PLEM Design Methodology

The pipeline end manifolds (PLEMs) should be designed to facilitate installation and allow them to land properly on the seabed. The following design and analyses should be performed during PLEM design:

• Structural design and analysis;
• Mudmat design and analysis;
• PLEM installation analysis.

Structure

The structural analysis software SACS/StruCAD, which is widely used in the analysis of subsea steel structures, is one of the most useful tools for the analysis of a PLEM’s steel frame structure. The SACS/StruCAD analysis provides loads, stresses, and deflections for all structural members of the PLEMs in various load conditions. The structural design of a PLEM should be performed in accordance with the latest editions of API-RP-2A and AISC’s Steel Construction Manual, Allowable Stress Design (ASD).

The structural analysis software SACS/StruCAD, which is widely used in the analysis of subsea steel structures, is one of the most useful tools for the analysis of a PLEM’s steel frame structure. The SACS/StruCAD analysis provides loads, stresses, and deflections for all structural members of the PLEMs in various load conditions. The structural design of a PLEM should be performed in accordance with the latest editions of API-RP-2A and AISC’s Steel Construction Manual, Allowable Stress Design (ASD).

The PLEM analysis for structural integrity should include the following conditions:

• Fabrication;
• Lifting;
• Transportation;
• Installation;
• In-place operating conditions, structural loads, and fatigue damage;
• Thermal and pressure-induced expansion of pipelines in the operating condition;
• Piping requirements, including bends, tees, reducers, and valves.

The PLEM is designed with padeyes and miscellaneous installation aids as required and has a sufficient number of padeyes for handling during various phases of installation. The design should ensure that the CG of the PLEM assembly, including the yoke, is near the geometric center of the mudmat, in line with the center of the pipeline (or slightly below) and below the pivot point of the yoke and slightly to the rear of the yoke pivot to ensure that the PLEM will stabilize with the correct orientation for landing.

Coordinates and CG Position of Second-End PLEM

The pivot is the local coordinate origin on the PLEM for dimensions and calculations. The offsets epipe and ePLEM are negative numbers. The PLEM is welded to the pipeline to minimize the cost and potential leak paths. The equipment size and location for a PLEM should take into consideration the need for ROV operability and visibility. The structural frame and mudmat for the PLEM are coated and cathodically protected for the design field life. The cathodic protection design and analysis may be carried out based on DNV RP B401 [6], “Cathodic Protection Design.”

Mudmat

A preliminary mudmat area for the PLEM is estimated based on the soil bearing capacity and the estimated weights of the hubs, valves, piping, jumper loads, and mat. The soil shear strength profiles are developed based on site-specific soil data obtained from drop cores. The design shear strength is obtained as the depth-averaged value over the shear strength profile at a depth equal to half the PLEM width. The foundation bearing stresses are calculated for the following anticipated jumper loading conditions:

• Jumper installation;
• Jumper operating;
• Jumper operating, future jumper installation;
• Future jumper operating.

The mudmat has a shallow foundation and may be designed to cover the following issues per API-RP2A WSD or DNV Classification Notes No.30.4, “Foundations”:

• Settlement;
• Eccentric loading;
• Overturning and cyclic effects;
• Safety factors for bearing, sliding, torsional, and overturn failures.

Mudmats with skirts are designed to keep the PLEM stability and to reduce settlement and movement on the seabed when in clay soil. However, PLEMs without skirts are often used in shallow water on sandy soil so it can move about on the seabed, because the penetration on sand is very small.

PLEM Installation

The pipe in aPLEMmust not be overstressed at any time during different load conditions. The PLEM must be stable during installation and configured to land on the seabed in the correct orientation. The PLEM used to initiate pipelay is termed a first-end PLEM. A PLEM installed on completion of the pipelay is termed a second-end PLEM. The second-end PLEM has a yoke that pivots near the pipe centerline and the PLEM center of gravity. The following problems could occur during installation and should be considered during the design procedure:

• Center of gravity too high due to late or unplanned equipment additions;
• Extra measures required to land upright due to pipe torsion;
• Bent pipe due to lowering too far with the PLEM held inverted by pipe torsion.

The installation of a second-end PLEM starts with the abandonment configuration of the pipeline. The pipeline can be laid by using an S-lay or J-lay process. On completion of pipe laying, the pipeline is fitted with an abandonment and recovery (A&R) head to prevent flooding and for attachment of the A&R wire. There are two reasons for not attaching the PLEM at this point:

• It is not practical to attach and maneuver the PLEM structure through the pipelay stinger or J-lay tower.
• It is prudent to lay the end of the pipe on the bottom and assess the unconstrained top-of-pipe orientation before attaching the PLEM.

The PLEM weight, balance, and geometry are all optimized during design to ensure that the PLEM has an intrinsic tendency to land in the correct orientation. Nevertheless, experience has shown that it is essential to attach the PLEM according to the observed natural top-of-pipe.

The rigging to lower the PLEM is connected to a yoke that applies a lift force to a pivot near the centerline of the pipe and above the CG of the PLEM. The pivot is located so as to:

Installation Sequence for a Second-End PLEM

• Control bending load on the pipeline throughout the installation sequence.
• Direct the force to correct the orientation of the PLEM as the PLEM approaches the seabed.

References

[1] M. Faulk, FMC ManTIS (Manifolds & Tie-in Systems), SUT Subsea Awareness Course, Houston, 2008.

[2] G. Corbetta, BRUTUS: The Rigid Spoolpiece Installation System, OTC 11047, Offshore Technology Conference, Houston, Texas, 1999.

[3] J.K. Antani, W.T. Dick, D. Balch, T. Van Der Leij, Design, Fabrication and Installation of the Neptune Export Lateral PLEMs, OTC 19688, Offshore Technology Conference, Houston, Texas, 2008.

[4] American Petroleum Institute, Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms–Working Stress Design, API RP 2A-WSD (2007).

[5] American Institute of Steel Construction, Manual of Steel Construction: Allowable Stress Design, nineth ed., AISC, Chicago, 2002.

[6] DET NORSKE VERITAS, Cathodic Protection Design, DNV RP B401 (1993).

[7] K.H. Andersen, H.P. Jostad, Foundation Design of Skirted Foundations and Anchors in Clay, OTC 10824, Offshore Technology Conference, Houston, Texas, 1999.

[8] DET NORSKE VERITAS, Foundations, DNV, Classification Notes No. 30.4 (1992).

[9] K.C. Dyson, W.J. McDonald, P. Olden, F. Domingues, Design Features for Wye Sled Assemblies and Pipeline End Termination Structures to Facilitate Deepwater Installation by the J-Lay Method, OTC 16632, Offshore Technology Conference, Houston, Texas, 2004.

[10] N. Janbu, L.O. Grande, K. Eggereide, Effective Stress Stability Analysis for Gravity Structures, BOSS’76, Trondheim, Vol. 1 (1976) 449–466.

[11] N. Janbu, Grunnlag i geoteknikk, Tapir forlag, Trondheim, Norway (in Norwegian). (1970).

[12] R.T. Gilchrist, Deepwater Pipeline End Manifold Design, Oil & Gas Journal, special issue (1998, November 2).

[13] D. Wolbers, R. Hovinga, Installation of Deepwater Pipelines with Sled Assemblies Using the New J-Lay System of the DCV Balder, OTC 15336, Offshore Technology Conference, Houston, Texas, 2003.