Experimental and numerical analysis of non-contact magnetic detecting signal of girth welds on steel pipelines.

The quality of the girth welds on pipelines is a critical point regarding the safe operation. Non-contact pipeline magnetic detection (NPMD) is a non-destructive detection technology based on the metal magnetic memory (MMM) method. However, present studies mostly focus on the qualitative analysis of girth welds instead of accurate quantitative analysis of the stress status. Here, many hydraulic tests in sealed pipelines are performed to investigate the magnetic signal under different internal pressures and detection heights. A numerical model of magnetic signal is established and verified by the experimental results. The results show the characteristics of the signal that the y component has sinusoidal fluctuations when the x and z component reach the extreme values. A new parameter Kvs is proposed to comprehensively reflect the stress status of the girth welds. It is founded that the residual strength ratio (RSR) reduces from 0.97 to 0.83 when the Kvs max increases from 7500 to 13500 nT/m The magnetic signals decay exponentially in the second order when the detection height varies within 0.1-1.0 m. This study provides a theoretical and experimental basis for identifying the stress status of the girth welds on pipelines.

Modeling three-dimensional underwater acoustic propagation over multi-layered fluid seabeds using the equivalent source method.

This paper develops an efficient three-dimensional (3D) underwater acoustic propagation model with multi-layered fluid seabeds based on the equivalent source method (ESM). It solves the Helmholtz equation exactly by a superposition of fields generated by equivalent sources. A linear system coupling ESM equations is derived by imposing boundary conditions and solved iteratively using the generalized minimum residual method. Unlike a direct ESM solver, matrix-vector products in each iteration are evaluated by a pre-corrected fast Fourier transformation (PFFT), significantly reducing the numerical cost and enabling efficient solution of 3D large-scale propagation. Moreover, sound speed profiles can be taken into account by dividing the water column into sub-layers, each of which requires an individual PFFT procedure using an FFT subgrid scheme. Simulations of propagation over a Gaussian canyon validate the PFFT-accelerated ESM (PFFT-ESM). The capability of the PFFT-ESM for 3D scattering problems is demonstrated by further presenting the Gaussian canyon simulations with corrugated surface waves.

Modeling propagation in shallow water with the range-dependent sea surfaces and fluid seabeds using the equivalent source method.

This paper describes a shallow water range-dependent propagation model (RPM) based on the equivalent source method (ESM). The proposed model allows both the sea surface and fluid seabed to vary with the propagation range. The proposed equivalent source method-based range-dependent propagation model (ESM-RPM) utilizes three sets of equivalent sources, placed above the sea surface, below the seabed, and above the seabed, which replace the sea surface reflection, seabed reflection, and seabed transmission, respectively. The unknown strengths of the equivalent sources can be determined by solving an inverse problem based on the boundary conditions. The capability of the ESM-RPM for propagation in refractive water is demonstrated by evaluating the Green's function using a modal projection method. Numerical simulations are conducted in iso-velocity and refractive shallow water with an underwater canyon and corrugated surface waves, including two-dimensional (2-D) propagation across the canyon and three-dimensional (3-D) propagation along the canyon. Further simulations demonstrate the 2-D across-canyon and 3-D along-canyon propagations with random rough sea surfaces. The results show that the proposed ESM-RPM provides efficient, benchmark-quality numerical solutions that accurately capture the mode coupling associated with the varying cross section of the waveguide. Thus, the model has great potential to be applied in benchmarking propagation in shallow water with the varying sea surface and seabed.

Three-dimensional sound scattering from transversely symmetric surface waves in deep and shallow water using the equivalent source method.

This paper proposes a propagation model to calculate the three-dimensional (3-D) sound scattering from transversely symmetric sea surface waves in both deep and shallow water using the equivalent source method (ESM). The 3-D sound field is calculated by integrating an assembly of two-dimensional (2-D) transformed fields with different out-of-plane wavenumbers through a cosine transform. Each 2-D solution is calculated using the ESM incorporating a complex image method that can efficiently and accurately solve the 2-D water/seabed Green's function. The oscillatory cosine integral is accurately calculated using a segmented integral scheme requiring relatively few 2-D solutions, which can be further improved through the use of parallel computation. The model is validated by comparison with a 3-D Helmholtz-Kirchhoff method for deep water and a finite element method for a shallow water wedge with both a fluid and an elastic seabed. The model is as accurate as the finite element approach but more numerically efficient, which enables Monte Carlo simulations to be performed for random rough surfaces in order to study the scattering effects at a reasonable computational cost. Also, 3-D pulse propagation in the shallow water wedge is demonstrated to understand the out-of-plane scattering effects further.