Review Reports - Re-Scour Below a Self-Buried Submarine Pipeline

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper investigates three-dimensional re-scour characteristics of a tilted submarine pipeline after self-burial, focusing on the impacts of the tilting angle and the flow incident angle on the scour topography, depth, and propagation rate. The study is of both scientific and practical interests, and it is suitable for publishing in Water . The following questions should be address:

Clearer definition of "sag pits" is advised.
In Section 2.1, elaborate the determination of 2° pipeline tilting.
In Section 3.3 Eq. (11), appropriate citation and discussions are needed.
Lines 386-391, provide the value of K in Equation (16) for the prediction of scour propagation rate.

Comments on the Quality of English Language

Some typos are found the context and the authors are advised to have a thorough check.

Author Response

Comments 1: Clearer definition of "sag pits" is advised.

Response 1: The intention of uing 'sag pit' is to imply possible sagging of pipeline after the re-scour, which keeps consistent with the topic of the present study. As a matter of fact, it is the same as 'scour pit' as we usually mean in previous references. To avoid confusion (both Reviewers mentioned the same question), we have changed 'sag pit' into 'scour pit' in the whole manuscript. All changes have been highlighted in the new manuscript.

Comments 2: In Section 2.1, elaborate the determination of 2° pipeline tilting.

Response 2: The whole pipeline can be considered as a rigid body and the deflection is small. We assume that the pipeline sagging symmetric and simplify it by using a pipe model tilted in a small angle β. This angle can be estimated by deflection formula of a pipeline (FREDSØE et al., Three-dimensional scour below pipelines, Journal of Offshore Mechanics and Arctic Engineering, 1988, 110: 373-379), considering the pipeline diameter, concrete weight coating and anti-corrosion layer thickness from worldwide database, which turns out to be about 2^° taking the general values of parameters. This has been added and highlighted in Section 2.1, Page 3, Lines 98-104 of the manuscript.

Comments 3: In Section 3.3 Eq. (11), appropriate citation and discussions are needed.

Response 3: According to the equation, the vertical scour rate can be described as the variation of scour depth with respect to time (Fredsøe et al., Time scale for wave/current scour below pipelines, Int. J. Offshore Polar Eng. 1992, 2(1),13−17). From Figure 15, it is seen that the slope of scour depth is significantly high at the initial scour stage and then becomes significantly small after the scour reaches the equilibrium state. This equation implies the variation rate of scour depth can be assumed as constant at the beginning of scour, which is similar to that of a horizontal pipeline scour (Chiew, Mechanics of local scour around submarine pipelines, J. Hydraul. Eng. 1990, 116(4), 515−529). Both citation and discussion have been added and highlighted in Section 3.3, Page 15, Lines 347-352 of the new manuscript.

Comments 4: Lines 386-391, provide the value of K in Equation (16) for the prediction of scour propagation rate.

Response 4: K=7 has been added in Section 3.3, Pages 17-18, Lines 408-409 the new manuscript. Thanks for pointing out this key information missing in the first copy.

General comments: Some typos are found the context and the authors are advised to have a thorough check.

Response: We appreciate Reviewer's comments and we have run a through check on the whole text.

Reviewer 2 Report

Comments and Suggestions for Authors

Reviewer: Manuscript # water-3997222

Title: Re-scour below a Self-buried Submarine Pipeline

Authors: Authors: Xiaofan Lou *, Yulong Hua, Lichao Chen

This study investigated the re-scour behavior of a tilting submarine pipeline with varying embedment to explore how the tilt angle influences three-dimensional scour characteristics. Two newly identified scour features, sand ripples extending downstream along the pipeline and sag pits forming beneath it, were found to significantly affect scour development, including both depth and propagation. The results showed that increasing the incident flow angle enhances scour propagation during the rapid scouring phase but suppresses it during the slow phase. Based on these findings, a new predictive model was proposed that relates scour propagation to sediment erosion characteristics and the magnified shear stress beneath the pipeline, providing accurate predictions under varying flow conditions. This study addresses a relevant and timely topic, offering valuable insights for readers of Water.

The manuscript is quite complete, clear, and concise. It is also of interest to those working in the management of hydraulic structures and environmental protection.

In my opinion, the manuscript is suitable for publication after minor revisions (see comments below).

Comments

Introduction: I suggest the authors consider the following studies:

Ben Meftah, M.; Mossa, M. New Approach to Predicting Local Scour Downstream of Grade-Control Structure. J. Hydraul. Eng. 2020, 146, 04019058.

Figure 1: needs slight improvement.
On lines 84-85: You mention “… the sand slope at the leading edge of the scour hole…”. It might be helpful to indicate the angle of the sand in Figure 2.
The various parameters shown in Figure 2 are not defined!
On lines 95-97: You mention “This angle can be estimated…which turns out to be about 2°”. But what angle are you talking about?
On lines 99-103: The parameter definitions should be moved to Fig. 2.
On line 107: What is the physical meaning of the time scale T?
On line 109: add the expression of the specific gravity s =(rs/rw).
(6): What are the limitations of this equation? Could you elaborate on this point?
On lines 131-135: The upstream and downstream slopes (1:20 and 1:10) of the “sand pit” are not clearly represented, as Figure 3 shows a horizontal bed, which may confuse readers.
Figure 8: Two velocity profiles are shown, but the definitions of u1 and u2, as well as their measurement locations, are not specified. Were these velocities measured inside the erosion hole in the equilibrium phase?
Figures 10 -12: Add the legend scale unit. It would be preferable to present these figures in dimensionless form, using the normalized coordinates x/D, y/D, and z/D.
Figures 13-14: It would also be preferable to present these figures in dimensionless form, using the normalized coordinates x’/D and z/D.
Figure 15: It would also be preferable to present this figure in dimensionless form, normalizing the scour depth by the maximum scour depth and the time by the total time (T-ti).
Figure 15: It would also be preferable to present this figure in dimensionless form, using the normalized coordinates t/T and L/Lmax.
On lines 298-300: Under a 45° incident flow, the velocity component parallel to the pipeline is the largest, so one would expect the longest scour hole. However, this does not appear in the results. How can this discrepancy be explained? Could you elaborate on this point?
On line 328: you mention “…which can be calculated by the derivation of Eq. (11)”. Do you mean Eq. (1)?
On line 332: “…model illustrated in Figure 17”.
Figure 18: It would be better to indicate which is the side view and which is the top view.
Could the development of scour downstream of the concrete structure (upstream of the test section) have influenced the experimental results? Could you elaborate on this point?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf