Elsevier

Applied Ocean Research

Volume 114, September 2021, 102766
Applied Ocean Research

The optimum mooring configuration with minimum sensitivity to remove a mooring line for a semi-submersible platform

https://doi.org/10.1016/j.apor.2021.102766Get rights and content

Abstract

Mooring lines may be damaged under severe conditions during the life of an offshore platform. In the event of a mooring failure, it is possible to break remaining mooring lines in a severe condition, which ultimately leads to the total structural failure; Therefore, the post-failure analysis is very important for the mooring system. In this study, the Amir-Kabir semi-submersible platform has been modeled which is installed in 700 m’ depth of the Caspian Sea, and random waves have been generated towards the structure in the form of the JONSWAP wave spectrum according to the conditions of the Caspian Sea. For analysis, four different mooring configurations have been considered. Then, in each analysis, one of the four mooring lines has been broken during the storm, and the effect of the failure has been analyzed on the platform responses. In other words, the responses of the six degrees of freedom have been compared with each other derived from the analysis. Finally, the optimum mooring configuration has been chosen to reduce structure motions after the mooring failure.

Introduction

In recent years, due to declining oil and gas reserves on land, efforts have been made to achieve hydrocarbon resources in deep waters. Thus, floating platforms have been constructed and used. Spar Platform, Tension Leg Platform (TLP), Semi-Submersible Platform, and Floating Production Storage and Offloading (FPSO) are some types of platforms used for oil and gas extraction in deep waters. All of these floating platforms have been exposed to the dynamic loads caused by sea currents, winds, and especially waves in deep waters.

On the other hand, the relevant company has to restrict the platform in a designated position for operational reason, including digging and extraction. Thus, the structure has been restrained with a suitable mooring system. As mentioned in API-RP-2SK, environmental events such as wave, wind, current, and tide cause steady and unsteady loads and hence platform response. Several subsystems performance including foundations, mooring system, risers, hull and deck equipment must be investigated in both normal and extreme environmental conditions. Typical environmental conditions frequently occur in the service life and construction period, whereas extreme environmental conditions have a low probability of occurrence in service life. All crucial subsystems, such as mooring system must withstand in this condition Design and Analysis of Stationkeeping Systems for Floating Structures, 2005.

Among all platforms, Spar Platform, Semi-Submersible Platform, and FPSO have a catenary mooring system. Generally, the mooring system should be designed to prevent riser damage in a harsh situation and allow the platform to operate in most environmental conditions Rashidi et al., 2015. Catenary mooring lines provide enough stiffness to restrain the platform by the weight of the suspended part of the steel chain. Therefore, the mooring system is able to control the horizontal responses of a floating platform and does not have a significant effect on its vertical responses. The size and the shape of the floating platform will be the main factor in vertical floating responses Sabzian et al., 2014. During the life of the platform, the mooring lines are subject to many loads, such as fatigue, overload, and other mechanical damage Kvitrud, 2014. As a result, during a time, the risk of damage and, consequently, single or multiple mooring failure will increase. It should also be noted that the cost of the mooring system is about 20 to 30 percent of the total cost of a project Ketabdari et al., 2016. Therefore, the analysis of the mooring system against damage is a crucial matter. over the past years, the experiences show that the mooring failure does not take place only during severe metocean conditions; on the other hand, repairing and replacing the broken line is a long and costly process. All these incidents and events in the offshore industry have caused concern in recent years.

Over the past years, numerous studies about the response of the semi-submersible platform and its mooring lines before and after the mooring failure have been investigated. In 2015, Rashidi and his colleagues examined the effects of different mooring patterns with 15, 45°, and a combined alignment of 15–45° on the response of the semi-submersible platform Rashidi et al., 2015. In 2015, Sabzian et al. examined the appropriate mooring arrangement for the Amir Kabir semi-submersible platform and optimized them for intact conditions to minimize the horizontal responses of the platform and the mooring lines forces Sabzian et al., 2014. In 2014, Kevitrud, an expert in the Norwegian oil company's safety department, presented the experiences from the mooring line failure in Norwegian offshore platforms from 2010 to 2013 Kvitrud, 2014. Ketabdari et al. analyzed the effect of the mooring lines pattern on the dynamic response of truss spar platforms by numerical methods in 2016 Ketabdari et al., 2016. In 2006, the National Oceanic and Atmospheric Administration of the United States analyzed the mooring lines of a buoy called KEO and studied the mooring system in terms of maximum breaking forces in a failed mooring line and generated forces in other lines after the failure Lawrence-Slavas et al., 2006. Zhang studied the situation of a failed mooring line for a semi-submersible drilling platform under the Gulf of Mexico storm conditions and presented the results of the analysis in a time history in 2009 Zhang et al., 2009. Han, with his colleagues from the University of Seoul and Boomer from the Hyundai Marine Industries Division, analyzed the catenary mooring system for an imagined platform in transitional conditions after the mooring failure in 2011 Han et al., 2011. In 2012, Kurian, Yassir, and Harahap analyzed the mooring failure for a semi-submersible platform in experimental conditions Kurian et al., 2012. in 2014, Razaghian et al. evaluated the hydrodynamic performance of a damaged ISSC TLP, which is caused by a tendon disconnection and analyzed the transient effect on the other tendons Razaghian et al., 2014. In 2015, Hajnorouzi studied fatigue in the catenary mooring system for the Amir Kabir semi-submersible platform in both time and frequency domains Tabeshpour and HajNorouzi, 2019. In 2015, Stendal examined the mooring failure for catenary and taut mooring systems in extreme environmental conditions in frequency and time domains using the MIMOSA and SIMO modules from the SESAM software package Stendal, 2015. In 2018, Tabeshpour, Ahmadi, and Malaijerdi studied a TLP platform behavior and stiffness matrix of the platform under tendon failure condition in terms of structural stability Tabeshpour et al., 2018. In 2016, Ahmed, Yenduri, and Kurian studied truss spar platform responses for various symmetric and asymmetric mooring configurations when one and two lines are damaged Ahmed et al., 2016. Bae et al. carried out series of numerical simulations of the performance changes for a Floating Offshore Wind Turbine (FOWT) with a broken mooring line and studied an aero-hydro-servo-elastic-dynamic mooring analysis in the time domain for this simulation in 2016 Bae et al., 2017. In 2017, Elhanafi and his colleagues investigated the influences of the mooring failure on different instants when an offshore floating-moored Oscillating Water Column (OWC) subjected to unidirectional regular waves; They simulated this device with CFD methods and carried out a test with (1:50) scale model to validate and develop their analysis Elhanafi et al., 2017.

Section snippets

Governing equations

In this paper, a semi-submersible platform has been modeled in ANSYS AQWA using the boundary element method. In the following, the diffraction theory has been introduced which has been used in the software.

This section addresses the hydrodynamic fluid loading of the diffracting bodies in regular waves. The velocity of the incident wave is assumed to be zero; fluid is ideal and irrotational, which allows the use of the potential theory. The wave velocity potential is given below Tabeshpour and

Equation of floating structure motion

Typically, the motion equation has been solved in the time domain by considering the mooring and uniform forces. In general, this equation is as follows Patel, 2013:MX¨+BX˙+BV|X˙|X˙+KX=F0(t)+Fw+Fc+T+Fm{X(t)}

The equation mentioned is a matrix equation and represents all horizontal movements of the platform. bv and K are the damping due to the drag force and the stiffness, respectively (regardless of the mooring system stiffness). Also, B is the linear damping due to the generated waves by the

Analysis of spread mooring system

The Relationship between mean external loads on the vessel and its position can be found by considering contributions from each cable line separately. The total forces from all mooring lines are F1M and F2M as horizontal forces in surge and sway direction, respectively, and F6Mas the yaw moment, which are given as follows Faltinsen, 1993:F1M=i=1nTHicosψiF2M=i=1nTHisinψiF6M=i=1nTHi[xisinψiyicosψi]Here THiis the horizontal force from the anchor line number i. Its direction is from the

Numerical analysis of the semi-submersible platform in the time domain

In this article, the Amir-Kabir semi-submersible platform has been Modeled in ANSYS AQWA. This GVA 4000 M semi-submersible platform has been designed by GVA (Sweden company) and built in the Sadra shipyard (Iranian company) in Neka under the DNV class. Dimensions of the platform are given in Table 1.

Also, there is a model of the Amir Kabir platform designed by AQWA (Fig. 3).

In the following, four different mooring configurations are suggested for analysis, as shown in Fig. 4.

Also, the

Conclusions

In this paper, the effect of the different mooring configurations was investigated on the platform performance after the mooring failure, and its responses were presented in the form of comparative graphs. Based on the study conducted, the following specific conclusions can be drawn:

  • 1

    Among all mooring line configurations, the 15-degrees configuration has a better response (less movement) in the surge. In this response, it should be noted that after the failure of the line no. 1, other

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (22)

  • J.S. Han et al.

    The Transient behavior of mooring systems in line-broken condition equation of mooring system

  • Cited by (0)

    View full text