A Study on the Holding Capacity Safety Factors for Torpedo Anchors

Abstract

The use of powerful numerical tools based on the finite-element method has been improving the prediction of the holding capacity of fixed anchors employed by the offshore oil industry. One of the main achievements of these tools is the reduction of the uncertainty related to the holding capacity calculation of these anchors. Therefore, it is also possible to reduce the values of the associated design safety factors, which have been calibrated relying on models with higher uncertainty, without impairing the original level of structural safety. This paper presents a study on the calibration of reliability-based safety factors for the design of torpedo anchors considering the statistical model uncertainty evaluated using results from experimental tests and their correspondent finite-element-based numerical predictions. Both working stress design (WSD) and load and resistance factors design (LRFD) design methodologies are investigated. Considering the WSD design methodology, the single safety is considerably lower than the value typically employed in the design of torpedo anchors. Moreover, a LRFD design code format for torpedo anchors is more appropriate since it leads to designs having less-scattered safety levels around the target value.

Avaliação de Juntas Soldadas de um Aço Fundido de Alta Resistência para a Indústria Offshore

“…Este é o caso de torpedos de ancoragem (Figura 3), que representam uma solução efetiva para a ancoragem do tipo vertical ("taut-leg") de plataformas de petróleo (Figura 1). Neste tipo de ancoragem, o raio de ancoragem é menor do que o das linhas em ancoragem convencional do tipo catenária, o que permite a instalação de mais plataformas e diminui a possibilidade de interferência com outros sistemas ou com equipamentos submarinos [33], visto permitir uma redução significativa (cerca de 3vezes) do raio de ancoragem, associado com menor custo de instalação em comparação com âncoras convencionais [34].…”

“…Para operação destas plataformas por longos períodos de tempo (em torno de 20 anos), os custos envolvidos na operação fazem com que a ancoragem seja realizada por sistemas compostos por âncoras, amarras e acessórios [4][5][6][7] conforme mostrado na Figura 1 [7], sendo que estas linhas de ancoragem e seus componentes tomam importância fundamental, no tocante à seleção e manutenção dos materiais envolvidos, uma vez que são importantes pontos de falha dos sistemas de ancoragem [8].…”

Evaluation of High Strength Cast Steel Welded Joints Suitable to Mooring Components

“…They are submitted to several loads as well as continuous exposure to a corrosive environment. The mooring systems consist of long lengths of steel chain links, wire or polyester ropes, and other accessories [6,7] designed for an operational life of about 20 years, and periodic inspections (5 years) are mandatory for monitoring the structural integrity of these components because they are an important point of failure of mooring lines [8]. Thus, the components of mooring lines have fundamental importance, concerning the selection and maintenance of the materials used [9].…”

Directional Stability Study of Torpedo Anchors

“…In 2009, Mohammad et al simulated the penetrating procedure by FEM and analyzed the velocity, forces, and other parameters during penetration [3]. In 2012, Luís et al studied the holding capacity safety factors of torpedo anchors to increase its reliability [4]. Most of the researches were about the procedure of penetrating into seabed, while less about the hydrodynamic characteristics during free fall.…”