Cutting umbilical costs

Ian Probyn outlines measures taken by Technip Umbilicals to reduce the cost of long-length umbilical projects.

A mid-line joint.

Despite the low oil price, a number of long-length tieback projects have been sanctioned over the last few months.

To safely control and supply power and chemicals to the remote trees and manifolds, an umbilical system is installed along the length of the tieback to a host vessel or platform. As the cost of the umbilical system is largely driven by material content, the overall system cost is approximately proportional to length. The trend for greater tieback lengths and the strong cost focus of the industry has brought the umbilical business into the spotlight of cost reductions.

Technip Umbilicals is addressing this challenge through a focused research and development (R&D) program to optimize the materials used within the umbilical structure, developing technologies to ease and save installation cost, and leveraging manufacturing asset capabilities to maximize efficiencies without compromising performance, quality or reliability.

This cannot be achieved by simply reducing requirements or using cheaper materials. Only through a structured engineering study can changes be justified and ensure the product remains reliable and fit for purpose.

The Skandi Africa subsea construction support vessel. Images from Technip.

Component savings

Hydraulic fluids and chemicals are transported through the umbilical in steel tubes or thermoplastic hoses. Typically, the bore size and pressure of the tube is set by the flow requirements and the umbilical manufacturer must calculate the wall thickness, based upon the requirements of the main umbilical design standard ISO13628 and clients’ specifications.

Technip Umbilicals has developed a suite of integrated design software tools that enable designers to rapidly optimize the tube wall thickness, based on material type, project environment, manufacturing and client requirements. However, working with clients to review their internal specifications has enabled conservatisms to be identified and justify material optimization resulting in cost savings. This has to be coupled with the ability to demonstrate the robustness of the product through the whole life cycle including installation and dynamic fatigue. Installability is proven through the use of validated analysis tools that predicts the behavior of the structure and confirms suitability without compromising the long-term reliability.

The latest developments in thermoplastic hose technology have seen temperature, pressure and water depths increase, enabling hoses to be used in more traditional steel tube applications allowing the inherent cost advantage of thermoplastic over steel tube to be exploited. Although the R&D focus has been to push the operating pressures higher beyond the previous ceiling limits, the knowledge gained can also be applied to lower pressure hose designs. Cost savings can be realized through more efficient material use, allowing savings of up to 20% over previous equivalent designs.

Manufacturing

The Maria development umbilical.

The capability and capacity of the lay-up machine used to bring the functional components together into the umbilical bundle can significantly affect the final cost. Large capacity machines, which can be operated with high proficiency, enable the assembly process to be both high quality and efficient with minimal downtime to reload components, bobbins, and perform tie-in welds and cable splices. Experience in positioning components and smart use of filler materials can deliver the required project performance requirements, while also easing the manufacturing process and reducing assembly time.

A common requirement of long-length tieback umbilicals is to incorporate a large central tube, typically 2-4in bore, known as a LBCT, to carry monoethylene glycol or methanol for flow assurance of the hydrocarbons. The inclusion of a LBCT can add complexity and cost to the umbilical because of the additional bending stiffness and complexity of tube welding required. Current 2D X-ray inspection can be ambiguous, and coupled with very stringent internal pass criteria, previous projects have suffered from high rejection rates, leading to the weld being cut-out and redone.

However, forensic investigation via microscopic material inspection and finite element structural analysis found a high proportion of the welds removed from the tube string would have been acceptable in service. Hence, a revised criterion was developed, which reflected a more realistic acceptance level, and was thoroughly justified via structural and fatigue testing of sample welds made with built-in flaws to prove suitability. The new acceptance criteria would have significantly reduced the unnecessary removal of sound welds and prevented the additional cost and time taken to replace them.

There could also be an impact from developments in the non-destructive testing technology used to assess the integrity of welds during production. A move from 2D radiographic inspection to 3D tomographic inspection developed in a recent R&D collaboration provides the basis for a quantitative inspection regime. The tomographic technology coupled with real-time finite element analysis of the welds has the potential to remove the ambiguity in interpreting conventional 2D images, while providing a clear service based acceptance criteria for any weld.

Termination

The umbilical termination assembly (UTA) hardware used to terminate the ends of the umbilical and enable connection to subsea equipment can influence the installation process and cost. Based on a standard modular design the hardware can be customized to fit the varying number and type of functional components. External size extremities are governed by the lay equipment to allow the hardware to pass through the installation caterpillars.

A useful adaption of the traditional UTA, which has been successfully deployed on a number of projects, is a midline joint that allows lengths of umbilical to be daisy chained together. Each half of the joint is fitted and tested at the factory and rapidly connected offshore. The midline joint could allow a long-length umbilical to be installed via reels rather than a carousel, opening up a greater choice of installation vessel, potentially offering a lower overall cost. Alternatively, a midline joint can be utilized to join two lengths of extra-long length umbilical together to provide flexibility with available weather windows.

Recent R&D studies have focused on the installability of the umbilical and development of a special tape layer that increases the frictional interface between the steel tube bundle and the outer sheath. While the umbilical is squeezed by the caterpillar tensioner, the tape interacts with the sheath and can effectively double the friction value. The upside is that either a greater installable water depth can be achieved, or a smaller, more cost-effective lay-spread can be utilized. Also, recent investigations into the umbilical behavior in axial compression, often experienced during installation, have allowed previously conservative limits to be raised, widening the allowable sea state at which the product can be installed.

Summary

The outlook remains strong for longer length umbilical projects and the main functional components can be optimized for material use through working with clients to reduce conservatism while remaining fit for purpose.

State-of-the-art lay-up assets enable efficient manufacturing and clever use of termination hardware can provide flexibility during offshore installation. New technologies and learnings have been deployed to ease installation and ensure the umbilical remains a reliable cost-competitive component of the subsea infrastructure.

Ian Probyn is global technology lead for Technip Umbilicals. Probyn graduated with a degree in automotive engineering from Loughborough University and spent his early career crashing numerous cars both physically and virtually using finite element analysis (FEA). Fifteen years ago, he entered the subsea industry, joining the R&D team at DUCO (now Technip Umbilicals). He represents the firm on the technical committee of the UMF (Umbilical Manufacturers’ Federation) and is a Technip Group Expert.