27.2.1 Design Stages

The design of subsea pipelines during subsea field development is usually performed in the following three stages:

The continuity of engineering design from conceptual engineering and preliminary engineering to the detailed design is essential to a successful project. The objective and scope of each design stage vary depending on the operator and the size of the project. However, the primary aims are discussed next.

27.2.2 Design Process

The object of the design process for a subsea pipeline is to determine, based on given operating parameters, the optimum pipeline size parameters. These parameters include:

The design process required to optimize the pipeline size parameters is an iterative one and is summarized in Figure 27-2. The design analysis is illustrated in Figure 27-3.

Figure 27-2 Subsea Pipeline Design Process

Figure 27-3 Subsea Pipeline Design Analysis

Each stage in the design should be addressed whether it is a conceptual, preliminary, or detailed design. However, the level of analysis will vary depending on the required output. For instance, reviewing the objectives of the detailed design (Section 27.2.1.3), the design should be developed such that:

27.3.1 Subsea Pipeline Design Basis Development

A design basis should be prepared to cover all necessary data, parameters, codes, and standards. All relevant data should be reviewed and evaluated and the criteria established for the detailed design. These should cover the following as a minimum:

27.3.2 Subsea Pipeline Route Selection

Develop a pipeline route selection based on route survey information and environmental data provided by the owner. The scope of work should include at a minimum the following tasks:

27.3.3 Steady-State Hydraulic Analysis

Prepare a steady-state hydraulic analysis study to confirm the selected pipeline sizes and define the operating envelope based on the production forecast given, selected pipeline route and overall pipeline configuration. Possible changes in flow rates and operational modes over the entire operational life of the pipeline should be included. The study should include:

27.3.4 Pipeline Strength Analysis

Prepare a mechanical design of the pipeline:

27.3.5 Pipeline Vertical and Lateral On-Bottom Stability Assessment

Perform pipeline on-bottom stability analysis in accordance with DNV RP E305 [2] (or DNV RP F109 [3]) design code or using AGA software (Subsea pipeline on-bottom stability analysis) [4]. The pipeline should be stable both laterally and vertically, incorporating “liquefaction” and self-burial consideration against the environmental loadings and soil condition. The ballast weight may need to be determined during the installation and operational phases.

An on-bottom stability analysis for the pipelines should include the following conditions:

27.3.6 Installation Method Selection and Feasibility Demonstration

The following studies should be performed when designing subsea pipelines:

Designers should consider pipeline installation via a conventional pipe-laying barge (S-lay, J-lay, and reel-lay), bottom tow, off-bottom tow, and surface tow or any other installation method.

27.3.7 Material Take-Off (MTO)

Prepare the MTO for the pipeline for the following items:

27.3.8 Cost Estimation

Prepare cost estimates for the flowlines, which include, but are not limited to:

27.4.1 Pipeline Spanning Assessment

Prepare a spanning analysis for the pipeline to include the following conditions:

and the following VIV limiting criteria for dynamic analysis:

27.4.2 Pipeline Global Buckling Analysis

Prepare a global buckling analysis for the pipeline for the following operating conditions:

27.4.3 Installation Methods Selection and Feasibility Demonstration

Perform for the pipelines:

Consultant should consider pipeline installation via a conventional pipe-laying barge, bottom tow, off-bottom tow and surface tow or any other installation methods.

27.4.4 Pipeline Quantitative Risk Assessment

The assessments carried out in this work are as follows:

27.4.5 Pipeline Engineering Drawings

Prepare a set of pipeline engineering drawings as follows:

27.5.1 Wall-Thickness Sizing

The wall-thickness level for pipelines should be able to withstand pressure and pressure effect (hoop and burst strength) requirements in accordance with API-RP-1111 [5] and API 5L [6] (or CFR [7], ASME B31 [8, 9], DNV-OS-F101 [10] depending on the project’s requirement). A stress analysis should be carried out for the wall-thickness check.

27.5.2 On-Bottom Stability Analysis

Subsea pipelines resting on the seabed are subject to fluid loading from both waves and steady currents. For regions of the seabed where damage may result from vertical or lateral movement of the pipeline, it is a design requirement for the pipe weight to be sufficient to ensure stability under the worst possible environmental conditions. In most cases this weight is provided by a concrete coating on the pipeline. In some circumstances the pipeline may be allowed to move laterally provided stress (or strain) limits are not exceeded. The first case is discussed briefly in this section since it is applied in the majority of design situations.

The analysis of on-bottom stability is based on the simple force balance or detailed finite element analysis. The loads acting on the pipeline due to wave and current action are the fluctuating drag and lift and inertia forces. The friction resulting from effective weighting of the pipeline on the seabed to ensure stability must resist these forces. If the weight of the pipe steel and contents alone or the use of rock berms is insufficient, then the stability design must establish the amount of concrete coating required. In a design situation, a factor of safety is required by most pipeline codes.

The hydrodynamic forces are derived using traditional fluid mechanics with a suitable coefficient of drag, lift and diameter, roughness, and local current velocities and accelerations.

The effective flow to be used in the analysis consists of two components:

The wave and current data must be based on extreme conditions. For example, the wave with a probability of occurring only once in 100 years is often used for the operational lifetime of a pipeline. A less severe wave, say, of 1 year or 5 years, is applied for for pipeline installation, in which the pipeline is placed on the seabed in an empty condition and has less submerged weight.

Friction, which depends on the seabed soils and the submerged weight of the pipe, provides the equilibrium of the pipeline. Remember that this weight is reduced by the fluid lift force. The coefficient of lateral friction can vary from 0.1 to 1.0 depending on the surface of the pipeline and on the soil. Soft clays and silts provide the least friction, whereas coarse sands offer greater resistance to movement.

Figure 27-7 illustrates the component forces for a subsea pipeline stability analysis. For the pipeline to be stable on the seabed, the following relationship must be satisfied:

(27-10)

where

Figure 27-7 Subsea Pipeline Stability Analysis

We can see that stability design is a complex procedure that relies heavily on empirical factors such as force coefficient and soil friction factors. The appropriate selection of values is strongly dependent on the experience of the engineer and the specific design conditions.

The goal of the subsea pipeline stability analysis described is to determine how much additional weight should be added via the coating. If the weight of the concrete required for stability makes the pipe too heavy to be installed safely, then additional means of stabilization will be necessary. The two main techniques are:

27.5.3 Free-Span Analysis

Over a rough seabed or on a seabed subject to scour, pipeline spanning can occur when contact between the pipeline and seabed is lost over an appreciable distance (see Figure 27-8). In such circumstances, normal code requirements are that the pipeline be investigated for:

Figure 27-8 Pipeline Span Analysis

Due consideration to these requirements will result in the evaluation of an allowable free-span length. Should actual span lengths exceed the allowable length, then correction is necessary to reduce the span for some idealized situations. This can be a very expensive exercise and, consequently, it is important that any span evaluations conducted be as accurate as possible. In many cases, multiple span analyses have to be conducted that account for a real seabed and in situ structural behavior.

The flow of waves and current around a pipeline span, or any cylindrical shape, will result in the generation of sheet vortices in the wake (for turbulent flow). These vortices are shed alternately from the top and bottom of the pipe, resulting in an oscillatory force being exerted on the span as shown in Figure 27-9. Vortex shedding means that cross currents will generate alternating loads on a pipe, resulting in vibration of the pipe.

Figure 27-9 Pipeline Vortex Shedding

If the frequency of shedding approaches the natural frequency of the spanning pipeline, severe resonance can occur. This resonance can induce fatigue failure of the pipe and cause the concrete coating to crack and possibly be lost. The evaluation of the potential of a span to undergo resonance is based on the comparison of the shedding frequency and the natural frequency of the spanning pipeline. The calculation of shedding frequency is achieved using traditional mechanics although some consideration must be given to the effect of the closeness of the seabed. Simple models have traditionally been used to calculate the natural frequency of the spanning pipeline, but recent theories have shown these to be oversimplified and multiple span analyses need to be conducted. Finite element analysis for a pipeline on a seabed is widely used for the natural frequency and VIV fatigue damage analyses.

27.5.4 Global Buckling Analysis

Global buckling of a pipeline occurs when the effective compressible force within the line becomes so great that the line has to deflect and, hence, reduces these axial loads (i.e., takes a lower energy state). As more pipelines operate at higher temperatures, the likelihood of buckling becomes more pertinent.

Global buckling analysis will be performed to identify whether the global buckling is likely to occur (see Figure 27-10). If it is, then further analysis is performed to either prevent buckling or accommodate it.

Figure 27-10 Global Buckling of Pipeline [11]

A method of preventing buckling is to dump rocks on the pipeline. This induces even higher loads in the line but prevents it from buckling. However, if the rock dump does not provide enough restraint, then localized buckling may occur (i.e., upheaval buckling), which can cause failure of the pipeline.

Another method is to accommodate the buckling problem by permitting the pipeline to deflect on the seabed, by using a snake lay or buckle mitigation methods such as sleepers or distributed buoyancies. This method is obviously cheaper than rock dumping, and results in the pipeline experiencing lower loads. However, the analysis will probably have to be based on the limit-state design, because the pipe will have plastically deformed. This method is becoming more popular. This method can also be used with intermittent rock dumping; by permitting the pipeline to snake and then to rock dump, the likelihood of upheaval buckling is reduced.

The methods employed in calculating upheaval and lateral buckling, as well as the pullover response, are detailed in Ref. [12].

27.5.5 Pipeline Installation

Various methods are used to install subsea pipelines. The method of installation determines the type of analyses that must be performed, as discussed next.

27.6.1 Flow Assurance

Flow assurance is one crucial factor for HP/HT pipeline design, which includes (1) steady-state performance of the flowline as governed by the overall heat transfer coefficient or U-value, and (2) transient performance of heatup and cooldown requirements. During the cooldown period after shutdown of the fluid flow, the product temperature must remain above a specified value to avoid hydrate formation. In deep water, pipeline design must seek a balance between the steady-state and transient cooldown requirements to meet the specific field conditions, along with issues of weight, installation needs, and other commercial and economic factors.

For the thermal insulation system, conventional single pipe and pipe-in-pipe (PIP) systems are two options and the final selection depends on a range of factors, such as cost, installation and thermal management. PIP systems may offer the best balance of thermal efficiency and steady-state performance for HP/HT pipelines; however, the increased weight is a factor of considerably greater significance for installation in deep water than in shallow.

The wet insulation materials used with HP/HT pipelines will result in an extremely thick coating for good steady-state performance, which introduces increased cost and seabed stability issues due to the low submerged weight. For the transient cooldown procedure for a flowline, low thermal conductivity and high thermal inertia are very important to determine the “no-touch” time during a shutdown and also the thickness of the insulation layers. The higher density also results in an increased submerged weight, which, in turn, eases seabed stability issues. Flow assurance for HP/HT pipelines is detailed in Part 2 of this book.

27.6.2 Global Buckling

HP/HT pipelines laid on the seabed are susceptible to global lateral buckling, resulting in deflections that can lead to the pipe cross section yielding. This is caused by compressive axial force building up as the pipeline tries to expand thermally but is restrained due to soil resistance between the pipe and the seabed.

The key to an effective solution is the pipe’s ability to extend axially. Control of the axial and lateral movements of a pipeline can be obtained by using either a buckle initiation device or expansion spools and it will result in economic relief of the axial force. Various approaches have been tried and applied to manage pipeline global buckling.

Finite element (FE) modeling of the pipeline installation and operation has been widely used to identify the issues related to the management of thermal buckling, which includes pipeline walking under cyclic thermal loading, global buckling interactions, etc. The detailed analysis and modeling of thermal buckling will enable cost-effective mitigation options.

27.6.3 Installation in Deep Water

The installation of HP/HT pipelines is more challenging and costly in deep water than in shallow water. As the reservoir and field go deeper, the weights involved with these pipeline systems may become too heavy for conventional installation methods. PIP systems are more installation-vessel dependent than conventional pipe, and this dependency is even more pronounced with increasing water depth.

Lay rates and reliability are key factors for the pipeline installation in deep water. The high tensions associated with the J-lay method and the high bending in the steep S-lay method may cause high stress in the pipeline. The strain-based design approach has been used in many installation analyses. Pipeline installation is detailed in Chapter 5.