Offshore Piping – Design, Installation, Inspection

David
Etukudo

Offshore piping refers to pipe installed in the subsea area. As mankind continually discovers and harnesses offshore mineral and energy resources, offshore piping continues to increase in popularity. Mostly, they serve as conduits for transporting hydrocarbons and other fluids related to crude oil production. But with the deployment of more sophisticated machinery on the seabed, as well as the prominence of offshore wind energy, offshore piping also serves as conduits for electrical and communication lines.

Subsea pipelines alongside various types of processing platforms
Courtesy: Oilstates

In this article, you will learn about offshore piping design, key considerations during installation, and inspection of pipelines.

Offshore Piping Design

The design considerations of an offshore pipe are much more than their inland counterparts because of the tough conditions. Operating at sea means pipelines are subject to hydrostatic pressure, hydrodynamic loads, and are more susceptible to corrosion. As a result, they should have a robust design that bears all operation loads, as well as withstand both internal and external corrosion.

In addition, as offshore projects venture into deeper waters, the installation loads are more significant and could have an impact on the fatigue performance of the pipeline. Carrying out maintenance at such depths is expensive as access is limited, so cannot be done as often as when the pipes are onshore. Industry principles that govern how the design should go are well documented in DNV, API, and ISO standards.

DNV Standard for Offshore Pipes

The DNV-OS-F101 is an offshore standard that applies modern limit-state-design principles to submarine pipeline systems. Moreover, these principles assign safety classes in line with possible consequences of failure and have several recommended practices (RP) to complement them. For example, DNV-RP-F101, DNVGL-RP-F103, and DNV-RP-F106 are the recommended practices bordering corrosion and coating of pipelines. DNV-RP-F110, DNV-RP-C205, and DNV-RP-F116 give guidance for design against buckling, environmental loads, and integrity management of the pipelines, respectively. Some unconventional factors also affect offshore pipes such as interference from fish trawling gear. DNV-RP-F111 recommends the use of protective gear such as mats, tunnels, and even burying the pipe in shallow areas.

Typical loads on an offshore piping
Courtesy: Researchgate

Another unique occurrence is the presence of free spans as a result of undulations, or seabed scouring under the pipe. When this occurs, the pipe section in suspension, or span, is subject to an increase in loading from its weight and design loads. Also, the interaction between the span and hydrodynamic forces could result in vortex-induced vibrations (VIV), where the pipe oscillates at or close to its natural frequency. Therefore DNV-RP-F105 recommends limiting the allowable free span length (L) below critical values where VIV can occur. Likewise, the maximum allowable stress should be in consideration when deciding the limit of the allowable free span length.

Free spanning of an offshore pipeline on the seabed
Courtesy: Researchgate

API Standard for Offshore Pipes

API also has a few standards and recommended practices that deal with offshore piping. The API 5L covers specifications for line pipes, while API RP 5LW offers recommendations on how to transport these pipes on marine vessels. Welding of pipelines and ancillary equipment is well documented in API 1104.

The most comprehensive of the standards is API 1111, which covers the design, construction, operation, and maintenance of offshore hydrocarbon pipelines. This standard entails pipeline design for the marine environment, fatigue analysis, and thermal expansion design. It also outlines corrosion control measures such as using coatings and cathodic protection systems. Guidelines on pipeline operations such as emergency plans and pipeline abandonment at the end of design life are detailed in this document.

Offshore Piping Installation

Offshore piping installation is a delicate process due to the challenges it presents. First, the pipeline experiences significant loading as the installation depth increases. This could result in damage or induce residual stress that limits its fatigue performance when in operation. Also, features on the seabed and characteristics of the soil could be detrimental to the pipeline and other equipment so requires investigation. As a result, there are several phases of installation which the following sections cover.

Site Investigation

Carrying out site investigation along the proposed route of the offshore piping is the first step of installation. Generally, this investigation is in two parts which are a geophysical survey and a geotechnical survey. The geophysical survey entails using non-invasive techniques with the aid of equipment such as echo sonar, side scan sonar, and seismic profiler to determine the following:

  • Seabed topography or bathymetry. Typically reveals areas of the seabed that are uneven where free spanning of the pipeline may occur.
  • Features and obstructions such as boulders, existing cables, and other pipelines.
  • Sub-bottom profile of up to 5m below the seabed. Moreover, this is useful especially when the intention is to bury the pipeline.

The geotechnical survey involves coring and sampling of the soil either in situ or in a laboratory. The essence is for accurate stratification of the soil, and to determine key engineering parameters such as its strength, bearing capacity, and the possibility of scouring.

Offshore Piping Initiation

When initiating a pipeline installation, the first end going to the seabed has an anchor to keep it in place. Because of the horizontal loads from the pipe-laying process, this anchor should have enough dead weight to ensure accuracy and safety. The type of anchor depends on the soil type as determined by the site investigation. Some types include suction piles, driven piles, and dead man anchors.

On-Bottom Stability

Every pipeline on the seabed is subject to hydrodynamic loading of which drag and inertia forces make up the horizontal force component. While the vertical component is the lift force that acts upwards to suspend the offshore piping from the seabed.

Hydrodynamic loading on an offshore pipe
Courtesy: AJER

None of these motions (horizontal/vertical) are desirable, so it is necessary to estimate these forces. First, operators ensure that resistance from the pipeline weight and friction with the soil are sufficient. If the pipe’s weight is not enough to ensure stability, then concrete coating can help increase the weight. When this is not technically or financially feasible, then concrete mats or tunnels can be put on sections of the pipe that are most at risk. In critical situations, burying the entire pipeline is a viable option. Its necessary to determine the course of action prior to installation.

Crossings

Whenever it is not possible to evade existing cables or other pipelines along the pipe route, there needs to be a crossing. At these points, the offshore piping takes off at some distance before the existing pipeline and lands at some distance after it. Also, industry codes recommend a minimum of 0.3m clearance between pipelines. A key thing about crossings is that the deflection makes the pipeline at risk of upheaval buckling, as it is an imperfection. In addition, it increases the risk of interaction with fish trawling gear because it is off the seabed. Assessing these risks is necessary during installation, and protective coverings such as mats should be made available.

Subsea pipeline crossing
Courtesy: Researchgate

Safety and Environmental Impact Assessment

The safety of offshore piping is key because any breach of its integrity will pollute the environment. The possibility of pipeline interaction with fishing gear in shallow waters requires assessment. Also, dropped objects along busy shipping routes are a common occurrence. Generally, the deployment of protective gear such as concrete mats and tunnels is sufficient in such zones.

Another aspect of EIA for consideration is decommissioning of pipelines. At the end of their operational life, governing regulations require their removal or abandonment in situ in a responsible manner. When decommissioning, the flushing of the pipeline should not release any corrosive or harmful substance into the environment. Also, disconnecting the pipelines from connecting equipment and blocking all entries into them is a necessary step.

Offshore Piping Inspection

Generally, divers carry out inspections of offshore piping up to depths of 200ft. Beyond this, remotely operated vehicles (ROVs) take the place of human divers. Basic checks during the inspection include corrosion level, cathodic protection measurement, free span measurement, and assessing the level of thermal expansion. It may also be important to remove marine growth from pipelines during the inspection. Other than hindering visibility, this growth can alter the dynamic behavior of risers if present in significant amounts.