FABIG Newsletter - Issue 65 - JAN 2015 9

Written by: M.Chalk, L. Arnaud

Solent Composite Systems Ltd.

sTrucTurAl coMposITe FIre & blAsT proTecTIoN eNclosure for reducing equipment skid weight

Abstract

This article describes the engineering design work for a project which aimed to reduce the weight of an equipment skid by using a structural composite fire and blast protection enclosure instead of a metallic structure.

1 Introduction

The project demonstrates how an equipment skid was developed and delivered for a new build platform, where weight was a critical consideration for a number of new equipment items being installed on a highly loaded deck area. As well as protecting the equipment (an important part of a gas turbine) against demanding explosion and hydrocarbon jet fire loads, the project was also seeking a way of minimising the additional weight of the protection enclosure.

2 Project Requirements

The appearance and principal dimensions of the required protection enclosure are shown in Figure 1, which also indicates a number of penetrations for tubes and cables connected to the equipment inside.

The dimensions shown in Figure 1 denote the maximum size for the enclosure. There was also a requirement for providing a minimal amount of clearance space for the equipment housed within the enclosure, which therefore imposes a restriction on the enclosure wall thickness. The requirement for a door to access to the equip-ment is essential for operational needs, and the entire structure had to weigh no more than 7 tonnes, including 1 tonne for the equip-ment within. Figure 1 Maximum dimensions for the protection enclosure

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The project challenge was therefore to design and engineer an enclosure of the required dimensions to protect against a maximum explosion overpressure of 0.5 barg and a 60 minutes hydrocarbon jet fire, as well as a 100 year wave slam load; all of this with an enclosure weight not exceeding 6 tonnes.

Other important project considerations included: Ô To provide a single point lift for placing the equipment skid

onto the platform Ô To provide insulation and ventilation in order to control the

equipment operating temperature within the enclosure Ô To accommodate the fixing of secondary items such as

lights, heaters and other essential equipment items to the enclosure walls

Ô To have each connection to the platform managed by a single bolt, if possible

Ô To achieve a 25 year design life as a minimum.

In addition to the fire and blast requirements, there were a number of different load cases to be considered in order to comply with NORSOK N-001 and N-003 as shown in Table 1 below:

2.1 Enclosure Options

The Client was considering two options to meet the above project requirements, which were either:

1. A metallic enclosure produced in a suitable marine grade stainless steel, or

2. An enclosure comprising lightweight composite panels on a structural metallic support frame.

Table 2 compares the estimated weight of each element of the equipment skid for the two options considered:

EquipmEnt Skid ElEmEntmEtallic EncloSurE

compoSitE EncloSurE

Support frame 3 tonnes 1.5 tonnes

Insulated panels 3 tonnes 1.5 tonnes

Equipment 1 tonne 1 tonne

Additional platform support structure 3 tonnes 1.5 tonnes

Total Additional Weight 10 tonnes 5.5 tonnes

SinGlE load caSES

combi- nation no.

dEScriptionlimit StatE

dEad wEiGht

100 YEar waVE

towinGEarth-quakE

1 YEar wind

100 YEar wind

10,000 YEar wind

EnViron-mEnt

blaSt

a1 a2 a3 a4 a5 a6 a7 a8 a9 w1 w2 w3 w4 w5 w6 icE Snow b1 b2 b3

1 Operation X-direction ULS-A 1.3 0.7 0.7 0.7 0.7

loa

d F

ac

tor

S

2 Operation X-direction ULS-B 1 1.3 1.3 1.3 1.3

3 Operation Y-direction ULS-A 1.3 0.7 0.7 0.7 0.7

4 Operation Y-direction ULS-B 1 1.3 1.3 1.3 1.3

5 Operation snow ULS-A 1.3 0.7

6 Operation snow ULS-B 1 1.3

7 Towout X-direction ULS-A 1.3 0.7 0.7 0.7

8 Towout X-direction ULS-B 1 1.3 1.3 1.3

9 Towout Y-direction ULS-A 1.3 0.7 0.7 0.7

10 Towout Y-direction ULS-B 1 1.3 1.3 1.3

11 Earthquake ALS 1 1 1 1

12 Blast North ALS 1 1

13 Blast Top, East, West ALS 1 1

14 Blast South ALS 1 1

1510,000 wind X-direction

ALS 1 1

1610,000 wind Y-direction

ALS 1 1

17 Lifting ULS TBA

SLS = Service Limit StateALS = Accidental Limit StateULS = Ultimate Limit State

Table 1 Load cases for the design of the enclosure

Table 2 Comparison of weight of metallic and composite enclosure options

sTrucTural coMposiTe fire & blasT proTecTion enclosure for reducing equipMenT skid WeighT

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An analysis of the two options was carried out to assess the installed cost and lead time. It was important to maintain the project time-scales, taking into account the restrictions for lifting and handling the equipment skid onto the platform. It was found that the costs and timescales were comparable for both options. Further con-sideration of the impact on the weight and cost of the additional structural supports needed on the platform itself led to the selection of the composite solution.

2.2 Lightweight Composite Solution

The developed enclosure design comprises composite panels weighing no more than 28 kg/m² and bolted onto a welded 316L stainless steel box section framework.

Figure 3 shows a diagram of the multi-layered structural composite panels used for the enclosure. These comprise a hydrocarbon jet fi re resistant ablative coating behind an environmentally resistant white gelcoat located on the external surface. These structural sandwich panels also provide protection against blast and wave slam, and incorporate insulation cores to provide a stable internal temperature and noise reduction.

The ability of the structural composite panels to withstand the blast and wave slam loads are assessed using FE analysis. For this pro-

ject, the panel defl ections resulting from those loads had to be less than 50 mm so as to not impact the equipment within the enclosure. Such calculations, which include some verifying hand calculations, also determine the structural loads transferred by the panels to the stainless steel frame for the correct engineering of the fi xings (connecting the panels to the steel frame).

The ability of the frame to resist the transferred loads is also deter-mined by FE analysis, which is run for all the NORSOK load cases listed in Table 1 to ensure that the structure is capable of withstand-ing the blast and wave slam. This also confi rms that details such as doorways, penetrations and connections to the platform deck are within safe design limits. A conservative approach was taken in the analyses by assuming that the energy of blast and wave slam was entirely transferred to the steel frame, whereas the panels are known to absorb energy via elastic response to these loads.

The engineering calculations were carried out to optimise the design of the 316L welded frame in terms of weight whilst ensuring that it withstands all the load cases. Using duplex bracing on the short edges is the most effi cient method for transferring loads between the frame sections.

A reduced weight for the support frame is possible with the composite enclosure because of the load distribution that occurs between the structural composite panels and the stainless steel support frame. This is assessed using Finite Element Analysis (FEA) that combines the structural capacity of the lightweight composite panels and the metallic frame. The blast loading transferred from the composite panels onto the metallic frame of the enclosure is shown on the FEA plot in Figure 2 (with the panels removed in order to see the stress distribution around the frame).

Figure 2 FeA plot of blast load on structural frame (panels removed for clarity)

Figure 3 Multi-layered structural composite panel

Figure 4 FeA plot of blast loaded structural frame showing stress distribution

around doorways and connection areas

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From these analyses, a maximum moment was calculated for each connection to the platform deck. This was provided to the clients’ engineers for them to ensure that sufficient supports are provided on the platform deck to deal with the applied loads. The welded frame structure and associated bolted connections were designed to support the maximum moments via single bolts located in each corner of the base of the enclosure.

The FE analyses and structural calculations were also used to design the lift points so as to deliver a single point lift for the entire equipment skid and meet offshore safety requirements.

An important aspect of the design development process was the use of three dimensional models of the equipment items, so as to develop a 3D model of the enclosure and entire equipment skid. This ensured that all items would fit inside the limited space available within the enclosure and also enabled the creation of an ergonomic layout inside that could accommodate the requirement for an operator to use and check the equipment. This approach allowed us to check the layout inside the enclosure for any clashes and to develop the runs of tubes and cables that would facilitate the rapid commissioning of the equipment. Furthermore, the 3D models facilitated the incorporation of operational features such as ventilation details and cable transits in the correct location to meet the project requirements.

The use of 3D models also allowed us to prepare for the assembly and commissioning of the equipment skid by developing a safe and suitable assembly sequence for the enclosure and all equipment items within the assembled skid (as shown in Figure 6). Figure 7 are photographs of the actual assembly process.

Figure 5 3D model of the equipment inside the enclosure showing tubes & cables and a photograph of the installed equipment

Figure 6 Assembly sequence of the ProTek® enclosure and equipment skid developed using 3D model

Figure 7 Assembly sequence of the ProTek® enclosure and equipment skid at Solent Composite Systems’ premises

3 Conclusion

An equipment skid was developed using a lightweight composite protection enclosure to meet the structural requirements from NORSOK load cases for wave slam, blast and other environmental loads. In addition, the enclosure protects the equipment located inside against 60 minute hydrocarbon jet fires whilst delivering operational benefits of temperature control, ventilation and person-nel access for maintenance purposes using a hinged composite door incorporating a safety panic bar.

The constraint of limited space on the platform was accommodated via the use of structural composite panels of the minimum thick-ness to keep the equipment skid within dimensional requirements. FE analysis techniques were used to demonstrate compliance with NORSOK load cases and to verify the single point lift and single bolt mechanical connections to the platform structure.

The use of 3D modelling techniques ensured that the equipment skid was able to accommodate all equipment items and their asso-ciated services (including tubes and cables) accurately within the

sTrucTural coMposiTe fire & blasT proTecTion enclosure for reducing equipMenT skid WeighT

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Figure 8 3D model of the enclosed equipment skid and a photograph of the skid after commissioning at SCS premises

Table 3 Final weight assessment of the enclosure and comparison to initial estimates

constrained space and permitted the development of the assembly and commissioning method prior to commencing work.

Table 3 provides the final weight assessment and compares it to the initial estimates for the metallic concept and proposed composite design. An overall saving of 5.5 tonnes has been achieved, corre-sponding to a 55% reduction in the target weight as compared to the initial estimate for the metallic concept.

EquipmEnt Skid ElEmEnt

mEtallic EncloSurE

compoSitE dESiGn

actual EncloSurE

Support frame 3 tonnes 1.5 tonnes 1 tonne

Insulated panels 3 tonnes 1.5 tonnes 1 tonne

Equipment 1 tonne 1 tonne 1 tonne

Additional platform support structure

3 tonnes 1.5 tonnes 1.5 tonnes

Total Additional Weight 10 tonnes 5.5 tonnes 4.5 tonnes

weight Saved = 5.5 tonnes

For further information, please contact:

Matthew Chalk

Solent Composite Systems Ltd. Victoria Barracks Cowes Waterfront Venture Quays,

Albany Road, East Cowes, Isle of Wight, PO32 6AA

T: +44 (0)1983 292 602

E: info@solentcomposites.com

W: www.solentcomposites.com