, accelerates the installation and decommissioning process, and offers potential long-term benefits [19]. Experimental and numerical analyses have shown that the structural design life can be extended, leading to fewer repairs and reduced operational risks [18; 21; 22]. With these advantages, PIP slip joints present an innovative and compelling solution for sustainable and efficient infrastructure. However, a comparative economic study between these joints has yet to be developed, and a significant research gap remains. Investigating this research question is crucial for the advancement of suitable tubular joints. Due to the high costs of offshore wind projects, it is necessary to explore a detailed lifecycle cost assessment (LCCA) while analyzing economic viability. This assessment examines the financial impact of different joint technologies over the project's entire lifespan. By analyzing costs related to material cost, fabrication, transportation, installation, maintenance, and decommissioning, this study aims to determine whether PIP slip joints provide a more cost-effective solution than traditional grouted or bolted flange joints. Ultimately, a detailed economic analysis will help investors, engineers, and policymakersmake informed decisions about selecting the most efficient and financially sustainable joint system for offshore wind applications [23; 24]. The initial investment costs for PIP slip joints may differ from those for traditional tubular joints due to the advanced technology and materials used in their construction. Also, these costs can be offset by the long-term benefits associated with their use. Moreover, PIP slip joints are designed to provide enhanced performance and durability, which can lead to significant savings in maintenance and repair costs over the lifecycle of the joint [13; 14]. Operational costs are another major consideration in lifecycle sustainability.